Refinement Mechanism of Centimeter-Grade Coarse Grains in As-Cast Ti2AlNb-Based Alloy Through Uniaxial and Multi-axial Compressions

Ti2AlNb-based alloy has great application potential in the aerospace industry due to its low density, high service temperature, and excellent mechanical properties. However, the ingots of this material are usually composed of coarse grains, even up to centimeter grade, which should be refined by deformation to avoid the deterioration of their workability and in-service performance. In this work, the grain refinement mechanism of as-cast Ti2AlNb-based alloy was investigated by uni- and multi-axial compressions at elevated temperatures. The results of uniaxial compression demonstrated that the grains were significantly elongated without volume change, indicating little recrystallization even after very large deformation. The investigation showed that the dislocations induced by deformation were prone to annihilate via dynamic recovery, while dynamic recrystallization (DRX) was inhibited due to insufficient dislocations. Two multi-axial compression schemes were compared, i.e., two-step compression with 60% reduction per step (Scheme 1) and three-step compression with 40% reduction per step (Scheme 2). The results indicated that the recrystallization fraction of deformed samples in Scheme 1 was much higher than that in Scheme 2, although the total strain of those two cases was similar. It is concluded that the formation of dislocation substructures in the former deformation, which strongly affects the intragranular misorientation, is the key factor to trigger the recrystallization in the processes followed.