Stability of lamellar microstructure consisting of γ/γ interfaces in Ti–48Al–8Nb single crystal at elevated temperatures

Change in the lamellar microstructure has been examined during aging of Ti–48Al–8Nb fully lamellar single crystal (PST) with a very few amount of α2 phase at 1473 K, and the role of γ/γ interfaces in thermal stability of lamellar microstructure was identified. The lamellar microstructure is maintained for a certain period of aging, but it collapses rapidly to become a γ grained microstructure. TEM analysis in the lamellae revealed three types of γ/γ interfaces: variant interface, perfect-twin and pseudo-twin boundaries, and the variant interfaces become extinct while perfect-twin boundaries increase drastically during the early stage of aging. Based on the evaluation of the interfacial energy, the variant interface has the highest among them, which gives rise to the reaction of variant interfaces with others, leaving the low energy perfect-twin boundaries. The difference in interfacial energy among the interfaces within the lamellae leads to the interface reaction, which is responsible for the thermal instability of this γ-TiAl lamellar microstructure.