Physical Metallurgy of Metals Belonging To Groups Va and Via of the Periodic Table S. Banerjee Metallurgy Division Bhabha Atomic Research Centre Bombay 400 085, India CONTENTS Page ABSTRACT 2 1. INTRODUCTION 2 2. INTERATOMIC BOND AND PHYSICAL PROPERTIES 2 3. PHASE DIAGRAMS 6 4. INTERACTION WITH INTERSTITIAL IMPURITIES 13 5. DUCTILE-BRITTLE TRANSACTION 14 6. RECOVERY AND RECRYSTALLIZATION 18 7. DEFORMATION MECHANISMS AND STRENGTHENING METHODS 20 8. PROTECTIVE COATINGS 26 9. SOME COMMON REFRACTORY ALLOYS 28 A. Vanadium Alloys 28 B. Niobium Alloys 29 C. Tantalum Alloys 29 D. Chromium Alloys 29 E. Molybdenum Alloys 31 F. Tungsten Alloys 31 10. SUMMARY 33 ACKNOWLEDGEMENTS 33 REFERENCES 33 Vol. 11, Nos. 1-4, 1993 Physical Metallurgy of Metals Belonging to Groups Va and Via of the Periodic Table ABSTRACT their applications have phenomenally extended and expanded to encompass almost every high-tech area: for Metals belonging to the groups Va and Via of the instance, the aerospace and nuclear industries, high periodic table have many common features. They have energy physics, electronics, energy conversion and the same crystal structure (bcc), high melting points, chemical processing. similar chemical interactions with other elements, In spite of the wide range of applications of generally high ductile-brittle transition temperature and refractory metals and alloys, the scientific literature similar alloying behaviour. This review gives a brief dealing with their physical metallurgy principles is account of the interatomic bond and physical properties rather limited. Information on specific metals or alloys of these elements and their relation with the position of are, however, available in scattered sources. The fact an element in the periodic table. The different classes of that, in the case of these metals, different physical phase diagrams these metals exhibit and their inter- phenomena like deformation, diffusion, recovery, actions with interstitial elements are summarized. The recrystallization, creep and oxidation stand on common problem of high ductile-brittle transition temperature and ground and bear a great deal of similarity has prompted the general principle of ductilization are discussed. In application of the same principles for alloying, the following sections the methods of strengthening strengthening, ductilizing and protecting these metals. both at ambient and at elevated temperatures and the In the present article, an attempt has been made to techniques of raising the recrystallization temperature are highlight the common features of these metals and discussed. The underlying principles for the develop- alloys and the basic physical metallurgy principles ment of protective coatings on these reactive metals, which influence the development of a wide range of especially for elevated temperatures service, are ex- alloys and which control the performance of these alloys plained. The last part of this review is devoted to the in different aggressive service conditions. application of the general principles of alloying for improving the strength, creep resistance and ductility of 2. INTERATOMIC BOND AND PHYSICAL refractory metal alloys. PROPERTIES 1. INTRODUCTION BRMs belong to the transition elements which are characterized by strong interatomic bonds and, con- Metals belonging to groups Va and Via of the sequently, by high melting points, mechanical strength periodic table have many commonalities. Similar and electrical resistance. The strength of the interatomic electronic configurations and identical crystal structure of bond depends mainly on the structure of the outer these metals are the two primary factors which make electron shells of the atoms and, therefore, the physical these metals possess remarkably similar physical, properties like melting point, boiling point, shear chemical and mechanical properties. The metals which modulus, coefficient of linear expansion, show a periodic are classified as refractory metals are those which have dependence on the atomic number of the element (Fig. 1) melting temperatures above 2400 'C. According to this /l/. The fact that the interatomic bond strength reaches a classification, a few members of the groups Va and Via, maximum in the group Via metals, chromium, such as vanadium and chromium, do not strictly qualify molybdenum and tungsten, is reflected in the peak in as refractory metals. In this paper, metals belonging to melting points of metals belonging to periods 4, 5 and the groups Va and Via will be abbreviated as BRMs (bcc 6, all of which exhibit a similar trend with a maximum Refractory Metals), unless otherwise specified. located in group Via. The melting point increases with The use of refractory metals and alloys was once an increase in the period number in a given group. limited to lamp filaments, electron tube grids, heating Elastic modulus and a number of physical properties elements, electrical contacts, etc.. At present, however, follow a similar behaviour while the coefficient of linear 2 S. Banerjee High Temperature Materials and Processes ΠΙΑ VA VBASBÄ IB ·Β VB HA TO OA vilA IB mB VB A Β • Po VIA «Α IB BB VB D Fig. 1: Variation of melting point (A), boiling point (B), shear modulus (C), and coefficient for thermal expansion (D) with group number for elements in periods 4,5 and 6. 3 Vol. 11, Nos. 1-4, 1993 Physical Metallurgy of Metals Belonging to Groups Va and Via of the Periodic Table thermal expansion and interatomic distance show an body centered cubic lattice while rhenium, ruthenium and inverse trend, minimum values being associated with osmium have a close packed hexagonal lattice. In group Via metals. Though there is good correlation general, the refractory metals situated on the right side of between these physical properties with their position in group Via have an hep or fee lattice while those on the the periodic table, the shear modulus alone falls some- left have a bcc lattice. In terms of the occupancy of d- what out of the general trend; its maximum value does shells, when more than half of the d-shells are filled, a not occur in tungsten, as was expected, but in osmium. close packed lattice is formed while a bcc lattice is The high bond energy of the transition metals is stabilized in metals with less than half the d-shells filled. believed to be due to the participation of d-electrons in Some important physical properties of group Va the bond formation by the hybrid spd-orbitals. The and Via refractrory metals are summarized in Table 1 higher values of the electronic specific heat of transition which shows some characteristic features like high metals is regarded as evidence of the contribution of d- density, high melting and boiling points, low thermal electrons to the hybridized spd-shells. In the elements and electrical conductivities of these metals. The appli- belonging to the first long period, the outer electrons are cations of refractory metals as heat resistant structural considered to be in two different but intersecting bands, materials, parts of electrovacuum instruments, heating formed of 3d- and 4s-states of isolated atoms. According elements and filaments are all possible because of these to Mott and Jones /2/, the bonding forces arise only as a characteristic properties. For example, high melting and result of 4s-electrons. Hume-Rothery /3/ has pointed boiling points, low vapour pressure, high resistivity, out that in transition metals of groups Ilia-Vila spearate low thermal expansion and high emissivity of tungsten s- and d-bands are not justified and both of them parti- are all attractive features for a filament material and cipate in bond formation. A6cording to Pauling /4/, the hence the universal acceptance of this metal in such electrons responsible for the formation of bonds in the applications. crystal lattice of a transition metal, belong to the hybrid The high electrical resistance of the refractory spd-orbitals. In the first long period from calcium to metals can be explained on the basis of the zone theory. vanadium, there is a continuous increase in the number The conduction of electrons in transition metals involves of electrons per atom participating in the bond formation s-d transitions which have higher probability of inter- (from one to five). In chromium this value reaches to actions with lattice vibration. This causes high resis- 5.78 (0.22 electron per atom being situated on 3d- tivity in refractory metals. Resistivity is a structure- orbitals). On further increase in the atomic number, sensitive property. Work hardening is known to increase electrons participating in the bond formation remain at the resistivity of metals. The extent of this resistivity 5.7. This scheme does not explain the occurrence of the increase with cold work is very high in tungsten. maximum of melting point in group Via metals in a Typically, the resistivity value increases from 5 μΩ cm given period. Hume-Rothery, Irving and Williamson to 7 μΩ cm with extensive cold working of wire. The /5/, therefore, proposed another valence system in which increase in resistivity is accompanied by a decrease in the the number of electrons per atom which participate in temperature coefficient of electrical resistance /I/. the covalent bond formation is considered. This valency The high resistivity of the refractory metals enables (as exhibited in the covalent compounds of these metals) them to be used in the production of high ohmic alloys. shows a maximum for group Via metals and then The most effective increase in resistivity is achieved decreases and also increases on passing to succeeding when palladium or platinum is alloyed with refractory long periods. metals. The crystal structure of the transition refractory Many of the refractory metals are superconductors, metals is determined by their electronic structure and is these include Ti, Zr, Hf, V, Nb, Ta, Mo, W, Tc, Re, consequently related to the position of these metals in Ru, Os, Ir. Niobium has the maximum superconducting the periodic table. BRMs are monomorphic and have transition temperature amongst all metals and the major 4 S.