The effect of crystal anisotropy and pre-existing defects on the incipient plasticity of FCC single crystals during nanoindentation

Abstract

Molecular dynamics simulation is used to identify and quantify the initial plastic deformation mechanisms of gold as a model face-centered cubic (fcc) metal in the nanoindentation process. The coupling effects of crystallographic orientation and internal structural defects on the resulting load distribution at the onset of plasticity are investigated to clarify the anisotropic characteristics of material responses to crystallographic orientation. Homogenous defect nucleation is studied by correlating the indentation force-displacement curve with the instantaneous defect structure. In the absence of pre-existing defects, nanoindentation deformation is dominated by nucleation of Shockley partial dislocations regardless of crystal orientation. Various forms of dislocation propagation are observed in different crystal orientations. The elastic-plastic transition point appears later for the [111]-oriented surface than the ones for [001]-, and [011]-oriented surfaces. The relation of hardening and dislocation density shows that conventional Taylor hardening captures the plasticity after a certain amount of indentation depth in the presence of enough dislocation density. The crystal's sensitivity to the presence of internal structural faults is strongly dependent on the crystallographic orientation. Nanoindentation simulations in the presence of sessile dislocation loops in the structure show that the most significant reduction in the pop-in load happens for the [111] oriented sample. Our simulations suggest that the indentation near a defect can lead to small, subcritical events that lead to a smoother "pop- in" at the onset of plasticity. Since internal defects in materials are nearly inevitable, a defect-based model can be useful to understand the stochastic pop- in loads in nanoindentation tests.