Micromechanical study on yield stress and the effects of twinning for γ-TiAl-based PST crystals

An analytical micromechanical method, based on the lamellar microstructure and dislocation slip and twinning deformation mechanisms of polysynthetically twinning (PST) crystal, is used to investigate the variation of the yielding stress as function of angle θ between the load axis and the lamellar boundaries for a PST crystal. The sub-domain microstructure of the γ-phase in PST crystal and the difference of CRSS in ordinary slip and true twinning have been considered in this paper. The results demonstrated that yielding stress relations on loading angle with respect to the microstructure and in soft mode of deformation, which is determined by ordinary dislocation or superdislocation slip systems in γ phase, shear deformation is parallel to the lamellar boundaries; in hard mode of deformation, shear deformation crosses the lamellar boundaries, which is controlled by twinning in the γ phase. In addition, the predicted yield stress values of PST crystals for three loading angles θ=45°, 0° and 90° have the relation as σ _y(45°)<σ_y(0°)< σ_y(90°). Furthermore, the active slip and twinning systems have also been analyzed systematically.