Stacking fault energy and dynamic recovery: do they impact the indentation size effect?

Aluminum, a high stacking fault energy (SFE) material and α-brass, a low SFE material were tested for the indentation size effect (ISE) using a combination of microhardness (high load) and nanoindentation (low load). Four samples from each material were tested; annealed electropolished, annealed mechanically polished, work-hardened electropolished, and work-hardened mechanically polished. The micro and nano indentations were made using a Vickers and a Berkovitch diamond indenter tips with indentation loads between 0.1–3 (microindentations) and 10−4–10−2 N (nanoindentations), respectively. Based on areas measured using optical and scanning electron microscopy in addition to contact stiffness, it is found that the calculated nanohardness increases monotonically with decreasing load or depth of indentation. The work-hardened samples are harder than the annealed ones except for aluminum at shallow indents where it is not possible to distinguish between differences in hardness. All eight samples regardless of the SFE, rate and intensity of cross-slip and dislocation climbing rates (dynamic recovery), and level of prior work hardening of the material, explicitly exhibited an ISE. The magnitude of the ISE is not influenced by SFE, dynamic recovery, or prior level of work hardening. The data are found to behave linearly consistent with the Strain Gradient Plasticity model (SGP) for micro and deep nanoindentations; the shallow nanoindentation data deviated into a second linear behavior constituting what we term a ‘bilinear behavior’.