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

Abstract

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 ωo variants and γ phase at 700 °C; but only one ωo variant would remain as the ageing time increased. The size of the ωo 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. γp 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 → ωo and B2 → γ, were discussed.