Фазовая диаграмма системы Al-Zn

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Al-Zn (Aluminum-Zinc) J.L. Murray Al-Zn is a eutectic system involving a monotectoid reaction and a miscibility gap in the solid state. The fcc (Al) solid solution has an extended homogeneity range, interrupted at lower temperatures by a miscibility gap. The fcc solid solution is denoted as (aAl) or (a›Al) on the Al-rich and Zn-rich sides of the miscibility gap, respectively. The (Al) liquidus and solidus descend to a eutectic equilibrium with cph (Zn) at 381 C, and at 277 C, a eutectoid (monotectoid) equilibrium of a, a›, and ( Zn) occurs. Near equiatomic compositions, the (Al) solidus has an inflection caused by the nearness of the fcc miscibility gap. The phase diagram of [Elliott] represents (a›Al) as two distinct fcc phases, separated by a narrow two-phase region at ~50 at.%. This two-phase region intersects the solidus at 443 C and the (aAl) miscibility gap at 340 C. The assessed diagram does not include these reactions, because the proposed two- phase region separates two structurally identical fcc solid solution phases of nearly equal compositions, which is thermodynamically implausible. The assessed phase diagram differs only slightly from the earlier version of [ Hansen]. The Al branch of the liquidus is based on a composite of the data of [1897Hey], [24Isi], [38Gay], [45But], [49Pel], and [49Sol]. All of these data lie within 5 C of the assessed liquidus curve. The (Al) solidus, which is the least accurately known phase boundary of this system, is based on microscopic studies [22Han, 38Gay, 39Mor, 45But, 49Geb] and high-temperature X-ray work [51Ell]. During solidification, large changes in the composition of the solid (~35 at.%) occur over a narrow temperature range. Alloys in the composition range 30 to 50 at.% are particularly resistant to homogenization treatments. Segregation causes low incipient melting temperatures in Al-rich alloys and the nonequilibrium extension of the eutectic arrests to Al-enriched compositions. The miscibility gap is based on data of [36Fin], [74Sim], and [56Mue]. Based on the careful resistivity measurements of [36Fin] and [56Mue], the miscibility gap is a smooth curve without any effect corresponding to a two- phase (a› + a››) region near 50 at.% Zn and above 351 C. The solubility of Zn in (aAl) increases from 2.2 at.% at 110 C to 16.5 at.% at the eutectoid temperature. Above 277 C, the solubility increases from 59 с 1 at.% Zn at 277 C to 67 с 1 at.% Zn at the eutectic temperature. The solubility limits in the assessed diagram are based on resistivity data [36Fin, 48Bor, 67Lar]. The maximum solubility of Al in (Zn) is 2.8 с 0.2 at.% (97.2 at.% Zn) at the eutectic temperature, decreasing to 1.6 at.% (98.4 at.% Zn) at 277 C and 0.07 (99.93 at.% Zn) at 20 C. The assessed low-temperature solubilities are extrapolations based on a straight-line fit through experimental data [36Aue, 40Loe, 50Hof]. The supersaturated fcc (aAl) solid solution can be retained at temperatures below the equilibrium solvus. Decomposition of the solid solution gives rise to a series of metastable structures: spherical and ellipsoidal GP zones; precipitates of rhombohedral structure, which take the form of platelets coherent with the fcc matrix; and an incoherent (a›Al). The sequence of structures observed during aging depends on the homogenization temperature, quenching rate, Zn content, and quenching procedure. At temperatures above 150 C, the coherent precipitate grows quickly, to become a coherent plate with a rhombohedral crystal structure. The formation of rhombohedral platelets is governed by the coherent solvus and is independent of particle size. These coherent platelets should be identified, thermodynamically, with the fcc solid solution. The critical point of the coherent miscibility gap is 40.1 с 0.8 at.% Zn at 324 C. The determination of the coherent solvus by aging at low temperatures and reheating to locate the reversion temperature of the GP zones is hindered by the complex precipitation kinetics in the temperature range 80 to 160 C. Thermal, resistivity, or hardness anomalies at temperatures much below the coherent solvus are probably not connected with the metastable phase equilibria. During rapid quenching from the liquid state, Al-Zn alloys do not form single- phase fcc solid solutions beyond the equilibrium maximum solubility of Zn in ( Al). The solubility of Al in (Zn), however, can be extended by rapid solidification. 1897Hey: C.T. Heycock and F.H. Neville, J. Chem. Soc., 71, 383-422 (1897). 22Han: D. Hanson and M.L. Gayler, J. Inst. Met., 27, 267-294 (1922). 24Isi: T. Isihara, Sci. Rep. Tohoku Univ., 13, 18-21 (1924). 36Aue: H. Auer and K.E. Mann, Z. Metallkd., 28, 323-326 (1936) in German. 36Fin: W.L. Fink, Trans. AIME, 12, 244-260 (1936). 38Gay: M.L.V. Gayler, M. Haughton, and E.G. Sutherland, J. Inst. Met., 63, 123- 147 (1938). 39Mor: T. Morinaga, Nippon Kinzoku Gakkaishi, 3, 216-221 (1939). 40Loe: K. Loehberg, Z. Metallkd., 32, 86-90 (1940) in German; Chem. Abstr., 35, 1745 (1945); Met. Abstr., 10, 139 (1943). 45But: E. Butchers and W. Hume-Rothery, J. Inst. Met., 71, 291-311 (1945). 48Bor: G. Borelius and L.E. Larsson, Ark. Mat. Astr. Fys., 35A(13), 1-14 (1948) . 49Geb: E. Gebhardt, Z. Metallkd., 40, 136-140 (1949) in German. 49Pel: E. Pelzel, Z. Metallkd., 40, 134-136 (1949). 49Sol: I.S. Solet and W.W. St. Clair, Bureau of Mines Report of Investigations 4553 (1949). 50Hof: W. Hofmann and G. Fahrenhorst, Z. Metallkd., 42, 460-463 (1950) in German. 51Ell: E.C. Ellwood, J. Inst. Met., 80, 217-224 (1951). 56Mue: A. Muenster and K. Sagel, Z. Phys. Chem., 7, 296-316 (1956) in German. 67Lar: L.E. Larsson, Acta Metall., 15, 35-44 (1967). 74Sim: M. Simerska, V. Synecek, and V. Sima, Czech. J. Phys., B24, 654-659 ( 1974). Published in Bull. Alloy Phase Diagrams, 4(1), Jun 1983. Complete evaluation contains 7 figures, 17 tables, and 194 references. Special Points of the Al-Zn System