Investigation of strain hardening near grain boundaries of an aluminum oligocrystal: Experiments and crystal based finite element method

Grain boundary mechanics plays a key role in the strengthening of polycrystals. In this study, deformation of aluminum grains is investigated to better understand the origins of strengthening mechanisms near the grain boundaries. For this reason a sample with nearly columnar structure was prepared and fully characterized by electron back scattered diffraction measurements using the two side surfaces of the sample before loading. The total strain distribution on one surface of the sample is measured during tensile loading using the digital image correlation technique. The numerical studies include implementation of crystal based finite-element model to understand the origin of grain boundary strengthening using both local phenomenological, local dislocation density based and non-local strain gradient based constitutive laws. The effects of mesh refinement, through thickness discretization, boundary condition, and orientation scatter are investigated. Comparison of experimental and simulated total strain distributions near grain boundaries suggests the existence of strain gradient hardening. For this reason, a novel non-local flux based model is developed and implemented with a consistent time integration scheme. Dislocation density based models reveal strain distributions that are in better agreement with the experimental results than the phenomenological model that was not able to capture grain boundary strengthening and experimental strain distributions. The non-local strain gradient based strain hardening law, which does not require separate definition of grain boundary elements, was successful in capturing the strengthening effect near grain boundaries.