Understanding imprint formation, plastic instabilities and hardness evolutions in FCC, BCC and HCP metal surfaces

Nanoindentation experiments in metal surfaces are characterized by the onset of plastic instabilities along with the development of permanent nanoimprints and dense defect networks. This investigation concerns massive molecular dynamics simulations of nanoindentation experiments in FCC, BCC and HCP metals using blunted (spherical) tips of realistic size, and the detailed comparison of the results with experimental measurements. Our findings shed light on the defect processes which dictate the contact resistance to plastic deformation, the development of a transitional stage with abrupt plastic instabilities, and the evolution towards a self-similar steady-state characterized by the plateauing hardness p(p) at constant dislocation density rho(p). The onset of permanent nanoimprints is governed by stacking fault and nanotwin interlocking, the buildup of nanostructured regions and crystallites throughout the imprint, the crossslip and cross-kinking of surfaced screw dislocations, and the occurrence of defect remobilization events within the plastic zone. As a result of these mechanisms, the ratio between the hardness p(p) and the Young's modulus Ebecomes higher in BCC Ta and Fe, followed by FCC Al, HCP Mg and large stacking fault width FCC Ni and Cu. Finally, when nanoimprint formation is correlated with the uniaxial response of the indented minuscule material volume, the hardness to yield strength ratio, p(p)/sigma(ys), varies from approximate to 7 to approximate to 10, which largely exceeds the continuum plasticity bound of approximate to 2.8. Our results have general implications to the understanding of indentation size-effects, where the onset of extreme nanoscale hardness values is associated with the occurrence of unique imprint-forming processes under large strain gradients. (C) 2021 The Author(s). Published by Elsevier Ltd on behalf of Acta Materialia Inc.

Automated summary

Nanoindentation experiments in various metals show the onset of plastic instabilities, formation of permanent nanoimprints and defect networks. Molecular dynamics simulations reveal the processes governing contact resistance, abrupt plastic instabilities, and the evolution towards a steady-state with plateauing hardness. The ratio of hardness to Young's modulus varies between different metals, with BCC Ta and Fe showing higher values. Factors such as stacking faults, nanotwin interlocking, and defect remobilization events contribute to the formation of permanent nanoimprints. Additionally, the correlation between nanoimprint formation and material response to indentation influences the hardness to yield strength ratio, exceeding the continuum plasticity bound.