Multiscale dislocation dynamics modeling of the high strain rate deformation in copper single crystal under monotonic, cyclic, and shock loading

Abstract

This work aims to investigate the deformation processes at high strain rates. Three types of high strain rate loading are investigated, 1) cyclic deformation 2) monotonic loading 3) shock and impact loading. Multiscale dislocation dynamics plasticity (MDDP) model is used to investigate the evolution of dislocation microstructure in copper single crystals subjected to low cycle fatigue loading. Half cycle total plastic strain simulations are carried out at strain amplitudes ranging from 1×10⁻³ to 8×10⁻³. The initial hardening is investigated and the micro-structural cause behind it is presented. In addition, the evolution of the microstructures is examined. In depth analyses of the dislocation microstructures show that: 1) dislocation walls that are parallel and very close to each other are formed, 2) these walls contain dipoles that keep on zipping and unzipping during the first few cycles until they reach some stable zipping configuration. We can see that the hardening rate decreases with the increase of the number of cycles where we have large hardening rate in the first cycles then we reach to somehow constant stress. Our results are qualitatively in good agreement with recent experimental results of low cycle fatigue deformation. In addition, MDDP is used to investigate the effect of strain rate on the behavior of copper single crystals subjected to monotonic loading. The simulations are carried out at strain rates between 1×10⁴ and 1×10⁹. From the stress-strain curves of the simulations, the yielding stress increases with the strain rate then it almost saturates. For this range of strain rates two competing dislocation multiplication mechanisms are present. For strain rates up to ≈ 1×10⁷, heterogeneous nucleation i.e. activation of Frank-Read sources is the governing mechanism for dislocation multiplication whereas above that homogenous nucleation is the governing mechanism. A comparison of yielding stresses for both tension and