Evolution of Metastable α₂ Phase in a Quenched High-Nb-Containing TiAl Alloy at 800 °C

Fast cooling from high temperatures is a common way in heat treatments and hot processing, which could introduce excess metastable α₂ phase in the microstructure. Herein, the evolution of the metastable α₂ phase in a water-quenched Ti–45Al–8.5Nb–(W, B, Y) alloy from different single-phase regions is experimentally studied. After water quenching from the β single-phase region, Widmanstätten α₂, as well as residual β_o, γ grains form from β phase and thin γ laths in α₂ phase are observed. However, the water-quenched microstructure from the α single-phase region consists of equiaxed α₂ grains, residual β_o, and massive γ phase. When annealing the quenched samples at 800 °C, similar microstructural evolutions are found in both samples quenched from different temperatures. 1) α₂ phase transform into α₂/γ nanolamellar structure immediately. 2) Recrystallization and discontinuous coarsening occur at the colony boundaries. 3) The nanometer-scale γ/γ_T lamellae transformed from α₂ grains are also unstable, which merge into thicker γ lamellae, and the interfaces of coarsened γ lamellae become curvy and blurry.