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

The nucleation of ω-related phases within B2-phase region in Ti-45Al-8.5Nb-(W, B, Y) (at.%) alloy were investigated by transmission electron microscopy (TEM) and high resolution electron microscopy (HREM). At 850 °C, the metastable B2 phase decomposes to ω particles quickly. Experimental evidence from high-resolution images revealed that first the atoms redistribute in the B2 structure, and then the original 11¯1_B2 planes collapse during B2 → ω″, which represents a “diffusion-collapse” pathway. The further collapse of (0001)_ω planes in [0001]_ω zone, that is ω″ → ω_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.