Pseudo Sub-grain Formation and Evolution in a Ti₃Al-Nb Based Alloy Induced by B2-BCC Disordering

The high temperature β/B2 phase field of Ti₃Al-Nb based alloys was rarely studied due to limitation in experimental technique, but different states of this phase field lead to difference in subsequent phase transformation path and resulting microstructure. By controlling the heating temperature and dwell time of α₂′ → β/B2 transformation and subsequent cooling rate, we achieved a new microstructure consisting of domains of the B2 phase with α₂′ martensite occupying the disordered BCC domain walls, effectively forming a pseudo sub-grain microstructure. It is shown that the domain walls evolved from the disordered β phase in the β/B2 phase field. Such pseudo sub-grain boundaries refine the prior β grains by subdividing them into smaller α₂ colonies that form during subsequent heat treatment in the α₂′ + β/B2 phase field upon reheating, and improve the mechanical properties of the alloy.