Indentation-based assessment of the dependence of geometrically necessary dislocations upon depth and strain rate in FCC materials

In the present paper, an assessment is presented of the indentation depth and strain rate dependency of “geometrically necessary” dislocations. Pyramidal indentation tests were performed at various loading rates from 1 to 1000 mN/s on annealed samples of pure copper, 70/30 brass, and 5052 aluminum alloy to study the effect of indentation strain rate on the indentation depth dependence of the average indentation stress. The model of Nix and Gao was applied to calculate the density of statistically stored dislocations (SSDs) and geometrically necessary dislocations (GNDs) as a function of indentation depth. The GND density displayed the characteristic decrease with increasing h. The average indentation stress, σind was observed to decrease with increasing h and, for any given h increase with increasing loading rate. This observed dependence of σind upon ε̇ind was analyzed for the data obtained from small indentation depths, up to 800 nm, to assess the operative mechanism of time-dependent deformation associated with the GNDs. It was observed that for the high and medium stacking fault energy (SFE) 5052 aluminum and pure copper, the thermal activation energy ΔGThermal of the deformation rate followed essentially the same dependence upon σind regardless of ε̇ind, however in the 70/30 brass test material, which possess a lower value of SFE, the ΔGThermal showed a dependence upon σind that was highly strain rate dependent. In the case of the high SFE material, the apparent activation volume, V⁎ of the deformation process was found to decrease with increasing ρGNDs in a way indicative of deformation occurring by a process that is listed by dislocation/dislocation interactions. Our data indicate that in fcc materials of low SFE (i.e. 70/30 brass) the deformation during nano/micro-indentation occurs by a more complex mechanism than simple time dependent dislocation glide limited by dislocation-obstacle interaction. The operative deformation mechanism most likely involves micro-twinning and geometrically necessary twins (GNTs).