Orientation effects of elastic–plastic deformation at surfaces: nanoindentation of nickel single crystals

Abstract

Molecular dynamics simulations were performed to study anisotropic features in nanomechanical properties at the surfaces of nickel single crystals as a function of indenter size and velocity for three crystallographic orientations: ⟨100⟩, ⟨110⟩, and ⟨111⟩. The tabular form of Voter's embedded atom method (EAM) potential was used to describe the interatomic interactions of nickel single crystals. A strongly repulsive potential model between the indenter tip and the metal surface was designed to address the effect of a passivation layer. The force vs. displacement curves for indentation followed the power law solution of F = k \delta ^{ \nu } for elastic deformation. The value of ν was dependent on the indenter velocity, following the Hertzian solution {\rm ( \nu = 1.5)} at the high velocity of approximately 670 m/s but showing a non-Hertzian power {\rm ( \nu = 2.5)} at the low velocity of approximately 67 m/s. The force fitted micro-modulus showed a dependency on indenter size and velocity for the three crystallographic orientations. The results of dislocation nucleation—the early stage of plasticity—in the different orientations showed anisotropy: stacking faults in the ⟨100⟩ orientation, deep partial dislocations in the ⟨110⟩ orientation, and shallow partial dislocations followed by stacking faults in the ⟨111⟩ orientation.

Keywords:

• Nanoindentation
• Anisotropy
• Indentation curve
• Dislocation nucleation
• Elastic–plastic deformation
• Nickel single crystal

Acknowledgements

Art Voter is thanked for the kindness of giving his package of EAM potential models for fcc metals and their alloys. Steve Stuart is thanked for his helpful conversations and support.