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

Introducing high symmetric cubic disordered beta or ordered beta(0) phases by alloying beta-stabilizers improves the strength and high-temperature deformability of intermetallic gamma-TiAl. However, the beta phase is prone to decomposition of lower symmetric hexagonal phases such as omega(0) and omega '', which are extremely brittle and harmful to the ductility of the alloy. Therefore, understanding the effect of the beta-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 beta, beta(0), omega '', and omega(0) of TiAl with some typical beta 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 beta < beta(0) < omega(0) < omega '' for Ti4Al3Nb and Ti4Al3Ta, beta < beta(0) < omega(0) < omega '' for Ti4Al3V, Ti4Al3Mo and Ti4Al3W, and beta < beta(0) < omega(0) < omega '' for Ti4Al3Cr and Ti4Al3Mn. The beta-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. (C) 2019 Elsevier B.V. All rights reserved.