Longitudinal twinning in a TiAl alloy at high temperature by in situ microcompression

The stress required to activate twinning of the longitudinal <112¯]{111} system in the lamellar γ-TiAl phase of the alloy Ti-45Al-2Nb-2Mn (at.%)-0.8 vol.% TiB2 was measured at several temperatures up to 700 °C by in situ micropillar compression of soft mode oriented γ-TiAl/α2-Ti3Al lamellar stacks. The lamellae undergoing deformation twinning were identified by electron backscatter diffraction orientation mapping. In some cases, such lamellae were not constrained by domain or colony boundaries and longitudinal twinning was the only deformation mechanism observed based on digital image correlation strain maps. The resolved shear stress for such unconstrained twinning was found to increase monotonically with temperature from 25 °C to 700 °C. This is consistent with the stacking fault energy increasing with temperature as found in many metallic alloys, suggesting that the increased ease of deformation twinning at high temperature in bulk TiAl alloys is due to the increased ease with which the twinning shear can be accommodated by the neighbouring domains and lamellae with increasing temperature, rather than a thermal softening of the intrinsic twinning mechanism.