Deformation of a 45° 〈321〉 oriented polysynthetically twinned TiAl crystal at high strain rate and high temperature

Abstract

Dislocation microstructures in individual domains in a 45° 〈321〉 oriented polysynthetically twinned TiAl crystal deformed in compression at a strain rate of 3000_s ⁻¹ and a temperature of 800°C were characterized. Ordinary dislocation slip was found to be the dominant deformation mode for the deformation of domains [II], [V] and [VI] where the transmission electron microscopy (TEM) foil normals were close to [011], [011] and [101], respectively; mechanical twinning and superdislocation slip were found to be the complementary deformation modes to accommodate the deformation of these domains. Domain [I], where the TEM foil normals were close to [110], was observed to be minimally (or barely) deformed under the deformation conditions imposed. Although mechanical twinning dominated the deformation of domains [III] and [IV] where the TEM foil normals were close to [101] and [110], respectively, dislocation slip within untwinned matrices was also observed. The majority of the dislocations observed in domains [III] and [IV] were the ½〈110] ordinary dislocations. A correlation of the dominant deformation modes in the individual domains with the mechanical response of the crystal suggests that the yield stress of Ti-rich TiAl alloys is determined either by ordinary dislocation slip or by mechanical twinning or by both, dependent upon the sample orientation.