Lattice and phase strain evolution during tensile loading of an intermetallic, multi-phase γ-TiAl based alloy

Intermetallic γ-TiAl based alloys are promising materials for lightweight high-temperature applications, but their limited room temperature ductility poses an obstacle to the exploitation of their full potential. Especially in the case of multi-phase TiAl alloys, such as the β-stabilised TNM alloy of a nominal chemical composition of Ti-43.5Al-4Nb-1Mo-0.1 B (in at.%), an understanding of deformation and load partitioning mechanisms is required that works at all scales and encompasses all phases, including e.g. β_o. In the present work, in situ high-energy X-ray diffraction measurements were conducted on a recent TNM sheet to study the load-bearing mechanisms and their sequential order upon tensile loading for the first time on the level of individual lattice planes and phases. Four specific stages of deformation were revealed. The direction-dependent analysis of the diffraction elastic moduli offered insights into the anisotropy of the individual phases and the initiation of intergranular and interphase stresses in the elastic regime. Plastic deformation was found to commence in the γ phase at applied stress levels of roughly 670–690 MPa. Load partitioning between differently oriented grains of the γ phase was observed, followed by a load transfer onto the α₂ and β_o phase. Further tensile loading entailed the onset of plasticity within favourably oriented α₂ grains. The globular β_o phase was found to deform elastically until failure. Differently oriented specimens of the weakly textured TNM sheet showed that the macroscopic mechanical properties can be assumed nearly isotropic.