Multi-scale simulations of plasticity in metal nanocontacts: Irreversibility of the deformation

A dual-scale scheme, comprising an atomistic and a continuum domain, is here developed to study contact deformation. The three-dimensional model allows to tackle with atomistic precision problems where contact occurs at the nano-scale, while the bodies have larger dimensions. The upper part of the body is modeled by means of an atomistic domain to capture non-linearities arising in the surrounding of the contact region, including defects formation and dislocation motion. The lower part of the body, away from the contact, is instead modeled as a continuum. The novelty of the dual-scale method lies in the way the solution to the continuum is obtained: analytically through a Green's function methodology, thus very time efficient. The model is first validated through direct comparison with full atomistic simulations and then applied to study light indentation and subsequent unloading of metal crystal by means of a spherical tip. The simulations intend to shed more light on the ‘reversed plastic behavior’ recently observed in nanocontacts. This is done by computing the irreversible plastic strain after unloading. Results show that if there is a significant deviation from the elastic curve upon loading, which manifests itself with a few drops in the force, there is also irreversible strain upon unloading. Only the nucleation of stacking faults that give negligible deviation from the load area curve is reversible. Classical theories like Hertz or JKR can be safely used when this initial stacking faults appear, but should be avoided when actual plasticity kicks in.