Фазовая диаграмма системы Co-V

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Co-V (Cobalt-Vanadium) J.F. Smith The assessed Co-V phase diagram is based primarily on the experimental data of [55Kos], with review of the work of [51Duw], [63Zeg], [79Aok], [81Ind1], [ 81Ind2], and [85Aok]. The major features of the Co-V system show qualitative similarity with the Ni-V system: (1) there is a first-order congruent transformation near 25 at.% V from a high-temperature terminal solid solution to an ordered intermediate phase; (2) there is a s phase with an extensive range of homogeneity in the central portion of each system that decomposes peritectically; (3) there is a eutectic reaction between the s phase and the Co-rich or Ni-rich terminal solution; and (4) there is an intermediate phase with Cr3Si-type structure and stoichiometry near 75 at.% V that forms by peritectoid reaction between the s phase and the V-rich terminal solution. However, there is no evidence of a phase in the Co-V system with stoichiometry near Co2V that corresponds to Ni2V. Although these features of the diagram are well established, a number of confusing aspects exist, particularly in the Co- rich region of the diagram. This results from the difficulty in achieving equilibrium, due to interference from coherency effects and from magnetic energy contributions. The assessed diagram has been drawn to run from the melting point through the experimental points of [55Kos] to terminate at a eutectic composition of 41.5 at.% V and temperature of 1248 C. The limiting slope of the solidus was matched to the liquidus in conformity with the constraints of the van't Hoff equation. At higher V concentrations, the solidus was curved to terminate at 35 at.% V and the 1248 C eutectic temperature; this composition corresponds to the maximum terminal solubility of the paramagnetic fcc Co solution, (apCo). This paramagnetic terminal solution transforms congruently near 25 at.% V and 1070 C to an ordered intermediate phase. Below the melting point of pure solid Co, a magnetic transition and an allotropic transition also occur. The Curie temperature for the magnetic transition is 1121 C [83Nis], and the ferromagnetic fcc terminal solution below this magnetic transition is designated (afCo). [81Ind1] and [81Ind2] indicated a steep decrease in Curie temperature with increasing V content. With composition dependence of the Curie temperature taken as TC (K) = 1395 - 4040 XV, where XV is the mole fraction of V and with the mean magnetic moment per atom being approximated in Bohr magnetons (mB) as = 1.7 - 5.7 XCoV, they calculated that the second-order magnetic transition should shift to a first-order transition at a tricritical point near 1023 C (1300 K) and 3 to 4 at.%. This shift begins a two-phase field between a V-rich paramagnetic phase and a Co-rich ferromagnetic phase, both with the same fcc crystal symmetry. [ 81Ind1] speculated that this first-order magnetic transition terminates in a eutectoid reaction. Such a eutectoid reaction has been included in the assessed diagram to meet the requirement of the phase rule. The allotropic transition between a low-temperature cph form of pure Co, eCo, and the higher-temperature afCo was reported [83Nis] as occurring at 422 C. However, the mechanism of the transformation is complex, because of the coherence between the cph and fcc structures. This leads to sluggish kinetics and makes it almost impossible to obtain the pure cph form because of extensive faulting, which generates local fcc layering. [38Kos] found the magnetic transition to decrease monotonically with increasing V content, but the allotropic transition was found to be ambivalent, with a monotonic decrease with increasing V content on cooling, but with a monotonic increase with increasing V content on heating. This must reflect the sluggish kinetics of the transition, because thermodynamic considerations based on the logic behind the van't Hoff equation would require that, under equilibrium conditions, both boundaries of any two-phase region emanating from an elemental phase transition should increase or decrease together in temperature with changing composition. [55Kos] reported that the allotropic transformation temperature upon heating increases with increasing V content and that the transformation temperature upon cooling decreases with increasing V content. This is characteristic of the metastability of a martensitic transition, with the cooling transition defining the Ms temperature and the heating transition defining the As temperature. The true equilibrium transition temperature for a martensitic material is generally considered to be near the mean of the Ms and As temperatures [58Kau]. Because the experimental data for the Co-V system indicate that this mean tends to decrease slightly with increasing V content, the transition in the assessed diagram is shown as dropping from 422 C at 100% Co [83Nis] to a eutectoid reaction with (afCo) decomposing to (eCo) and Co3V. This is shown by dashed lines because the details must be considered speculative. The assessed diagram shows the paramagnetic fcc terminal solid solution near 25 at.% V as transforming during cooling through 1070 C [55Kos]. [59Sai] found an ordered hexagonal superlattice, with the ordering within a close- packed layer being the same as in AuCu3, but with a different layering sequence. [74Aok] reported the transformation at a slightly lower temperature of 1035 C. Both investigators reported two transformations, with the upper transformation near 1035 C and the lower transformation near 1010 C and with the phase between the transformations having cubic AuCu3 structure. The assessed diagram shows this second transformation with dashed lines, because it has been shown that equilibrium in this general region of the phase diagram is difficult to obtain [85Aok]. Long-term equilibration [85Aok] (greater than 3 weeks at 800 C) of test specimens of an 18.2 at.% V alloy showed phase separation markedly different from that developed during equilibration for only 1 week. This separation after 3 weeks indicates that the limiting phase compositions at 800 C are 14. 5 and 22 at.% V. This result has led to the conclusion that the equilibria in this composition region in the assessed diagram are most likely correct, and that a metastable structure is produced after long-term equilibration. Two metastable fcc phases have been reported by [79Aok]. A phase that occurs below ~700 C was reported to be isomorphous with AuCu3 in the L12-type structure. The phase that occurs above ~700 C also has a structure based on the L12 type. [80Aok] revealed satellite spots around the fundamental reflections, and these were considered to be due to a lattice modulation of the ordered L12 lattice. 38Kos: W. K”ster and K. Lang, Z. Metallkd., 30, 350-352 (1938). 51Duw: P. Duwez, Trans. AIME, 199, 191 (1951). 55Kos: W. K”ster and H. Schmid, Z. Metallkd., 46, 195-197 (1955). 58Kau: L. Kaufman and M. Cohen, Prog. Met. Phys., 7, 165-246 (1958). 59Sai: S. Saito, Acta Crystallogr., 12, 500-502 (1959). 59Stu: H.P. StЃwe, Trans. AIME, 215, 408-411 (1959). 63Zeg: S.T. Zegler and J.W. Downey, Trans. AIME, 227, 1407-1411 (1963). 74Aok: Y. Aoki, K. Asami, and M. Yamamoto, Phys. Status Solidi (a), 23, K167- K169 (1974). 79Aok: Y. Aoki, Y. Obi, and H. Komatsu, Z. Metallkd., 70, 436-440 (1976). 80Aok: Y. Aoki, H. Hiroyoshi, and M. Kawakami, J. Magn. Magn. Mater., 15-18, 1179-1180 (1980). 81Ind1: G. Inden, Physica, 103B, 82-100 (1981). 81Ind2: G. Inden, Scr. Metall., 15, 669-671 (1981). 83Nis: T. Nishizawa and K. Ishida, Bull. Alloy Phase Diagrams, 4(4), 420 (1983) . 85Aok: Y. Aoki and J. Echigoya, Scr. Metall., 19, 639-642 (1985). Published in Phase Diagrams of Binary Vanadium Alloys, 1989. Complete evaluation contains 6 figures, 5 tables, and 54 references. Special Points of the Co-V System