Atomic-scale observations of B2 → ω-related phases transition in high-Nb containing TiAl alloy

The nucleation of omega-related phases within B2-phase region in Ti-45A1-8.5Nb-(W, B, Y) (at.%) alloy were investigated by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). At 850 degrees C, the metastable B2 phase decomposes to omega particles quickly. Experimental evidence from high-resolution images revealed that first the atoms redistribute in the B2 structure, and then the original (111)(B2) planes collapse during B2 -> omega, which represents a diffusion-collapse pathway. The further collapse of (0001)(omega) planes in [0001](omega) zone, that is omega -> omega(o), is a sequential and uneven process rather than an instantaneous transition, which may be attributed to that an uneven structure transition could decrease the energy barrier.