Фазовая диаграмма системы Mg-Si

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Mg-Si (Magnesium-Silicon) A.A. Nayeb-Hashemi and J.B. Clark [09Vog] determined the liquidus across the Mg-Si phase diagram by thermal analysis of 18 Mg-Si alloys made of low-grade Si. [40Ray] accurately determined the (Mg) liquidus. [68Gef] and [77Sch] redetermined the liquidus by thermal analysis of high-purity alloys in the composition ranges 10 to 63 and 0 to 100 at.% Si, respectively. The liquidus curves of these investigators in the common range of composition are only a few degrees apart. Consequently, in the assessed phase diagram, the (Mg) liquidus has been taken from [40Ray], and the liquidus for the remainder of the diagram has been based on select data of [68Gef] and [77Sch]. The work of [26Woh] and [81Rao] was also evaluated in the construction of the assessed diagram. The general characteristics of the Mg-Si equilibrium phase diagram, first reported by [09Vog], are well established and generally accepted. The controversy of two different liquidus curves 30 C apart, as reported in [ Hansen], has been settled. [35Elc] noted the existence of a metastable (Mg)-MgZn2-Mg4Si system and concluded that the (Mg)-Mg4Si system is metastable with a eutectic at 2.34 at.% Si and 575 C. [77Sch] reported the possible existence of the MgSi compound that would decompose on heating by a peritectoid reaction (Mg2Si + (Si) = 2MgSi) at ~805 C. At high pressure, metallic Si has a tetragonal structure. After being subjected to 16.0 or 20.0 GPa pressure at room temperature, Si had a bcc structure. When heated in air at 200 to 600 C for up to 3 days, bcc Si formed a new hexagonal structure. At pressures above ~2.5 GPa and temperatures above 900 C, Mg2Si transforms from a cubic fluorite structure to a hexagonal structure. The high-pressure hexagonal phase was noted as being indefinitely metastable. Following the release of pressure, the hexagonal structure did not change after more than a year at ambient conditions. An alloy of Mg-50 at.% Si, subjected to 3.0 GPa pressure at 900 C, exhibited a texture similar to that of martensitic steel. At ordinary temperatures (25 to 400 C) and high pressures (up to 10.0 GPa), the Mg2Si compound undergoes polymorphic transformations. The phase change ( from cubic to hexagonal) was accompanied by a large hysteresis. The region where the two phases (cubic and hexagonal Mg2Si) coexisted extended up to 10.0 GPa at 400 C. Similar transformations were found in intermetallic compounds of Mg with other elements of group IVB (Mg2Sn and Mg2Ge). 09Vog: R. Vogel, Z. Anorg. Chem., 61, 46-53 (1909) in German. 26Woh: L. Wohler and O. Schliephake, Z. Anorg. Chem., 151, 1-20 (1926) in German. 35Elc: E. Elchardus and P. Laffitte, C.R. Hebd. Seances Acad. Sci., 200, 1938- 1940 (1935). 40Ray: G.V. Raynor, J. Inst. Met., 66, 403-426 (1940). 68Gef: R. Gefficen and E. Miller, Trans. Metall. Soc. AIME, 242, 2323-2328 ( 1968). 77Sch: E. Schurmann and A. Fischer, Giessereiforschung, 29, 111-113 (1977) in German. 81Rao: Y.K. Rao and G.R. Belton, Chemical Metallurgy-A Tribute to Carl Wagner, N.A. Gokcen, Ed., The Metallurgical Society of AIME, 75-96 (1981). Published in Phase Diagrams of Binary Magnesium Alloys, 1988, and Bull. Alloy Phase Diagrams, 5(6), Dec 1984. Complete evaluation contains 2 figures, 8 tables, and 49 references. Special Points of the Mg-Si System