Adiabatic shear banding in FCC metallic single and poly-crystals using a micromorphic crystal plasticity approach

Finite element (FE) simulations are performed for hat-shaped specimens made of face-centered cubic (FCC) metallic single and poly-crystals in order to investigate strain localization phenomena under adiabatic conditions which are related to adiabatic shear band (ASB) formation process. A micromorphic crystal plasticity model is used to overcome the main limitation of classical plasticity models, namely the mesh size dependency in strain localization problems. A thermodynamically consistent formulation of the constitutive equations is proposed for micromorphic thermo-elasto-viscoplasticity of single crystals. The temperature evolution under adiabatic conditions is derived from the competition between plastic power and energy storage. The micromorphic crystal plasticity model is used first to simulate strain localization induced by thermal softening in a metallic single crystal strip loaded in simple shear undergoing single-slip. The FE solution of this boundary-value problem is validated using an analytical solution. Regarding single crystal hat-shaped specimen simulations, five different crystal orientations are considered to study the formation, intensity and orientation of shear bands. In particular, one special crystal orientation is found resistant to shear banding. In addition, the formation of shear bands in hat-shaped polycrystalline aggregates is investigated. The specimens are polycrystalline aggregates with different grain sizes, namely the coarse-grained and fine-grained specimens with random crystal orientation distribution. Furthermore, several realizations of the microstructures are taken into account for statistical considerations. The micromorphic crystal plasticity model incorporates a characteristic length scale, which induces a grain size effect in the simulation of polycrystalline specimens. The grain boundaries act as obstacles against shear band formation. A significant grain size effect, namely the finer the grain size the higher the resulting load, is predicted by the simulations under isothermal conditions. However, the fine-grained specimens are found to fail earlier by shear banding than some coarse-grained samples, the latter being associated with significant dispersion of the results depending on grain orientations. The effect of grain size on the width of the shear band is also analyzed. The temperature-dependent material parameters and shear band widths considered in the paper correspond to Nickel-based superalloy Inconel 718 in a large temperature range. No strain hardening was considered in the hat-shaped specimen test to simplify the interpretation of the results.