Nonequilibrium α₂/γ interfacial structures in a γ-TiAl-based two phase alloy after hot-working and annealing

A transmission electron microscope investigation has been performed on the thermal instability of the nonequilibrium α₂/γ interfacial structure in a hot-forged Ti-45Al-10Nb-2.5Mn-0.05B alloy. The distinctiveness of these nonequilibrium α₂/γ interfaces in the hot-deformed sample is the misorientation from the conventional orientation relationship {111}_γ//{0001}_α2, <11̄0> γ//<112̄0>_α2. The structure of this nonequilibrium interface boundary is characterized by numerous interfacial ledges containing 1/3[111] Frank partial dislocations. The analysis points towards the formation of this type of nonequilibrium α₂/γ interface being caused by the relative rotation of α₂ and γ plates in the lamellar structure. Further TEM studies of microstructural changes during annealing for 4 min/1000°C of the nonequilibrium α₂/γ lamellar structure generated by hot-forging revealed that many low-angle grain boundaries are formed in the γ plates of lamellar colonies, resulting in a γ subgrain microstructure; necking of α₂ plates occurs preferentially at α₂ subgrain boundaries; subsequently disintegration of the original α₂ plates takes place resulting in spheroidized α₂ particles which are aligned at the prior α₂/γ boundaries. (C) 1997 Published by Elsevier Science Ltd.