Indentation size effect in polycrystalline F.C.C. metals

Abstract

High purity aluminum and alpha brass samples were tested for the indentation size effect (ISE) using a combination of microhardness (high load) and nanoindentation (low load). We employed rate effects to study low temperature deformation mechanisms using nanoindentation creep and load relaxation. Based on these rate effects which are conspicuous in terms of the rate sensitivity of the hardness, ∂H /∂ ln ε ̇ eff , we calculated the activation volume, V ∗ , and compared data from indentation creep with data from uniaxial loading. The data from nanoindentation for alpha brass when plotted, V∗ vs H or τ (flow stress) , extrapolated into literature data from conventional tensile testing, while the aluminum nanoindentation data exhibited an offset. We propose some mechanisms for this offset. We demonstrate that the trend of V ∗ vs H, resulting from the ISE in a single specimen, concurs with that obtained from testing specimens with various levels of work hardening. This suggests that the ISE is driven by a dislocation mechanism. Additionally, when the results are fitted to a strain gradient plasticity model (SGP), the data at deep indents (microhardness and large nanoindentation) exhibited a straight-line behavior closely identical to literature data. However, for shallow indents (nanoindetation data), the slope of the line severely changes, decreasing by a factor of ten, resulting in a “bilinear behavior”. We also propose some possible mechanisms for this behavior.