Formation of Fine γ Grain Structure Through Fine α2 / γ Lamellar Structure in Ti-Rich TiAl Alloy

Abstract

ABSTRACTMicrostructural development of an extremely fine α2-Ti3Al / γ-TiAl lamellar structure, which was formed by ice water quenching after solution-treatment in a high-temperature α-Ti phase field for a long period of time, was examined during isothermal aging treatment. In an as-quenched Ti-48at.%Al alloy, the massively transformed γ (γm) and untransformed (meaning massively untransformed) fine α2/γ lamellar regions were observed. Fine γ grains, which were similar to γm, were generated both within the fine α2/γ lamellae and at the boundary area between the γm and the fine α2/γ lamellar regions by aging at a low-temperature (1173K) for a short time (180s). Further aging (1.8ks) caused the coarsening of these newly generated fine γ grains. On the other hand, the coarsening of the γ grains occurred by a high-temperature (1323K) aging treatment even for 180s. Fine α2 plates and particles, which were aligned to a particular direction, were observed in the γ grain interiors, indicating that the newly generated γ grains grew at the expense of the fine α2/γ lamellae. It can be considered that the γ grain formation through the fine α2/γ lamellae is closely related to the α2→γ reaction of the α2 plates sandwiched by the γ plates, and needs the fast heating rate enough to overcome the α2/γ → γ/γ lamellae reaction.