Atomistic Processes of Phase Transformation and Dynamic Recrystallization during Hot-Working of Intermetallic Titanium Aluminides

Abstract

Intermetallic titanium aluminide alloys are multiphase assemblies with complex microstructure and constitution, involving gamma-TiAl, alpha-2, beta, B2 and orthorhombic phases. The earlier stages of phase transformation and dynamic recrystallization occurring upon hot-working were investigated at the atomic scale by high-resolution electron microscopy. Accordingly, the conversion of the microstructure is triggered by a particular deformation state and non-equilibrium phase composition. The beta/B2 is apparently unstable under tetragonal distortion, which gives rise to the formation the B19 phase via distinct shuffle displacements. These processes lead to a modulated microstructure, which is comprised of several stable and metastable phases. The phase transformations are accomplished by the propagation of ledges and the formation of twin structures. Large ledges can apparently easily be rearranged into intermediate metastable structures, which serve as precursor for the nucleation of new grains.