The role of slip and twinning in the deformation behaviour of polysynthetically twinned crystals of TiAl: A micromechanical model

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Abstract
This micromechanical study investigates the deformation behaviour of TiAl based alloys with a two-phase microstructure composed of the γ-Ti3Al phase and a small volume fraction of the α2-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 γ-TiAl phase, ordinary slip and true twinning, are considerd in the model. Slip is modelled by crystal plasticity. Deformation twinning is implemented by a constitutive law analogous to Schmid's law for crystallographic slip. This micromechanical model is applied to simulate uniaxial tests of so called polysynthetically twinned (PST) crystals of TiAl which contain only a single set of parallel γ-TiAl and α2-Ti3Al lamellae with a specific orientation. Stress-strain curves are calculated numerically for various lamellae orientations. The computed yield stresses are compared with experimental ones. The model is able to explain the deformation mechanisms in detail. Specifically it predicts the twin volume fractions in the γ-TiAl phase and confirms the importance of twinning as a deformation mechanism.