Origin of the depth dependence of the apparent activation volume in polycrystalline 99.999% Cu determined by displacement rate change micro-indentation

Micro-indentation with displacement rate changes were performed at 300 K on high purity polycrystalline copper rods in the as-swaged and fully recrystallized condition to evaluate the influence of indentation depth on the apparent activation volume, V′, and the indentation hardness, H. Both H and V′, initially increase with indented inelastic depth and then approach plateau-values with H being higher and V′ being lower for the swaged compared to the annealed copper. The average inter-obstacle spacing, ℓ∗, characteristic of the annealed copper, was estimated to be 2800b, which corresponds to an average activation distance, d∗, of 0.49b. For the swaged copper, ℓ∗ was estimated to be 490b for the same d∗. The variation in H and V′ with indentation depth indicates that the dislocation density, and hence the average plastic strain, around the indentations, increases as the plastic zone size decreases. H and V′ at various indentation depths were used to construct part of the dislocation-obstacle force versus distance diagram for copper.