THE ROLE OF SLIP AND TWINNING IN THE DEFORMATION-BEHAVIOR OF POLYSYNTHETICALLY TWINNED CRYSTALS OF TIAL - A MICROMECHANICAL MODEL

This micromechanical study investigates the deformation behaviour of T iAl based alloys with a two-phase microstructure composed of the gamma -TiAl phase and a small volume fraction of the alpha(2)-Ti3Al phase at room temperature. The modelling is based on the unit cell technique. A three-dimensional unit cell represents the microstructure. The main deformation mechanisms in the gamma-TiAl phase, ordinary slip and true twinning, are considerd in the model. Slip is modelled by crystal pla sticity. Deformation twinning is implemented by a constitutive law ana logous to Schmid's law for crystallographic slip. This micromechanical model is applied to simulate uniaxial tests of so called polysyntheti cally twinned (PST) crystals of TiAl which contain only a single set o f parallel gamma-TiAl and alpha(2)-Ti3Al lamellae with a specific orie ntation. Stress-strain curves are calculated numerically for various l amellae orientations. The computed yield stresses are compared with ex perimental ones. The model is able to explain the deformation mechanis ms in detail. Specifically it predicts the twin volume fractions in th e gamma-TiAl phase and confirms the importance of twinning as a deform ation mechanism.