Dislocation Nucleation and Source Activation during Nanoindentation Yield Points

The onset of plasticity during nanoindentation of a tungsten single crystal was examined as a function of pre-existing dislocation density. Vickers indentations were used to generate a spatially varying dislocation density, and nanoindentation was then carried out at regions of high and low dislocation densities. Even with dislocation densities as high as 1.8 × 10¹³ m⁻², a sharp elastic-plastic transition was observed during some indentations. At lower dislocation densities, 3.5 × 10⁹ m⁻², the shear stress at the elastic plastic transition increased and approached the theoretical shear stress of the crystal. A first-order model that predicts the load required for the onset of plasticity during nanoindentation from the activation of a dislocation source within a critical volume of material, rather than homogeneous dislocation nucleation, is developed. The model correlates well with experimentally measured loads at the onset of plasticity for dislocation densities of 10¹² m⁻² and higher for these nanoindentation conditions.