# An Atomistic Investigation of Cobalt’s Nanoindentation Response with An Angular Dependent Potential

## TL;DR

This paper studies the mechanical behavior of cobalt at the nanoscale using a new atomic-level model to understand how it deforms under pressure.

## Contribution

A novel angular-dependent potential for cobalt is developed and validated for nanoindentation simulations.

## Key findings

- Plastic deformation in cobalt starts with dislocation nucleation followed by phase transformation under high pressure.
- The critical shear stress for dislocation nucleation decreases with larger indenter radius, converging to 13.7 ± 0.6 GPa.

## Abstract

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.