Creep-Induced Phase Instability and Microstructure Evolution of a Nearly Lamellar Ti–45Al–8.5Nb–(W, B, Y) Alloy

Microstructure degradation and stress-induced transformation of a high Nb-containing TiAl alloy with nearly lamellar microstructure during creep were investigated. Tensile creep experiments were performed at 800, 850 and 900 °C under 150 MPa in air. Microstructures before and after creep tests were examined using scanning and transmission electron microscopy (SEM and TEM). Dislocations within the lamellar structure and β_o(ω) region and twin intersection in massive γ grains were investigated. Dislocation sliding played a critical role in the deformation of ω_o phase, which preferentially occurred on the (0002)ω_o plane. Possible deformation mechanisms were revealed. A stress-induced γ → α₂ phase transformation took place during the creep test at 850 and 900 °C. α₂ lamella could directly decompose into the ω_o phase at 850 °C. The instability of high-temperature microstructure can weaken the creep resistance and promote the plastic deformation of the lamellar matrix, thus could be detrimental to the creep properties. The correlations between creep properties and microstructure instability were discussed.