Invar and Elinvar Characteristics in Nonferromagnetic Cr-Co Dilute Binary Alloys*

By Kazuaki Fukamichi,** Norio Fukuda*** and Hideo Saito**

Invar and Elinvar type alloys are important materials for precision instruments. Practical applications of these alloys are, however, often restricted to within narrow limits because of their ferromagnetism. Therefore, researches to develop nonferromagnetic Invar and Elinvar type alloys have recently received considerable attention. Chromium is an antiferromagnetic metal and its physical properties in the neighborhood of the Neel temperature are drastically affected by addition of solute atoms. The present authors have investigated the thermal expansivity Δl/l, the relative change in the electrical resistivity Δρ/ρ and the temperature dependence of the magnetic susceptibility X for Cr-Co dilute alloys. The thermal expansivity of these alloys in the vicinity of room temperature is very small, showing the Invar characteristic. Some alloys show also the Elinvar characteristic in the same temperature range where the Invar characteristic occurs. Both Invar and Elinvar characteristics have for the first time been found in chromium dilute binary alloys. The magnetic susceptibility of these alloys is less than 5×10-6emu/g, indicating that they are practically nonferromagnetic. The Neel temperature of the Cr-Co dilute alloys varies irregularly with increasing cobalt con- centration;itdecreases slightlywith cobalt concentration up to 2.0%Co, increasesup to 2.5%Co, and decreases with further increase in cobalt concentration. But the Neel temperature determined from the temperature dependence of Δ ρ/ρ and X does not coincide with an inflection point on the thermal expansion curve. (Received April 22, 1975)

thermal expansion of the alloy due to the tem-

I. Introduction perature increase of 1℃. Consequently, it results from the fact that the alloy cannot be Invar and Elinvar type alloys are widely used used as materials of electromagnetic and preci- in the field of electromagnetic and precision sion instruments which are often put in a instrumentation engineering. These alloys, how- static magnetic field and, moreover, the alloy ever, have been all ferromagnetic and applica- exhibits a magnetostrictive oscillation in an tions are often restricted to within narrow alternating magnetic field. limits because of their ferromagnetism. The elastic moduli of ferromagnetic Elinvar Ferromagnetic Invar type alloys exhibit a type alloys are affected remarkably in a mag- large magnetostriction in a static magnetic netic field, as known as the ΔE and ΔG effects(2). field. For example, Fe-36%Ni Invar alloy For example, when the Elinvar alloys are used shows a magnetostrictive elongation of about as hair springs of watches, the rate is affected 2×10-5 per unit length in a magnetic field of by a magnetic field, and the oscillation is 80 Oe(1) and this value is larger than the stopped at only 80 Oe(3). Therefore, the dis- * This paper was presented at the Spring Meeting covery of nonferromagnetic Invar and Elinvar type alloys has long been awaited for. of the Japan Institute of Metals (1973), Tokyo, Japan, and published originally in Japanese in The elastic moduli of metals and alloys J. Japan Inst. Metals, 38 (1974), 327. exhibit remarkable crystal anisotropy, and so ** The Research Institute for Iron , Steel and Other we can comparatively easily obtain Elinvar Metals, Tohoku University, Sendai 980, Japan. type alloys of polycrystalline materials by ap- *** Graduate School , Tohoku University, Sendai 980, Japan. Present address: Nippon Gakki plication of cold working or heat treatment. Co. Ltd., Metal Division, Iwata, Shizuoka 438, Therefore, the nonferromagnetic Elinvar char- Japan. acteristic has often been reported on paramag-

Trans. JIM 1976 Vol. 17 126 Kazuaki Fukamichi, Norio Fukuda and Hideo Saito

netic or antiferromagnetic metals and al- found that these alloys have both Invar and loys(3)～(8). On the contrazy, no crystal aniso- Elinvar characteristics. tropy should exist in the thermal expansion of cubic crystals. Therefore, the only way to obtain II. Experimental nonferromagnetic Invar type alloys should be based on the anomalous volume effect of an As starting materials for the alloys, Cr antiferromagnet. (99.99%) and Co (99.99%) were used. The Mn-base antiferromagnetic alloys of Mn-Cu, alloys were prepared in the form of buttons by Mn-Ni, etc. cause the martensitic transforma- repeating arc-melting three times in argon tion and the magnetic transformation in the atmosphere. They were remelted in a strip- vicinity of room temperature, but their thermal shaped form in the same atmosphere. The expansion coefficient changes only slightly at specimens were vacuum-sealed in quartz am- the transformation temperature, showing no poules, homogenized for 3 days at 800℃ and Invar characteristic(9) Other antiferromag- then cooled in a furnace. netic Elinvar type alloys containing Mn gener- The temperature dependence of the electrical ally exhibit a large thermal expansion coeffi- resistivity was measured by the four-terminal cient(6); the minimum coefficient obtained so method using a direct current potentiometer. far is about g×10-6for a binary Fe-34.8%Mn The thermal expansion curves were determined alloy. The value is too large to be called the with a roller-mirror type dilatometer. The Invar type and it should be less than 4×10-6 temperature dependence of the magnetic sus- to be Invar type alloys(10) ceptibility and the magnetization curve were The study of physical properties of Cr-base measured with a magnetic balance. The tem- primary solid solution alloys was for the first perature dependence of the proper frequency time made by Newmann et al. for the Cr-Fe of vibration was determined with a vibrator system, and it was found that the thermal controlled oscillator system. expansion and the electrical resistivity change remarkably in the neighborhood of the Neel III. Results and Discussion temperature(11). The spin density wave theory was then proposed by Overhauser(12), and Figure 1 shows thermal expansion curves of since then Cr and its primary solid solution alloys have been investigated by many workers(13)～(20). Magnetic phase diagrams of Cr-V(13), Cr-Fe(14) and Cr-Mn(13) systems have been determined from the experimental results of neutron diffraction. The spin structure differs between these phase diagrams depending on the kind of additional elements and tem- perature. Thermal expansion, elastic modulus and electrical resistivity exhibit anomalies along with such variations of the spin structure. The present authors have studied many Cr- base alloys taking these unusual phenomena into account and have already discovered many Cr-Fe base ternary Invar type alloys in the primary solid solution range of Cr-Fe-Mn, Cr-Fe-Ru and Cr-Fe-Sn(21)～(25). In the present study, the thermal expansion, electrical resistivity, elastic characteristic and magnetic property of Cr-Co binary primary solid solu- Fig. l Temperature dependence of the thermal tion alloys have been measured and it has been expansivity for some Cr-Co alloys. Invar and Elinvar Characteristics in Nonferromagnetic Cr-Co Dilute Binary Alloys 127 some Cr-Co binary solid solution alloys. The thermal expansivity Δl/l becomes smaller at room temperature with increase in Co con- centration. Especially, the thermal expansion coefficient α of a Cr-2.12%Co alloy is almost zero from -10°to 30℃ showing an excellent Invar characteristic. With further increase in Co concentration, conversely,Δl/l becomes gradually larger, but as shown in the figure, in the case of a Cr-3.44% Co alloy, the value of a at room temperature is about 3.4×10-6 which is only a few tenths of the values of usual metals and alloys. In the high temperature range, the thermal expansion coefficients of all Cr-Co alloys are almost the same as that of pure Cr. In the case of Cr-Fe binary solid solution alloys, the thermal expansion curve and the Neel temperature are remarkably Fig. 2 Relative change in proper frequency of vibra- changed by the addition of third elements such tion for Cr-Co alloys. as Mn, Sn, Ru, etc. and this fact results in a wide temperature range of the Invar char- solution alloys. These properties are important acteristic(21)～(25). As mentioned above, how- in designing precision instruments. ever, in the present experiments, the Invar In a Cr-2-0% Co alloy, the Δf/f takes a characteristic has been also obtained in the minimum value at -30℃, and it is possible to Cr-Co binary alloys. The temperature range in obtain the Elinvar characteristic in the vicinity which the above alloys exhibit the Invar char- of room temperature with addition of third acteristic is narrower than that of Cr-Fe-Mn elements such as Mn and Ru which should ternary primary solid solution Invar type raise the Neel temperature, and some ternary alloys(21)～(22), but the range is extended by alloys showing the excellent Elinvar character- control of Δl/l and the Noel temperature with istic have thus been found. Detailed results addition of third elements(26) . will be reported elsewhere in the near future. Figure 2 shows the temperature dependence Figure 3 shows the temperature dependence of the relative change in proper frequency of vibration Δf/f in Cr-Co binary primary solid solution alloys. Conventional practical Invar type alloys exhibit a maximum value for the temperature coefficient of elastic modulus e in the composi- tion where the thermal expansion coefficient a becomes minimum. Accordingly, we cannot realize the alloy composition which exhibits both Invar and Elinvar characteristics in the same temperature range. A Cr-2.7% Co alloy exhibits, however, the small change in Of/f from -20°to 20℃ showing the Elinvar char- acteristic as shown in the figure; moreover, this alloy exhibits the Invar characteristic. Both Invar and Elinvar characteristics have for the first time been found in the same temperature Fig. 3 Temperature dependence of the magnetic range in the Cr-base binary primary solid susceptibility for Cr-Co alloys. 128 Kazuaki Fukamichi, Norio Fukuda and Hideo Saito

Fig. 4 Magnetization curve of a Cr-2.12%Co alloy at room temperature. of the magnetic susceptibility x in a magnetic field of 4000 Oe. With increasing Co con- centration, the value of x becomes larger and the peak of the curve becomes more pronounced. Fig. 5 Temperature dependence of the relative The value of x is negligibly small in the order of change in the electrical resistivity for some Cr-Co 4.6×10-6emu/g in a Cr-2.03% Co alloy, alloys. indicating that the Cr-Co binary alloys are magnetically insensitive Invar-Elinvar type of Δρ/ρ. The Neel temperature of the alloys is alloys. easily determined from the relative change in Figure 4 shows the magnetization curve for a the electrical resistivity, because Δ ρ/ρ shows a Cr-2.12% Co alloy, and the curve is slightly distinct minimum at the Neel temperature. convex upward. A similar phenomenon has The composition dependence of the Neel been observed in Cr-Fe and Cr-Fe-Sn antifer- temperature determined from the Δρ/ρ curves romagnetic primary solid solution alloys(24)(27). is compared with other investigators' results in This alloy system may also have a property of Fig.6(29)～(31). In general, the additioll of superantiferromagnetism. transition metals such as Mn, Ru, Os, etc. It is well known that the electrical resistivities which are situated on the right of Cr in the of Cr, Cr-base primary solid solution alloys and periodic table increases the Neel temperature, rare earth metals take a minimum value at the and conversely the addition of V, Nb and Ti Neel temperature because of the change of situated on the left of Cr depresses It(32)～(35). carriers due to the partial truncation of the Fermi surface causing the appearance of antifer- romagnetism. In a strict sense, since an anoma- lous transport phenomenon occurs in the vicinity of the transition temperature, the Neel temperature determined from the temperature dependence of the electrical resistivity deviates approximately 1℃ from the temperature at which the magnetic moment disappears, or the temperature determined from the specific heat curve (28), but the relative change in the electrical resistivity Δ ρ/ρ of Cr-base primary solid solu- tion alloys in the vicinity of the Neel tempera- ture is remarkable. Therefore, the Neel tem- perature is often determined from the tempera- ture dependence of the electrical resistivity. Fig. 6 The Neel temperature of Cr-Co alloys as a Figure 5 shows the temperature dependence function of Co concentration. Invar and Elinvar Characteristics in Nonferromagnetic Cr-Co Dilute Binary Alloys 129

However, when the ferromagnetic elements Fe, Co and Ni are added, the concentration de- pendence of the Neel temperature is peculiar though these elements are situated on the right of Cr in the periodic table; that is, both Fe and Ni depress linearly the Neel temperature, and Co affects irregularly it as shown in Fig. 6. With increase in Co concentration, the Neel tem- perature decreases at first, increases abruptly in the concentration range from 2 to 2.5%Co, and decreases again gradually. It is known that the abrupt change in the Neel temperature around 2%Co corresponds to the change in the spin structure from the transverse sinusodial spin density wave to the commensurable antiferromagnetism, and the increase of the

Neel temperature may be caused by the stabili- Fig. 7 Temperature dependence of thermal ex- zation due to the transition (31). pansivity, relative change in the electrical resistivity The results mentioned above do not agree and magnetic susceptibility for a Cr-0.97% Co alloy. with those obtained by other investigators as seen in Fig. 6. The present study has confirmed As is well known, the residual strain by that the Noel temperature does not change working in Cr increases the Nt el temperature independent of the heat treatments at 800, 900 and changes the spin structure (36). A similar and 1100℃, and consequently the disagreement phenomenon is observed in Cr-base primary may not be caused by the difference in the heat solid solution alloys. The maximum point of x treatment condition. shifts to the higher temperature by working. As is well known in the thermodynamics, In the case of a Cr-0.97% Co alloy, it shifts by whether phase transition is of the first or the about 60℃ and the value of x becomes smaller second kind, an anomalous volume change after pulverization into fine particles. takes place at the transition temperature; that The present study has determined the tem-

is, the first kind phase transition causes a dis- perature dependence of Δl/l, Δ ρ/ρ and x in the continuous volume change, and the second selfsame-conditioned specimens, and hence, the kind phase transition does a discontinuous disagreement in Fig. 7 between the inflection change in the thermal expansion coefficient. temperature of the curves cannot be attributed Therefore, one can determine the magnetic to the difference in the condition of specimens transition temperature from the temperature preparation. dependence of the thermal expansivity Δl/l, A similar phenomenon has also been ob- and the Neel temperature of some Cr-base served for Cr-Ni(31) and Fe-Mn(37) antiferro- alloys has often been determined by this magnetic alloys. In the former, the transition method(14)(31). While, in the case of a Cr- temperature determined from the thermal 0.97% Co allov. the temperature of the maxi- expansion curves is higher about 10℃ than that rnum of X and that of the minimum of Δρ/ρ determined by neutron analysis, and the agree well with each other, the temperature of latter, the temperature at which the specific the abrupt change of Δl/l does not agree with heat curve changes discontinuously is lower them, as shown in Fig. 7. The same phenomena about 10～20℃ than that determined from the have been observed in all Cr-Co binary primary temperature dependence of the magnetic sus- solid solution alloys in the present study. The ceptibility and the electrical resistivity. temperature dependence of Δl/l is reversible, The experiments are in progress in many and Δρ/ρ exhibits no anomaly at the tempera- other Cr-base alloys containing nonferro- ture where Δl/l changes abruptly. magnetic elements, whether the above-men- 130 Kazuaki Fukamichi, Norio Fukuda and Hideo Saito tioned anomalous phenomenon occurs or not. Comm., 4 (1966),519. (5) H. Masumoto, S. Sawaya and M. Kikuchi: J. Japan Inst. Metals, 33 (1969), 121. IV. Conclusions (6) H. Masumoto, S. Sawaya and M. Kikuchi: J. Japan Inst. Metals, 35 (1971),723. The temperature dependence of thermal (7) H. Masumoto, S. Sawaya and M. Kikuchi: J. expansivity, electrical resistivity, magnetic sus- Japan Inst. Metals, 35 (1971), 1143. (8) H. Masumoto, S. Sawaya and M. Kikuchi: J. ceptibility and proper frequency of vibration of Japan Inst. Metals, 35 (1971), 1150. Cr-Co binary primary solid solution alloys (9) K. Fukamichi and H. Saito: unpublished. were studied, and significant conclusions ob- (10) Y. Tanji and Y. Shirakawa: J. Japan Inst. Metals, tained are as follows: 34 (1970), 897. (11) M. M. Newmann and K. W. H. Stevens: Proc. (1) Cr-Co binary primary solid solution Phys. Soc., 74 (1959),290. alloys exhibit the Invar characteristic in the (12) A. W. Overhauser: Phys. Rev. Lett., 4 (1960),462. vicinity of room temperature. This characteris- (13) T. J. Bastow: Proc. Phys. Soc., 88 (1966), 935. tic has for the first time been found in Cr-base (14) Y. Ishikawa, S. Hoshino and Y. Endoh: J. Phys. Soc. Japan, 22 (1967), 1221. binary alloys, and the magnetism of the alloys (15) S. Komura, Y. Hamaguchi and N. Kunitomi: is neglected in practical applications. J. Phys. Soc. Japan, 23 (1967), 171. (2) Some of the Cr-Co alloys exhibit the (16) Y. Hamaguchi and N. Kunitomi: J. Phys. Soc. Elinvar characteristic at room temperature. The Japan, 19 (1964), 1849. (17) M. A. Mitchell and J. F. Goff: Phys. Rev. B, 5 alloys have both Invar and Elinvar characteris- (1972), 1163. tics and are very useful for precision instru- (18) I. R. Herbert, P. E. Clark and V. H. Wilson: ments. J. Phys. Chem. Solids, 33 (1972),979. (19) A. Arrott, S. A. Werner and H. Kendrick: Phys. (3) The Noel temperature determined from Rev., 153 (1967), 624. the temperature dependence of Δρ/ρ or X does (20) S. Arajs and G. R. Dummyer: J. Appl. Phys., 38 not agree with the inflection point of the (1967), 1892. thermal expansion curve. A similar phenome- (21) K. Fukamichi and H. Saito: Phys. Status Solidi, non has been observed in a few other alloy (a), 10 (1972), K129. (22) H. Saito and K. Fukamichi: Solid State Physics systems. (in Japanese), 7 (1972), 411. (4) With increase in Co concentration, the (23) H. Saito and K. Fukamichi: IEEE, Trans, Magn., Neel temperature decreases irregularly though 8 (1972), 687. (24) K. Fukamichi Y. Suzuki and H. Saito: J. Japan Co is situated on the right of Cr in the periodic Inst. 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