Phase stability of TiAl-X (X=V, Nb, Ta, Cr, Mo, W, and Mn) alloys.

Introducing high symmetric cubic disordered β or ordered β 0 phases by alloying β -stabilizers improves the strength and high-temperature deformability of intermetallic γ -TiAl. However, the β phase is prone to decomposition of lower symmetric hexagonal phases such as ω 0 and ω ' ' , which are extremely brittle and harmful to the ductility of the alloy. Therefore, understanding the effect of the β -stabilizers on the phase stability of TiAl is crucial for the rational design of TiAl based alloys. In the present work, the equilibrium lattice structures, bulk moduli, and heats of formation of β , β 0 , ω ' ' , and ω 0 of TiAl with some typical β stabilizers V, Nb, Ta, Cr, Mo, W, and Mn were calculated by using a first-principles method, with which the relative phase stability were discussed. We showed that the phase stability sequence is β < β 0 < ω 0 < ω ' ' for Ti 4 Al 3 Nb and Ti 4 Al 3 Ta, β < ω 0 < β 0 < ω ' ' for Ti 4 Al 3 V, Ti 4 Al 3 Mo and Ti 4 Al 3 W, and β < ω 0 < ω ' ' < β 0 for Ti 4 Al 3 Cr and Ti 4 Al 3 Mn. The β -stabilizing effect of the alloying element X increases from Nb/Ta to V/Mo/W to Cr/Mn. The relative stability of the various phases is roughly in accordance with the trends of the unit volume and bulk modulus against the phases with a few exceptions. The phase diagrams of the alloys associated with these phases were estimated by comparing the free energies against the temperature of various phases, evaluated with the calculated unit volumes and bulk moduli. • Phase stability of TiAl alloying with transition metal elements is determined. • Phase stability is roughly in accordance with the unit volume and bulk modulus. • Strategy for the composition design of TiAl based alloys is presented. [ABSTRACT FROM AUTHOR]