Influence of grain boundary activites on elastic and plastic deformation of nanocrystalline Cu as studied by phase filed and atomistic simulaiton

Abstract

When the grain size is below a critical value, the deformation mechanism of nanocrystalline (NC) metal shifts from dislocation- to grain boundaries (GBs)-mediated activities. However, GB activities including deformation and rotation in the elastic and plastic stages have not given detailed analysis because of the limitation of current experimental techniques, as well as the unrealistic model with regular boundaries in most molecular dynamics (MD) simulation reducing the activities. To quantitatively analyze GB activities, we recently developed an algorithm based on moving least-squares interpolant. Besides, the phase field model was used to build the nanocrystalline (NC) Cu with more natural GBs for MD tensile simulation. The results of temperature dependence of Young's modulus (E), strain rate sensitivity (SRS), and variation of stress at GBs (σGB) and at grain interiors (σGI) are in good agreement with previous researches. In the elastic stage, the σGB fluctuates in a larger range than σGI. The vibrated σGB is the results of micro-elastic and micro-plastic deformation of GBs and the increased stage of σGB in one cycle can lead to increasing the σGI. Thus, the temperature dependence of E is affected by the frequency of vibrated σGB at relative low temperatures (T ≤ 450 K). The E is strain rate independent because of the same vibration of σGB. In the plastic stage, the flow stress strongly depends on the size of twinning boundaries (TBs) structures. The fraction of GB activities improves with increasing temperature and decreasing strain rate. GB activities can attenuate stress concentration on boundaries and then results in the formation and growth mechanism of TB changing from Shockley partial dislocation gliding on adjacent plane of stacking faults (SFs) to GB deformation. Thus, the size of TB at low temperatures and high strain rate is larger than that at the opposite conditions.