Phase transformation mechanisms in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy after subsequent annealing at 800 °C

During the processing of high Nb-containing TiAl (Nb-TiAl) alloys, the cooling rate at certain parts of the components can be very high, especially at thin parts. Moreover, annealing treatment must be applied to TiAl alloys to improve their mechanical properties. In this study, the phase transformation mechanisms in a quenched Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y alloy during subsequent annealing were characterized using a scanning electron microscope (SEM) and a transmission electron microscope (TEM). The results show that βo, α2, and massively transformed γ phases co-exist in the as-quenched microstructure. Some fine γ laths nucleated in the primary α2 phase in the quenched samples. After annealing at 800 °C for 1 h, numerous extremely fine γ laths precipitated in the bulk α2 phase and could only be recognized using TEM imaging. The ωo particles at sizes of 0.5-1 μm precipitated in the retained βo phase and nearly consumed all of the βo areas. More interestingly, some coarsened γ grains in true-twin relationship were observed at the boundaries of the lamellar colony and βo(ω) regions. The orientation relationship between βo(ω) and coarsened γ was confirmed to be the following: [110]β//[112¯ 0]ω//[111]γ, (111¯)β//(0001)ω//(11¯ 0)γ. After annealing at 800 °C for 100 h, the βo phase region transformed into small ωo particles and equiaxed γ grains and still followed the above-mentioned orientation relationship. The α2 phase only existed as thin laths in the lamellar structures in a small volume fraction. These results indicate that the ωo phase is stable at 800 °C. Possible mechanisms of these phase transformations are discussed.