Phonon instability of Co single crystal in uniaxial tension and nanoindentation

Abstract

Although hexagonal close-packed (HCP) metals have significant industrial importance, little attention has been given to their crystal instability. In this work, the uniaxial tension and nanoindentation of Co single crystal were simulated by a multiscale lattice dynamical finite-element method (LDFEM) and molecular dynamics (MD) to investigate its crystal instability. Simulation results show that the crystal instability of Co is associated with loading modes and crystallographic orientations. In uniaxial tension, the stress–strain curves obtained from LDFEM simulations are in good agreement with those in MD simulations before crystal instability. The critical stress for crystal instability in [0001] tension is 21.04GPa, larger than 16.00GPa in [011¯0] tension and 8.31GPa in [21¯1¯0] tension. In nanoindentation, the crystal instability and mechanical behavior of Co are affected by crystallographic orientations. The load–displacement curves and stress distribution obtained from LDFEM simulations match well with those in MD simulations. The crystal instability of Co mainly leads to dislocation nucleation on basal plane. Six symmetric dislocation nucleation sites appeared in the spherical nanoindentation of Co (0001) surface.