Numerical simulation of yield loci for PST crystals of TiAl

The yield strength of polysynthetically twinned (PST) crystals of TiAl depends on the orientation angle between the lamellar boundaries and the load axis. It is the aim of this work to model this anisotropic behaviour. A micromechanical model is employed which considers the lamellar microstructure and the deformation mechanisms such as ordinary slip and deformation twinning in γ-TiAl. Since the calculated and measured yield strength data show good agreement in the case of uniaxial loading, there is a real motivation to study numerically the behaviour of a PST crystal under biaxial loading. Yield points are calculated for various proportional stress paths defined by the ratio of the principal stresses σ1/σ2 in relation to the orientations of the load axes towards the lamellae. These predicted yield loci are compared with the von Mises and the Hill yield criteria.