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

A study of the size-dependent hardness in aluminum and a-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, partial derivativeH/partial derivativeln(epsilon)over dot(eff), where H is the hardness and (epsilon)over dot(eff) is an effective strain rate in the plastic zone beneath the indenter. partial derivativeH/partial derivativeln(epsilon)over dot(eff) is measured using indentation creep, load relaxation, and rate change experiments. partial derivativeH/partial derivativeln(epsilon)over dot(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 alpha-brass when plotted V* vs. H (hardness) or a (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 partial derivativeH/partial derivativeln(epsilon)over dot(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. (C) 2002 Elsevier Science B.V. All rights reserved.