Microstructure change in fully lamellar Ti-48Al-8Nb single crystal during aging at elevated temperatures

The change in a γ-TiAl lamellar microstructure during aging at 1273 and 1473K has been investigated using Ti-48Al-8Nb fully lamellar single crystal, which consists mostly of three different types of γ/γ interfaces (variant interface, perfect-twin and pseudo-twin boundaries) with a very small amount of α₂ phase. The γ lamellar microstructure collapses abruptly after a certain incubation period of aging, and subsequently changes to a γ grained microstructure at both temperatures. In as-grown crystal, the variant interface is dominant among the γ/γ interface. During the incubation period, interfacial reactions of energetically unstable variant interfaces with others take place, leaving either no boundaries or low energy perfect-twin boundaries. These interfacial reactions give rise to the coalescence of the same variant domains across the lamellae, resulting in a locally coarsened γ plate. Repeating these reactions is responsible for the formation of γ grains. Thus, the resulted γ grained microstructure becomes a particularly oriented polycrystal by following the orientation of the six variant domains of the original lamellar microstructure.