Modelling high temperature deformation of lamellar TiAl crystal using strain-rate enhanced crystal plasticity

Abstract

Multi-phase lamellar gamma -TiAl alloys are highly anisotropic and rate sensitive. At high temperature, the strain rate sensitivity (SRS) increases, which eventually influences the yield behaviour of the alloy. With increasing temperature, the onset of yield changes anomalously showing an increasing or sometimes a decreasing trend. This anomalous behaviour depends on alloy chemistry, lamellar microstructure and anisotropy due to lamellar orientations. To capture the strain rate sensitive deformation behaviour over a wide temperature range and to understand distinct nature of yield anisotropy, a new formulation of the strain rate sensitivity (SRS) expressed by a temperature dependent exponent has been proposed within the framework of the classical power-law based visco-plastic Crystal Plasticity Finite Element Method (CPFEM). In the presented formulation, the evolution of SRS parameter was deduced from laws of thermally activated dislocation motion to obtain temperature-enhanced strain rate sensitivity. Moreover, a dislocation pile-up assisted mechanical threshold stress was incorporated to obtain an anomalous yield response. The model was validated for a lamellar PST-TiAl, which is a single crystal consisting of gamma-TiAl and alpha2-Ti3Al lamellar phases. Detailed predictive numerical analysis was performed for an in-depth understanding of temperature enhanced strain-rate sensitivity during local deformation behaviour of TiAl alloy based on slip activities.