Microstructural evolution of PST-TiAl during low-rate compressive micro-straining at 1023 K in hard and soft orientations

Scanning and transmission electron microscopy (SEM and TEM) have been combined with quantitative image analyses to document systematically the microstructural evolution of polysynthetically twinned Ti-48at.%Al (PST-TiAl) during low-rate compression in hard and soft orientations at 1023 K to micro-strains (0.2%<ε<0.4%). The PST-TiAl contained excess volume fraction of α2-phase after conventional homogenization. The PST-TiAl maintained a lamellar morphology, the excess α2-phase lamellae transformed to γ-phase lamellae and both the populations of α2- and γ-phase lamellae coarsened during elevated temperature straining for hard and soft orientations. The microstructural changes were accomplished mainly by the instability mechanism of termination migration. Plastic strain accommodating defects assisted in the formation of lamellar terminations. The rates of the microstructural evolution were faster for the hard orientation tests than for the soft orientation tests, which has been attributed to the more frequent interactions between hard deformation modes and the lamellar interfaces in the former. It has been proposed that microstructural changes similar to those reported here are suitable to rationalize the large primary creep strains characteristic of fully lamellar TiAl alloys.