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 alpha(2) phase in the microstructure. Herein, the evolution of the metastable alpha(2) 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 beta single-phase region, Widmanstatten alpha(2), as well as residual beta(o), gamma grains form from beta phase and thin gamma laths in alpha(2) phase are observed. However, the water-quenched microstructure from the alpha single-phase region consists of equiaxed alpha(2) grains, residual beta(o), and massive gamma phase. When annealing the quenched samples at 800 degrees C, similar microstructural evolutions are found in both samples quenched from different temperatures. 1) alpha(2) phase transform into alpha(2)/gamma nanolamellar structure immediately. 2) Recrystallization and discontinuous coarsening occur at the colony boundaries. 3) The nanometer-scale gamma/gamma(T) lamellae transformed from alpha(2) grains are also unstable, which merge into thicker gamma lamellae, and the interfaces of coarsened gamma lamellae become curvy and blurry.