Nucleation behavior of ω_o phase in TiAl alloys at different elevated temperatures

In this study, the location, morphology and composition of the ω_o phase, which transformed from B2 phase in Ti–45Al–8.5Nb-(W, B, Y) (at.%) alloy, were investigated by short-time heat treatments in an intermediate temperature range. The results showed that the characteristics of B2 → ω_o phase transformation differ with the reduction in temperature. Below the ω_o solves, the B2 → ω_o phase transformation will take place within B2 area. There are existing two stages in the phase transformations during cooling from 850 to 700 °C. When the temperature is high (850 °C), granular ω_o populated along the dislocations existing in B2 phase and B2 grain boundaries, due to higher energy and the enrichment of Nb element at the B2/γ boundaries. It is worth noting that only two ω_o variants nucleate at 850 °C. With the decrease in temperature, the nucleation sites of ω_o phase would be migrated from the B2 boundaries to the inner area. At the temperature of 700 °C, four ω_o variants with different orientations nucleate homogenously within B2 grain. The ordered ω_o formation process of Ti–45Al–8.5Nb-(W, B, Y) alloy during cooling was postulated in this study.