Phase transformation and microstructures in Ti-Al-Nb-Ta system

A variety of heat treatment has been carried out to explore the transformation and microstructure in a Ti-22Al-20Nb-7Ta. Special emphasis focused on the effect of solution and/or aging in O + B2 (orthorhombic + bee) phase field. Two forms of the transformation of B2 to O phase were observed. They are grain size change that is controlled by grain boundary diffusion and Widmanstatten precipitation of O phase from the B2 matrix. It has been found that Ta addition in Ti2AlNb-based alloy could increase the Widmanstatten transformation starting temperature W-s, which determines the transformation mode of B2 to O phase. During recrystallization of O grain, it evolves into lath microstructure. The primary alpha(2) phase is relatively stable when Ti-22Al-20Nb-7Ta was heat treated in O + B2 phase field, while the transformation of alpha(2) to O phase occurred in this study. (C) 2002 Elsevier Science B.V. All rights reserved.