Heterogeneous deformations in copper single crystals at high and low strain rates

Abstract

In polycrystalline materials, the microscopic strain field is always highly heterogeneous during inelastic deformation, even when the macroscopic strain field is uniform. This is particularly true near potential damage and failure nucleation sites, such as voids and inclusions. Also, in the context of high-strain-rate plastic flow, the only currently practical way to induce large strains and high strain rates simultaneously under experimental conditions is by means of heterogenous deformations. For these and other reasons, experiments involving highly heterogeneous deformations are of considerable interest. A joint experimental/computational study of heterogeneous deformations in copper single crystals will be presented. Carefully oriented, monocrystalline copper specimens were subjected to large-strain indentation at both high and low strain rates. The high-rate experiments were performed using a split Hopkinson pressure bar high-strain-rate apparatus. Scanning and transmission electron microscopy performed on the deformed specimens revealed strikingly different deformation morphologies and dislocation substructures between the two strain rates. In general, the substructure of the high-rate specimens showed relatively little evidence of dynamic recovery processes as compared to that of the low-rate specimens. This finding tends to support the notion of strain-rate sensitivity of structure evolution in face-centred cubic metals. A series of finite element simulations of the experiment was performed using the explicit finite element code PRONTO2D, into which a rate-dependent crystal plasticity constitutive theory has been implemented. In the constitutive model, the effects of strain-rate-dependent substructure evolution were simulated through the inclusion of a strain-rate-history-dependent hardening parameter. Comparison of the experimental and computational results reveals that strain-rate history has a profound influence on deformation morphology at medium to high strain rates. Also, the comparison serves to highlight the importance of proper characterization of both instantaneous rate-sensitivity and strain-rate-history effects.