Pair vs many-body potentials: influence on elastic and plastic behavior in nanoindentation

TL;DR

This study compares the effectiveness of simple pair potentials versus a realistic many-body potential in molecular dynamics simulations of nanoindentation in copper, revealing that pair potentials capture many qualitative features but fail quantitatively in modeling dislocation processes.

Contribution

It demonstrates that simple pair potentials can qualitatively reproduce nanoindentation behavior but have significant quantitative limitations compared to many-body potentials.

Findings

Pair potentials capture elastic regime and yield points.

Pair potentials underestimate stacking fault energy.

Quantitative discrepancies include overestimated work hardening.

Abstract

Molecular-dynamics simulation can give atomistic information on the processes occurring in nanoindentation experiments. In particular, the nucleation of dislocation loops, their growth, interaction and motion can be studied. We investigate how realistic the interatomic potentials underlying the simulations have to be in order to describe these complex processes. Specifically we investigate nanoindentation into a Cu single crystal. We compare simulations based on a realistic many-body interaction potential of the embedded-atom-method type with two simple pair potentials, a Lennard-Jones and a Morse potential. We find that qualitatively many aspects of nanoindentation are fairly well reproduced by the simple pair potentials: elastic regime, critical stress and indentation depth for yielding, dependence on the crystal orientation, and even the level of the hardness. The quantitative deficits of the pair potential predictions can be traced back (i) to the fact that the pair potentials are unable in principle to model the elastic anisotropy of cubic crystals; (ii) as the major drawback of pair potentials we identify the gross underestimation of the stable stacking fault energy. As a consequence these potentials predict the formation of too large dislocation loops, the too rapid expansion of partials, too little cross slip and in consequence a severe overestimation of work hardening.