Molecular dynamics and first-principles studies on the deformation mechanisms of nanostructured cobalt

Deformation mechanisms of nanostructured cobalt are investigated by classical molecular dynamics (MD) simulation and first-principles calculation. In MD simulation, deformation twinning and HCP-to-FCC transformation are found to play important roles during the deformation of nanostructured cobalt. At high stress levels, the HCP-to-FCC transformation seems to overwhelm the deformation twinning. In particular, when deformation occurs in nanocrystalline cobalt with pre-existing twins, (0 0 0 1) twins are transformed into FCC structures through the deformation mechanism of HCP-to-FCC transformation. The generalized planar fault energy (GPFE) curves calculated by density functional theory are used to elucidate the deformation processes such as stacking faults, deformation twinning and HCP-to-FCC allotropic transformation observed in nanostructured cobalt. It is demonstrated by the GPFE curves that HCP-to-FCC transformation is more favorable than deformation twinning when hydrostatic pressures or shear stresses are applied on cobalt.