Thermal stability of deformation substructure of cyclically deformed TiAl PST crystals

The effect of deformation mode on the deformation substructure and its thermal stability of TiAl polysynthetically twinned (PST) crystals was investigated using cyclically deformed and monotonically compressed crystals. In the fatigued TiAl PST crystals, two types of deformation substructure of deformation twins and a vein-like structure containing very tangled ordinary dislocations were formed depending on the type of ordered domains in gamma phase. In contrast, numerous deformation twins and a large number of homogeneously distributed dislocations were observed in the compressed crystals. There was a great difference in the softening process and the thermal stability of deformation substructure between the fatigued and compressed crystals. Recovery occurred in the fatigued crystals even at low temperature (similar to 400 degrees C) due to the climb motion of dislocations assisted by vacancy diffusion but recrystallisation could not be confirmed after annealing at 1000 degrees C for 10(5) s. In contrast, recrystallisation was observed in the compressed crystals at 1000 degrees C and deformation twins had an important role in the nucleation of new domains. (C) 1996 Elsevier Science Ltd.