Microstructure Evolution of Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y Alloy during Long-Term Thermal Exposure

The hot-rolled alloy Ti-45Al-8.5Nb-0.2W-0.2B-0.02Y was exposed to 700 °C air for up to 10,000 h. The changes in microstructure were observed using scanning and transmission electron microscopies. It was found that the α₂ laths, α₂ + γ lamellae, and B2(ω) structure of the alloy showed thermodynamic instability. There were three types of phase transformation in the alloy during long-term thermal exposure. The first was α₂ → γ, which occurs in the interior and boundary of the α₂ + γ lamellae. The second was α₂ + γ → B2(ω), which occurs on the α₂ + γ boundary. In addition, B2(ω) also precipitates on the γ/γ interfaces. The third was B2(ω) → γ, which describes the precipitation of micron-scale γ phases in the B2(ω) area after thermal exposure of 5000 h. The volume fraction and size of the B2(ω) area and equiaxed γ grains continued to increase throughout the exposure process. Large-sized γ grains and a B2 area of tens of microns appeared in the microstructure after long-term thermal exposure. The volume fractions of the B2 area and the equiaxed γ grains after thermal exposure of 10,000 h reached 16.8% and 63.2%, respectively.