The evolution of deformation substructure in a creep deformed fully-lamellar TiAl alloy

The evolution of deformation substructure in a powder metallurgy (P/M) TiAl alloy creep deformed at 760°C has been examined using transmission electron microscopy. Since the multiplication of lattice dislocations within γ lamellae becomes limited as a result of a refined fully-lamellar (FL) microstructure (i.e. refined lamellar spacing) within the P/M TiAl alloy, the deformation of the alloy at the primary creep stage is mainly accommodated by soft lamellar grains through the movement of interfacial dislocations in the γ–α2 and γ–γ interfaces. The mobility of interfacial dislocations is primarily impeded by grain boundaries, interface ledges and dislocation barriers formed by the impingement of lattice dislocations to the lamellar interfaces. When the alloy is deformed into the secondary creep stage, the density of interfacial dislocations increases and deformation twins nucleate and grow from the lamellar interfaces. It is suggested that deformation twinning in FL-TiAl results from a stress-relief process due to the pile-up of interfacial dislocations during deformation. The alloy becomes more resistant to creep deformation as a result of the deformation twinning because the deformation twins further restrict the dislocation motion within the γ lamellae, and the formation of sessile dislocations at the intersections between the deformation twins and the γ or α2 lamellae provides effective barriers for the movement of interfacial dislocations.