Evolution of B2(ω) region in high-Nb containing TiAl alloy in intermediate temperature range

The thermal stability of the B2(ω) region within an intermediate temperature range in a Ti-45Al-8.5Nb-(W, B, Y) (at.%) alloy was investigated. First, the “pure B2 region” without the ω phase was obtained after 1100 °C/1 h followed by water quenching. Then, the samples were heat treated at 700 °C, 800 °C and 850 °C for 1 h and 96 h respectively. The results show that B2 decomposes into four types of ω_o variants and γ phase at 700 °C; but only one ω_o variant would remain as the ageing time increased. The size of the ω_o particles obviously increases with the increase of heating temperature and treating time. It is also found that the phase transformation of B2 → γ occurred at 800 °C and the γ phase prefers to nucleate at B2 region boundaries. γ_p would further coarsen to micron size after 96 h. In contrast, no γ phase in the B2 matrix is observed when the annealing temperature increases to 850 °C, which could be attributed to the influence of W element on the formation of γ phase in B2 phase. Phase transformation mechanisms in the B2(ω) region, involving B2 → ω_o and B2 → γ, were discussed.