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Phase Diagram Evaluations: Section II

Al-Mo-Ti (Aluminum-Molybdenum-Titanium) V. Raghavan

The compilation of data on this ternary system by [1995Vil] includes a liquidus projection, a partial solidus projection, partial isothermal sections for Ti-rich alloys at 1100, 1000, 950, 925, 900, 850, 800, 700, and 600 °C, and vertical sections at 5, 10, 15, 20, 25, 30, and 40 wt.% Al, at 0.5 and 5 wt.% Mo and at 7 at.% Ti. In the review by [1993Bud], isothermal sections at 1600, 1300, and 900 °C, partial sections at 700 and 400 °C, and a vertical section at 7 at.% Mo are given. Recently, partial isothermal sections for Ti-poor alloys were reported by [2003Nin] between 1400 and 1100 °C.

Binary Systems The Al-Mo phase diagram [1997Sau], with an added update for the Al-Mo3Al8 region by J.F. Smith (Editor of the

Journal of Phase Equlibria and Diffusion), shows the following intermediate phases: MoAl12 (WAl12-type cubic); MoAl5 (with three modifications); Mo5Al22; Mo4Al17; MoAl4 (monoclinic); Mo1 − xAl3 + x (cubic); MoAl3 (monoclinic); Mo3Al8 (monoclinic); MoAl (body-centered cubic [bcc]); and Mo3Al (A15, Cr3Si-type cubic). The updated version of the Al-Ti phase diagram [2005Rag] depicts a number of intermediate phases. TiAl3 has two crystal modifications: TiAl3 (high temperature [HT]) (D022-type tetragonal) forms peritectically at 1387 °C and decomposes eutectoidally at 735 °C. TiAl 3 (low temperature [LT]) (tetragonal) forms at ∼950 °C and is stable at low temperatures. Ti5Al11 is a superstructure based on the AuCu-type tetragonal phase. It forms peritectically at 1416 °C and decomposes eutectoidally at 995 °C to TiAl2 and TiAl3 (HT). TiAl2 (HfGa2-type tetragonal) forms congruently at

Fig. 1 Al-Mo-Ti partial isothermal sections for Ti-poor alloys at (a) 1300 °C, (b) 1200 °C, and (c) 1100 °C [2003Nin]

Journal of Phase Equilibria and Diffusion Vol. 26 No. 4 2005

357

Section II: Phase Diagram Evaluations 1215 °C from Ti5Al11 and is stable at low temperatures. Ti1 − xAl1 + x (AuCu-type tetragonal) is stable between 1445 and 1170 °C. Ti3Al5 is an LT phase that is stable below 810 °C. TiAl, often designated ␥, has the L10, AuCu-type tetragonal structure and forms peritectically at 1460 °C. (␤Ti) (bcc, also denoted ␤) and liquid undergo a peritectic reaction at 1490 °C to yield (␣Ti) (close-packed hexagonal, also denoted ␣). Ti3Al, commonly labeled ␣2, has the D019, Ni3Sn-type hexagonal structure and forms congruently from (␣Ti) at 1176 °C. The Mo-Ti phase diagram [Massalski2] depicts a continuous bcc solid solution between ␤Ti and Mo over a wide range of temperatures. A miscibility gap occurs in this solution at lower temperatures, with a monotectoid reaction (␤Ti) → (␣Ti) + (Mo) at 695 °C.

Ternary Isothermal Sections [2003Nin] arc-melted under Ar atmosphere 11 Al-Mo alloys with Ti in the range of 3 to 17 at.%. The samples were annealed at 1400 to 1500 °C for 1 day and were quenched in water. Some quenched samples were isothermally aged at 1150 to 900 °C for 1 to 3 days and were quenched in water. The phase equilibria were