A molecular dynamics (MD) simulation study to investigate the role of existing dislocations on the incipient plasticity under nanoindentation

Incipient plasticity is realized in the nanoindentation load-depth response as a load-drop under a displacement controlled loading condition or as a displacement-burst under a load controlled loading condition. Experimental results indicate that the characteristics of load-drop or displacement-burst such as the critical indentation load, critical indentation depth, magnitude of load-drop, or magnitude of displacement-burst, can vary substantially, depending on the microstructural conditions of the deformation volume. In this article, a molecular dynamic (MD) simulation study to investigate the role of existing dislocations, particularly the role of mutual interaction of individual dislocations, on incipient plasticity under nanoindentation is reported. Dislocations are introduced into the perfect lattice structure of a copper sample by removing two adjacent layers of atoms. Subsequent stabilization of the this system results in extended edge dislocations (EEDs) consisting of two partial dislocations and a stacking fault. The nanindentation is simulated with three different tip radii on these samples consisting of various number of EEDs. Results show that, a substantial cross-slipping of atoms at constriction points formed by the motion of partial dislocations toward each other, driven by the indenter stress field, is a potential deformation mechanism that yield load-drop (or displacement-burst). The effect of partial dislocations on critical indentation load, critical indentation depth, and magnitude of load-drop (or displacement-burst) is studied. The mutual interaction of the local stress fields of neighboring partials have significant influence on the deformation kinematics, and consequently, on the various characteristics of incipient plasticity. (C) 2014 Elsevier B.V. All rights reserved.