Stress-induced phase transformation during superplastic deformation in two-phase Ti-Al-Fe alloy

Ti-5.5Al-1Fe alloys consisting of the h.c.p.-alpha phase and the b.c.c.-bet a phase were investigated for microstructural changes during superplastic d eformation at temperatures from 1050 to 1200 K. Observed changes occurred i n two steps: (1) agglomeration of the beta phase to grain boundaries perpen dicular to the tensile axis and (2) subsequent increase of the beta volume fraction. The beta volume fraction after failure was found to increase with increasing deformation temperature. The first step was considered to be in duced by the gradient of traction force acting upon grain boundaries. The s econd step was considered to be induced by stress concentration at grain bo undaries of the 2 phase where the beta phase was depleted by agglomeration to the perpendicular boundaries. The phase equilibrium under stressed condi tion was calculated by increasing the Gibbs energy of the alpha phase by 50 0 J/mol relative to that of the beta phase. An excellent quantitative agree ment was found between calculated results and experimental results of the b eta volume fraction and the Fe composition in each phase. The present work indicates that the phase transformation accompanied by diffusion can be ind uced by application of stress of the order of 100 MPa. This new type of str ess-induced phase transformation can decrease the beta transus temperature by more than 100 K. (C) 2000 Acta Metallurgica Inc. Published by Elsevier S cience Ltd. All rights reserved.