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.-beta phase were investigated for microstructural changes during superplastic deformation at temperatures from 1050 to 1200 K. Observed changes occurred in two steps: (1) agglomeration of the beta phase to grain boundaries perpendicular 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 induced by the gradient of traction force acting upon grain boundaries. The second step was considered to be induced by stress concentration at grain boundaries of the 2 phase where the beta phase was depleted by agglomeration to the perpendicular boundaries. The phase equilibrium under stressed condition was calculated by increasing the Gibbs energy of the alpha phase by 500 J/mol relative to that of the beta phase. An excellent quantitative agreement was found between calculated results and experimental results of the beta volume fraction and the Fe composition in each phase. The present work indicates that the phase transformation accompanied by diffusion can be induced by application of stress of the order of 100 MPa. This new type of stress-induced phase transformation can decrease the beta transus temperature by more than 100 K. (C) 2000 Acta Metallurgica Inc. Published by Elsevier Science Ltd. All rights reserved.