STUDY ON PHASE-STABILITY IN TI-AL-X SYSTEMS AT HIGH-TEMPERATURES

The effect of additional elements X(X = V, Cr, Mn, Nb, Mo and Ta) on gamma(TiAl)-based titanium aluminides has been investigated. The addition of elements X caused a change in phase equilibria among the gamma, alpha-2(Ti3Al) and beta (b.c.c. Ti) phases; the beta-phase is stabilized by the addition (except X = Ta), resulting in a drastic change in microstructures. The Ti-Al-Mo system has shown unique behavior; the beta-phase region protrudes toward the a phase region around 1200-degrees-C in the ternary phase diagram. The beta-phase contained in specimens quenched from high temperature has a long-range ordering structure that is different from the B2 structure. The chemical potentials of the beta, alpha and gamma-phases may have close values around 1200-degrees-C and their phase stabilities are strongly influenced by processing or heat treatment.