Фазовая диаграмма системы Ag-Ti

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Ag-Ti (Silver-Titanium) J.L. Murray and K.J. Bhansali The equilibrium solid phases of the Ti-Ag system are (1) the bcc (bTi) solid solution, with a homogeneity range of 0 to 15.5 at.% Ag at the peritectic temperature of 1020C; (2) the cph (aTi) solid solution, with a homogeneity range of 0 to about 4.7 at.% Ag; (3) the fcc (Ag) solid solution, with a homogeneity range of 95 to 100 at.% Ag; (4) the essentially stoichiometric compound Ti2Ag with the MoSi2 structure; and (5) the compound TiAg with the gTiCu structure and a composition range of approximately 48 to 50 at.% Ag. Major studies of the Ti-Ag system were conducted by [53Ade], [53Wor], [60Mcq], [69Ere], and [78Pli]. The assessed diagram is drawn from thermodynamic calculations, which are consistent with the assessment of the experimental data within the experimental accuracy. A difficulty encountered in experimental work on Ti-Ag alloys is the loss of Ag by volatilization during melting. The only data on the liquid/(bTi) phase relationships are incipient melting and metallographic determinations of the ( bTi) solidus [53Ade] and the peritectic composition [69Ere]. The flattening of the liquidus was observed by [53Ade] as a change in volume fraction of liquid as a function of the temperature of the heat treatment. The peritectic composition was estimated to be about 94 at.% Ag, where peritectic arrests were observed by thermal analysis [69Ere]. Because of the small volume fraction of (bTi), this may underestimate the Ti content of the liquid. Solidus data of [53Ade] and [60Mcq] are mutually consistent. They reported the maximum solubility of Ag in (bTi) at the peritectic temperature as 16.3 and about 15 at.%, repectively. The present estimates for the liquidus and solidus are based on thermodynamic optimization of the Gibbs energies with respect to experimental data on the solidus and peritectic reaction. The type of reaction involving (Ag), L, and TiAg is difficult to determine experimentally. [53Ade], [53Dec], and [69Ere] reported that the reaction is probably of the eutectic type and occurs close to the melting point of pure Ag. The eutectic reaction L = (Ag) + TiAg at ~960 C [69Ere] is accepted. For the assessed (bTi) transus and eutectoid reaction, the work of [78Pli] is preferred, because of the high purity of the materials used to prepare alloys. Conflicting results all tend to place the phase boundaries at higher Ag content, due to the effect of oxygen contamination, because oxygen is a stabilizer of the cph structure. The bcc (bTi) phase can transform to cph (aTi) partitionlessly during quenching from the (bTi) field. The mechanism of the transformation is massive or martensitic, depending on cooling rate and composition. [78Pli] demonstrated the existence of the massive transformation by microstructural and microprobe analysis. Higher cooling rates or higher Ag content favored the martensitic over the massive mechanism. 53Ade: H.K. Adenstedt and W. Freeman, WADC Tech. Rep., 53-109, Part 1 (1953). 53Dec: N.A. Dececco and J.M. Parks, Welding J., 1071-1073 (1953). 53Wor: H.W. Worner, J. Inst. Met., 82, 222-226 (1953). 60Mcq: M.K. McQuillan, J. Inst. Met., 88, 235-239 (1960). 69Ere: V.N. Eremenko, Y.I. Buyanov, and N.M. Panchenko, Poroshk. Metall., (7), 55-59 (1969) in Russian; TR: Sov. Powder Metall. Met. Ceram., 8(7), 562-566 ( 1969). 78Pli: M.R. Plichta and H.I. Aaronson, Acta Metall., 26, 1293-1305 (1978). Published in Phase Diagrams of Binary Titanium Alloys, 1987. Complete evaluation contains 2 figures, 7 tables, and 16 references. Special Points of the Ti-Ag System