Фазовая диаграмма системы Be-Cu

К оглавлению: Другие диаграммы (Others phase diargams)

Be-Cu (Beryllium-Copper) D.J. Chakrabarti, D.E. Laughlin, and L.E. Tanner The equilibrium phases of the Cu-Be system at 1 atm are (1) the liquid, which is miscible at all compositions; (2) the fcc terminal solid solution, (Cu), which is stable up to the melting point of Cu at 1084.87 C; (3) the bcc disordered solid solution, b, which is stable above ~620 C and is formed by a peritectic transformation at 866 C (and probably also at 930 C), or congruently from the liquid at 860 C, or by direct solidification from the liquid; (4) the simple, cubic g (B2, CsCl-type) ordered phase, which is stable over a narrow phase field at off-equiatomic Be-deficient compositions; this phase is formed presumably by a higher order transition from b above the maximum in the b + g two-phase region at 880 C and below that by spinodal transformation; it is also formed eutectoidally at ~620 C and below that by precipitation from (Cu) via several intermediate metastable phases; (5) the Laves-type fcc phase, d, with a broad phase field that extends from ~64.3 at.% Be at 930 C to ~81.5 at.% Be at 1190 C and up to 1219 C where it is formed by congruent transformation; (6) the bcc (bBe) allotropic modification, which is stabilized in the presence of Cu from its melting point at 1289 C to 1109 C at ~13.7 at.% Cu; and (7) the cph (bBe) terminal solid solution, which extends up to 9.5 at.% Cu at 1109 C and transforms congruently to (bBe) at ~1275 C. The assessed phase diagram is based primarily on the work of [16Oes], [29Mas], [32Bor], [41Iwa], [52Abr], [63Gel], and [82Jon]. The accepted temperatures have not been adjusted to IPTS-68. The minimum in the b liquidus-solidus curves is at 28.1 at.% Be and 860 C. The solidus and the liquidus between 65 and 75 at.% Be for d are tentative, because very few data are available. The maximum solubility of Be in (Cu) is 16.5 at.% (2.73 wt.%) at 866 C, and the maximum solubility of Cu in (bBe) is 17.3 at.% at 1199 C. The solid solubility of Cu in Be and in the Be-rich end of the Cu-Be intermediate phase, d, is reduced in the presence of impurities. Further extensions of both single- phase fields, d and (aBe), are possible. At the eutectoid temperature (1109 C) , the solubility of Cu in Be reaches a maximum of 9.5 at.%. The b + g two-phase field is taken from [70Rau] and is characterized by a markedly triangular shape with a maximum at 45.8 at.% Be and 880 C. Within the two-phase region, several sequences of phase transformation can be obtained. Above the extrapolated b = g continuous transition boundary, the b phase is metastable with respect to either ordering or phase decomposition. Below the extrapolated continuous transition boundary, the b phase is unstable with respect to continuous ordering. After the b phase orders to the g phase, the homogeneous ordered solid solution becomes unstable with respect to phase decomposition. The g/(Cu) + g boundary shifts to higher Be content immediately below the eutectoid temperature (620 C), thus narrowing the g field considerably. The tentative boundary is indicated by a dotted line. The b phase cannot be retained in the completely disordered state, irrespective of the composition or the severity of quenching rate. The initial rate of decomposition is slower for hypereutectoid (<31.5 at.% Be) alloys, because the proeutectoid fcc (Cu) phase requires more atomic rearrangement to form from the bcc b phase. In contrast, the reaction rate is very rapid for hypoeutectoid compositions, where the proeutectoid g phase has the CsCl-type structure, which is a crystallographic derivative of the bcc structure. The precipitation of the ordered g phase from the b phase is preceded by the formation of the metastable ordered phase b› of stoichiometry Cu2Be, having a tetragonal structure. The formation of the metastable b› phase was observed only after severe quenching, such as splat cooling [73Tya]. Eventually, this reaction leads to the formation of the transitional aT phase, which has a tetragonal structure. Depending on the quenching rate or annealing conditions of the quenched alloy, the transition from the metastable aT to the equilibrium (Cu) phase proceeds through a series of intermediate stages of successive misorientations and variations in lattice parameters. The low-temperature decomposition of the supersaturated solid solution of Be in Cu passes through several intermediate metastable phases prior to the formation of the equilibrium precipitate phase. The sequence of phase separation is: Supersaturated solid solution: (Cu)s <259> (Cu) + GP zone <259> (Cu) + g› <259> (Cu) + g› <259> (Cu) + g In the commercial Cu-2.1 wt.% Be-0.4 wt.% Ni alloy, application of pressure ( 70 to 76 kbar) reduced the solubility of Be in Cu by half at 825 C ( solutionizing temperature) and by one third at 396 C (aging temperature). On rapid solidification, the Cu-rich boundary of b was found to extend to ~19 at.% Be (as compared to equilibrium 23.5 at.%), and the CsCl-type ordered structure corresponding to the g phase was observed in quenched off- stoichiometric alloys with 32 at.% Be. Depending on composition and thermomechanical history, both continuous and discontinuous modes of transformation of the supersaturated solid solution of Be in Cu have been observed. The nose of the TTT curve for the discontinuous reaction shifts to higher temperatures and to shorter times with increasing Be content and to longer times with increasing grain size of the (Cu) phase. The reaction can be inhibited by deformation or by large random precipitates formed prior to or during the process. Quenching from high solutionizing temperatures is beneficial, as it aids in the growth of continuous precipitates by thermal diffusion due to excess vacancies. Both Co and Ce are effective in suppressing the cellular reaction, Ce being more effective for higher aging temperatures. 16Oes: G. Oesterheld, Z. Anorg. Allog. Chem., 97, 13-27 (1916) in German. 29Mas: G. Masing and O. Dahl, Wiss. Veroffentl. Siemens-Konzern, 8, 94-100 ( 1929) in German. 32Bor: H. Borchers, Metallwirtschaft, 11, 317-321, 329-330 (1932) in German. 41Iwa: K. Iwase and M. Okamoto, Nippon Kinzoku Gakkai-Shi (J. Jpn. Inst. Met.), 5, 82-84 (1941) in Japanese. 52Abr: N. Kh. Abrikosov, Anal. Akad. Nauk SSSR, 21, 101-115 (1952) in Russian. 63Gel: S.H. Gelles, J.J. Pickett, E.D. Levine, and W.B. Nowak, Proc. Conf. on the Metallurgy of Beryllium, Inst. Met., London, No. 28, 588-600 (1963). 70Rau: R. Rautioaho and E. Suoninen, Phys. Status Solidi, 2(a), 493-496 (1970). 73Tya: Yu.D. Tyapkin and V.A. Golikov, Fiz. Met. Metalloved., 35, 336-346 ( 1973) in Russian; TR: Phys. Met. Metallogr., 35(2), 101-110 (1973). 82Jon: S. Jonsson, K.Kaltenbach, and G. Petzow, Z. Metallkd., 73(8), 534-539 ( 1982). Published in Phase Diagrams of Binary Beryllium Alloys, 1987, and Bull. Alloy Phase Diagrams, 8(3), Jun 1987. Complete evaluation contains 6 figures, 11 tables, and 101 references. Special Points of the Cu-Be System