Фазовая диаграмма системы Be-Co

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Be-Co (Beryllium-Cobalt) H. Okamoto, L.E. Tanner, and T. Nishizawa The assessed Be-Co phase diagram is adopted primarily from [77Ald]. A partial diagram based on [77Pug] proposes a possible alternative for the composition range between 0 and 50 at.% Co. Although [77Ald] and [77Pug] agree reasonably well as far as the overall form of the liquidus is concerned, the disagreement between them regarding other sections of the diagram is so significant that a compromise appears to be quite difficult to accomplish. As a whole, the liquidus of [77Ald] is tentatively accepted in the assessed phase diagram because of its coverage of the entire composition range. Both diagrams contain some unlikely features when considering the thermodynamics of the system. Further investigation is obviously necessary to resolve the contradictions. Alternative phase relationships have been derived in this evaluation by means of thermodynamic modeling. The equilibrium phases of the Be-Co system are (1) the liquid, L; (2) the bcc ( bBe) phase with a maximum solubility of ~6 at.% Co; (3) the cph (aBe) phase with a maximum solubility of ~3 at.% Co; (4) the g brass-type g phase and fcc g› phase in the composition range 7.5 to 20 at.% Co with the congruent melting point of 1400 C at 14 at.% Co; (5) the hexagonal e phase with an approximate stoichiometry Be7Co; (6) the hexagonal z phase in the composition range 23 to 28 at.% Co, forming by a peritectoid reaction at 1092 C; (7) the CsCl-type b phase in the composition range 25 to 53 at.% Co with a congruent melting point of 1420 C at 47 at.% Co; (8) the bcc b1 phase, of uncertain structure, in the composition range 70 to 85 at.% Co in a limited temperature range between 1262 and 1230 C, with a minimum melting point of ~1215 C at ~74 at.% Co; and (9) the fcc terminal solid solution, (aCo), with a maximum solubility of 12 at. % Be. The effect of ferromagnetic interaction in (aCo) reduces the Be solubility significantly. The cph terminal solid solution phase (eCo) exists below 422 C with a maximum solubility of ~0.5 at. B . The metallurgical structures of (eCo) formed by martensitic transformation are closely related to the distribution of magnetic domains of the high- temperature (aCo) phase [76Fuk]. The maximum solubility limit of Co in (bBe) is ~6 at.% [77Ald, 77Pug]. The superconducting transition temperatures of 4.2 and 5.6 at.% Co alloys as arc melted or fast quenched into water from 1200 C are about 2.5 K [67Ols]. The superconductivity was attributed to the stabilized (bBe) phase; [61Pic], on the other hand, could not suppress the b to a transformation in a 5.3 at.% Co alloy by quenching from 1200 C even at -16 000 C/s. The superconductivity found by [67Ols] was probably due to the e phase. The maximum solubility of Co in (aBe) is approximately 3 at.%. The transition from g to g› has not been well defined but appears to be of higher order. [77Ald] and [77Pug] disagreed regarding the solidus composition by up to ~4 at.%. The g = (aBe) + e eutectoid transition temperature was reported quite differently: 993 с 5 [77Ald] and 860 C [77Pug]. No information is available to corroborate either of these results. According to [77Ald], the hexagonal z phase forms by a peritectoid reaction from g› and b at 1092 с 5 C. [77Pug] also found a thermal effect at a similar temperature, 1100 C, but attributed it to a disordering reaction of z to a high-temperature orthorhombic phase, z›, which then decomposes into L and g› at 1284 C on further heating. This disagreement has not been resolved. According to [77Ald], the CsCl-type b phase extends from 25 to 53.5 at.% Co, has a minimum melting point of 1240 C at about 29 at.% Co, and has a congruent melting point of 1420 C at 47 at.% Co. On the other hand, [77Pug] observed that this phase field consists of the CsCl-type b phase in a composition range from 34 to at least 50 at.% Co (the maximum composition investigated) and the bcc W-type b› phase in a composition range from 26.5 to 38.5 at.% Co. The maximum temperature of b› is 1344 C at the L + b = b› peritectic, and the minimum temperature is 980 C at the b› = z + b eutectoid [ 77Pug]. Again, the disagreement between [77Ald] and [77Pug] remains unresolved. Because of the similarity in phase diagrams, it is tempting to assume that b1 is part of b, separated by an order-disorder transformation, by analogy with the Be-Cu system, or that b1 is disordered bcc, similar to the (aFe) phase in the Be-Fe phase diagram. However, it was found that there is a two-phase field between b1 and (aFe) in the ternary diagram (present investigation). Therefore, the nature of b remains unresolved. A solubility of Be vs 1/T plot of this solvus indicates that the solubility limit of Be at the allotropic transformation temperature of Co is about 0.5 at.%, which is considerably less than the value of ~6 at.% estimated by [37Kos] and [38Has] from the aCo = eCo transition temperatures. Study of the aging process of (aCo) revealed that the precipitation sequences are similar to those observed for the aging of (Cu) in the Be-Cu system. The AuBe5-type d phase found by [57Bat] and [66Bec] may exist at low temperatures or as a metastable phase because similar phases are found in Be- Mn, Fe, and Cu systems. [57Bat] reported finding a Mn12Th-type Be12Co phase, but there is no further confirmation. As of this writing, it is not known whether this phase exists, even metastably. [57Bat] also reported a Mn12Th- type Be12Fe, which does not exist as a stable phase. During a segregation process in a 90.9 at.% Co alloy, a metastable bct phase appears after 1 to 3 min tempering at 650 C [71Zak]. It is similar to the metastable g› phase (~50 at.% Cu) in the Be-Cu system. [77Var] found a tetragonal metastable phase in rapidly quenched alloys with compositions ranging from 60 to 84 at.% Co. The Curie temperature of aCo is 1121 C [83Nis]. The composition dependence of the Curie temperature in (aCo) was measured by [37Kos], [38Has], [76Jon], and [ 77Ald] and is assumed to be ~1062 + 2183 X C in the present thermodynamic modeling. The magnetic moment per atom is 1.731 - 3.10 (1 - X) mB [67Her], where X is atomic fraction of Co. 36Mis: L. Misch, Metallwirtschaft, 15(6), 163-166 (1936) in German. 37Kos: W. Koster and E. Schmid, Z. Metallkd., 29(7), 232-233 (1937) in German. 38Has: U. Haschimoto, J. Jpn. Inst. Met., 2, 70-71 (1938) in Japanese. 51Ven: G. Venturello and A. Burdese, Alluminio, 20, 558-564 (1951) in Italian. 57Bat: F.W. von Batchelder and R.F. Raeuchle, Acta Crystallogr., 10(10), 648- 649 (1957). 61Pic: J.J. Pickett, E.D. Levine, and W.B. Nowak, U.S. At. Energy Comm., NMI- 1252, 34 p (1961). 66Bec: H.J. Becher and H. Neidhard, Z. Anorg. Allg. Chem., 344, 125-139 (1966) in German. 67Her: A. Herr and A.J.P. Meyer, Compt. Rend. B, 265, 1165-1168 (1967) in French. 67Ols: C.E. Olsen, B.T. Matthias, and H.H. Hill, Z. Phys., 200, 7-12 (1967). 70Joh: Q. Johnson, G.S. Smith, O.H. Krikorian, and D.E. Sands, Acta Crystallogr. B, 26(2), 109-113 (1970). 71Zak: M.I. Zakharova, V.V. Korchazhkin, and V.V. Moshkov, Dokl. Akad. Nauk SSSR, 201(4), 894-896 (1971) in Russian; TR: Dokl. Phys. Chem., 201(4), 1020- 1022 (1971). 76Jon: S. Jonsson, Diss., Max Planck Institute (1976) in German. 77Ald: F. Aldinger and S. Jonsson, Z. Metallkd., 68(5), 362-367 (1977) in German. 77Pug: M.S. Pugachev, L.F. Verkhorobin, M.M. Matyushenko, and I.V. Aleksenko, Dop. Akad. Nauk Ukr. RSR, A, Fiz. Mat. Tech., (12), 1135-1137 (1977) in Ukrainian. 77Var: N.I. Varich, V.I. Savich, and A.N. Petrunina, Izv. V.U.Z. Tsvetn. Metall., (5), 113-116 (1977) in Russian. 83Nis: T. Nishizawa and K. Ishida, Bull. Alloy Phase Diagrams, 4(4), 387-390 ( 1983). Published in Bull. Alloy Phase Diagrams, 9(5), Oct 1988, and Phase Diagrams of Binary Beryllium Alloys, 1987. Complete evaluation contains 5 figures, 9 tables, and 30 references. Special Points of the Be-Co System