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Atomistic Model Analysis of Local and Global Instabilities in Crystals at Finite Temperature

There have been a lot of studies dedicated to structural instability in solids. For local instability, theoretical (ideal) strength of crystals has been extensively studied with ab initio calculations. Global instability taking into account the collective motion of atoms involved in deformation has also been investigated. However, these studies have usually been done at 0 K and little has been understood about the effect of temperature. In this study, we demonstrate computational approaches to the effect of temperature on local and global instabilities. Ideal shear strength (ISS) of silicon at finite temperatures is calculated by molecular dynamics (MD) simulations with an empirical potential. ISS is obtained as a function of temperature. Our results imply that, unlike metals, the reduction in ISS by temperature cannot be estimated simply by taking into account thermal expansion of volume. In addition, global instability for dislocation nucleation in a Cu thin film model under tension is investigated. We first evaluated instability modes at 0 K with increasing strain, and then performed MD simulations at 50 K. After the nucleation of a partial dislocation, the second dislocation can be one to create a twin or one to create another partial dislocation. These different deformations can be understood as the competition of latent instability modes that have relatively small eigenvalues.