Dislocation nucleation in a thin Cu film from molecular dynamics simulations: Instability activation by thermal fluctuations

Abstract

To elucidate the mechanism responsible for structural instability at the atomic level, atomistic modeling simulation of tension in a Cu thin film containing a notch was performed using an embedded-atom method potential and dislocation nucleation was observed. Mechanical stability during tension was analyzed by solving the eigenvalue problem of the Hessian matrix taking into account all the degrees of freedom of the atoms in the system. Since an eigenvalue designates the curvature of the potential energy landscape in the direction of the corresponding eigenvector, which indicates a deformation mode, the system is unstable under vanishing temperature at the critical strain (εc) when any eigenvalue is zero or negative. At a strain smaller than εc where all the eigenvalues are positive, atomic fluctuations due to finite temperature may cause structural instability. We found that the path of activated instability (dislocation emission from the notch) could be written with a linear combination of the eigenvectors having small eigenvalues obtained under a corresponding external strain at zero temperature. The energy landscape has a much lower hill along the mixed-mode path than along any single-mode paths. In a molecular dynamics simulation under finite temperature, components of deformation modes having small eigenvalues fluctuate at low frequency, which dominate the activation of instability.