Crystal plasticity FE modeling of Ti alloys for a range of strain-rates. Part I: A unified constitutive model and flow rule

The deformation and failure response of many polycrystalline metallic materials is strongly dependent on the strain-rate. For an applied strain-rate, different points in the material microstructure can undergo strain-rates that differ by orders of magnitude depending on location and deformation history. Dislocation motion in metals is governed by the thermally-activated and drag-dominated processes under low and high rates of deformation, respectively. Commonly used flow rules, e.g the phenomenological power law or the linear viscous drag models are generally applicable to a limited range of strain-rates, without transcending these rates. To enable this transition in a seamless manner, this two part paper develops a unified constitutive model and an image-based crystal plasticity finite element model for polycrystalline hcp metals. The first part develops a dislocation density-based crystal plasticity constitutive relation with a unified flow rule by combining the thermally-activated and drag-dominated stages of dislocation slip, suitable for modeling deformation at a wide range of strain-rates. The model is explicitly temperature dependent, making it appropriate for simulating high rate deformations, where temperature increases locally with plastic deformation due to the adiabatic heating. The unified model is used to study rate-sensitivity of flow stress in single crystal and polycrystalline titanium alloy, Ti-7Al. An elastic overshoot, followed by a stress relaxation is observed at high strain-rates in single crystals. For the polycrystalline simulations, the model effectively captures the increase in rate sensitivity at high strain-rates. (C) 2016 Elsevier Ltd. All rights reserved.