Regulating Phase Ratios and Mechanical Properties of Polysynthetic Twinned TiAl Single Crystals Via Annealing

Polysynthetic twinned (PST) TiAl single crystal specifically refers to a fully lamellar TiAl single crystal with parallel phase interfaces and twin interfaces grown by directional solidification. In this paper, PST single crystals with different phase ratios are obtained by annealing at specific temperatures and holding times. The results show that the diffusion rates of Ti and Al elements at various temperatures directly trigger and propel the surface recrystallization and variation in the internal phase ratio. When the temperature is lower than 1448 K, the diffusion rate of Ti is obviously higher than that of Al, which causes one dense α2 recrystallized layer to form on the surface of TiAl single crystals. Meanwhile, as more Ti elements migrate to the surface, the α2 phase ratio inside the TiAl single crystal thereby decreases. When the temperature exceeds 1448 K, the diffusion rate of Al gradually reverses to exceed that of Ti, which forms the surface sandwiched recrystallization dominated by γ phase and simultaneously increases α2 phase ratio inside the TiAl single crystal. The variation in the two-phase ratio directly induces a significant change in the lamellae thickness, which exhibits different tensile behaviors of PST-TiAl single crystal. When the α2 phase content is less than 20 %, wider γ lamellae make it easier for dislocations to be activated within its lamellae and continuously move across the γ/α2 interfaces, thereby obtaining better tensile plasticity. As the α2 phase content exceeds 30 %, finer γ lamellae inhibit the dislocation initiation, resulting in the fracture occurrence of TiAl single crystal before yielding. No matter how the phase ratio changes, the crack preferentially initiates within α2 lamellae. However, the crack propagation follows different paths based on various γ lamella thicknesses. The fracture mode of PST-TiAl single crystal also changes from shear fracture along slip bands within the γ lamella to brittle fracture along the {11¯00} planes within α2 lamella.