Size-dependent hardness in annealed and work hardened α-brass and aluminum polycrystalline materials using activation volume analysis

Abstract

A study of the size-dependent hardness in aluminum and α-brass is presented. The study employs rate-effects to examine the fundamental mechanisms responsible for the indentation hardness size dependence (effect), or (ISE). These rate effects are characterized in terms of the rate sensitivity of the hardness, ∂ H/∂ln ε eff, where H is the hardness and ε eff is an effective strain rate in the plastic zone beneath the indenter. ∂ H/∂ln ε eff is measured using indentation creep, load relaxation, and rate change experiments. ∂ H/∂ln ε eff is used to calculate the activation volume, V*; activation volume data measured using conventional uniaxial testing are compared with activation volume data measured using nanoindentation. The data for α-brass when plotted V* vs. H (hardness) or σ (flow stress), extrapolated into literature data from conventional uniaxial testing, while the aluminum data suffered an offset. We propose some mechanisms for this offset. Using V* formalism, we demonstrate using materials with different stacking fault energy (SFE) and specimens with different levels of work hardening how increasing the dislocation density affects V*; these effects may be taken as a kinetic signature of dislocation strengthening mechanisms. We depicted an ISE in both H and ∂ H/∂ln ε eff( V*). The trend of V*-vs.- H as a result of the ISE is consistent with the trend of testing specimens with different levels of work hardening. This indicates that a dislocation mechanism drives the ISE.