Refinement mechanism of centimeter-grade coarse grains in as-cast Ti2AlNb-based alloy during multi-directional forging

Abstract

Centimeter-grade coarse grains commonly exist in heavy ingots of Ti2AlNb-based alloy and severely deteriorate their workability. To investigate the refinement of coarse grains, multi-directional isothermal forging (MDIF) was carried out on samples cut from an ingot weighing about three tons. The results showed that the alloy can be refined by MDIF after 4 passes with 40% reduction per compression at 1200℃, while it cannot be refined even after 10 passes with 30% reduction per compression at 1200℃ or with 40% reduction per compression at 1050℃. The investigation showed that, as an intermetallics with low strain hardening rate, the material is very difficult to be refined by recrystallization if only increasing the total strain. The special refinement mechanism, i.e., grain thinning and shearing (GTS) induced recrystallization, is proposed. The reduction per compression is crucial to refine the coarse grains of this alloy. Coarse grains should be firstly squashed and thinned by large reduction, and then bent and bowed to form shear bands in the following perpendicular compressions. Coarse grains are truncated by repeated shearing, and the truncation increases interfaces within the coarse grains and contributes to the local misorientation accumulation, which in turn promotes the completion of recrystallization.