Large scale atomistic simulation of size effects during nanoindentation: Dislocation length and hardness

GZ Voyiadjis and M Yaghoobi, MATERIALS SCIENCE AND ENGINEERING A-STRUCTURAL MATERIALS PROPERTIES MICROSTRUCTURE AND PROCESSING, 634, 20-31 (2015).

DOI: 10.1016/j.msea.2015.03.024

The present paper studies the size effects during nanoindentation in Ni thin films using large scale atomistic simulation. The main focus of this paper is to evaluate the available theoretical models of size effects during nanoindentation using atomistic simulation. First, the dislocation nucleation and evolution in the simulated samples are studied. In the next step, the plastic zone size is obtained for each sample at several indentation depths incorporating the dislocation visualization. The results show that the plastic zone size divided by the contact radius is not a constant factor and varies as the indentation depth changes. The total length of dislocations located in the plastic zone is measured in the simulated samples and compared to that of the corresponding theoretical models. The results obtained from the atomistic simulation verify the theoretical predictions of the dislocation length. Next, the variation of hardness obtained directly from the molecular dynamics outputs, which is the indentation force over the contact area, is studied. In the case of conical indenter, the theoretical predictions of hardness have been verified using both experiments and simulations, and the current simulation shows the same trend, i.e. the hardness decreases as the indentation depth increases. However, in the cases of flat indenters, the theoretical models have not been validated using any experiments or simulations. Here, in the cases of flat indenters, the simulation results verify the theoretical predictions of hardness. They show that the hardness increases as the indentation depth increases. The variation of dislocation density as a function of indentation depth is then studied. In the case of nanoindentation experiment, the validity of Taylor hardening model, i.e. the relation between the hardening and dislocation density, which has not been previously studied with full atomistic details, is investigated. Accordingly, the hardness obtained directly from the simulation is compared with the one calculated from the dislocation density and theoretical size effects models. (C) 2015 Elsevier B.V. All rights reserved.

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