The Properties of Metallic Cobalt
Progress in Materials Science, Vol. 24. pp. 51-142. Pergamon Press Ltd. 1979. Printed in Great Britain.
THE PROPERTIES OF METALLIC COBALT
W. Betteridge formerly Chief Scientist International Nickel Ltd., London, England (Submitted 7 February 1979)
CONTENTS 1. INTRODUCTION 52 2. ATOMIC AND NUCLEAR PROPERTIES 52 2.1 Atomic properties 52 2.2 Nuclear properties 54 3. CRYSTAl_ STRUCTURE AND ALLOTROPIC TRANSFORMATION 56 3.1 Allotropes 56 3.2 Allotropic transformation 58 3.3 Deformation textures, recovery and recrystallization 60 4. PHYSICALPROPERTIES 60 4.1 Optical properties 60 4.2 Thermal and thermodynamic properties 64 4.3 Electrical properties 73 4.4 Magnetic properties 77 5. MECHANICAl_ PROPERTIES 91 5.1 Density 91 5.2 Elastic properties 91 5.3 Hardness 95 5.4 Tensile properties 96 5.5 Compressive properties 97 5.6 Formability 98 5.7 Creep properties 101 5.8 Frictional properties 101 6. SPECIAL FORMS OF COBAI.T 104 6.1 Single crystals 104 6.2 Whiskers 106 6.3 Small particles 107 6.4 Thin films 110 7. ELECTROCHEMICAL PROPERTIES 112 7.1 Reversible potentials 112 7.2 Irreversible potentials 113 7.3 Cathodic processes 113 7.4 Anodic processes 114 7.5 Potentials in fused salts 115 8. CORROSION RESISTANCE AND GAS SOLUBILITY 116 8.1 Oxidation and hot corrosion 116 8.2 Wet corrosion 120 8.3 Gas solubility in cobalt 122 9. LIQUID COBALT 124 9.1 Density 124 9.2 Emissivity 124 9.3 Viscosity 125 9.4 Surface tension 125 9.5 Miscellaneous properties 126 ACKNOWLEDGEMENTS 127 REFERENCES 128
~.P ~,t S 24 2 A 51 52 PROGRESS IN MATERIALS SCIENCE
i. INTRODUCTION
The present article deals with the physical, mechanical and chemical properties of unalloyed cobalt. The subject is not new, and mny compilations have been devoted to it. The most comprehensive of these I-3 date back to 1960-1961, with one excep- tion 4 dating back to 1965. While the following sections draw to some extent on these earlier reference works as well as on general collections of numerical data S-?, they also embody, in a condensed form, the wealth of information that has become available up to 1975. In some cases, corresponding data on iron and nickel are quoted with a view to comparing and contrasting cobalt with these two associated elements. The reader's attention is drawn to the fact that there are occasionally wide variations among the values reported for a given property of cobalt. Many such discrepancies can be accounted for by the varying purity of the metal used for the determination. An uncertain metallurgical structure, resulting mainly from the sluggishness of the cubic-to-hexagonal transformation, may also be responsible for some of the differences, while, particularly with a hexagonal crystal structure, the orientation of the crystals may actually have a marked effect on directional properties.
2. ATOMIC AND NUCLEAR PROPERTIES
2.1. Atomic properties
Cobalt is one of the transition metals, appearing between iron and nickel in the first long period of the periodic table. Its symbol and general atomic properties are as follows: ? Symbol Co Atomic number 27 Atomic mass 58.9332 based on carbon 12.
The electronic structure of iron, cobalt and nickel atoms are compared in Table i. The energy levels of cobalt have been derived from characteristic X-ray frequencies and from photoelectric data; they are as follows : K L I LII LII I M I MII,III MIV,V 7708.9 925.6 793.6 778.6 100.7 59.5 2.9 eV. ±0.3 ±0.4 ±0.3 ±0.3 ±0.4 ±0.3 ±0.3 The temperatures at which electronic transitions occur in atoms have been found by Ham and Samans 8 to be expressed by the equation given below :
where Tn is the absolute temperature of a transition, A is a constant for a given element and n is an integer greater than n o . They report the following constants for iron, cobalt, and nickel : n o A, K Fe 3 12,500 Co 3 13,870 Ni 3 8,910 PROPERTIES OF METALLIC COBALT 53
TABLE i. Electronic structure of iron, cobalt and nickel atoms *
Iron Cobalt Nickel
Ground state -3d64s 2 --3d74s 2 --3d84s 2 Ground state of the atom 5D 4 4F3/2 3F 4 First ionization potential, eV 7.87 7.864 a 7.633 b Ion ground state 6D9/2 3F 4 2D5/2 Second ionization potential, eV 16.18 17.05 18.15 C C C Resonance potentials, eV, c 2.39 2.91 3.18 3.20 3.50 3.64 Resonance lines, A d 5166.29(7D5) 4233.99(6FII/2)3884.58(SD4)
3859.91(5D4) 3526.85(4F9/2) 3670.43(3P2)
~Amer. Inst. Phys. Handbook 7 aNormal state of ion 3d 8. bNormal state of ion 3d 9. CResonance potential is the energy in electron volts (eV) required to raise an atom from the ground state to the lowest excited state. The letter C indicates that there are states of the same parity as the ground state between it and the first resonance state. dThe resonance line is the spectrum line absorbed or emitted in this or the reverse transition.
The characteristic X radiation from cobalt is given in Table 2. The values of the atomic scattering factor are given in Table 3. and the mass absorption coefficients for X-rays and y-rays will be found in Table 4. The more accurate coefficients determined for characteristic X-rays of different target materials are those rele- vant to X-ray crystallographic studies I0
TABLE 2. Characteristic X radiation of cobalt 7
Line A(0.1nm) Line A(0.1nm)
Ka I 1.788965 La I 15.972 K~ 2 1.792850 L~ 1 15.666 KS1, 3 1.62079 L[ 18.292 K65 1.60891 L 17.87 K absorption edge 1.60815 LS3,4 14.31
With regard to the emission spectrum of cobalt the sensitive lines are listed in Table 5, with the relative intensities in both arc and spark spectra. Details of further spectral lines are given in Reference 6, in which over 330 lines varying in wavelength from 9746 to 2011 A are listed. The magnetic properties of the cobalt atom are dealt with in Section 4.4. 54 PROORESS |N MATER;ALS SC;ENCE
TABLE 3. Atomic scattering factor, a(% in A)*
Sin 0/~ f Sin e/% f 0.0 27 7 9.3 0.i 24.1 8 8.3 0.2 19.8 9 7.3 0.3 16.4 0 6.7 0.4 14.0 1 6.0 0.5 12.1 2 5.5 0.6 10.7
*Metals Reference Book 6 aThe atomic scattering factor, f, of an atom of atomic number Z is defined as the ratio of the amplitude of the wave scattered by the atom (taken as the resultant of that scattered by its Z electrons) to that scattered by the electron. It is a function of the number of electrons, Z, the angle of diffraction, 28, and the wavelength, %.
TABLE 4. Mass absorption coefficients of cobalt
For l-rays* For X-rays Energy, Coefficient, a Wavelength, Energy, Coefficient, a Ref. MeV ~/p , cm2/g A keY ~/p, cm2/g 0.5 0.0828 0.098 127 0.287 1 0.0598 0.130 96 0.46 2 0.0418 0.175 75 0.92 3 0.0356 0.200 62.5 1.26 [9] 4 0.0326 0.260 48 2.60 6 0.0303 0.417 30 9.45 8 0.0298 Radiation i0 0.0300 0.560 Ag Ks 21.14 ± 0.ii 14 0.0310 0.712 Mo Ks 40.40 ± 0.32 18 0.0322 1.54 Cu Ka (350) [ i0] 20 0.0328 1.79 Co Ka 61.56 ± 0.50 1.94 Fe Ka 78.0 ± 0.6 2.29 Cr Ka 126.0
*Handbook of Chemistry and Physics s aThe mass absorption coefficient, ~, is defined by the relation I =Io exp ~t, often rewritten I =Io exp ~ pt, ~here I is the intensity of a beam of rays, of incident intensity Io, afte~ passing through a thickness, t, of a material of density p.
2.2 Nuclear properties
The only naturally occurring isotope of cobalt, 59Co, is stable. There are, however, twelve other known isotopes, all of them radioactive. Their mass numbers range from 54 to 64. Their main characteristics are listed in Table 6.
60Co, by virtue of the reasonable half-life, its ready production by neutron bombard- ment of natural cobalt and the general physical and mechanical properties of cobalt metal, is the most widely-used source of radioactivity for such purposes as radio- graphy, radiotherapy, tracer applications, control devices, food sterilization, etc. Details of the production and use of 60Co are given in Chapter 17 of the Cobalt Monograph I and in Chapter 16 of Young's monograph 3. PROPERTIES OF METALLIC COBALT 55
TABLE 5. Sensitive lines of emission spectrum of cobalt 6
Wavelength, A Intensity Arc Spark
3529.813 i000 30 3465.800 2000 25 3453.505 3000 200 3405.120 2000 150 2519.822 40 200 2388.918 i0 35 2378.622 25 50 2363.787 25 50 2307.857 25 50 2286.156 40 300
R = wide self-reversal
TABLE 6. Isotopes of cobalt
Mass Half Radiation Particle energy, b Gamma ray energy, c number life, emitted a MeV MeV T
54 0.2 sec B+ _ M 55 18.2 hr B+,EC 0.26(2.3+) ,0.53(4.9+), 0.253(2+) ,0.477(28+) ,0.935(156+), 1.03(39.5+) ,1.50(53.3+)1.41 (26+) ,1.84(0.6+) ,2.17(4+) 56 80 days B+,EC 0.44(4+),1.50(96+) 0.85(100+),1.24(55+),1.75(24+), 2.30(12+), 2.60(14+), 3.25 (24+) 57 270 days B+ 0.320 0.014(~ = Ii x I0-8sec),0.123(15+) I K 0.011 ]M 1,0138(1+)[ K0.14 IE 2 58m 9 hr no B+,IT -- 0.025 [ large]M 3 58 72 days B+,EC 0.472(15%) 0 805 [ 2.9 x 10 -4[E 2 59 stable none 60m i0.i min B- 1.56(0.28%) 0.059 [K 35]M 3 60 5.3 years B- 0.31,1.48(0.15%) 1.1728 [1.6 x 10-41, 1.3325 [1.2 x i0 -q] 61 99 min ~- 1.00(45%),1.42(55%) =0.5 62m 1.6 min B- 62 13.9 min B- 2.8 I .0(40+), 1.17(100+) ,i. 7(10+), 2.0(15+) 64 <28s B-
*The Reactor Handbook II and Amer. Inst. Phys. Handbook 7 aB+ = positive beta particle e(+) : relative number of gamma rays. 8- = negative beta particle. [ ] : number of conversion electrons per EC = electron capture. gamma ray. IT = isomeric transition. [K] : number of K conversion electrons per b(+): relative number of B+,8-transitions in gamma ray. the group. (%): number of B+,B-transitions per I00 MI,E2: multipolarity of the transition. desintegrations.
Important nuclear properties of the naturally occurring isotope 59Co are as follows: 7
Spin 7/2 (in h/2z units) Abundance 100% 56 P R O G RESS IN M A TERIA LS SCIE N C E
Magnetic dipole moment + 4.62 nuclear magnetons Electric quadrupole moment + 0.4 barns Thermal neutron absorption cross-section (2,200 37.2 barns m/s) Nuclear reactions reported for 59Co are given in reference II
3. CRYSTAL STRUCTURE AND ALLOTROPIC TRANSFORMATION
3.1. Allotropes
There are two allotropic modifications of cobalt, a close-packed-hexagonal form, c, with space group P6J~c, stable at temperatures below about 400°C (673 K), and a face-centred-cubic form, a, with space group Fm3m, stable at higher temperatures up to the melting point. For the latter form, however, a controversy exists as to its stability; several investigators 12-14 believe that there is a second allotropic transformation of cobalt (f.c.c. to h.c.p.) at or near the Curie point, I121°C (1394 K), while others 15-18 have presented evidence that the f.c.c, phase is stable from 450°C (723 K) to at least 1350°C (1623 K). There are indications that the occurrence of an h.c.p, phase at high temperatures is dependent on the presence of interstitial elements such as nitrogen 17 or carbon 14.
Table 7 19-23 summarizes the lattice parameters of cobalt as proposed by various authors for both allotropic forms. They are mostly described as having been deter- mined at ambient temperature. The values quoted by Vincent and Figlarz 22 were obtained on high-purity cobalt (99.999% purity sponge or metal produced by thermal decomposition of salts); they are in reasonable accord with the other values and may be accepted as representative. Table 8 24-25 shows the effect of temperature on the lattice parameters of both polymorphs. For a-Co, the relation between temperature and lattice parameter has been described 16 by the expression
a T = a ° (i + 12.997. i0 -6 T + 2.042.10 -9 T2).
Additional crystallographic parameters deduced from the above data are given in Table 9 26, while the atomic and ionic radii of cobalt are listed in Table I0 6 The stacking fault energy in both allotropes has been determined as a functior of temperature by the node method. The results are listed in Table ii 27. The low values reported confirm the qualitative estimation of Seeger 28, who remarked that the major part of the binding energy in the case of cobalt was associated with the directional (homopolar) interaction of d electrons, which implies that the stacking fault energy be small.
TABLE 7. Lattice parameters of cobalt at ambient temperature in angstrom units (0.I nm)
sCo aCo Reference a c c/a a 2.5074 4.0699 1.6239 3.5442 19 2.5059 4.0695 1.624 - 20 2.5071 4.0686 1.6228 - 21 2.5071 4.0695 1.6233 3.5446 22 - - - 3.5441 23 PROPERTIES OF METTALIC COBALT 57
TABLE 8. Variations of lattice parameters of cobalt with temperature, in angstroms
Temperature s Co ~ Co Reference °C a c a -176.5 2.5022 4.0592 -127.0 2.5033 4.0616 - 75.5 2.5034 4.0655 24
- 14.5 2.5053 4.0685 22.5 2.5064 4.0714 520 3.5688 598 3.5720 1003 3.5900 25 1398 3.6214
TABLE 9. Additional crystallographic data Co ~ Co Ref.
Volume of unit cell, A 3 22.15 44.54 Atomic volume, A 3 11.09 - 11.12 11.13 26
I A = 0.i nm
TABLE i0. Atomic and ionic radii of cobalt, in angstroms*
Cobalt (h.c.p.) a Cobalt (f.c.c.)
Coordination number 6-6 a 12 Interatomic distance 2.49, 2.51 2.51 Goldschmidt atomic radius 1.25 1.26 Pauling singlebond radius 1.157 Goldschmidt ionic radius Co ++ 0.82 Goldschmidt ionic radius Co +++ 0.65 Empirical ionic radius Co ++ 0.72
*for definitions and complementary data, the reader is referred to Smithells' Metals Reference Book 6 athe symbol 6-6 indicates that a given atom has 6 equidistant nearest neighbours and 6 equidistant neighbours lying a small distance farther away. 58 PROGRESS IN MATERIALS SCIENCE
TABLE ii. Variation of stacking fault energy of cobalt with temperature*
Tempe ra ture, SFE, o C erg/cm 2
20 31 ± 5 h.c.p. 150 24.5 ± 5 370 20.5 ± 5
500 13.5 ± 5 f.c.c. 710 18.5 ± 5
*After Ericsson 27
An X-ray method for measuring the proportion of the f.c.c, and h.c.p, phases in metallic cobalt was developed by Sage and Guillaud 29. Subsequently, Lambert and Economopoulos adapted their method for determining the residual austenite content in ferrite or martensite aggregates to the case of the f.c.c, phase in h.c.p./f.c.c. cobalt aggregates 30.
3.2. Allotropic transformation
3.2.1. Thermodynamics and kinetics. The temperature of the allotropic transforma- tion depends on the initial condition and purity of the metal. Dilatometric measure ments on > 99.99% purity metal prepared by electron-beam melting gave the following start temperatures, M s and As, for the transformations on heating and cooling: g ÷ a, 422±8°C (695±8 K); a ÷ E, 387±8°C (660±8 K) 18. More recently 31 tempera- tures of 440±I°C (713±i K) and 405±2°C (678±2 K) were obtained by differential thermal analysis of an annealed 99.9985% purity metal. These values are in agree- ment with the results of extensive M s and A s measurements on 99.998% purity single and "multivariant" crystals 32, which gave an average transformation temperature, ½ (M s + As) , of 421.5°C (694.5 K). They are slightly higher than those determined earlier on lower-purity metal I. It is interesting to note that the allotropic transformation temperature corresponds to the first term of the temperature series (n = 4) given for the electronic transitions (see Section 2.1).
The allotropic transformation is associated with a very low free-energy change, which accounts for its sluggishness. Values of 473 J/mol and 352 J/mol for the ÷ ~ and ~ ÷ E transformations have been reported 32. These are in good agreement with later determinations, which gave 511 and 377 J/mol, respectively 33
The low energy involved in the structural change of cobalt accounts for the fact that the allotropic transformation is often affected significantly by modifications of the testing or processing parameters. For instance, the a ÷ z and ~ ÷ ~ trans- formation temperatures have been shown to depend on the cooling and heating rates through the transformation zone; fast cooling rates depress the transformation temperature34,35; increasing the heating rate has no effect in the case of quenched specimens, but raises the transformation temperature in the case of deformed specimens 36. The relative stability of the two phases has also been shown to depend on the grain size23: when the grain size is very small, as in cobalt sponge, the f.c.e, structure is stable down to room temperature. This size effect was confirmed in later work on aerosol particles 37, and on thin (1500 A thick) cobalt samples 38 and on evaporated cobalt fibres 39 P R O P E R TIES O F M E T A L LIC C O B A L T 59 The question of the influence of plastic deformation on the allotropic transformation of cobalt has been extensively studied. Bibring and Sebilleau 40, in an exhaustive investigation on ~ 99.5% purity metal, demonstrated that, when not preceded by recovery at 300°C (573 K), recrystallization of hexagonal cold-worked cobalt resulted in the formation of a certain amount of cubic phase which never returned to the hex- agonal state at room temperature. These results were contradicted in recent work on 99.999% purity cobalt 41 which was examined in the as-cold worked, fully recovered and fully recrystallized conditions. It was shown that entirely h.c.p, cobalt was never observed at room temperature, a significant amount of me tastable cubic phase being present, independently of the prior thermomechanical treatment applied. For fully recrystallized cobalt, a pronounced reduction in the amount of the latter phase is observed on cycling through the transformation. A much less pronounced effect is observed on cycling the recovered material.
Finally, the effect of the electrodeposition ~arameters on the phase transition in cobalt electrodeposits was recently reviewed 4L. Cobalt deposited on copper single crystals from sulphate solutions having a pH of 2.3 is face-centred cubic. When the pH of the solution is raised to 4, the phase is critically dependent on the thickness, changing from a to E as the thickness exceeds about 350 ~. This trans- formation appears to be instantaneous, and to be caused by the need to relieve pseudomorphic stresses which cannot be released via the initial stacking faults. At low solution pH values, codeposited hydrogen probably inhibits dislocation motion, hence preventing the phase change from taking place:.
According to Bibring and Sebilleau ~0, the [ ÷ a transformation leads to an expansion of 4.2 x 10 -3 in the direction normal to the basal plane and to a contraction of 3 x 10 -4 in the basal plane; the corresponding volume expansion is 3.6 x 10 -3 . Measurements of the density of cobalt from 300 to ]40~C by Lucas 43 revealed a rise in specific volume of 1.4-]..8 x 10 -3 on transformation from ~ to a.
3.2.2. Mechanism. The martensitic nature of the allotropic transformation of cobalt was first identified by Troiano and Tockich ~4, who showed its athermal character Gaunt and Christian45,'by kinetic and crystallographic observations on single crystals, supported this conclusion, and although the details of the mechanism were questioned later by Bibby and Parr 34, observation of a surface relief due to trans- formation shear 46,~7 confirmed that the transformation was martens itic. The orientation relationship between the two phases is: (iii) a //(0001)E <112>~ II
Pole mechanisms for the transformation have been discussed by Sebilleau and Bibring 40, Bilby 48, and Seeger 49. These authors have proposed that the cooling transformation is effected by rotation of a Shockley partial dislocation around a pole dislocation with a screw component of 2a/3
Faulting in relation with the transformation has been investigated by Houska et al. 52 on 99.99% purity cobalt powder obtained by grinding. Both growth faults (which contain three planes of f.c.c, stacking) and deformation faults (which contain four such planes) were found in the h.c.p, phase generated either by the cold work or the cooling transformation. Faulting in the f.c.c, phase could be detected with assurance only after the cooling transformation was under way, but the extent of such faulting is small compared to that in the h.c.p, phase. The observed faulting in both phases is produced mainly by the transformation, and is not inherited from the parent phase. 60 PROGRESS IN MATERIALS SCIENCE
3.3 Deformation textures, recovery and recrystallization
Beckers et al. $3 observe that the pole figures for the (0001) planes of high-purity, deoxidized "malleable" cobalt sheet (cold rolled from 1 to 0.25 mmwith intermediate anneals) showed a definite tendency to crystal orientation in the rolling plane. This had already been observed by Bibring and Sebilleau 40 on cold-rolled metal as well as on turnings.
A mere complete study of the deformation texture developed in cold-worked cobalt has been made by Eppelsheimer and Wilcox 54. Starting from sintered powder or sponge and cold rolling to about 50% reduction developed textures described as (0001)
As regards recovery and recrystallization, the most systematic study seems to be that of Feller-Kniepmeier 56 on high-purity cobalt (99.995%) cold rolled 30% and subjected to 30-min anneals at temperatures from ambient to 600 °C (873 K). Hardness measurements indicated that recovery started at 200°C. Electron microscope examina- tions confirmed that polygonization set in at this temperature, while recrystalliza- tion started at 35~C. Work by Plewes and Bachmann 41 on 99.999% cobalt cold rolled 56% confirmed that temperatures in excess of 35~ C had to be reached to cause primary recrystallization; on the other hand, a treatment of 46 h at 30~C produced a fully recovered structure.
Electrical resistivity measurements by Sharp et al. 57 on 99.999% purity cobalt wire deformed at -18fC (90 K) indicated that some recovery occurs in the temperature region immediately above the deformation temperature, while a second region of recovery is found at -73 to -2~C (200-250 K). Neither of these stages corresponds to any recovery of the mechanical properties. The initial flow stress of a deformed specimen remained unchanged after anneals at all temperatures up to 39~C (666 K) but some recovery was found at this temperature and at 419 °C (692 K). Electron microscopy gave some evidence of recovery after long periods at 27~C (543 K).
Dubois et a1.$8 have indicated a marked influence of purity on the recrystallization temperature of cobalt; 99.7% cobalt recrystallized at 650°C, but after electro- chemical purification and electrodeposition it recrystallized at 400°C.
4. PHYSICAL PROPERTIES
Most determinations of the properties of metallic cobalt have been carried out on relatively massive specimens of cast or wrought metal and while the purity and the method of preparation of the specimens are not always stated, it is assumed that the properties are representative of the normal metal. Special forms of the metal, e.g. powders, thin films, whiskers, etc. are dealt with later.
4.1. Optical properties
The optical properties of metals, particularly the reflectance and the emittance, are of practical importance in some applications, but the more fundamental optical constants are mainly of interest in that they enable the electron structure of the metal to be studied. All optical properties depend critically on the condition of the metal surface. PRO P E RTIES O F M ETA L LIC C O B A L T 61
4.1.1. Reflectance. Reflectance values for cobalt are listed in Table 12. The results of an investigation by Bueche 59 are given in Fig. i; they show the effect of wavelength and angle of incidence on the reflectance of cobalt. Skornyakov and Yefremova 60 have shown that the reflectance for "white light" of electrolytic cobalt, annealed in vacuum at 1000°C and polished under benzol is 63.3%, and that it remains unchanged for at least 22 h when in contact with the atmosphere. The effect of temperature up to 1200°C (1473 K) on reflectance has been studied by Grasse 61, who found that after initial recrystallisation of cold-worked surface had reduced the reflectance from 1.00 to about 0.80 there was no further effect of temperature.
TABLE 12. Reflectance of cobalt Polished; near normal incidence Wavelength, Reflectance, pm %
1.0 67 3.0 77 4.0 81 7.0 93 I0.0 97 12.0 97
Amer. Inst. Phys. Handbook 7
80 50* / =< / ;oo 6O / /f 70°
~: 40 so~ / I -I
20.4 o ,8 1.2 1.6 2.o 2,4 .... Wavelength, ~m
Fig. i. Reflectance of cobalt at various angles of incidence (Bueche59).
Yu et al. 62 measured the optical constants of high purity cobalt in vacuum at normal temperature for photon energies ranging from 0.05 to 12 eV - corresponding to wavelengths from the far infra-red to ultra-violet, and showed that reflectance fell roughly linearly, but with some fluctuations, from almost 100% at the lowest photon energy to about 12% at 12 eV.
4.1.2. Emittance. Emittance is defined as the radiant energy emitted normal to the surface divided by the corresponding emission from a black body. The spectral and total emittances of cobalt are given in Table 13. The spectral emittance in the infra-red region as a function of temperature has been studied by Ward 63 and by Wahlin and Knop64; a change was noted at or near the Curie temperature (Fig. 2). 62 PROGRESS IN MATERIALS SCIENCE
TABLE 13. Emittance of cobalt 7
Temperature, Emittance K
Spectral emittance at 1180 0.39 = 0.665 ~m 1530 0.37
Total emittance 350 0.20 600 0.28 1030 0.74
0.27
1.2 Fm
0.25
___.,.___------
0.23 ,?. E I LU I 0.21
/ J 0.19 ff / / 1 o.f; l 600 800 I000 1200 1400 Tempelalure, °C
Fig. 2. Effect of temperature on spectral emittance of cobalt (Ward63).
Lange and Schenck 65 have measured the spectral emittance of cobalt and other metals over a range of temperatures embracing the melting points, and shown a sharp increase as the metals melt. With the specimens in an atmosphere of purified hydrogen and using a wavelength of 0.65 ~m the emittance of cobalt was 0.20 between 1200 °C and the melting point, and 0.32 to 0.33 between the melting point and 1700°C (1973 K).
A more complete study of the spectral emittance of high-purity cobalt over the temperature range 1200-1500 K and between wavelengths of 0.50 and 0.65 ~m, and also of the total emittance has been reported by Jain et al. 66 The values depend critically on the surface condition and after evaporating some metal from the surface in vacuum the results shown in Fig. 3 were obtained. The total emittance was found to be almost constant throughout the temperature range covered, rising from about 0.21 at 1200 K to about 0.213 at 1500 K. PROPERTIES OF METALLIC COBALT 63
0.45 [_1200 K
1250 K "~
0.40 ~,
~ 0.35 "%~ ~" 03~- 15oo K
0.30 052 0.54 0.56 0.58 0.60 0.62 0 64 0.66 Wavelength, #m
Fig. 3. Spectral emittance of cobalt as a function of wavelength (Jain et al.66).
4.1.3. Optical constants. The fundamental optical constants of a metal are the index of refraction, n, and the extinction coefficient, k, which are related to the conductivity, o, and the dielectric constant, s, by the relationships: g = n2 - k 2
and o = nkv where v is the frequency of the light.
Care must be taken not to confuse k, the extinction coefficient, with K, the absorption index used in older literature, which is related by the expression k = n.K.
Table 14 gives these constants for polycrystalline cobalt.
TABLE 14. Optical constants for cobalt 7
Wavelength, Refractive index, Extinction coefficient, Reflectance, ~m n k calculated
2.50 5.10 7.80 0.7919 4.00 4.70 Ii.00 0.8775 6.00 5.00 17.50 0.9416 8.00 5.80 24.00 0.9627 i0.00 7.10 29.50 0.9697 15.00 11.20 40.00 0.9750 20.00 15.20 51.70 0.9793
Values for evaporated cobalt films over the wavelength range 0.113 - 2.16 vm are also given in reference 7. Johnson and Christy 67 have made similar measurements on evaporated films of a number of transition metals, including cobalt, and have studied the effects of oxidation, film thickness and evaporation rate.
Measurements of this nature are mainly of interest in studies of the outer electron configuration of metals.
4.1.4. Magneto-optical rotation. When a linearly polarized ray of light of wave- length ~o in vacuo, traverses an inactive medium of thickness t, in the direction 64 P R O G R E SS IN M A T E RIALS S CIE N C E
of an external magnetic field of strength H, the rotation, ~, which the ray exhibits as a result of the field is expressed by : = VtH where ~ is generally expressed in minutes of arc, t in cm, and H in oersteds; V is the so-called "Verdet Constant" of the medium. In ferromagnetic media the rotation tends to reach a saturation value a~ as the external field is increased. For the films of the ferromagnetic metals magnetized to saturation and using red light, the values of ~/t expressed in degrees per cm are 7 : iron 209,000 cobalt 198,000 nickel 89,000
4.2. Thermal and thermodynamic properties
4.2.1. Melting and boiling points. Reported values of the melting point of pure cobalt range from 1492 to 1495°C (1765 to 1768 K) with a preponderance of opinion favouring 1495°C as the most likely value 4. The differences are probably due to impurities and to the effect of supercooling, which has been shown by Turnbull and Cech 88 and by Fehling and Schei169 to depend on the extent of the heating above the melting point. The reported values of the boiling point at normal pressure are far more variable, whether determined directly or by extrapolation of vapour pressure measurements. W1nterhager• and KrUger. 4 summarizedo results ranglng. from 2477 to 3100 o C (2750 to 3373 K) and selected the value given by Vintaikin and Tomash 70 (2802°C; 3075 K) as the most reliable. A more recent value of 275~C (3032 K) derived from the vapour pressure measurements of Winters et al. 71 is in reasonable agreement with this. A value of 2800 ± 50°C (3075 ± 50 K) would seem to be a fair assessment.
4.2.2. Vapour pressure. Measurements of the vapour pressure of cobalt have been made by a number of investigators, generally over quite restricted ranges of temp- erature. Table 15 summarizes the results obtained. For the temperature range II00-1250°C (1373-1523 K) Winterhager and KrUger prefer the results of Vintaikin and Tomash, and the results of Winters et al. for a higher temperature range are consistent with these. The interpolated values given by Hultgren 76 are quoted in Table 16.
4.2.3. Heat capacity. The most reliable measurements of specific heat of cobalt between normal temperature and about 1600°C (1873 K) are those reported for 99.5% cobalt by Braun and Kohlhaas 77. Their values do not differ appreciably from the values included with other thermodynamic data in Table 18. For sub-normal tempe- ratures the values quoted by Hultgren et al. 76 are given in Table 17. Measurements over the range 1.2 to 4.2 K by Dixon et al. 78 gave a value of 4.38 ± 0.01 mJ/K mole.
Kubaschewski et al. 79 gave the following empirical expression for the heat capacity of cobalt: Cp = a + bT + cT -2 with T in kelvins, and the constants for Cp in J/K mole are as follows : Temperature Range, K a b.lO -3 c.lO 5
440 - 650 21.4 14.3 - 0.88 718 - 1400 13.8 24.6 1400 - m.p 40.2 m.p - 1900 40.4 PROPERTIE OF METALLIC COBALT 65
% % I I I~ oJ o ,q~ o8 o o
C~
M ~ M r~
I
CO O-J '~1 II 0,1
1 ~ I
~I" 0 M '--' M P"
o ~1 0,1 ,'~ II I • I ~-~
C::' 0 M ~ ~q h-
o 4J o t~
,--4 II
q~
~ 0
0 ~., o4 I ~ r-. ~ °
,--4 % ~ ,---I II
,,-.i 0-, 0 ¢1 ~D ~1 .,,,-I
0 ~1 ~ ~ ~ .~ o 66 PROGRESS IN MATERIALS SCIENCE TABLE 16. Vapour pressure of cobalt 76 Vapour pressure, atm. .Temperature °C K
I0 -I0 983 1256
10 -9 1061 1334
10 -8 1148 1421
i0 -? 1249 1522
10 -6 1365 1638
10 -5 1501 1774
10 -4 1670 ]943
10 -S 1876 2149
10 -2 2133 2406
i0 -I 2462 2735
1 2901 3174
TABLE 17. Heat capacity of cobalt at low temperature 76 Temperature Heat capacity, Cp K J/Kmol 2 0 0102 3 0 0157 5 0 0278 7 0 0427 i0 0 072 15 0 149 20 0 318 25 0 569 3O 0 980 40 2 34 5O 31 70 8.58 85 ii .4 i00 13.9 125 17.2 150 19.5 175 21 .I 200 22.4 250 24.0 298.15 24.8 PROPERTIES OF METALLIC COBALT 67
4.2.4. Heats of transformation, fusion and vaporization. The heat of transformation for the ~ ÷ ~ phase change will be found in Section 3.2.1. The clearly marked peak in specific heat in the region of the Curie temperature , which is shown by the figures in Table 18, makes it extremely difficult to identify any heat emission associated with the proposed structural transformation in the vicinity of that temperature (see Section 3.1). Values of this heat of transformation have been quoted in earlier literature 79, but their reliability is very uncertain. Khandros and Bogolyobov 80 determined the heat of vaporization above and below the Curie point and found that although the absolute values varied considerably from sample to sample the differential heat of vaporization was almost constant at 50 kJ/mole. The heats of fusion and of vaporization at the normal boiling point given by Hultgren et al. 76 were: Heat of fusion 17.2 kJ/mole Heat of vaporization 424.9 ± 4 kJ/mole
4.2.5. Thermodynamic functions of cobalt. Data on the heat capacity, heat content, entropy and other thermodynamic functions of cobalt in t he solid and liquid states up to 3200 K and also as an ideal gas phase are given in Tables 18 and 19. These are taken from Hultgren et al. 76 and converted to S.I. units.
4.2.6. Thermal expansion. The most complete direct measurements of the thermal expansion coefficients of polycrystalline cobalt are still those of Fine and Ellis 81 given in Table 20. Results recently quoted by Krajewski et al. 82 and by Arbuzov and Zelenkov 83 are also given in this Table. A sharp peak in the thermal expansion coefficient occurs at the allotropic transformatiov temperature; it is related to the volume expansion which accompanies the e ÷ a transformation (see Section 3.2.1). Krajewski 84 using a high-temperature vacuum dilatometer failed to detect any anomaly associated with the proposed transformation near tile Curie temperature.
Masumoto et al. 85 have studied the expansion characteristics of single crystals of hexagonal cobalt at temperatures between -I00 and 350°C (173 and 623 K) and derived the coefficients given in Table 21. It is clearly possible that some of the differences revealed in Table 20 might be due to variations in preferred orientation of crystals in the samples studied.
4.2.7. Thermal conductivity. Powel186, 87 and his co-workers have made measurements on high-purity material (99.92%) at temperatures between i00 and 423 K and the results, given in Table 22 are closely confirmed by more recent work by Laubitz and Matsumura 88 on 99.999% cobalt, extending the temperature range to 1250 K.
Measurements at very low temperatures have been recorded by Rosenberg 89 and at temperatures from 400 to 1600 K by Zinovev et al. 90, and these are also included in the Table. The results of the latter workers show a minimum in conductivity in the vicinity of the Curie temperature. The results quoted form a reasonably consistent set of values.
4.2.8. Diffusion coefficients. The rate of diffusion of one material into another at a constant temperature is given by J = D. d__cc dx where J is the transport velocity, dc/dx the concentration gradient and D the diffusion coefficient. The variation of D with temperature follows an Arrhenius- type equation : D = Doe-Q/R'~r"
Where D o is the diffusion constant (or frequency factor) for the particular process and Q is the activation energy. The self-diffusion of a metal is determined by the diffusion of a radio-active isotope in an inactive matrix of the same metal. Values of D o and Q quoted for the self-diffusion of cobalt and for its diffusion
~.M s 24,2 . 68 PROGRESS IN MATERIALS SCIENCE
TABLE 18. Thermodynamic functions for solid and liquid cobalt 76
Heat capacity Heat content Entropy Free energy function Temperature 6~ H T - Sst ST -- SSt - (FT - Hst) T K J/K mol kJ/mol J/K mol J/K mol
298 24.8 0 0 30.06 4OO 26.5 2.62 7.54 31.07 5OO 28.2 5.36 13.65 32.99 600 29.9 8.27 18.92 35.22 700 31.6 11.33 23.66 37.52 700 31.2 11.76 24.33 37.52 8OO 32.8 14.99 28.60 39.00 900 34.9 18.38 32.58 42.08 i000 37.4 21.99 36.38 44.42 ii00 40.1 25.84 40.07 46.59 1200 43 5 30.06 43.66 48.70 1300 48 2 34.62 47.35 50.75 1400 49 8 39.69 51.17 52.67 1500 41 0 44.00 54.10 54.79 1600 39 9 48.02 56.69 56.73 1700 39 1 52.00 59.07 58.58 1800 (37 7) 72.96 71.02 60.50 1900 (37.7) 76.76 73.05 62.73 2000 (37 7) 80.52 74.98 64.78 2200 (37 7) 88.05 78.54 68.58 2400 (37 7) 95.58 81.84 72.05 2600 (37 7) 103.1 84.87 75.26 2800 (37 7) 110.7 87.62 78.15 3000 (37 7) 118.2 90.22 80.90 3200 (37 7) 125.7 92.70 83.45
Heat capacity at i atm. pressure Standard state at 298.15 K PROPERTIES OF METALLIC COBALT 69
TABLE 19. Thermodynamic functions for cobalt as an ideal monatomic gas 76
Heat capacity Heat content Entropy Free energy function
Temperature Cp H T - SSt S T - SSt - (fT - HSt ) T K J/K mol kJ/mol J/K mol J/K mol
298 23.05 0 0 179.53 400 24.52 2.425 6.99 180.46 500 25.45 4 ~28 12.56 182.24 600 25.91 7 499 17.25 184.29 700 26.12 i0 i0 21.28 186.37 800 26.21 12 72 24.74 188.39 900 26.28 15 34 27.84 190.33 1000 26.34 17 97 30.61 192.18 Ii00 26.42 20.61 33.12 193.93 1200 26.50 23.26 35.43 195.59 1300 26.58 25.91 37.56 197.16 1400 26.64 28.58 39.53 198.66 1500 26.69 31.24 41.37 200.08 1600 26.73 33.91 43.12 201.44 1700 26.74 36.58 44.72 202.73 1800 26.73 39.26 46.26 203.97 1900 26.72 41.93 47.69 205.15 2000 26.69 44.61 49.08 206.29 2200 26.60 49.92 51.58 208.44 2400 26.49 55.24 53.94 210.42 2600 26.39 60.53 56.02 212.28 2800 26.31 65.80 58.00 214.02 3000 26.26 71.05 59.78 215.64 3200 26.26 76.32 61.52 217.17
Heat capacity at i atm. pressure
Standard state at 298.15 K 70 PROGRE~SS IN MATERIALS S'CIENC'E
TABLE 20. Linear thermal expansion of cobalt
Temperature Coefficient of expansion °C K -I x 106
Ref. 81 Ref. 83 Ref. 82
I00 - 200 14.2 - - i00 - 300 - - 12.7 200 14.2 12.8 - 200 - 300 14.2 - - 300 - 13.7 - 300 - 400 14.8 - - 300 - 800 - - 17.4 400 15.7 14.9 - 600 16.0 15.2 - 750 16.8 16.5 -
TABLE 21. Thermal expansion of single-crystal hexagonal cobalt 85
Crystal direction
Temperature °C 0001 i012 I010 Ii~0 Polycrystal
Coefficient of expansion, K -I x 106
- I00 14.26 12.85 10.86 10.85 11.94
20 14.62 12.95 10.97 10.96 12.14
350 15.61 13.70 11.30 11.29 12.71
in iron and nickel are given in Table 23; they have been selected on the basis of the purity of the starting materials used. Results reported by Lange et al. 99 and by Elyutin I00 have shown that the self-diffusion of cobalt is very much faster in powder metallurgy samples than in normal melted samples, the magnitude of the difference falling with increasing temperature; according to the former authors it is about i0 times faster at 1000°C (1273 K). Elyutin finds that both the constant D o and the activation energy Q are very markedly changed. The difference is attributed to high diffusion rates along grain boundaries and the internal surface~ of pores. It is to be expected that diffusion rates in cast cobalt will also vary with grain size.-
Some investigators have concluded that the diffusion constants are anomalously affected by the magnetic transformation of the matrix in which diffusion is proceeding, the activation energy in particular being greater below the Curie temperature than above (see Table 23). Hirano et al. 92 suggested that for self- diffusion of cobalt the change in activation energy was given by : A Q = Qferromagnetic - Qparamagnetic ~ RTc. PROPERTIES OF METALLIC COBALT 71 TABLE 22. Thermal conductivity of cobalt
Temperature, Thermal conductivity, Lorenz function, Reference K W/mK 108V2/K 2
2 29 - 89
4.2 56 -
I0 128 -
15 181
20 224
25 260
30 280 -
35 286
40 273
43 262
i00 150 87
150 130
200 117 1.90
300 i00 1.95
350 93 - 323 92 2.11 86
373 85.5 2.11
423 79 2.13
400 90 - I00 90
I000 ~ 50 -
1600 % 40 -
i00 164.3 88
150 134.1
200 119.3
250 108.4
300 98.9
400 83.1
500 71.5
700 56.9
i000 51.1
1200 46.4 ?2 PROGRESS IN MATERIALS SCIENCE
TABLE 23. Diffusion of cobalt
Diffusion Activation Temperature Purity Systema constant, Do, energy, Q, range, Reference of cm2/s kcal/mol °C solute, %
D Co 0.83 67.7 ii00 - 1400 91 99.9 Co 0.50 65.4 772 - 1048 92 ~ 99.5 0.17 62.2 1192 - 1297 2.20 70.5 1057 - 1306 93 ~ 99.95
D Co 64.4 64.6 800 - 905 94 Armco ~Fe 9.5 62.3 800 - 900 95 > 99.9 7.19 62.2 683 - Tc 96 ~ 99.95 6.38 61.4 Tc - 884
D Co yFe 1.0 72.1 1130 - 1360 97 99.999
Co D6F e 6.38 61.4 1428 - 1521 96 ~ 99.95
DCo Ni 0.59 64.4 850 - 1370 97 99.999 0.55 63.4 580 - 980 98 99.99
a ~ represents the diffusion of A in B
To convert kcal/mol to kJ/mol multiply by 4.19
Hirano and Cohen TM have also indicated that diffusion varies with co,position, both the activation energy and the frequency factor reaching maxima in a and X phases of the Co-Fe system at about 50 at % cobalt.
4.2.9. Surface energies. The surface energy of cobalt in hydrogen and the energies of grain and twin boundaries have been measured at 1345°C (1618 k) by Bryant eta/. I02 who reported values of 1.970, 0.650 and 0.0127 J/m 2 respectively. More recent work by Roth I03 gave values at 1454°C (1727 K) of 2.250 ± 7% and 1.020 ± 7% J/m 2 for the surface and grain-boundary energies respectively; in helium instead of hydrogen the energies increased to 2.670 and 1.220 J/m 2 respectively.
Wawra I04 has determined the free surface energy of isotropic polycrystalline cobalt by an ultrasonic technique with the following results, which extrapolate to give close agreement with the value of Bryant et al. Temperature Surface energy K J/m e 4 2. 766 173 2. 735 273 2. 704 323 2.685 423 2.644 523 2.596 PROPERTIES OF METALLIC COBALT 73
4.3 Electrical properties
4.3.1. Electrical resistivity. Many studies of the electrical resistivity of cobalt have been made and all the measurements are in close agreement at temperatures above normal. Recent measurements on cobalt containing only about i0 ppm total impurities have been reported by Kierspe eta/. I05 and by Laubitz and Matsumural06; their results are given in Table 24. Cubic s-cobalt has a lower resistivity than hexagonal e-cobalt and hence hysteresis may be shown, particularly with commercial- purity metal because of the sluggishness of the allotropic transformation. An inflection in resistivity is also shown in the vicinity of the Curie point.
TABLE 24. Electrical resistivity of cobalt
99.999% Co I05 99.999% Co I06
Temperature, Resistivity, ~p/6f Temperature, Resistivity, °C lO-8~m 10-11~m/°C K I0-8~ m
-196 0.44 90 0.744 -i00 2.65 - I00 0.939
- 50 3.84 125 1.461 0 5.25 30 150 2.018 50 6.81 34 200 3.214 i00 8.52 39 250 4.827 200 12.80 50 300 5.995 300 18.38 60 400 9.542 400 24.72 71 500 14.118 500 30.65 75 600 19.872 600 39.05 87 700 26.590 700 48.20 97 701 25.015 800 57.92 105 800 32.056 900 68.20 105 900 40.377 i000 78.45 i00 i000 49.562 ii00 87.45 81 Ii00 59.259 1200 92.65 41 1200 69.116 1300 96.52 34 1250 73.995 1400 99.95 34 1300 78.78 1400 87.17 1500 91.37 1600 94.86 1700 97.62
Cobalt does not show superconductivity at temperatures down to 0.06 K according to Thomas and Mendoza I07, and other workers confirm this conclusion. Measurements of resistivity at sub-normal temperatures supplementing those included in Table 24, are given in Table 25. ?4 PROGRESS IN MATERIALS SCIENCE TABLE 25. Low-temperature resistivity of cobalt
Temperature, Resistivity, Purity Reference K 10-8~m
0 0.0904 Spectro Ii0 4 0.215 99.998 iii 20 0.198 Spectro 89 295 5.85 Spectro ii0
Equations have been proposed for the resistivity of cobalt at very low temperatures, including that of Radhakrishna and Nielsen I08 for the temperature range 1.2 to 6 K: R = R o + RIT + R2 T2 where R o = 870.004 x 10-12~m R I < 0.0313 x 10-12f~n T -I R 2 = 0.09892 x 10-12~m T -2
Over the temperature range for which the expression applies the first term is by far the most important.
The effect of impurities on the low-temperature resistivity has been emphasized by the work of Dubois eta/. I09 who used ion exchange methods to purify cobalt. This reduced the nickel content of the metal from 1500 ppm to 0.5 ppm and reduced the ratio of the resistivity at 20.3K to that at 294 K from 170.10 -4 to 62.10 -4 . Rosenberg 89 (see Table 25) using spectrographic standard metal found the ratio to be 340.10 -4 . More recently Dubois 31 has indicated that carbon and oxygen play a predominant role in the effect of impurities in the low-temperature resistivity.
Measurements by Masumoto eta/. I12 of the resistivity at room temperature of single crystals of hexagonal cobalt of 99.8% purity showed marked differences in different crystal directions. In the <0001> direction the resistivity was 10.280 x 10-8~m while in the <|0~0> direction it was 5.7 x lO-S~m. At an angle ~ to the <0001> direction the resistivity is given by the expression 0~ = 0± + (~ - 01) Cos 2 where 0~ and 0i are the resistivities in the <0001> and <1050> directions respectively.
The effect of pressure on the electrical resistivity has been determined at 2~C (293 K) by Bridgmanll3; his results are given in Table 26.
The results of an investigation by Bates I14 of the magneto-resistance effect in cobalt are shown in Fig. 4. The material used was 98.4% cobalt, containing as the major impurities 0.45% nickel, 0.13% iron and 0.19% carbon, and was in the form of 2.6 mm diameter hard-drawn wire. Two points at the left of the curve for the transverse field lie above the H axis, but this is merely because the specimen cannot be placed absolutely at right angles to the applied field. Masumoto eta/. I12 studied the magneto-reslstance effect in their work on single crystals of 99.8% cobalt and observed similar results to those of Bates, although in the <0001> direction a saturation value of A0/p of about 0.4% was reached at a field strength of only about 200 Oe (16,OOO A/m): Bates' results showed a maximum of about 0.35% at a field strength of about 2500 Oe (200 kA/m). PROPERTIES OF METALLIC COBALT 75
TABLE 26. Relative resistivity and compression of cobalt, as a function of pressure
Pressure, Cobalt kg/cm 2 Pl~o VlVo
0 i .000 1.000 i0000 0.991 0.99496 20,000 0.983 0.98994 30000 0.976 0.98508 40000 0.970 50 000 0.965 60 000 0.961 70 000 0.958 80 000 0.955 90 000 0.953 i00,000 0.951
*After Bridgman 113
c Longit-udinol fields u} --... ~ ,o 1 g g al o o .~
o_ Transverse fields
-2 0 4 8 12 16 Field strength, I0 5 A/m
Fig. 4. Magneto-resistance of cobalt (Batesll4).
The magneto-resistance properties of single-crystal cobalt whiskers in different crystallographic directions have been studied by Coleman and Morris I15 at temperature below 4.2 K. The ratio Ap/p reaches saturation for most field and current directions
4.3.2. Thermoelectric properties. The thermoelectric properties of a metal can be expressed by the equation = dE/dT where e is the absolute thermoelectric power, E is the thermoelectric potential and T the temperature. The thermoelectric voltage developed between two different metals with junctions at temperatures T 1 and T2, i.e. the Seebeck effect, is given e I and e 2 are the absolute thermoelectric powers of the individual metals and are normally expressed in units of DV/K. The values for pure cobalt have been reported by Laubitz and Matsumara I08 over the temperature range 90-1250 K and by Howe and Enderby I16 in the vicinity of the melting point. The results are given in Table 27. Similar results have been reported by Rudnitskii 117 using spectroscopically pure material who confirmed the jump at the ~ + a phase change and observed an inflection at the Curie point. His results are shown in Fig. 5 76 PROGRESS IN MATERIALS SCIENCE
TABLE 27. Absolute thermoelectric power of cobalt
Temperature, K g,pV/K Structure Reference
90 -7.49 ~ 106 i00 -8.43 E 106 125 -10.97 ~ ]06 150 -13.77 s 106
200 -19.75 c 106
250 -25.54 E 106 300 -30.82 E 106 400 -39.22 ~ 106 500 -44.84 c 106
600 -47.78 c 106
700 -48.05 ~ 106
701 -42.56 ~ |06 800 -41.82 ~ 106 900 -39.60 ~ 106
i000 -35.88 ~ 106 ii00 -30.69 ~ 106 1200 -24.00 ~ 106 1250 -20.10 a 106
]~n solid -i0 116 Tm liquid - 3 116
Temperoture, °C 0 200 400 600 800 IO00 1200
I ! ! ! i m
x~f -IO / o~
:::L -20 / -30 x~--x-x/
-40
Fig. 5. Absolute thermal e.m.f, of cobalt (RudnitskiillT). PROPERTIES OF METALLIC COBALT 77
For practical purposes the thermoelectric force developed between cobalt and platinum as a reference metal is required, and values over the temperature range 0-1200°C (273-1473 K) are given in Table 28. The negative sign indicates that the current flows from the platinum to the cobalt at the cold junction.
TABLE 28. Thermoelectric force of cobalt V platinum I18
Temperature, °C Thermoelectric force, mV
0 0
i00 - 2.51 200 - 5.51 300 - 9.03 400 -12.81 500 -16.18 600 -18.80 700 -21.13 800 -22.81 900 -23.70 I000 -23.75 ii00 -23.00 1200 -21.75
The associated Thomson effect is the evolution or absorption of heat when current flows along a thermal gradient in a single conductor; the coefficient ~ is defined by the relation Q=-" o I "~7dT where Q is the heat evolved or absorbed and I the current. The Thomson coefficient for cobalt has been reported by Rudnitskii 117 and his results are given in Table 29.
4.3.3. Electron work function. The electron work function of cobalt measured by different effects as given in reference 5 is as follows : Thermionic effect 4.40 eV I19 Photoelectric effect 3.90 eV 120 4.12-4.25 eV 121 Contact potential 4.21 eV 122
Wahlin's data 119 were determined above 850°C (1123 K), where cobalt is face-centered cubic. The photoelectric emission of cobalt (99.9+% purity) as determined by Cardwel1121 showed very abnormal behaviour at a temperature of approximately 850°C (1123 K), probably due to the presence of surface oxide films.
4.4. Magnetic properties
4.4.1. Spontaneous magnetization, Curie temperature, paramagnetic properties and gyromagnetic ratio. The spontaneous magnetization of cobalt at absolute zero is equal to 1.72 Bohr magnetons (~B), which corresponds to an absolute saturation 78 PROGRESS IN MATERIALS SCIENCE TABLE 29. Thomson coefficient for cobalt II?
T Thomson coefficient, °C ~V/°C
i00 -39.5 200 -34.0 E phase 300 -22.9 400 -12.8 447 -I0.i
447 0 500 0 600 3.5 phase 700 16.5 800 36.5 900 62.2 i000 90.5 1085 129.2
1085 21.8 ii00 22.0 Non-nmgnetic 1150 22.8 1200 23.6 magnetization, Oo, of 162.5 G g-I (1625 T kg-l) 123. The temperature dependence of this spontaneous magnetization is shown in the reduced co-ordinate plot of Fig. 6124; as is the case for other ferromagnetic materials (see Table 30), this property first decreases very slowly, then more rapidly until it vanishes at the ferromagnetic Curie temperature. No change in saturation is apparent at the allotropic transformation temperature, although a 1.5% increase has been reported for a single crystal of 99.9% purity on transforming from h.c.p, to f.c.c. 124 The effect of pressures up to 430 MPa (4.3 kbar) on the room-temperature saturation magnetization, Us, of cobalt has been measured125; the relative change of u s with pressure (dos/dP)Os -I has been found to be equal to -2.18 x 10 -4 kbar -I.
The Curie temperature has been determined by direct observation on high-purity cubic cobalt as 1121 ± 3°C (1394 K) 124; observation based on resistivity, X-ray diffraction, specific heat and thermal e.m.f, all support this value, Pressures up to 6 GPa (60 kbar) have no effect on the Curie temperature 126. By extrapolation of low-temperature data, values for the Curie point of hexagonal cobalt of 79~C (1070 K) 127 and 877°C (1150 K) 128 have been obtained.
At temperatures above the Curie point, cobalt is paramagnetic. The temperature dependence of the reciprocal paramagnetic susceptibility is depicted by a straight line (Fig. 7) 129 , corresponding to the equation i _ T - 0p X C where Op, the paramagnetic Curie temperature, is I140°C (1413 K) and C, the atomic PROPERTIES OF METALLIC COBALT 79
Curie constant, is 1.36130 . No inflection is observed at the melting point. These results are in agreement with those determined by Kohlhaas 131 between 1250 and 1600°C (1523 and 1873 K). Paramagnetic susceptibility measurements over a much narrower range of temperature just above the Curie point 132 have led to the somewhat different expression (!) n X = p(T - Op) with 8_ = II14°C (1387 K), I/n = 1.32 + 0.02, and ~ = 5.73 -+ 0.08 Similar work by Gelssler et al. 133 gave ep = I122°C (1395 K) and i/n = 1.20_+ 0.04.
>, ~t b a
g
I I i i i 300 400 500 GO0 Temperoture, °C
Fig. 6. The variation of the spontaneous magnetization at the transformation temperature of cobalt (Myers and Sucksmith124). Polycrystalline: a, heating; b, cooling. Single crystal: c, heating.
TABLE 30. Saturation magnetic moments and Curie temperatures of the ferromagnetic elements*
Number of Magnetic moments Curie temperature, Bohr magnetons at Element per atom, a 20°C(os). 0 K(oo). no gauss/g gauss/g °C K (10-1T/kg) (10-1T/kg)
Fe 2.218 217.75 221.89 770 1043 Co 1.714 161 162.5 1121 1394 Ni 0.604 54.39(15 °C) 57.50 358 631 Gd 7.1 -- 253.5 16 289
*After Bozorth 123 alBohr magneton = 0.927 x 10 -20 erg/gauss. 80 PROGRESS IN MATERIALS SCIENCE 12 /
,¢ , 8 0 × / 4 / 0 1373 1573 17"73 1973 Tempereture, K
Fig. 7. Variation of the reciprocal paramagnetic suscepti- bility of cobalt as a function of temperature (Wachtel and Urbain129).
Gyromagnetic ratio. The gyromagnetic ratio, g', is defined by the relation: J mc 2 M e g'' where J/M is the ratio of the angular momentum to the dipole moment of the electron which contributes to the spontaneous magnetization as measured by the Einstein-de Haas effect or a similar experiment. Lande's splitting factor, g, is defined by the same relation but the measurement rests with a ferromagnetic resonance experi- ment. The difference between g and g' is probably accounted for by the fact that the spin-orbit coupling is different in each case. For iron, cobalt and nickel, the factors g and g' are as follows ? :
Iron Cobalt Nickel Lande's splitting factor, g 2.17 2.22 2.19 Gyromagnetic ratio, g' 1.93 1.837 1.91
The value of g' for cobalt is unaffected by changing from the hexagonal to the cubic structure 134.
For other magnetic characteristics, the reader is referred to Section 4.1.4 (Magneto- optical rotation) and Section 4.3.1 (Magneto-resistance).
4.4.2. Magnetization, magnetocrystalline anisotropy and domain structure. Single crystals of ferromagnetic materials are magnetically anisotropic, including even those with cubic crystal symmetry. The magnetization curves, obtained in different crystallographic directions on single crystals of hexagonal cobalt are shown in Fig. 8. The work that must be done to magnetize a specimen is proportional to the area comprised between the magnetization curve and the I axis. Thus it clearly appears that, for cobalt, the hexagonal axis <0001> is the direction of easiest magnetization at room temperature.
On heating hexagonal cobalt, the magnetization in the easy (axial) direction becomes more difficult, while in the perpendicular direction <1010> it becomes easier (Fig. 9). When a temperature of about 250°C (523 K) has been reached, hexagonal cobalt is nearly isotropic with regard to its magnetization123, 136. At higher temperatures, the hexagonal axis becomes the direction of most difficult magnetization.
For cubic cobalt magnetization curves are similar to those for nickel in that the crystal directions in the order of increasing difficulty of magnetization are PROPERTIES OF METALLIC COBALT 81
I I I I m 8 < 0001 > - 1200
N 800
400
i I I I 2000 4"000 6000 8000 I0000
Field strength, Oe
Fig. 8. Magnetization curves of single crystals of hexagonal cobalt at 20°C (Kaya135).
. 1600 I I I I I I <0001 > <~oTo% ' 0= .~-- <,, ~o> ._N 1200' <0001> I= ~10[0> / ~ 8o0
200oc 500 °C ~" 40o
E I I I I f J I I 2000 4000 6000 8000 10000 0 2000 4000 6000 8000 I0000 Field strength, Oe FLeld strength, Oe
Fig. 9. Effect of temperature on the magnetization of sin$1e crystals of hexagonal cobalt (Honda and Masumotolo6).
The subject has been studied by many workers both by direct magnetic measurements on single crystals 137-139 and in relation to the patterns of magnetic domains revealed by Bitter powder techniques 140 and by the Kerr magneto-optical effect 141,142 To understand the magnetization processes fully it is necessary to have a detailed knowledge of the easy directions of magnetization and the corresponding domain structures. At normal temperatures, where the hexagonal axis is the easy direction of magnetization, it is established that the magnetic domains consist of simple plane layers parallel to the basal planes, magnetized anti-parallel to each other; the formation of these is connected with planar dislocation arrangements (dislocation lattices) which develop during the ~ ÷ E transformation and are enhanced by plastic deformation. The structure is unchanged as the temperature rises to about 245°C (518 K), except for a small variation in dimensions. The domain width also increases slightly on straining along the c axis within the elastic range 143 82 PROGRESS IN MATERIALS SCIENCE
1400 I I I <111>
1200
IO00
800
600 550oc
1200
4- .~ I000 \<110> g -<100> o E 8O0 % 750°C :~ 600 800
soo
400 IO00°C 2OO I I I 0 250 500 750 I000 Field strength, Oe
Fig. I0. Magnetization curves of single crystals of face- centred-cubic cobalt (Sucksmith and Thompson137).
Within the termperature range about 245-32~C (518-598 K) the directions of easy magnetization become a cone around the hexagonal axis with the angle between the easy direction and the c axis increasing from 0 ° to 9~, while the domain structures are dependent on the applied field and internal stresses. Above 32~C (598 K) the easy direction is perpendicular to the c axis and there are wide curved domains determined by the internal stress pattern in the cobalt.
For hexagonal crystals, the equation expressing the energy of magnetization is : F c = E o+ K 1 sin2@ + K 2 sin4@ where K0,Kl,and K 2 are the anisotropy constants and @ is the angle which the magnetization vector makes with the hexagonal axis. Its azimuth around the axis has a negligible effect. K o can be ignored if a comparison of energy differences in different directions is being made. Values of the anisotropy constants are given in Table 31. The values determined by Sievert and Zehler 144 were extrapolated to correspond to zero field. The variation of the constants with temperature is shown in Fig. 11. 138 The sum K 1 + f 2 is the energy necessary to change the magne- tization from along the hexagonal axis to the perpendicular direction; at -19~C (77 K) the sum is reported to be 8.1 x 106ergs/cm 3 145 and the temperature at which it changes sign is about 25~ C (523 K). PROPERTIES OF METALLIC COBALT 83
TABLE 31. Anisotropy constants for cobalt
Anisotropy Constants, Structure Temperature, 105 ergs/cm 3 Reference °C (104 J/m 3) £1 K2
h.c.p. - 273 68 17 138
- 176 79 i0 137
15 45.3 14.4 138
20 53 I0 137
room 41.2 ± .02 14.3 ÷ .05 144
f.c.c. 500 - 2.30 - 4.31 600 - 1.09 - 3.21
700 - 0.60 - 1.91 137
800 - 0.23 - 1.03
900 - 0.09 - 0.57
I000 0 0
S I i 1
2 4 © ~J2 Z, K~
o
o 200 400 600 8OO Temperature, K
Fig. ]i. Crystal anisotropy constants ef cobalt (Barnier et ~Z.138).
The effect of cold rolling on the magnetic anisotropy of b.c.p, cobalt has been studied by Takahashi ~ G~.146 who found it to be a maximum at about 5% reduction, the direction of the easy axis being parallel to the rolling direction.
For cubic crystals the energy of magnetization can be expressed as :
~c = Xo + KI (~ + ~2~:~22 + ~3~i2 2 ) + ~:2 ((~2 ~2~3J, .2^ 2~
,.M>, 242 ( 84 PROGRESS IN MATERIALS SCIENCE
1,2,3 being the direction cosines of the magnetization vector with respect to the principal crystal axes <100>. The anisotropy constants for cubic cobalt for temperatures in the range 500-I00~C (773-1273 K) are given in Table 31. Cubic cobalt becomes isotropic at 100~C (1273 K). Below this temperature, K 1 and K 2 are both negative, whereas for nickel K I is negative and ~2 is generally positive. For cobalt, K 2 > K I (in absolute value), though extrapolation suggests that K 1 might be greater than K 2 for lower temperatures than those at which measurements could be made; for nickel, K I > K 2.
The magnetic properties of polycrystalline cobalt are compounded of those of single crystals in a manner dependent on the orientation of the individual crystals, their purity and physical condition. A typical magnetization curve for a polycrystalline specimen, compared with that of a hexagonal single crystal magnetized along the hexagonal axis is shown in Fig. 12. Saturation is reached in this case at a field strength of 13 kOe (about i x 106 A/m), but details of the purity and physical condition of the specimen are not stated. IO ingleI r~, ,,~'~'''~'-T ~ 0.8 crystal ~ _ <000r>/
Polycrystel _ ~ 0.6
+- o N ~- 0.4
:~ o.2
I I I I 0 20 40 60 80 100 Field strength, kA/m Fig. 12. Magnetization of single crystal and polycrystalline cobalt (Kouvel and Hartelius125).
Magnetic properties are quoted by Bozorth 123 for a specimen in the form of rolled tape 0.006 in. (0.15 mm) thick annealed in a closed pot at IOOPC and cooled with the furnace. It contained 98.6% cobalt (chief impurities 0.6 iron, 0.9 manganese, 0.i silicon and 0.02 carbon). The maximum permeability of this specimen was about 250 G/Oe (3.1 x 10 -4 H/m) a value approximately one tenth that of nickel and one twentieth that of ordinary iron. The initial permeability was correspondingly low as indicated in Fig. 13. The magnetic constants for this specimen were as follows: Maximum permeability 250 G/Oe (3.1 x 10 -4 H/m) Coercive force 8.90e (710 A/m) Remanence 4.900 G (0.49 T) Hysteresis loss, W h 6.900 ergs/cm 3 cycle (690 J/m 3 cycle) Hysteresis constant, n 0.008 (Wh = n.B 1"6 where B is the maximum induction)
The effect of mechanical strain on the magnetic properties of 99.5% pure polycrys- talline cobalt has also been examined by Carmichae1147, who found that the coercive force was increased from ii00 A/m (14 Oe) in the initial state to 2400-3200 A/m (30-40 0e) on torsional straining of rods; the remanence was also affected. PROPERTIES OF METALLIC COBALT 85
175 f t I
E 150 w
>~ i25
E
O- qoo
75 i I i 0 I00 200 300 400 Field strength, A/m
Fig. 13. Permeability of cobalt in low fields (Bozorth123).
It has been shown by Graham 148 and others 149 that pronounced magnetic anisotropy can be developed in polycrystalline cobalt by cooling slowly (~ 2-5°C/min) through the a + E transformation temperature in a strong magnetic field (3-8 kOe; 240-640 kA/m). The magnitude of the anisotropy is very sensitive to the specimen history but Graham has convincingly shown that the effect is due to the development of crystalline texture although this could not be detected by X-ray methods because of the large grain size. However, the temperature dependence of the induced magnetic anisotropy is the same as that for crystal anisotropy established in single crystals.
4.4.3. Ma~etostriction. Magnetostriction is the term applied to the dimensional change observed when a body is magnetized. The relative change in length, &L/L, during magnetization from the demagnetized state to saturation is denoted by the symbols h or It, respectively used when it is measured in the direction parallel or perpendicular to that of the field.
Much of the early work on the magnetostriction of cobalt was carried out on material of doubtful purity and structure, and hence is of limited value. However, the curves of longitudinal and transverse effects shown in Fig. 14 are representative of the behaviour of polycrystalline electrolytic cobalt. Recent observations by Alberts and Alberts 152 on spectroscopically pure cobalt showing no texture confirm that a maximum longitudinal value of I = -50 x 10 -6 is obtained in fields up to 17 kOe (1360 kA/m).
Studies by Buravikhin et al. 153 using rod specimens of 99.99% cobalt cooled from I150°C at different rates have shown that the maximum magnetostrictive effect is obtained with slowly cooled specimens. With rapidly cooled specimens the retention of some cubic phase reduces magnetostriction. The saturation value of I of -75 x 10 -6 for slowly cooled specimens was reached in a field of only about 2 kOe (160 kA/m).
Cobalt exhibits a rather large volume magnetostriction. The results of its measure- ment by Masiyama 154 and by Kornetzki 155 are shown in Fig. 15. Richter and Lotter 156 have studied the thermal expansion characteristics of iron, nickel and cobalt in the vicinity of the respective Curie points and derived values of the volume magne- tostriction for the isothermal transformations from the non-magnetic to the magnetic state. For cobalt (ca. 99.9%) the value was given as -2.5 ± 0.3 x 10 -4 . 86 PROGRESS IN MATERIALS SCIENCE
20 i I I I I
0 Trans. I0 (Masiygma)~--"-- Trans. disc
s" o \\'x_.. Long. -~o £ -20 8, 8 ~ Long.disc 5 -30
. -40 Long. rod ~ (Nish]yoma),
-50 : 0 I00 200 300 400 500 600 Field strength~ kA/m
Fig. 14. Longitudinal and transverse magnetostriction of annealed electrolytic cobalt (Nishiyama 150, Masiyamalgl).
o 0.5 I I I I I I f l
0 E
> -0.5 c
g~ -I .0 fi ~ -t.5
~ -2.0 ] I I I 1 I J I 0 I00 200 300 400 500 600 700 800 900 Field strength, kA/m
Fig. 15. Volume magnetostriction of annealed cobalt (Kornetski155; Masiyama154).
As would be expected the magnetostrictive properties of cobalt are strongly aniso- tropic, and the behaviour of polycrystalline material is not readily related to that of single crystals. Fig. 16 shows the longitudinal magnetostriction for the three principal crystallographic directions in a single crystal, while Fig. 17 shows the transverse magnetostriction for a number of crystallographic directions. The contraction of the polycrystalline material (Fig. 14) is larger than that occurring in any of the three principal crystallographic directions.
Bozorth 1S7 measured the magnetostriction of single crystals of hexagonal cobalt and found that magnetization causes contractions as large as I00 x 10 -6 and expansions PROPERTIES OF METALLIC COBALT 8? of up to 150 x 10 -6 , depending on crystallographic direction. Figure 18 shows the longitudinal and transverse effects as a function of the angle between the direction of measurement and the hexagonal axis.
0 I I P
0 ' -- -5
c:~ -IO
~_ -~5
c -20 u
~> -25
-30 I I I I I I I00 200 300 400 500 600 Field strength, kA/m
Fig. 16. Longitudinal magnetostriction of single crystal hexagonal cobalt (NisiyamalS0).
0 I Fie - ~c" F
~ I~ -~ <0001:> in (I010)_
C -20~
~0 0 iO0 200 300 400 500 600 Field strength, kA/m
Fig. 17. Transverse nmgnetostriction of single crystal hexagonal cobalt (Nisiyamal50).
The expression for magnetostriction at saturation in a hexagonal crystal is158:
: ~A~IB1 + ~2B2) 2 - (~IB1 + ~262)~353] + ~B[ (I - ~)(i - ~) - (~iB1 + ~2B2) 2] + IC[ (1 - ~)B~ - (~1~1 + ~262)~3631 + 4kD(~161 + ~262)~3~3, where ~ is the relative change in length in the direction (BI,62,~3) when the crystal is magnetized to saturation in the direction (~l,~2,~3);_the direction cosines are referred to axes x, y and z respectively in the <]120>
160 i J I I ~ I I I o 120 ~ 80
~ 4o
F_
Parallel
-so
r~ -120 I I I I I / I I 0 I0 20 30 40 50 60 70 80 90 Angle between measurement direction and hexagonal axis, degrees Fig. 18. Magnet£striction of single crystal hexagonal cobalt in (i010) plane, parallel and perpendicular to applied field of 16 x 105 A/m. (Bozorth157).
The constants derived by Bozorth for cobalt are : hA = -- 45 X 10 -6 %B = -- 95 X 10 -6 hC = +ii0 X 10 -6 hD =--i00 X 10 -6
Studies of the temperature dependence of these constants have been made by several investigators 159-162 over the temperature range -200 to 400°C (73-673 K). Their results have been in fair agreement with Bozorth's so far as the room temperature values are concerned, but the constants generally show a strong dependence on temperature, the details of the variations not being confirmed by all investigators.
Domyshev and Buravikhin 162 using polycrystalline cobalt, have extended the tempera- ture range of the studies to 120~C, (1473 K) covering also the behaviour of cubic cobalt. The magnetostriction of this form reaches a maximum at about 54~ C (820 K) and then falls to zero near to the Curie point. Figure 19 shows the variation of the saturation magnetostriction over the temperature range -200-1200°C (73-1473 K); the results of Zubov 163 are also included in this diagram.
4.4.4. Magnetothermal effect. In the successive stages of the magnetization cycle of a ferromagnetic material heat is emitted or absorbed. The magnitude of the effect depends on the field strengths and on the temperature. Tebble and Teale 164 studied cobalt and nickel, and their results for cobalt are given in Fig. 20. The effect of temperature has been examined by Ivanovskii 166 who found that with fields up to 8 kOe (640 kA/m) cooling was produced on magnetization up to 22~ C (500 K) and heating above 327°C (600 K). Rocker and Kohlhaas 167 studied the effect at tempera- ture in the region of the Curie point and found that the heat produced was a maximum close to the Curie point as shown in Fig. 21.
4.4.5. Hall effect. When a magnetic field is applied to a conductor perpendicular to the direction of the current flow, an electric field is developed at right angles to the current and the magnetic field. For ferromagnetic materials the following equation expresses the magnitude of the effect: E = (RoB +RIM ) T PROPERTIES OF METALLIC COBALT 89
I I I J I i I
50
0 E
.£ g E ~ -5o u
- I00 , i I J I ~ [ -200 0 200 400 600 800 ,000 1200 1400
Temperatur e, o C
Fig. 19. Saturation magnetostriction of cobalt (Domyshev and Buravikhin162).
801
60
b
4o
o 2O g
J 0 I I ~ I I I 40 30 20 I0 0 I0 20 30 40
Field strength, kA/m
Fig. 20. Magnetothermal effect in annealed cobalt (Tebble and Tealel6~). where E is the Hall voltage, B the magnetic induction, M the magnetization, I the current, t the thickness of the sample in the direction of the magnetic field, and R o and R 1 are respectively the ordinary and spontaneous, or ferromagnetic, Hall constants. For cobalt of 99.1% purity the values of the constants in m3/coulomb (10 -2 ~ cm ) areT: Oe 90 PROGRESS IN MATERIALS SCIENCE
Temperature R o x i0 II R 1 x 1011 K
293 - 12.4 8.4 273 - 13.6 7.2 193 - 11.8 - 0.9 77 - 13.2 - 4.2
The dependence on temperature is related to the changes in electrical resistivity, p, and the following expression has been suggested for the ferromagnetic Hall constant: R I = ap + bp 2 Over the temperature range 18-650°C (291-923 K) the constants for cobalt are168:
a b hexagonal cobalt - 3.10 -6 0.9 cubic cobalt 4.10 -6 0.49
Kondorskii et al. 169 have confirmed that this relationship with resistivity holds for cobalt and cobalt-nickel alloys over the temperature range O-500°C (273-773 K), except for an anomaly in the region of the phase transformation temperature, but the values of the constants for pure cobalt were not quoted. Tsoukalas 170 also measured the spontaneous Hall constant and the resistivity for 99.99% cobalt over the temperature range 0-1200°C (273-1473 K) and again confirmed agreement with the above expression.
The Hall effect shows anisotropy in single crystals 171,172 and because it is sensitive to the phase transformation, it also shows hysteresis around the transformation temperature 173 .
1 I I
3
J
2 <1
0 1050 1075 I100 1125 tl50 Temperature, °C
Fig. 21. Magnetothermal effect for cobalt (Rocker and Kohlhaas167). Field strength: i, 1.72 MA/m; 2, 1.08 MA/m; 3, 0.61 MA/m; 4, 0.42 MA/m. PROPERTIES OF METALLIC COBALT 9l
5. MECHANICAL PROPERTIES
5.1. Density
The most recent and extensive studies of the density of cobalt in both the solid and liquid states has been reported by Lucas 174, who used cobalt better than 99.95% pure. The derived value for hexagonal cobalt at 20°C (293 K) was 8832 kg/m 3, very close to the value of 8834 kg/m 3 calculated from the best values of the lattice parameters (see Table 7) and the atomic mass of cobalt. The density falls with increasing temperature with an anomalous reduction at the transformation temperature of 0.14-0.18% (See Section 3.2.1) and by extrapolation to the melting point a value of 8180 ± 20 kg/m 3 was obtained. On melting an expansion of (5.5 ± i)% gives a density for the liquid metal of 7730± 50 kg/m 3. For changes in density with further increase in temperature see Section 9.1.
5.2. Elastic properties
Measurements of the elastic properties of cobalt have been made both on wrought polycrystalline nmterial, usually drawn wire, and also on single crystals. With drawn wire the material has been treated as an isotropic medium to which the usual relationships between the elastic properties apply, viz: E = 37~ (i-2~) ~= 2G (i +V) Where E is Young's modulus, K is the bulk modulus, G is the shear modulus and is the Poisson ratio. Little regard has been paid to the possible effects of preferred orientations developed during working, or remaining after annealing, or to the grain size in relation to the size of the test specimen. The work on single crystals has shown, however, that the elastic properties of cobalt are markedly anisotropic and hence the above relationships should not apply.
5.2.1. MassiVe cobalt. Young's modulus of cobalt and its temperature dependence have been determined by K~ster 175 using a resonant-bar method at about 1300 Hz, and his results are shown in Fig. 22. They relate to sintered and wrought cobalt containing only traces of impurities in the order of a few hundredths of a percent after annealing at 1000°C. The room temperature value of the modulus is approxi- mately 21500 kg/mm 2 (21.1 x 1010 N/m 2) and this has been confirmed by more recent measurements on high purity cobalt. RNdiger eta/. 176 using vacuum-melted high- purity powder (> 99.9%) forged, annealed at 850°C (1123 K) and furnace cooled, quoted a mean value of 21315 kg/mm 2 (20.903 x I0 I0 N/m2). K~ster's curve of modulus against temperature shows an anomalous fal] at about 250-300°C (520-570 K) which may be related to the change in magnetic anisotropy occurring at this temperature, and a further inflection occurs due to the allotropic transformation at about 47~C (740 K) on heating and 385°C (660 K) on cooling. RNdiger's work showed similar inflections and indicated that the ratio ET/E20 had fallen to 0.80 at 600°C (870 K). However, the results of Fine and Greener 177 on cobalt about 99.9% pure with a grain size of 1.5 x lO-3mm show no marked inflections, the relationship between modulus and temperature being linear with a change in slope at about 400°C (673 K) - see Fig. 23.
The shear modulus of cobalt was determined by Maringer and Marsh I78 on material of 99.5% purity furnace-cooled from 820°C (1093 K), using a torsional pendulum at frequencies from 1 to 1.4 Hz. The values of modulus were calculated from the natural frequency and were not corrected for changes due to thermal expansion. The results have been confirmed by Selle 179 on material of unstated purity (See Fig. 24). The inflections in the region of the transformation were closely repro- duced. The value of G at normal temperature is 8350 kg/mm 2 (8.2 x I0 I0 N/m2). 92 PROGRESS IN MATERIALS SCIENCE
24oi / 50
~- 22o~ 40
0 30 --
c ~ 180 20 E m
I0 >-
D~ampi ~ 140 0 I I I I I -I00 0 200 400 600 800 I000
Temperature, °C
Fig. 22. Temperature dependence of Young's modulus and damping of cobalt (K~ster175).
0.020 ~ i t i i I l
210 0.016 L' / z 200
190
180 ° ° ° /x,,x E 170 "~ o.oo,, / \ 160 )- Internal / fric?,'on ~ I l" X 0 I00 200 300 400 500 600 700
Temperature, °C
Fig. 23. Young's modulus and internal friction of cobalt (Fine and Greener177). PROPERTIES OF METALLIC COBALT 93
I I I I I I ,90
80
% 70 Z % 0.016 !60 o -- 0.012 50 =o E .o_J 4- 0.008 Internal fri 40 O3
L 0.004 30 c
I I I I I I 20 0 }00 200 300 400 500 600 700 Temperature, °C
Fig. 24. Internal friction and shear modulus of cobalt determined dynamically (Selle179)°
Bridgman 180 gives the following equation for the volume compressibility of cobalt : AV V -- ap - bp 2 where, at 30°C(303 K) : a = 0.539 X 10 -6 k--~cm 2 (5.49 x i0-12 m2/N)
b = 2.3 X 10 -12 cm4 (2.4 x 10 -22 m4/N 2)
This leads to a value for the bulk modulus, K, of 1.86 x 106 kg/cm 2 (18.3 x I0 I0 N/m2), which is confirmed by the results of Schramm 181 who quoted 18.30 x 1011 dyn/cm 2 (18.30 x i0 I0 N/m2). Some compressibility values are included in Table 26. Schram~ plotted the variation of bulk modulus with temperature over the range 0-900°C (273-1173 K) and indicated that the modulus fell to about 14.5 x i0 II dyn/cm 2 (14.5 x I0 I0 N/m 2) at 900°C (1173 K) with a number of inflections in the curve in the region of the allotropic transformation. The temperature coefficient of compressibility has been determined by Batalov and Peletskii 182 as follows: I00-400°C 500-700°C 710-750°C 980-I150°C -2.23 x i04/°C -2.87 x I04/°C -3.26 x I04/°C -8.60 x I04/°C
According to K~ster 183 the most reliable value of Poisson's ratio for cobalt is 0.32. Calculations from the modulus relationships for isotropic materials quoted above, and using the best values of the different moduli at normal temperature, give values for the Poisson ratio of 0.617 and 0.286 respectively for the E/K and E/G relationships; it is clear that the isotropic relationships do not apply.
5.2.2. Single-crystal cobalt. Single crystals with hexagonal symmetry, such as cobalt, require five independent elastic constants to specify the relationships between stresses and strains at a given point. In terms of stiffness moduli they 94 P R O G R ESS IN M ATE RIALS SCIE N C E
may be listed as Cll , c12 , c13 , c33and c~4, or in terms of compliance coefficients as Sll , s12 , s13 , s33 and s44. By studying the velocity of transmission of sonic waves through single crystals, grown by the Bridgman method with different orien- tations, Masumoto et al. 85 have derived Young's modulus and the shear modulus for different directions in the cobalt crystal, and the temperature coefficients of these meduli. These are given, together with the derived values for polycrystalline cobalt in Table 32. The basic elastic constants are given in Table 33. The latter values are in reasonable agreement with the earlier work of Honda and Schirakawa 18~, but show significant variations from the values quoted by McSkimin 185 Using McSkimin's values for the elastic constants of single crystals Evenschor et al. 186 have calculated the polycrystalline elastic constants for several different (h.k.1.) planes; they are necessary for stress determinations by X-ray diffraction methods. The elastic anisotropy ratios quoted by Masumoto et al. 85 for 20°C are E///E i 1.22 and G///G± 1.00.
TABLE 32. Elastic moduli for different directions in cobalt crystals 85
Young's Modulus Shear Modulus Direction E at 20°C Temp. Coeff., I0-5/°C G at 20°C Temp. Coeff., I0-5/°C i0 I0 N/m 2 -50/200°C 200/350°C 1010N/m 2 -50/200°C 200/350°C
<0001> 21.3 - 52.10 - 137.80 6.24 63.80 - 119.70 <10~2> 16.9 - 74.60 - 114.90 7.41 - 39.70 - 129.70 <10~0> 17.5 - 33.55 - 116.70 6.22 - 60.65 - 119.90 <1120> 17.4 - 37.30 - 118.82 6.22 - 60.00 - 116.00 Polycrystal 17.4 - 43.40 - 115.90 6.48 - 49.78 - 125.10
TABLE 33. Elastic constants for hexagonal cobalt at 20°C 85
Stiffness moduli Compliance coefficients 1011N/m 2 10-11m2/N
Cll 2.708 Sll 0.572
c12 1.485 s12 - 0.246
c13 1.280 s13 - 0.143
c33 2.912 s33 0.469
c4~ 0.631 s~4 1.584
5.2.3. Internal friction. The internal friction of cobalt, measured by the damping of oscillations or the attenuation of sound waves has been studied by a number of workers and the results of K~ster 175, of Fine and Greener 177, of Maringer and Marsh 178 and of Selle 179 are included in Figs. 22, 23 and 24. These results and those of others indicate that internal friction generally increases with increase of temperature, but a number of inflections are observed, particularly in the region of the transformation temperature. The inflections have been further examined by Selle, who made measurements of internal friction both under dynamic conditions of temperature change at about 0.5 degrees/min, and under static conditions in which PROPERTIES OF METALLIC COBALT 95
the temperature was stabilized for 2½-3 h before measurements. These enabled the peaks in internal friction in the region of the transformation temperature to be identified with the actual process of transformation, and support the view that a peak at 14~C (413 K) is due to anisotropic thermal expansion. Other peaks in the curve have been assigned to recovery and recrystallization or to the presence of impurities, dislocations, and point defects or metastable a-phase nuclei 187,188
Dubois and Bouquet 189 have studied the effects of annealing on the room-temperature internal friction of cobalt and have confirmed that the phase transformation influences damping through its effect on the mobility of dislocations.
5.2.4. Sound tr~smission. The velocity of sound in a metal is given by the equation 02 =~ 1 - ~ for longitudinal waves (I + ~)(i - 2~) and by 1 v 2 =-~ for transverse waves p 2(1 + ~) where E is Young's modulus and p is the density. Schramm 190 derived values of 5732 and 3002 m/s respectively for these velocities in cobalt at normal temperatures. Gobran and Youssef 191 have measured the velocity, using ultrasound at 2-6 MHz, over the temperature range - i00 to 1000°C (170-1270 K) and have plotted the attenuation over the same range. A number of peaks in the attenuation curve in ranges of both hexagonal and cubic structures have been identified as associated with the redistri- bution of spins amongst the stable directions of spontaneous magnetization inside the domains.
AE Effect. The variation with magnetization of the modulus of elasticity (the so- called &Eeffect) for specimens of annealed and unannealed cobalt has been investi- gated by Street 192. The material used contained 98.4% cobalt, the principal metallic impurities being nickel and iron, and after annealing for 2 h at 1000°C (1273 K) in vacuum and cooling to room temperature in about 8 h, the virgin curve of magnetization was given by the empirical relationship AE - 0.023 + 0.67 13 - 0.22 14 E o I being the intensity of magnetization measured in kilogauss. Eo, the minimum value of Young's modulus, occurring at the coercive force points of the hysteresis cycle (/max z I000 gausses) was 21 x 1010 N/m 2.
5.3. Hardness
Reported values of the hardness of massive cobalt vary quite widely, no doubt due to variations in purity and in processing history. The results of a number of investigators are shown in Fig. 25 in which hardness is plotted against the tempe- rature of measurement; the figures relate to electrodeposited, to sintered or to zone-refined high-purity metal. Altmeyer and Jung 193 gave the following relation- ship between hardness and temperature between 300 and 600 K In HV = In A 1 - BIT where A 1 = 260 kg/mm 2 and B I = 85 x 10 -5 K -I.
The results of microhardness determinations made with a Knoop indentor on zone- refined material depend on crystal orientation 194, values ranging from 81 to 249 HV. If the plane of measurement is near to the basal plane of the hexagonal lattice, the results are sensibly independent of the azimuth of the indentor, while if the measurement plane is at an angle to the basal plane, the size of the impression 96 PROGRESS IN MATERIALS SCIENCE
400 I t I I I I ! 300 "'i 200
> I00 "r "...... "-.. 8O 70 60 50 "% ~E 40 -- Electrodeposited, 99.7% "" T 3O (Chubb 195) "% % -- Sintered (Alfrneyer ond 20 dung 193 ) ..... Zone-refined, 99.98% (Morrol eto'/r94) I0 I i i i I I I I 0 IO0 200 300 400 500 600 700 800 900
Temperel"ur e, °C
Fig. 25. Temperature dependence of the hardness of cobalt.
depends upon the orientation of the indentor with respect to the intercept of the slip plane with the surface. Morral associated high indicated hardness with twinning and low hardness with extensive basal slip. Eppelsheimer and Wilcox 196 using vacuum- melted sponge, of purity better than 99.5% hot-rolled and annealed at I1760C, confirmed these conclusions.
Arnel1197 reached similar conclusions using both Knoop and Vickers indentors in five prominent crystal planes, and compared his results, quoted only in normalized terms, with the predictions of various theories. The Knoop values on planes of high symmetry were more consistent with theories based on compressive forces than those based on tensile forces, but for planes of low symmetry and all Vickers values the results conformed to no theory.
Lozinsky and Fedotov 198 correlated the indentation hardness of metals with the elasticity modulus at 20°C (293 K), the data for most metals falling on a straight line. Hexagonal cobalt was an exception, its hardness being too high for its modulus, but at temperatures above 480°C (753 K), when the cobalt had transformed to the cubic form, its hardness has reverted to the common relationship.
5.4. Tensile Propertie~ The tensile properties of cobalt, as of all metals, depend critically upon the purity and thermal history of the material, and with continuing improvement in techniques of melting and consolidation a closer approach to the properties truly representative of pure cobalt is being obtained.
Table 34 summarizes the tensile properties determined by a number of investigators, and it is apparent that with cobalt of purity better than 99.5% a room temperature tensile strength within the range 80-85 kg/mm 2 (say 800-850 MN/m2), associated with an elongation between 20 and 25% can be obtained.
More recently MUller 204 using seven grades of cobalt with purities ranging from 98.67 to 99.89%, examined the effects of grain size, varied by annealing at different temperatures, on the tensile properties. His results are summarized in Table 35. For the finer-grained material, produced by annealing at about 800°C (1073 K) the results are similar to those given above. PRO PE RTIES OF MET A LL1C C O BA LT 97 The effect of temperature on tensile properties is shown in Fig. 26 and all three sources, on various grades of cobalt, confirm the maximum in ductility shown at about 500°C (773 K).
TABLE 34. Tensile properties of cobalt at room temperature
0.2% Tensile Form Purity Proof stress strength Elongation Ref. MN/m 2 MN/m 2 %
Vacuum-melted, forged 99.6 325 800 29.4 176 Annealed 85~ C Zone-refined, hot-rolled 99.98 516 807 6.9 194 Annealed 50~ C Electron-beam melted 317 807 21 199 Electron-beam melted - 875 21 200 Hot-rolled Electron-beam melted 313 805 21 201 Hot-rolled Vacuum-melted, deoxidi- 99.65 310-345 815-865 18-28 202 zed, cold-rolled sheet Annealed 70~ C Powder-metallurgy strip 99.88 305-345 755-865 15-22 203 Annealed IOOPC
TABLE 35. Effect of grain size of cobalt on tensile properties 20~
Property Grain size ~m 30 3 x I0 ~
Annealing temperature, =C 800 ~ 1300 0.2% proof stress, MN/m 2 510 195 Tensile strength, MN/m 2 780 275 Elongation, % 20 5
5.5. Compressive properties
The compressive properties of pure cobalt at normal temperature have been reported by Dawihl and Frisch 205 for two grades of material and are summarized in Table 36. The differences between the two materials are presumably due to the effect of the porosity in the sintered material restricting grain growth. Dawihl and Frisch quote results by K. M. Mal indicating that the 0.2% compressive yield strength of cobalt reduces from 196 N/mm 2 at 20°C to I0 N/mm 2 at 500°C and to 3 N/mm 2 at ll00°C.
The high-temperature plasticity of cobalt has been studied by Jacquerie and Habraken 206 using a technique of uniaxial compression testing at controlled strain rates between 900 and 1200°C (1170 and 1470 K). The results for 1200°C (1470 K) are shown as true stress-strain curves in Fig. 27. A range of high formability of the metal corresponds to the minima in these curves at true strains between 0.35 and 0.51. 98 PROGRESS IN MATERIALS SCIENCE 5.6. Formability
It was indicated in the previous sections that the mechanical properties, particu- larly the ductility, of cobalt are considerably influenced by the purity and the
IO0 aI t i I L 1 [ I ( tO00 90
~b Tensi I 900 80 e strength B00
70 700 z~;
C fl 60
50
40 tlongatlon ~ ~ xt.x " '~ '" 30 ..... ~__.,~\ ~-~ 300 ®
20
~O I00
I I I l } I I I I 0 I00 200 300 400 500 600 700 800 900 I000 Temperature, °C
Fig. 26. Elevated temperature tensile properties of cobalt, a, Morral et a/.194; b, Fraser et a/.203; c, Beckers et al. 202
TABLE 36. Compressive properties of cobalt at normal temperature 205
Form True compressive stress (MN/m 2 ) for permanent deformation of 0.2% 1% 2% 3% 5% 10% 15% Pure cast 196 251 302 343 402 480 490 cobalt Sintered at 1200°C 295 388 579 589 736 7% porous
I I I I
~ SO a Z 60
40
~ 20
i I i i 0 010 022 035 0.51 0.69 True strain Fig. 27. True compression stress-strain curves at 1200°C for wrought 99.78% cobalt (Jacquerie and Habraken206). Strain rate: a, 19.6 s-l; b, 12.7 s-l; c, 7.1 s-l; d, 3.4 s -I . PROPERTIES OF METALLIC COBALT 99
thermal history of the material. Progressively more successful efforts have there- fore been made to improve the ductility and formability of cobalt in the conventional forms of bar, wire~ sheet and strip.
Levinson 200 showed that melting in vacuum by electron-beam techniques improved the hot-workability of cobalt at 930°C (1200 K). The tensile properties at normal temperature of the vacuum-melted material are given in Table 34. The elongation was 21%, compared with 3% for arc-melted material and at 930°C (1200 K) the elongations were 43 and 13% respectively. The sulphur and oxygen contents were slightly reduced by the vacuum melting but the metal was microscopically much cleaner, possibly due to reduced contents of hydrogen and nitrogen. Peckner 201 (see Table 34) quoted similar results, and further found the improved hot-workability only after slow electron-beam melting in vacuum, not after fast melting - again perhaps associated with gas evolution.
The work of Beckers et a~. 202 describes a very co~)let:e study of the subject. They found that lead, zinc and sulphur were mainly responsible for the poor ductility of cobalt, and that phosphorus, silicon, nitrogen and hydrogen also exert some influence. Oxygen and carbon, however, can reach higher contents without detrimental effects on hot and cold malleability. Cobalt granules, nominally of 99.5% purity, could readily be purified of the deleterious elements by conventional refining operations in oxidizing and reducing atmospheres, and the hydrogen and nitrogen contents could then be reduced to below the harmful levels by vacuum degassing. The oxidizing-refining step eliminates most of the impurities, the sulphur forming sulphur dioxide which is removed by vacuum treatment, leaving an oxidized refined product. Treatment with hydrogen followed by further vacuum degassing yields a deoxidized metal. Both grades of refined metal were studied by Beckers and his colleagues and were found to be readily hot-rolled to sheet in the temperature range I000-60~C (1270-870 K). They were also extruded at II00°C (1370 K) with extrusion ratios in the range 12-22, and in this case a marked preferred orientation, with the hexagonal basal planes perpendicular to the extrusion axis, was found in the cooled rod, although no preferred orientation was found in hot-rolled sheet. The hot-rolled sheets were cold-rolled, with reductions of 30-35% between anneals at 700 or 800°C (970 or 1070 K), and Sheet down to 0.5 mm in thickness produced - this showed preferred orientation with the hexagonal axis concentrating in two positions at angles to the normal to the sheet, and in the rolling direction.
The behaviour on annealing cold-rolled sheet at te1~eratures within the range 300-1000 °C (570-1270 K) could be explained by the varying proportions of ~ and E phases in the resultant structure, and by the grain size - the hardness increases with increasing proportion of e and with decreasing grain size. The proportion of residual ~ phase in annealed cold-rolled sheet depends to some extent on the thicknes~ of the sheet, the annealing time, and the cooling rate, but is mainly influenced by the annealing temperature - a maximum content of ~ phase of about 50% occurs after annealing in the range 500-700 ° (770-970 K), and this favours ductility and workability of the material.
There is little residual preferred orientation re~ining after annealing at 500°C (770 K) or above.
The room-temperature tensile properties measured by Beckers ~ al. on sheet annealed for 1 h at 700°C (970 K) were as given in Table 34; There was little difference between the properties in longitudinal and transverse directions. The properties at elevated temperatures are included in Fig. 26. The work-hardening behaviour of the sheet on further cold-rolling is illustrated in Fig. 28. Erichsen tests on sheet of various thicknesses after different annealing treatments showed reasonable ductility. In this field Derricott 207 has reported Erichsen tests on cobalt sheet of lower purity (99.7%) containing 30% of ~ phase after annealing, and shows that
J.P.M', 24 2 I~ 100 P R O G RESS IN M ATE RIA LS SCIE N C E
maximum cupping depth was obtained when the test was carried out at 500°C (770 K). He indicated that acceptable drawability was obtained between 500 and 600°C (770 and 870 K).
1300 I I , I t [
1200
I100
I000
.~ 900
I..-- 80o
700 i 1
ff 20
~ ~o o 1 0 5 I0 15 20 25 30 35
Cold deformation, %
Fig. 28. Work-hardening behaviour of deoxidized annealed cobalt sheet (Beckers et a/.202).
An alternative method of producing ductile cobalt strip has been described by Fraser st al. 203. This is based on the hydrometallurgical recovery of high-purity cobalt powder from aqueous solutions by high-pressure hydrogen reduction in the presence of a catalyst, followed by direct roll compacting. The green compacted strip is annealed in hydrogen at 925-950°(1198-1223 K), which reduces the sulphur content to about 0.005%, and is then hot- and cold-rolled to the required thickness. The tensile properties of the strip, after annealing at 100~C (1270 K), are given in Table 34.
The properties, particularly the elongation, are detrimentally affected by the method of specimen preparation and the highest ductility is obtained by annealing after preparation. In general, the observations of Fraser and his colleagues were similar to those of Beckers et al. in that the ductility of the strip was related to the proportion of ~ phase remaining in the strip and that, after annealing, there was no preferred orientation; the mechanical properties and work-hardening behaviour of the two products were similar. Of particular note is the low value of Young's modulus, in the range 9-14 x i0 I0 N/m 2, obtained by both groups of workers, compared with the value of about 21 x i0 I0 N/m 2 for high-purity bar material. The difference cannot be explained simply on the basis of differing proportions of the ~ and E phases, since the change in modulus on transformation (see Fig. 22) is too small; it seems possible that anelastic phenomena associated with the two-phase structure might be the cause. PROPERTIES OF METALLIC COBALT 101
5.7. Creep properties
The most comprehensive measurements of the creep behaviour of pure cobalt have been made by Feltham and Myers 208, who used 99.999% cobalt annealed at 850 °C (1123 K) to give a grain size of about 30 pm. It was tested in vacuum at temperatures between 450 and 75~C (723 and 1023 K), i.e. mainly in the cubic form. The results, plotted as the steady-state creep rate against stress, are shown in Fig. 29. On similar material but with a grain size of ii0 pm Feltham 209 determined the stress- strain curves at temperatures between -196 and III°C (77-384 K) shown in Fig. 30, and derived the parabolic work-hardening coefficients plotted in Fig. 31. Kamel and Halim 210, using 99.99% cobalt in the form of 0.5-rmn-diameter wire, found that the work-hardening coefficient for hexagonal cobalt was 4 times greater than that for cubic cobalt at the same temperature, and in studying the creep properties in the region of the transformation temperature found that the activation energy derived from the variation of creep rate with temperature was 5.6 eV for hexagonal cobalt and 2.0 eV for cubic cobalt. The creep properties show hysteresis due to the sluggishness of the phase change. Myshlyaev c~ ~.211 have confirmed that the activation energy for creep is dependent on the crystal structure.
10 4 750 --600
, I0 s 550 500
I0 6
10`7 a; k~
10-8 / Tempera'ture, °C rO-~ t I I I I 0 40 80 120 160 200 240
Stress, MN/m ~
Fig. 29. Creep rate of pure cobalt (Feltham and Myers208).
5.8. Frictional properties
The frictional and wear characteristics of metals under normal circumstances in air or in the presenc~ of lubricants are determined more by the influence of chemically formed films such as oxides or of adsorbed molecules, than by the surface properties of the metals themselves. Under these conditions the behaviour of dif- ferent metals has been correlated with the hardness, or the elastic properties, which determine the specific surface loads developed. For example, Khruhchov and Babichev 212 found, for a wide range of metals and alloys, that wear was proportional to E 1"3, where E is Young's modulus. In the local regions of true metallic contact, however, the structure of the metal has a marked influence, lower values of frictional coefficient and of wear being found for hexagonal metals than for cubic metals, particularly those with simple basal-plane slip systems. Measurements made with carefully cleaned metallic surfaces tested in vacuum reveal these difference
The results of Buckley 213 for cobalt shown in Fig. 32 are particularly interesting in showing the increase in frictional coefficient associated with the phase change. 102 PROGRESS IN MATERIALS SCIENCE
A cobalt hemisphere was slid on a cobalt surface in vacuum at 198 cm/s with a load of 1 kg and the increase in friction from 30~C (570 K) upwards was ascribed to the actual interface temperature being considerably above the bulk temperature of the sliding members, due to the high velocity of sliding.
I I 77 I
300
E
200 384 ^
I00
I I I 0 2 4 6 8
Strain, %
Fig. 30. Stress-strain curves for annealed cobalt at strain rate 0.I % s -I (Feltham209).
20 E Z v
~5 .i,-
u ~ ~o
I
5 T I I 0 I00 200 300 400
Temperature, K
Fig. 31. Parabolic work-hardening coefficient for annealed cobalt (Feltham209).
Bowden and Childs 214 examined the frictional behaviour in vacuum of a number of metals sliding on flats of the same metal at temperatures between 25 and 293 K. For all the face-centred-cubic metals the coefficient was in the range 2.4-1.7, for the body-centred-cubic metals between 1.4 and 1.0, and while the hexagonal metals zirconium and titanium behaved similarly to the cubic metals, cobalt and beryllium had significantly lower coefficients. For cobalt the coefficient was about 0.6 over the whole temperature range and there was little wear or surface damage. PROPERTIES OF METALLIC COBALT 103
1.6 I i i i I
g :~ 1.2
'Z 0.8 i1; / ,~ 0.4 <>
I i r I 0 200 400 600
Temperature, °C
Fig. 32. Coefficient of friction for cobalt sliding on cobalt. Vacuum 10 -9 torr; 198 cm/s; 1 kg (Buckley213).
Buckley 213 explained the low friction of cobalt as being due to easy crystallographic slip on the basal plane and made a direct comparison of the behaviour of single crystals of cobalt and copper, In each case both the slider and the surface were single crystals, with the slip plane and the easy direction of slip favourably oriented. The results are given in Table 37.
TABLE 37. Coefficients of adhesion and friction for cobalt and copper single crystals 213
In vacuum i0 -II torr; velocity O.OO1 cm/s; load 50 g; 20°C.
Coefficient of Metal couples Adhesion Friction Adhesion before sliding after sliding
Cobalt (0001)[ II~OJ < 0.05 0.35 < O.O5 Copper (111)[ II01 0.30 21.0 10.5
The frictional behaviour of cobalt sliding against itself in various gases at temperatures up to 760°C (1033 K) was examined by Foley ~ G~. 215 At hi~er temperatures they found that the friction was controlled by oxidation, while at lower temperatures the friction was low, either due to adsorbed gases or to the inherent low friciton of the cobalt-cobalt combination. The results in air at normal pressure are shown in Fig. 33.
Recent work by other investigators 216-218 has confirmed that cobalt, whether sliding against itself or against hardened steel, generally shows a low coefficient of friction in the region of 0.4, and that this is associated with the formation of a surface layer with a preferred orientation of the hexagonal crystal lattice. 104 PROGRESS IN MATERIALS SCIENCE
0.8 I i I
0.6 o=
'S 0.4
u
o.2 k)~
0 200 400 600 800
Temperoture, °C
Fig. 33. Coefficient of friction for cobalt sliding on cobalt. Air 760 torr (Foley et al.215).
6. SPECIAL FORMS OF COBALT
6,1. Single crystals Study of the properties of single crystals of cobalt is of particular interest since the metal is hexagonal at normal temperature and displays a degree of aniso- tropy in many of its properties, the effect of which in polycrystalline material can only be explained by the careful study of single crystals. Furthermore, it has a low stacking-fault energy and therefore provides valuable fundamental informa- tion on deformation mechanisms in hexagonal metals. Many of the physical and mechanical properties of single crystals have already been referred to in previous sections (thermal expansion 4.2.6; electrical resistivity 4.3.1; thermoelectric power 4.3.2; magnetic properties 4.4.2; magnetostriction 4.4.3; Hall effect 4.4.5; elasticity 5.2.2; hardness 5.3; friction 5.8) in explanation of the properties of massive cobalt, and in this section attention is concentrated on methods of production of crystals and their deformation characteristics.
6.1.1. Preparation of single crystals. The methods most commonly used are the Bridgman method of growing from the melt and the more recent floating-zone method. In both of these a cubic crystal is first grown and this must be transformed under controlled conditions to the hexagonal form. It is surprising that many of the published papers on growth methods pay little attention to the latter stage of the process.
Davis and Teghtsoonian 219 found the electron-beam-heated floating-zone method satisfactory, where strain/anneal and growth from the melt methods were unsuccessful They used cormnercial cobalt rod 3.2 rmn in diameter and grew crystals up to 200 mm long by a single pass of the molten zone at 250 mm/h in a high vacuum. The growth axes clustered around the [ ]0TO] pole, the basal planes being at least 60 ° from the axis. Kralina 220 used the same method to grow crystals 3-8 mm in diameter and 60-120 mm long at growth rates between 45 and 380 mm/h. Both hexagonal and cubic crystals were obtained, but no indication was given of how the cubic structure was retained; the structure was not controlled by the growth rate. With increase in the growth rate the block size of the crystallites forming the single crystal decreased from 2 x 10-2rmm to 1.5 x 10-3mm, and their disorientation increased from 13' to 30'
Hayashi eta/. 221, also using the floating-zone method grew hexagonal crystals with the
Hudson 222, using 99.99% cobalt grew single crystals 9.5 ran diameter and 62.5 mm long by melting and progressive solidification in an alumina crucible. Induction heating through a graphite susceptor was used and the crucible remained stationary while the induction coil moved at 25 mm/h. After solidification, the temperature was maintained between I000 and 600°C (1270 and 870 K) for a time, and then a traverse through the transformation temperature range was made at the same speed. The crystal was finally cooled overnight in vacuum.
Thieringer 223 grew cubic crystals by the Bridgman method, and after annealing for 12-14 h at 1300°C (1570 K), passed them through a temperature gradient at 120 mm/h to transform to the hexagonal form.
6.1.2. Deformation of single crystals. Deformation of hexagonal cobalt single crystals is confined to slip on the basal plane, unlike other hexagonal metals with similar or lower axial ratios (magnesium, beryllium, titanium) which also slip on non-basal planes. Davis and Teghtsoonian 22~ attribute this behaviour to the low stacking-fault energy. These authors measured the critical resolved shear stress for commercial-purity and high-purity cobalt crystals and obtained the following results : 99.5% cobalt 18 °C (298 K) 9.5 N/mm 2 -196°C ( 77 K) 16.5 N/mm 2 High-purity cobalt 18°C (298 K) 6.4 - 6.9 N/mm 2 (~ i0 ppm impurities)
The slope of the resolved shear stress-shear strain curve in the linear portion of the curve is a measure of work hardening and Davis and Teghtsoonian 224 found the slope, O, although subject to wide variations, to have a mean value of 13.7 N/mm 2 at normal temperature.
Boser 225 carried out similar studies on single crystals of high-purity cobalt (22 ppm impurities) over the temperature range 4.2-398 K and obtained the results plotted in Fig. 34. Thieringer 223 also confirmed the general level of the critical shear stress, obtaining values at normal temperature in the range 10.1-10.3 N/mm 2 for 99.93 and 99.998% cobalt crystals, although his values of work-hardening factor, 6, were lower than those of Davis and Teghtsoonina, viz: 3.6-5.1 N/mm 2.
Holt and Teghtsoonian 226,227 have studied the effect of elevated temperature on the deformation behaviour of hexagonal crystals and also the behaviour of cubic crystals in the temperature range 430-600°C (700-870 K). The critical resolved shear stress of pure hexagonal cobalt falls from 7 N/mm 2 at 20°C (293 K) to 5 N/mm 2 at 380°C (650 K) and to 4 N/mm 2 at 420°C (690 K). For cubic cobalt easy glide is not observed, and the critical resolved shear stress is about 3 times that for the hexagonal form. The deformation behaviour is sensitive to what is termed the transformation factor - the behaviour of a {Iii} plane depends on whether or not it has transformed from the hexagonal basal plane. Hence dislocations which operate during the transformation process may continue to multiply and glide under the applied stress after transformation is completed. Conversely dislocations produced during deformation may activate the transformation when the temperature is reduced. If a crystal is prestrained in the hexagonal form it may continue to deform when cubic on the same slip plane with little change in flow stress, even though other (iii} planes may be more favourably oriented. However, the work-hardening rate 0/G, where G is the shear modulus, is low for the hexagonal form, (~ 3 x 10-4N/rmn 2) but much higher (5-28 x 10 -3 N/mm 2) for the cubic form, depending on the transformation and deformation history.
A review of the deformation of single crystals of cobalt has been published by Holt 228 . 106 PROGRESS IN MATERIALS SCIENCE
I I I I I I Z
I0
2 o
+r. 6 0 200 400 600
Temperal-ure, K
Fig. 34. Critical resolved shear stress of hexagonal cobalt (Boser225).
6.2. Whiskers
Metal whiskers are fine single crystals usually in the order of i-i00 ~m in diameter and a few rmn long grown from gaseous or liquid media so that they contain few lattice defects and hence show abnormally high strength. Cobalt whiskers were first grown by Brenner 229 by reducing cobalt bromide with hydrogen, and Dragsdorf and Johnson 230 used the same method from the chloride. The whiskers grew at about 600°C (870 K), and hence were in the cubic form, and were cooled through the transformation range at about 6°C/min. They were mainly hexagonal, but contained some cubic form, and the growth axis was predominently the
A different method was used by Luborsky et al. 234 who grew cobalt whiskers by evaporation of the metal on to a molybdenum substrate, and found that the growth axis was in the <1120> direction. Oxide on the whisker surface was epitaxially related to the metal, the (ii0) plane of the oxide being parallel to the basal plane and the
Measurements by Bokshtein eta/. 236 of the tensile properties of cobalt whiskers grown by reduction of the halides, have indicated strengths 8-10 times that of ordinary bulk single crystals, associated with extensions of up to 500%. Bibby et al. 237 also found high, but very variable, extensions in the range 22-650%, and related the breaking strength to the cross-sectional area, but later work by Nemeth PROPERTIES OF METALLIC COBALT 107 eta/. 231 led to the conclusion that there was no relationship between tensile properties and size. For the commonest orientations of whiskers grown from halides the average tensile strength is in the range 960-1140 N/n~n2 with elongations of about 25%. These are generally low strength levels compared with those of whiskers of other metals.
6.3. Small particles
Elemental cobalt in the form of powder is of considerable technological importance for the production of powder metallurgical articles, particularly carbide cutting tools. However, the present section is largely confined to a survey of the prepa- ration and study of small particles for their fundamental interest as a hexagonal magnetic metal. The most important methods are as follows: (I) Reduction of the oxides; (2) pyrolysis of organic salts; (3) hydrogen reduction of aqueous salt solutions under pressure; (4) condensation of vapour; (5) electrolysis; (6) electrolytic reduction.
The first three are widely applied in industrial practice. For instance hydrogen reduction of oxides yields a commercial powder of 99.5% purity with an average grain size of 4 ~m 238. Decomposition of organic salts, particularly cobalt oxalate, yields a finer powder better suited to the fabrication of cemented carbides; its purity is 99.9% and its average grain size i ~m 238. Finally, hydrogen reduction of cobaltic pentammine solutions yields a powder which contains 99.9% cobalt + nickel, and exhibits an irregular, cemented-chain appearance which is beneficial in the production of strip by direct rolling 239.
By variation of the conditions of production, the characteristics of the powders can, in most cases, be varied within quite wide limits. The properties of a selection of grades of cobalt powder produced by methods i, 2 and 3 were studied by Meyer et al. 240 who found that they all had mean grain sizes (determined by the Fisher subsieve sizer) between i.O and 2.2 Dm and specific surface areas (determined by nitrogen adsorption by the B.E.T. method) between 0.75 and 1.45 m2/g. The coercive force lay in the range 70-230 Oe (5.6-18.4 kA/m). The major difference: lay in the proportion of retained cubic phase, which varied from 5 to 95%, but in a manner not directly related to the method of production.
6.3.1. Reduction of the o~des. Vincent ctal. 241 have shown that the crystal habit and texture of cobalt powder produced by the hydrogen reduction of CoO at 330°C (600 K) depends on the morphology of the oxide powder, which is in turn dependent on the conditions of thermal dehydration of the hydroxide. Borchert and Carl 242 measured the sintering behaviour of powder reduced from oxide, 90% of which had a particle size less than 3 ~m, after pressing at 2.6 and 5.2 t/cm 2 to give a green density of 55.6% and 66.3% of theoretical. The sintering curves determined in purified argon at 720 mm Hg (96 kPa) are shown in Fig. 35.
Fedorchenko and Kostyrko 243 studied the effects of prior annealing of powders produced by the reduction of oxides in hydrogen at 700°C (970 K). In the unannealed condition the specific surface area was 0.76 m2/g and the bulk density 1.31 g/cm 3, while after annealing at 1000°C (1270 K) these values became 0.18 m2/g and 1.79 g/cm 3 respectively. When pressed at 4.4 t/cm 2 and sintered at Ii00°C (1370 K), the volume shrinkage and porosity values were: Volume Shrinkage, % Porosity, % Unannealed 31.5 10.2 Annealed 26.9 15.7 108 PROGRESS IN MATERIALS SCIENCE
i I ] v I t I i I
Temperature, °C 2
° 4 i
E E w
rr 10
12 a I i~l I I 0 2 4 6 8 10
Time ~ hours
Fig. 35. Shrinkage of cobalt compacts on sintering after cold pressing at 2.6 t/cm 2 (Borchert and Car1242).
The differences were correlated with changes in the surfaces of the powder particles - prior annealing reduces the risk of excessive dimensional changes during sintering.
6.3.2. Pyrolysis of organic salts. Figlarz and his colleagues 244,246 have closely studied the production of cobalt powder by thermal decomposition of pentammine cobalt nitrates with the general formula [ Co(NH3)sR]N03, where R is carbonate or other acid radical. On heating these salts in hydrogen or nitrogen they decompose explosively at about 20~C (470 K) and on further heating to about 30~C (570 K) yield agglomerated powders with a crystallite size around 120 nm and a specific surface area in the range 2-10 m2/g. The proportion of cubic cobalt lies between 0 and 77%, and ~' ne the rate of flow of the gas cover. The powder is not pyrophoric, a : ~ ~iv[ty as a catalyst (measured by the chemisorption of nitrogen at -195°C (78K) is ~oportional to the content of cubic cobalt; Figlarz identifiedt the active catalytic points as stacking faults in the cubic lattice.
Colloidal powders of a similar type were produced by Thomas 247 by thermal decompo- sition of cobalt carbonyl in a hydrocarbon solvent containing polymeric material; the temperature was raised until stoichiometric evolution of carbon monoxide showed that decomposition was complete. The particles then ranged in size from 2 to 30 nm, with a mean diameter of 20 nm, and had a coercive force of 550 Oe (44 kA/m).
6.3.3. Hydrogen reduction of salt solutions. This process, developed by Sherritt Gordon Mines, involves the reduction of aqueous cobaltic pentar~nine solutions by hydrogen under pressure, and by variation of the conditions a range of characteristics may be obtained. Thus the sieve analysis of such powders can range from 98% coarser than 150 pm, to 100% finer than 20 pm, while special grades can have a Fisher sub- sieve size of about I pm 239.
Soubirous et al. 248 have shown that hexagonal single crystal platelets about i ~m in diameter can be produced by hydrogen pressure reduction of an aqueous suspension of cobalt hydroxide.
6.3.4. Condensation of vapour. The condensation of metal vapour on to a cold subs- trate in an inert atmosphere yields very fine powders. Tasaki eta/. 249 showed that the grain size of the powder depends on the pressure of argon, rising from PROPERTIES OF METALLIC COBALT 109
8 nm at 0.55 mm Hg (66 Pa) to 300 um at 3 mm Hg (400 Pa) and to 200 nm at 35 mm Hg (4600 Pa). The structure of the powder was cubic and the magnetic properties varied with the grain size. A coercive force of about 1600 Oe (130 kA/m) was obtained with the powder deposited at a pressure of 400 Pa. Palatnik et el. 250 obtained similar results and showed that the particle size was also affected by the temperatures of the evaporating source and of the surface on to which the powder condensed. With the latter at 40~C (670 K) some hexagonal cobalt was present.
Gen and his colleagues have studied the properties of condensed aerosol particles of cobalt 251-253 The particles ranged from 15 to 120 nm in diameter and were normally single domain particles with a cubic structure. The coercive force rises with decrease in particle diameter while the saturation magnetization falls. The lattice parameter also decreases with decreasing particle size.
6.3.5. Electrolysis. Electrolytic production of cobalt powder has been reported from sulphate 25q, sulphamate 255, and chloride 258 electrolytes. The conditions of deposition, e.g. electrolyte concentration, current density, pH, etc., control the powder characteristics. Shvets et el. 257 reported a maximum residual magnetic induction of 8000 gauss (80.106 T) and a coercive force of 1400 Oe (ii0 kA/m) for powder from the chloride bath.
6.3.6. Electrola3s reduction. Sub-microscopic particles of cobalt may be produced from a sulphate solution by auto-catalytic reduction by hypophosphite in a protein- containing solution 258. By variation of the protein and of both composition and pH the size and properties of the particles can be varied. Particle sizes varied from i0 to 50 nm. The maximum coercive force of 1500 Oe (120 kA/m) was obtained at pH 8.5 into a hypophosphite concentration of 20 g/l.
6.3.7. Sintering behaviour. The sintering of cobalt powder after cold pressing depends upon the initial characteristics of the powder, but in general sintering starts at about 700°C (970 K) and is effectively completed at about 1000°C (1270 K), although to reach full theoretical density in a reasonable time temperatures as high as 130~ C (1570 K) are needed. The behaviour of the different powders ~xamined by Meyer et el. 240 , which he followed by dilatometric and density measurements, were all in agreement with this general pattern. Borchert and Carl 2~2 and Fedorchenko and Kostyrko 243 made sintering studies on oxide-reduced powders (see Section 6.3.1) which also conform to this pattern°
A more complete study of the sintering of cobalt and other metal powders has been made by Tikkanen and M~kipirtti 2S9. A general phenomenological sintering equation was developed based on the densification parameter c~: = "/o - Vs V ° - Vte where Vo, Vs and Vte are the volumes of the pressed specimen respectively before, during and after sintering to full density. The sintering equation has the general form ~_ (Kt)n 1 - where t is the time and K and n are constants.
For cobalt powder with a particle size of 7-30 Dm pressed to a green density of 4.92 g/cm 3, the constants for the temperature range 665-885°C (938-1158 K) are K = i.I x 1014 exp (- 70570/RT) n = 0.50 with the time measured in minutes. 110 PROGRESS IN MATERIALS SCIENCE
6.4. Thin films
The unique characteristics of cobalt in respect of hexagonal structure, magnetic properties and anisotropy have stimulated study of thin films of the metal produced by various methods, predominantly by vacuum evaporation, but also by electrodeposition chemical deposition, ion plating and thermo-chemical reaction. The principal tech- niques for study of the films have been by electron microscopy, and by measurement of electrical resistance, of magnetic properties and of the domain structure.
The phase composition of evaporated fihns has been found to be independent of the substrate but to vary with the substrate temperature 260. For a substrate tempera- ture between 20 and 200°C (290 and 470 K) the film is predominantly hexagonal, but above 20~C (470 K) the proportion of cubic phase increases. The nature of stacking faults and other lattice defects in the films depends on the growth conditions and subsequent annealing treatment 261 . Antipova c~ a~. 262 examined the transformation characteristics of vacuum evaporated films up to 20 nm thick and found that in the range 4-20 nm the transformation temperature was the same as for massive cobalt. For films 2 nm thick (fixed with added carbon) the temperature at which the trans- formation begins on heating is increased by 40°; this change was explained in terms of a dislocation mechanism. Votava263, 264 made similar studies by electron micros- copy of films thinned from high-purity foils of cobalt, and found that the cubic form can be more readily retained in thin films than in massive metal; the trans- formation temperature on cooling is lowered to about 200°C (470 K). This was explained on the basis of Seeger's mechanism (see Section 3), the normal propagation of stacking faults from one {Iii} plane to another being prevented so that only a limited number of perfect dislocations contribute to the transformation.
Epitaxial thin films can be grown by vacuum evaporation on to suitable single- crystal substrates. Thus Honma and Wayman 265 have shown that cubic films can be grown, regardless of the condensation temperature, on cleavage faces of KCI and NaCI crystals; semi-epitaxial hexagonal cobalt films were grown on a graphite (0001) substrate. Doyle and Flanders 266 grew cubic cobalt single-crystal films by vacuum deposition on to the (Ii0) face of a MgO substrate, with the substrate above 40~C (670 K); if the substrate were below 340°C (610 K) no cubic cobalt was formed. The anisotropy constant was measured and was associated with stress due to differential thermal expansion between the film and the substrate.
Livesay and Spooner 267 produced cubic films with six-fold magnetic symmetry by deposition on to {iii} faces of copper single crystals.
Epitaxial films have also been grown by electrodeposition on to copper or nickel substrate by Wright 268. The structure of the film and the phase transformation depend on film thickness and pH, as well as on the substrate.
Amorphous films of cobalt have been formed by low-temperature condensation in ultra- high vacuum on to liquid-helium-cooled substrates; the films showed a rapid drop in electrical resistivity due to crystallization and crystallite growth 269.
Measurements of the electrical resistance and the temperature coefficient of resistanc~ have been made on films evaporated in vacuum on to glass or quartz, and the mean free path of the conduction electrons deduced. Early work by Colombani 270 on films obtained by cathodic evaporation in hydrogen included a study of the effect of heating to 850°C (1120 K). As deposited, the resistance depended markedly on thickness; below 20pm they did not follow Ohm's law and above 36 pm the resistivity was much higher than that of the bulk metal. It is probable that the films were by no means pure metallic cobalt. On heating in vacuum the resistance diminished, and after full annealing the resistivity was equal to that of the massive metal. Savornin 271-273 studied films with thicknesses of 2-21 x 10-11m evaporated on to PROPERTIES OF METALLIC COBALT 111
glass with the substrate either at room temperature or at 175°C (448 K), and found marked differences in the initial resistance in the two cases, which diminished as the thickness increased. The differences were identified with the predominantly hexagonal or cubic structure of the two types of film. The thermoelectric power of the films was also measured and found to depend on thickness. The mean free path for films 15-70 nm thick, deduced from the temperature coefficient of resis- tance, was 13 ± 2 nm at 0°C (270 K) and I0± 1.5 nm at 150°C (420 K), i.e. of the same order of magnitude as the theoretical value of 8.2 nm at 0°C (270 K) given by Fuch's theory. Daridon and Columbani 274 made similar measurements on films evapo- rated on to quartz and found changes of slope in the resistance/temperature curve at 200-300°C (470-570 K) associated with the allotropic transformation; for thicker films (> 20 nm) the resistivity became constant at 23 x 10 -9 ~m and a mean free path of about 14 nm was deduced. The effect of film thickness and of temperature on the Hall coefficients was also measured. The vslues of both coefficients reach those for massive cobalt at a film thickness of about 75 nm. More recent work on this topic has been reported by Whyman and Aldridge 275 and by Galepov 276. Ginzburg and Polyakov 277 measured the electrical resistance of vacuum evaporated films of different thicknesses at temperatures down to 1.65 K; no evidence was found of superconductivity.
Studies of the magnetic properties of films have mainly been devoted to the nature of the domains and the movement of domain boundaries on magnetic reversal, in different types of film and using a variety of techniques to observe them. Bates and Spivey 2~8 prepared films varying from 8 to 150 nm in thickness by vacuum evaporation on to glass with a magnetic field of 400 Oe (3200 A/m) parallel to the surface. The reversal of magnetization in different fields and at different temp- eratures was studied. It was found that uniaxial films exhibit almost square hysteresis loops when the reversing field is applied parallel to the easy axis. Jablonowski 279 prepared films by thermal decomposition of cobaltous acetylacetonate; metallic films could be formed at decomposition tenperatures between 318 and 352°C (591 and 625 K) under appropriate conditions of vaporization, carrier-gas flow, etc, and these displayed B-H loops characteristic of bulk cobalt, with coercive force increasing with decreasing film thickness. No magnetic anisotropy was observed, unlike the behaviour usually found with vacuum evaporated or electro- deposited films. For example. Goddard and Wright 280 deposited single-crystal films of both cubic and hexagonal cobalt by electrodeposition on to copper single crystals; the cubic films showed uniaxial anisotropy which decreased with increasing film thickness, while the hexagonal films showed exceptionally high anisotropy.
Thin self-supporting films obtained by electropolishing from annealed high-purity cobalt foils have been examined by electron microscopy. Grundy and Tebble 281 reported on the structures observed in films containing regions of both cubic and hexagonal cobalt. The domain structure in hexagonal regions is related to the uniaxia] anisotropy with, at room temperature, the easy direction of magnetization along the <0001> direction, but on heating it changes at about 275°C (550 K) to a direction lying in the basal plane. In cubic regions the domain structure is also in agreement with anisotropy measurements, but in a region of mixed phase the easy direction in the cubic areas changes from
Similar work has been reported by Schmitt and Gantois 282 who confirmed the obser- vations in the single-phase structures, but found that the mixed-phase region, occurring between 380 ° and 450°C (650 and 720 K) showed two types of orientation for each phase.
Silcox 283 also made similar studies and found in the hexagonal phase the domain boundaries to be close to the projection of the easy magnetization axis <0001> with the plane of the foil. 112 PROGRESS 1N MATERIALS SCIENCE
Films formed by ion plating in argon on to aluminium foil or polyamide substrates have been reported by Watanabe et al. 284 as having coercive forces of 250-600 Oe (20-48 kA/m). The direction of magnetic anisotropy was perpendicular to the plane of the films.
7. ELECTROCHEMICAL PROPERTIES
7.1. Reversible potentials
Reference to a reversible-electromotive-force series shows that cobalt is more active than nickel, and is more noble than cadmium. This relationship is main- tained in many irreversible systems also, but the relative degrees of activity or nobility will vary. Normally, cobalt is classified as "passive" although it actually falls in the transition group between the "passive" and "nonpassive" metals.
For a valence of +2, the electrochemical equivalent of cobalt is 0.3054 mg per coulomb (1.099 g/Ah).
The potential-pH equilibrium diagram of the system Co-H20 at 25°C (298 K) has been established by Deltombe and Pourbaix 285. Eighteen different reactions were consi- dered involvin~ cobalt metal, the solid oxides CoO, Co30~, Co203 and CoO 2 and the ions Co 2+, Co 3 and HCoO~ . The numerical relationships controlling these reactions were established and the equilibrium zones plotted. Figure 36 derived from the results represents the theoretical conditions for corrosion, irmnunity and passivity of cobalt. The equations governing the reactions between cobalt metal and its ions were given as follows: Co 2+ = Co 3+ + e- E o = 1o808 + 0.0591 io~
Co = Co 2+ +2e- E o = -0.277 + 0o0295 log(Co 2+)
2.2 i ~ i i i f I i
1.8
1.4 Jvetion 1.0 ~ > 0.6 Corrosion ] 0.2 "6 :.p -0.2
-0.6
-I.0 Immunity -I.4
-I.8 I I I I I I I I -2 0 2 4 6 8 I0 12 14 16 pH
Fig. 36. Theoretical domains of corrosion, in~nunity and passivation for cobalt at 25°C (Deltombe and Pourbaix285). PROPERTIES OF METALLIC COBALT 113
7.2. Irreversible potentials
The irreversible potentials considered here are those arising when a cobalt elec- trode dips into an electrolyte originally containing no cobalt ions. They are often referred to as "corroding potentials". Akimov and Clark 286 have measured the potentials of cobalt in several solutions. The values given in Table 38 were obtained after one and five minutes, then while stirring the solution, and finally while rubbing the surface of the cobalt with an abrasive. It is presumed that these measurements were made at room temperature.
TABLE 38. Irreversible potentials of cobalt in various solutions* Potential (hydrogen scale), volts Corroding solution i min 5 min Stirred Abraded 3% maC1 -0.146 -0.144 -0.166 -0.382 0.1N HCI -0.147 -0.166 -O.110 -0.108 0olN HNO 3 -0.019 -0.065 -0.046 -0.015 0.1N NaOH -0.147 -0.093 -0.082 -0.532
*After Akimov and Clark 286
7.3. Cathodic processes
7.3.1. Oeposition potential. Glasstone 287 has measured the deposition potentials of cobalt from N-CoSO~ solutions at 15°C and at various acid concentrations and current densities. His results are given in Table 39. The deposition potentials of cobalt from chloride and bromide solutions have been determined by Verdieck et a!.288 The potentials were measured over the temperature range 30-90 °C (300- 360 K) using solutions containing 0.5 mol % of CoCI 2 or CoBr 2. Cobalt was depo- sited more easily from hot than from cold solutions and in hot solutions higher current densities; may be reached before hydrogen is evolved. At lower current densities a plot of log. current against voltage is linear, but above 0.2 A/dm 2 the current/voltage plot is linear.
Fischer 289 has discussed the origin of the high ow~rpotential in the electrodeposition of metals of the iron group compared with that for other metals. Thus with a current density about 1 x 10 -2 A/cm 2 the overvoltage for precipitation from normal solutions of salts at 20°C is about 0.2 V for iron, 0.25 V for cobalt and 0.3-0.4 V for nickel, while for other metals, e.g. copper, cadmium, zinc, they are about 0.07-0.08 V. He concluded that it is a complex phenomenon which may be related to pH.
TABLE 39. Deposition potentials for cobalt* Potentials in J-CoSO 4 (hydrogen scale), volts Current density. mA/cm 2 ~V/10 H2SO 4 pH 2.8 pH 4.0 pH 5.0 pH 6.0
0.014 -0.19 -0.17 -0.i c~ -0.22 -0.24 0.056 -0.22 -0.22 -0.34 -0.39 -0.44 0 . 14 -0 . 31 -0.42 -0 .49 -0 .56 -0 . 56 0.28 -0.36 -0.46 -0.56 -0.57 -0.57 0.56 -0.42 -0.51 -0.57 -0 •59 -0.59 1.2 -0.49 -0.56 -0.59 -0.61 -0.61 4.0 -0.56 -0.57 -0.61 -0.62 -0.63 8.0 -0.57 -0.59 -0.62 -0 °63 -0.64 16.0 -0.58 -0.60 -0.63 -0.64 -0.65
*After Glasstone 287 114 PROGRESS IN MATERIALS SCIENCE
7.3°2. Hydrogen overvoltage. Using a commutator method, Newbery 290 has measured the hydrogen overvoltage on cobalt in both acid and alkaline solutions° He used commercial sheet cobalt which, according to today's standards, would be considered of low purity. However, the major impurity was probably nickel, and this would not introduce too great an error. Nevertheless, the data should be treated with some restraint. The values of overvoltage, measured at 15°C, are given in Table 405 they represent averages of four sets of measurements and show the combined effect of time and exposure. The hydrogen overvoltage on cobalt in a 2N-H2SO 4 solution at 25°C has been determined by Pecherskaya and Stender 291 for current densities ranging from i to 200 mA/cm 2.
Piontelli 292 and his colleagues have studied the anodie and cathodic behaviour of single crystals of cobalt in a number of electrolytes, both acid and alkaline, and found that the overvoltages are practically unaffected by crystal orientation° The overvoltages were generally influenced by pH, and decreased with increasing temp- erature. Ryachevski and Vitkova 2q3 have shown that the hydrogen overvoltage of cobalt is little affected by the presence of about 25% of s-phase in predominently
TABLE 40. Hydrogen overvoltage on cobalt in N-H2SO4and N-NaOH solutions* Current Overvol tage, volt dens i ty mA/cm 2 N-H2SO 4 N-NaOH
2 0.23 0.48 4 0.24 0.53 6 0.24 0.56 i0 0.24 0.61 20 0.25 0.67 50 0.25 0.69 i00 0.26 0.69 200 0.26 0.69 400 0.25 0.68 i000 0.24 0.65 2000 0.20 0.64
*After Newbery 290
z-phase. However, both hydrogen overvoltage and anodic polarization increase with proportion of a-phase present.
7.4. Anodic processes
7.4.1. Behaviour of cobalt as an anode. The anodic behaviour and passivation of cobalt has been reviewed by Schwabe 294 who found that it may be active or passive as an anode, depending on types and concentrations of electrolytes, current densitie and temperature. E1 Wakkad and Hickling 295 studied the behaviour of cobalt in sodium hydroxide solution at 18°C and found a 3-step polarization curve, very dependent on experimental variables. The first step was associated with formation P R O P E R TIES O F M E T A L LIC C O B A L T 115 of CoO, the second with further oxidation to Co203 and the third to CoO 2 before oxygen evolution commenced. The potential of the last step was + 0.58 V. Tikkanen and Tuominen 296 studied the behaviour of cobalt in both alkaline and acidic solution~ by means of potentiokinetic, galvanostatic and potentiometric measurements, and showed that the rate-determining step of the dissolution is the reaction : (CoOH)ad s ÷ (CoOH) + + e- The potentiokinetic polarization curves show three distinct maxima, probably representing the equilibrium reactions : Co ÷ CoO and/or Co ÷ Co (OH) 2,
CoO ÷ Co30~, and Co304 ~ Co (OH) 3 and/or Co(OH) 2 ÷ Co(OH) 3. Cowling and Riddiford 297 working in alkaline solutions found only two stages in the oxidation process, which they identified with the reactions. Co ÷ Co(OH) 2 and Co (OH) 2 ÷ Co (OH) 3
Studies of anodic polarization have also been reported in sodium perchlorate 298, trichloracetic acid 299, liquid armnonia 300 and sulphuric acid 293. The effects of complexing agents have been reported by Gusev and Drozdova 301. The passivation of cobalt in sulphuric acid has been studied by Epelboin et al. 302 using a device with a negative internal resistance to measure the current potential characteristics The co-existence of passive and active states was shown within a trarsition range between two potential plateaux.
7.4.2. Oxygen oVervoltage. Hickling and Hill 303 have determined the oxygen over- voltage at 20°C on cobalt which was electrodeposited on platinum. The electrolyte was N-KOH saturated with oxygen. Both the interrupter and direct methods were used; the results are given in Table 41.
The oxygen overvoltage on cobalt is lower than that on most metals. For example, it is only half of the oxygen overvoltage on platinum, and is somewhat lower than that on nickel; it is even lower than the overvoltage on platinized platinum.
TABLE 41. Oxygen overvoltage on cobalt in N-KOH solution. * Oxygen overvoltage, volt Method 10 -2 i0 -I i i0 102 10 3 mA/cm 2 mA/cm 2 mA/cm 2 mA/~m 2 mA/cm 2 mA/cm 2
By interrupter method, values extrapolated 0.27 0.32 0.39 0.46 0.54 0.61 By direct method 0.27 0.32 0.39 0.46 0.54 0.83
*After Hickling and Hill. 303
7.5. Potentials in fused salts
7.5.1. Static potent~ls. Rempel and Ozeryanaya 304 have measured the potentials of metals in a fused salt cell of the type M/KCI + NaCI (equimolar), 0.I-N MC12/C12, in which M is the metal under study. At 690°C (963 K), cobalt had a potential of - 1.514 V in the above cell (chlorine electrode taken as zero). The metals in this series were in about the same order as those in the aqueous series, except that nickel was more noble than silver.
J.p ~s. 24;2F 116 PROGRESS IN MATERIALS SCIENCE
Flengas and Ingraham 305 have also measured electrode potentials in equimolecular sodium/potassium chloride mixtures at temperatures between 650 and 90~C (920 and 1170 K) against an Ag/AgCI reference electrode. The system Co/CoCI 2 was included and the temperature coefficient of the electrode potential was also measured. Gaur and Jindal 3u6 measured the electrode potentials of a number of systems, including Co(II)/Co(0), in molten chloride mixtures at 475°C (740 K) using Ag(1)/Ag(0) as a reference electrode, and have converted the results to a standard chlorine reference. The electrode potentials of the above cobalt system in three different mixtures were as follows: MgCI 2 - NaCI - KCI (50 : 30 : 20 mol %) - 1.046 V MgCI 2 - KCI (32.5 : 67.5 mol %) - 1.1689 V LiCI - KCI (59 : 41 mol %) - 1.207 V
Delimarskii and Khaimovich 307 have determined the potential of cobalt in a molten AIBr3-NaBr electrolyte at 300°C (570 K). A thin-walled glass bulb with 0.2% NaHg was used as a reference electrode. In this series, cobalt was slightly more noble than silver, having a potential of 2.53 V. The other metals fell pretty well in line with the aqueous series.
Studies of the electrochemical behaviour of cobalt have also been reported in molten sodium nitrite 308, alkali nitrates 309 and sodium tetraborate 310
7.5.2. Deposition potential. Wade et al. 311, have measured the deposition potentials of various metals from fused salt baths. The electrolyte was composed of AICI 3 (66 mol%), NaCI (20 mol%), and KCI (14 mol %). Cobaltous chloride was added so that a I mol % solution resulted. With a platinum reference electrode and at a temperature of 156°C (429 K) the deposition potential for bright cobalt was -- 1.15 V while that for gray crystalline cobalt was -- 1.34 V.
8. CORROSION RESISTANCE AND GAS SOLUBILITY
8.1. Oxidation and hot corrosion
The oxidation of cobalt has been studied by many investigators and has been found to be parabolic in nature over a wide range of temperatures and oxygen partial pressures. Thus the weight gain on heating isothermally at any temperature is given by the expression: (Am) 2 = kt + c where t is the time, k is the parabolic rate constant and c is a constant covering irregularities frequently observed in the initial stages. Hence a protective oxide scale is formed which progressively reduces the oxidation rate, although the oxidation resistance of cobalt is much less than that of nickel. In 1965 Wood et al. 31 published a review of work on the oxidation of nickel, cobalt and nlckel-cobalt alloys, and although the values of the rate constant, k, varied considerably with different investigators, there appeared to be an order of magnitude difference between the values for nickel and cobalt at about 80~C (1070 K). The values given for cobalt are included in Table 42.
Subsequent to this review Paidassi 316 and his co-workers made a full gravimetric and micrographic study of the oxidation of cobalt in air at atmospheric pressure over the temperature range 400-135~C (670-1620 K). The parabolic law was followed over the whole range and the initial condition of the surface was only important below 50~C (770 K). Between 400 and about 90~C (670 and about 1170 K) the scale consists of two layers, an inner one of CoO and an outer one of Co304. Above about PROPERTIES OF METALLIC COBALT 117
o
"I.
o £
o x 'E.
"r
T ? '8 c o % × R ~ <
7 %
x x ~2 8 -£
0 u £ 8
× t £
' % c o
c
8 X
.X
i o
iI i
× 118 PROGRESS IN MATERIALS SCIENCE
900 °C (1170 K) the scale consists only of CoO. Apart from early irregularities the thicknesses of both constituents in the duplex scale follow a parabolic law with time and the ratio of thicknesses Co304/Co0 falls uniformly from 0.6 at 500 °C (770 K) to 0 at 900 °C (1170 K). Two activation energies were derived from the oxidation process as follows : 550 - 750°C -25.0 kcal/mol (105 kJ/mole) 750 - 1350°C -41.0 kcal/mol (172 kJ/mole)
KrUger et al. 317 made further determinations of oxidation rates of 99.999% cobalt in the temperature range 800-II00°C (1070-1370 K) and at oxygen partial pressures from 0.019 to i atm. Values of the parabolic rate constant derived from their results for 0.21 atm. (i atm. air) are included in Table 42. Snide eta/. 318 also studied the oxidation behaviour of high-purity cobalt between 950 and 1250°C (1220 and 1520 K), over a similar range of oxygen partial pressures and their rate constants for i atm. of air are also included in Table 42. The more recent results on high-purity material are reasonably consistent.
Krllger et al. 317 found that the rate constant at 800°C (1070 K) was little affected by the partial pressure of oxygen, that it increased slightly with increasing pressure at 900°C (1170 K), while at i000 and II00°C (1270 and 1370 K) it conformed to the following expression :
log k = A log p02 + B The values of the constants A and B were 0.31 and 1.84 at 1000°C (1270 K) and 0.34 and 2.35 at II00°C (1370 K). Snide et al. 318 found the exponent of the pressure term to be 0.35 over the complete range covered by his work and proposed the following empirical formula for the rate constant : k = 6.0 x 10 -2 (P02)0.35 e -37,100/RT where k is in g2cm-4s-l, p in atm., R = 1.987 cal/mol K and T in kelvins. The values given by this expression agree within 3-8% with those calculated from the diffusion coefficients of cobalt in CoO determined by Carter and Richardson 319, using Wagner's 320 rate equation. It was concluded that the oxidation mechanism within this range of conditions was controlled by the rate of cationic diffusion in the scale. Mrowec et a~. 321 earlier reached similar conclusions and found that at 1000°C (1273 K) the rate constant was proportional to (Pn J " • Small amounts of impurities play ..... %7" ° a declslve role In the mechanlsm of oxlda~lon.
KrUger et a~. 317 presented an Arrhenius plot of the rate constant against the reci- procal of temperature, which is reproduced in Fig. 37. This shows two activation energies, the lower value of 38 ± 2 kcal/mol (159± 8 kJ/mol) being operative over the temperature range above 900°C (1170 K) in which k is sensitive to the partial pressure of oxygen, and the higher value of 59± 2 kcal/mol (247± 8 kJ/mol) applying to the lower temperature regions (800-900°C)(I070-I170 K) where the rate constant is independent of oxygen pressure. The lower value is close to the value for diffusion of cobalt in CoO (34.5 kcal/mol). It is to be noted that Snide et al. 318 found the activation energy for 950-1250°C (1220-1520 K) to be 37.1 kcal/mol (155 kJ/mol). More recent work by Morin and Rigaud 322 in oxygen pressures up to I atm. has given activation energies of 37.5 kcal/mol (157 kJ/mol) for the range 950-1325°C (1220-1600 K) and 21 kcal/mol (88 kJ/mol) for the range 475-750°C (750-1020 K). Below 45~C (720 K) in the region of the allotropic transformation, parabolic, logarithmic and linear laws were found to apply depending on temperature, oxidation time and metal structure; activation energies were not determined 323.
KrHger eta/. 317 confirmed that when oxidation took place at 800°C (1070 K) the scale consisted of a double layer, and concluded that the presence of the outer Co30 ~ layer was responsible for the rate constant being independent of the external PROPERTIES OF METALLIC COBALT 119
oxygen pressure. It was hypothesised that at the Co0/Co304 interface the effective oxygen pressure was the dissociation pressure of Co304 and since this remained constant at a constant temperature the concentration of Co 2+ vacancies in the CoO remained constant. At higher temperatures (above about 900°C)(i170 K) Co30 ~ is not formed and the scale consists of CoO only, and the scaling rate is then pressure dependent. Vallee 32q'325 had also found that at above 900°C (1170 K) Co304 is not formed, and investigated the nucleation and growth of the surface layers of this oxide at temperatures just below 900°C. Reti 326 studied short-time oxidation of cobalt at 100~C (1270 K) and found reaction rates in the i0-i00 s range conformed to those predicted from longer-time studies, but in the 0.01-I0 s range rates were much slower.
Temperature, °C -4 ,2tO ,;50 '~0 ,050 ,000 9~0 900 8~0 800
-5
/ 0.5) 15 t:n c -6
~c po2~ otto. J -7
-8 I I I I I 0 65 0,70 0.75 0,80 0.85 0,90 095
Reciprocal $emperuture, i/T x I0 -= (T in K )
Fig. 37. Parabolic rate constant for the oxidation of cobalt in oxygen at different partial pressures (KrUger et ai.317).
Pettit and Wagner 327 studied the oxidation kinetics of cobalt in mixtures of carbon monoxide and carbon dioxide over the temperature range 920-1200°C (1190-1470 K). Within the range I000-1200°C (1270-1470 K) and for scale thicknesses below 0.7 vm the rate-controlling step of the process is the dissociation of carbon dioxide into carbon monoxide and adsorbed atoms or ions of oxygen, so that scale growth followed a linear law. For greater thicknesses of scale the diffusion of Co 2+ ions outwards through the oxide layer became the rate-determining step and the process became parabolic.
Reactions between the metals nickel, cobalt and iron and sulphur are influenced by the formation of low-melting-point eutectics between the metals and their sulphides. Clearly if the reaction takes place above the eutectic melting point very rapid attack can occur since the reaction product cannot form a solid protective scale. The sulphide eutectics and their melting points for the three metals are as follows: Ni-Ni3S 2 64~ C (916 K) Co-CoqS 3 877°C (1150 K) Fe-FeS 988°C (1261 K)
At temperatures between 500 and 70~C (770 and 970 K) Mrowec and Werber 328 have shown that when cobalt is corroded by sulphur vapour at 1 atm. pressure (~I05 Pa) 120 PROGRESS IN MATERIALS SCIENCE
a protective sulphide scale is formed, giving a parabolic law, and the activation energy controlling the process is about 34 kcal/mol (143 kJ/mol). The rate constants are given in Table 43. Davin 329 confirmed that a parabolic law was followed in sulphur vapour at ~ressures between i00 and 400 mm Hg (13-52 x 103 Pa) but at i0 mm Hg (1.3 x i0 ~ Pa) a linear law was found to apply. The rate constants for both laws are given in Table 43. Activation energies of 32.6 kcal/mol (137 kJ/mol) for the parabolic law and 18.6 kcal/mol (78 kJ/mol) for the linear law were derived, and the rate-controlling mechanisms were identified as outward diffusion of metallic ions in the former case and inward diffusion of sulphur through a porous scale in the latter. Two sulphides Co4S 3 and Co9S 8 were observed in the scales.
Davin and Coutsouradis 330 have shown that sulphidation in hydrogen sulphide at normal pressure also proceeds according to a parabolic law, and the rate constants derived were as follows : Temperature °C Rate constant, g2cm-4s-I 600 1.76 x 10 -7 650 3.86 x 10 -7 700 6.95 x 10 -7 750 8.85 x I0 -?
An activation energy of 23.8 kcal/mol (i00 kJ/mol) was derived. At 800°C (1270 K) the attack was not parabolic, possibly due to the heat of reaction raising the temperature locally above the melting point of the eutectic.
When cobalt is corroded by sulphur dioxide or air containing sulphur dioxide the scale may contain both CoO and CoS 331 . Arkharov Gt al. 332 found the CoO to be at the surface and the CoS in contact with the metal. Similar reactions may occur in sulphate salt mixtures and have significance in regard to the hot corrosion of cobalt-base high-temperature alloys 333. Konev ~t a~. 334 have studied the kinetics of corrosion of cobalt in mixtures of oxygen, sulphur dioxide and carbon dioxide, and found that reaction was always parabolic with the formation of a scale consisting only of CoO, but the rate constants depended on the concentration of sulphur dissolved in the oxide.
Fleitman 335 has reported on the behaviour of cobalt in liquid sodium at 660-760°C (930-1030 K), while Rosenblatt and Wilson 338 have studied the solubility of cobalt in lead-bismuth eutectic in the temperature range 300-550°C (570-820 K). Resistance to molten gallium has been studied by Yatsenko ~t al. 337. Muroi et a~. 338 have studie~d the attack of cobalt by molten vanadium pentoxide and showed that at 850°C (1120 K) a porous, thick scale of Co3V208 is formed.
8.2. Wet corrosion
Corrosion-rate data reported by Young339, 340 for cobalt are given in Table 44. The corrosion resistance of cobalt was found to be of the same order as that of nickel in dilute sulphuric acid, a~m~onia, and acetic acid solutions.
Hedges 3~I reported that cobalt was vigorously attacked by concentrated nitric acid at room temperature, but became passive at a temperature of about -II°C (262 K).
Corrosion by organic acid vapours has been studied by Donovan and Stringer 342. Acetic and formic acid vapours at a concentration of 5 ppm in air at 3~ C and 100% relative humidity for a period of 3 weeks produced total corrosion levels of 0.$i and 0.06 g/dm 2 respectively, compared with 0.002 g/dm 2 in acid-free air. PROPERTIES OF METALLIC COBALT 121
4..i
o o
c.~ 4_1 Y Y ~ co t~ O O O
,--I O .~ r~
O ~ on ~ c-4 O co
o o o r~
,.~ O ~8
O 4D O ~ m o o o
O O of -~- o u'3 e-i ~.1 r-4 c,l
co ~J~ 4_1 c~ O O O O O ~-~ q~ N N N N u-3
¢.~ o ,--4
O
co ? o4 co ;--~ O O O
X
O0 cq
~D O0
o 4J
O O O O O O L~ O O O O
.g 122 PROGRESS IN MATERIALS SCIENCE
TABLE 44. Corrosion of cobalt in aqueous media at 25°C *
Reagent Corrosion rate mg/dm 2 day
5 vol.% Acetic acid 12.5 5 vol.% Ammonium hydroxide 5.3 5 vol.% Sulphuric acid 56.8 i0 vol.% Sodium hydroxide 5.6 1 : 1 Hydrofluoric acid 178.6 conc. Hydrofluoric acid 101.5 1 : 1 Phosphoric acid 65.1 conc. Phosphoric acid 7.4 5 vol.% Hydrazine 7.8 Distilled water i.I
*After Young339, 3~0
Corrosion rates are markedly affected by many factors. Among those studied for cobalt are the effects of crystal orientation on the acid attack of single crystals by La Vecchia Gt al. 343 L~dion and Talbot 344 have shown that mechanical working or polishing of a cobalt surface increases the rate of corrosion in acid media in comparison with electrolytic polishing. Kurilovich and Klyuchnikov 345 examined the effect of pressure of oxygen or nitrogen on corrosion rates in hydrochloric acid; with oxygen, increase of pressure increased the corrosion rate, but with nitrogen the effect depended on whether the gas was bubbled through the solution.
8.3. Gas solubility in cobalt.
The solubility of hydrogen and nitrogen in cobalt and cobalt alloys has recently been reviewed by Blossey and Pehlke 346. Whereas no solubility for nitrogen in solid cobalt has been reported, the results of a number of investigators plotted in Fig. 38, are consistent in showing a solubility in liquid cobalt which rises from about 40 ppm at the melting point to 60-70 ppm at 1750°C (2020 K).
Hydrogen is soluble in cobalt to the extent of 1-2 ppm at 70~C (970 K), increasing to about 8 ppm at the melting point - see Fig. 39. In liquid cobalt the results of different investigators are again in excellent agreement, the solubility increasing from 18-20 ppm at 150~C (1770 K) to 25-28 ppm at 1750°C (2020 K) as shown in Fig. 40.
Seybolt and Mathewson 347 reported the solubility of oxygen in cobalt over the temperature range 600-1200°C (870-1470 K) as follows: temperature, °C 600 700 810 875 945 i000 1200 solubility, wt% 0.006 0.009 0.016 0.010 0.007 0.008 0.013
The inflections were thought to indicate a phase change in the cobalt at about 850 ° C PROPERTIES OF METALLIC COBALT 123
Reciprocal temperoture~ 10-4K -'
5.6 ,5.5 5.4 5.3 5.2 5. I 5.0 70 I I I I I f I
E 6o
Schenck e?" (2/ ~~"~e T dJ - 50 ~5
~ 4o
o
Z
3c I L t p ]500 t550 1600 t650 t700 t75o
Temperoture, %
Fig. 38. Nitrogen solubility in pure molten cobalt (I atm. N2) (Blossey and Pehlke346).
Reciprocal "temperature, 10-4K
I0 9 8 7 6 8 ~ t l r I
6
=
~ ] I i I t 1 700 Boo 900 Iooo ,00 ,200 ,3001400
Temperature, °C
Fig. 39. Hydrogen solubility in pure solid cobalt (i arm. H 2 (Blossey and Pehlke346).
The solubility of oxygen in liquid cobalt has been measured by Belov e~ aI.348 and the results compared with those of earlier workers - agreement was generally satisfactory, the maximum disagreement being about 25% at the higher temperatures. The solubilities determined by Belov ~ a~. were as follows : Temperature, °C Solubility, wt. % oxygen 1510 0.125 ± .006 1550 0.164 ± .008 1600 0.22] ± .011 1650 0.306 ± .015 1700 0.408 ± .020 124 PROGRESS IN MATERIALS SCIENCE
Reciprocal temperature, 10 -4 K-I
5.6 5.5 5.4 5.3 5.2 5.1 5.0 30 J I I I I I I
25 C Busch of el ~ d'
g
o~ 20 . I "1- /8ags~ow eta/
17 I I I I 1500 1550 1600 1650 1700 1750
Temperatur% °C Fig. 40. Hydrogen solubility in pure molten cobalt (i atm. H2) (Blossey and Pehlke346).
The values are expressed by the relationship 9470 log [0] =---~-- + 4.408 where T is in kelvins.
9. LIQUID COBALT
9.1. Density
The density of high-purity (99.99%) liquid cobalt has been determined by Lucas I?~ using the maximum bubble pressure technique. The formula proposed for the specific volume was : F = 0.1304 + 20.9 x 10 -6 (T - 1766) cm3/g (10-3m3/kg) where T is in kelvins. This gives a density at the melting point of 7670 kg/m 3. A number of other investigators have proposed formulae $iving a linear relationship between density and temperature, that of Levin et al. 34~ giving results very close to those of Lucas 174. Their formula was of the form: D = 9.92 x 103 - 1.25T kg/m 3
A recent review by Crawley 350 selected the results of Saito eta/. 351 as being based on a combination of experimental data, and these were expressed by the formula: D = 7740 - 9.50 x 10 -2 (T - 1766) kg/m 3 with T in kelvins.
Density values derived from the formulae of different investigators are given in Table 45.
9.2. Emissivity
The spectral emissivity of liquid cobalt near to its melting point has been measured by Samarin and Svet 355 and the results are plotted against wavelength in Fig. 41. PROPERTIES OF METALLIC COBALT 125
TABLE 45. Density of liquid cobalt in 103 kg/m 3
Temperature °C Investigators Reference 1500 1600 1700 1800 1900 2000 2100 2200
Lucas 174 7.67 7.54 7.42 ..... Frohberg and Weber 352 7.75 7.58 ..... Kirshenbaum and Cahill 353 7.98 7.87 7.77 7.66 7.55 7.44 7.33 7.22 Saito and Sakuma 354 7.77 7.67 7.56 7.46 7.36 7.25 7.15 7.05 Levin et al. 349 7.70 7.58 7.45 7.33 ....
0.45
0.40
0.35
E a30 LU
0,25
t l , K i i 0.5 1.0 1,5 2.0
Wavelength, /zm
Fig. 41. Emissivity of liquid cobalt near to the melting point (Samarin and Svet355).
9.3. Viscosity
Cavalier 356 has measured the viscosity of liquid cobalt with the results given in Table 46. The activation energy derived from these values was 8.4 kcal/mol (35.2 k J/tool). Samarin 357 reported that the viscosity conformed to the formula : 2500 log ~ = ~ - 0.675 although it was not explicitly stated whether this was the dynamic viscosity, n, or the kinematic viscosity, ~/p (p is the density). Assuming it to be the dynamic viscosity in centipoises, with the temperature in de~rees Celsius, the formula gives values about 30% higher than those of Cavalier °56, but with a similar tempe- rature dependence. These higher values conform more closely to those of Schenck et al. 358, which are also given in Table 46.
9.4. Surface tension
Winterhager and KrNger ~ made a comprehensive review of the published values of the surface tension of liquid cobalt and little new work has been published since that time. The values are sensitive to impurities, particularly oxygen, and it was concluded that the most reliable results were those of Kozakevitch and Urbain 359, and of Allen 360. The former recorded a value of 1886 dyn/cm (10 -3 N/m) at 15500C (1820 K) in hydrogen, while the latter reported values between 1855 and 1900 dyn/cm 126 PROGRESS IN MATERIALS SCIENCE
TABLE 46. Viscosity of liquid cobalt
Temperature Viscosity, cP (mPa s) °C Cavalier 356 Schenck et al. 358
1450 4.46 - 1495 4.18 - 1500 4.14 5.21 1550 3.85 4.75 1600 3.61 4.36 1650 3.40 - 1700 3.20 - 1750 3.03 -
at about 150~C (1770 K) in vacuum. The effect of temperature has not been widely explored, but Allen and Kingery 361 reported values of 1870, 1820 and 1600 dyn/cm at 1500, 1635 and 1800°C (1770, 1900 and 2070 K) respectively for 99.5% cobalt. Winterhager and KrUger 4 corrected the figures to 1760, 1700 and 1493 dyn/cm respec- tively, by using the more accurate density figures determined by Lucas 174. Levin et al. 349 found the following relationship for surface energy of cobalt: o = 2170 - 0.2 T MJ/m 2 where T is the temperature. This corresponds to the same numerical expression for surface tension in N/m (103 dyn/cm). It agrees with Allen and Kingery's value at 150~C (1770 K) but indicates much lower temperature dependence.
9.5 Miscellaneous properties
The magnetic susceptibility of liquid cobalt, amongst other metals, was measured by Urbain and Ubelacker 362 at temperatures up to 1800°C (2070 K) in argon. Similar work in the vicinity of the melting point has been reported by Dovgopol et al. 383
Fehling and Schei169 studied the supercooling of molten metals and their findings have been referred to earlier in connection with the determination of the melting point of cobalt (Section 4.2.1). They related supercooling to the presence or absence of nuclei which may be removed from the molten metal by appropriate glassy slags. For cobalt, a maximum supercooling of 310°C (580 K) was obtained after heating to only 5° above the melting point, and it remained unchanged with higher degrees of overheating. Supercooling has virtually no effect on the macrostructure since it does not affect crystallization mechanisms 364.
Gas solubilities in liquid cobalt have been dealt with in Section 8.3.
The thermodynamic functions of liquid cobalt are dealt with in Section 4.2.5, Table 18. PROPERTIES OF METALLIC COBALT 127
CONVERSION FACTORS
Physical quantity e.g.s, units S.I. units
Wavelength 1 A I0 -I nm Cross-section 1 barn 10 -28 m 2 Energy 1 erg 10 -7 J 1 eV 1.602 10 -19 J Surface energy 1 erg cm -2 10 -3 J m -2 Energy density i erg cm -3 i0 -I J m -3 Heat energy 1 cal 4.187 J Heat capacity 1 cal/°C g mole 4.187 J/K mol Heat content I cal/g mole 4.187 J/mol Thermal conductivity 1 cal/cm s °C 418.7 W/m K Pressure 1 rmn Hg 133.3 Pa I torr 133.3 Pa 1 bar 105 Pa 1 atm 1.013 105 Pa Stress 1 dyn cm -2 I0 -I N m -2 1 kgf mm -2 9.807 MN m -2 (N rmn-2) Surface tension 1 dyn cm -I 10 -3 N m -I Viscosity 1 cP 10 -3 Pa s (I mPa s) Electrical resistivity 1 microhm cm 10-8~m Magnetic field 1 oersted 79.58 A m -I Magnetic flux density 1 gauss 10 -4 T Magnetic permeability 1 gauss/oersted 12.57 10 -7 H m -I
Acknowledgements. This article was prepared for the Cobalt Information Centre and was intended to be published as a further item in the Cobalt Monograph Series. It appears in its present form by courtesy of the Cobalt Information Centre and its former member companies.
The author's thanks are due to the former staff of the Cobalt Information Centre for help in the preparation of the article, particularly to M. J. Dumont for his valued advice and guidance. 128 PROGRESS IN MATERIALS SCIENCE
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