Lattice Rotation Patterns and Strain Gradient Effects in Face Centered Cubic Single Crystals Under Spherical Indentation

Abstract

Strain gradient effects are commonly modeled as the origin of the size dependence of material strength, such as the dependence of indentation hardness on contact depth and spherical indenter radius. However, studies on the microstructural comparisons of experiments and theories are limited. First, we have extended a strain gradient Misesplasticity model to its crystal plasticity version and implemented a finite element method to simulate the load–displacement response and the lattice rotation field of Cu single crystals under spherical indentation. The strain gradient simulations demonstrate that the forming of distinct sectors of positive and negative angles in the lattice rotation field is governed primarily by the slip geometry and crystallographic orientations, depending only weakly on strain gradient effects, although hardness depends strongly on strain gradients. Second, the lattice rotation simulations are compared quantitatively with micron resolution, threedimensional Xray microscopy (3DXM) measurements of the lattice rotation fields under 100 mN force, 100 خ¼m radius spherical indentations in م€ˆ111م€‰, م€ˆ110م€‰, and م€ˆ001م€‰ oriented Cu single crystals. Third, noting the limitation of continuum strain gradient crystal plasticity models, twodimensional discrete dislocation simulation results suggest that the hardness in the nanocontact regime is governed synergistically by a combination of strain gradients and sourcelimited plasticity. However, the lattice rotation field in the discrete dislocation simulations is found to be insensitive to these two factors but to depend critically on dislocation obstacle densities and strengths.