An Atomistic Investigation of Cobalt's Nanoindentation Response with An Angular Dependent Potential.

Cobalt and its alloys are essential in many advanced technologies and understanding their mechanical properties at the nanoscale is crucial for designing next-generation materials. In this work, an angular-dependent potential for cobalt was developed by fitting to a reference data set of atomic forces, energies, and stress tensors derived from first-principles density functional theory calculations. The potential's performance was systematically evaluated against experimental data and two established classical potentialsan embedded-atom method potential and a modified embedded-atom method potentialacross a range of structural, mechanical, thermal, and defect properties for both HCP and FCC phases, as well as the liquid state. The ADP model demonstrates a favorable balance between accuracy and computational cost, exhibiting a mean absolute percentage error of 6.3% for mechanical and elastic properties. Large-scale molecular dynamics simulations of nanoindentation on the (0001) basal plane of HCP cobalt were performed to investigate the atomistic mechanisms of plastic deformation. The simulations reveal that plasticity initiates with the nucleation of <a>-type dislocations on basal planes, followed by the activation of pyramidal <c+a> slip and a localized, reversible HCP-to-FCC phase transformation under high pressure. The critical shear stress for dislocation nucleation was found to decrease with increasing indenter radius, converging to a value of (13.7 ± 0.6) GPa.