Superplasticity Assisted by Stress-Induced Phase Transformation in Ti-5.5Al-1Fe Alloy

摘要

Ti-5.5Al-lFe alloy exhibits an excellent superplasticity. In this alloy, the volume fraction of the β phase is much less than the optimum value reported in other superplastic Ti alloys. Since dislocation creep or diffusion creep in the β phase has been regarded as a major stress accommodation mechanism, the accommodation mechanism may differ in this alloy because of the less amount of the β phase. In the present work, microstructure changes were investigated to understand the stress accommodation mechanism. The microstructure was found to change with strain in two steps: (1) redistribution of the β phase to perpendicular grain boundaries to the tensile axis and (2) subsequent increase of the β volume fraction. We consider that the first step accommodates traction force acted upon grain boundaries of the α phase and that the second step accommodates stress concentration at grain boundaries and triple junctions of the α phase by volume change associated with phase transformation.