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Section II: Phase Diagram Evaluations

Al-Ti-V (Aluminum-Titanium-Vanadium) V. Raghavan

The room temperature ductility of TiAl is significantly improved by V. The previous review of this system by [1995Hay] presented a schematic liquidus projection, a partial reaction scheme, isothermal sections at 1400, 1200, 1100, 1000, and 800 °C, and partial isothermal sections for Ti-rich alloys at 900, 800, 700, and 600 °C. The work of [1991Par], [1994Ahm1], and [1994Ahm2] (not covered in the review of [1995Hay]) is discussed in this update, adding two full isothermal sections at 900 and 600 °C and clarifying the liquidus projection near the Al corner.

Binary Systems 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; and TiAl3 (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 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 a low-temperature phase that is stable below 810 °C. TiAl, often designated ␥, has the L10, AuCu-type tetragonal structure and forms peritectically at 1460 °C. (␤Ti) (body-centered cubic [bcc], also denoted ␤) and liquid undergo a peritectic reaction at 1490 °C to yield (␣Ti) (close-packed hexagonal, also denoted ␣). Ti3Al, commonly called ␣2, has the D019, Ni3Sntype hexagonal structure and forms congruently from (␣Ti) at 1176 °C. The Al-V diagram [2000Ric] depicts five intermetallic compounds: V5Al8 (D82, Cu5Zn8-type cubic); VAl3 (D022, TiAl3-type tetragonal); V4Al23 (hexagonal); V7Al45 (monoclinic); and V2Al21 (cubic). The results of [2000Ric] give lower peritectic formation temperatures of 1408 °C (1670 °C in [Massalski2]) for V5Al8 and 1270 °C (1360 °C in [Massalski2]) for VAl3. [1994Ahm1] considered VAl3 to be distinct from TiAl3, quoting the results of [1955Car] of a “face-centered tetragonal” structure for VAl3. In a bodycentered tetragonal setting (which is the correct unit cell), the lattice parameters given by [1955Car] tally with those listed by [Pearson3]. TiAl3 and VAl3 are isomorphous D022 compounds. The Ti-V phase diagram [1981Mur] has no intermediate phases. ␤Ti and V form a continuous solid solution (␤Ti,V) over a wide range of temperatures. It is not clear whether a stable miscibility gap (shown in [Massalski2]) exists in this solid solution at lower temperatures [1989Wei].

Liquidus Projection With starting metals of 99.999% Al, 99.98% Ti, and 99.7% V, [1991Par] induction-melted 28 alloy composi-

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tions under an Ar atmosphere and used 10 additional alloys obtained from other sources. The phase equilibria were studied by metallography, x-ra