Assessment of the depth dependence of the indentation stress during constant strain rate nanoindentation of 70/30 brass

The results of constant strain rate nanoindentation tests performed on annealed and 80% cold-worked 70/30 brass are presented which address the effect of indentation strain rate and indentation depth upon the indentation stress and the kinetics of the deformation process. At any given indentation depth from 0.2 to 2.0 μm the strain rate sensitivity m, and the apparent activation energy ΔG0 of the deformation process show a clear dependence upon indentation depth and prior cold-work. The data from all indentation tests, regardless of indentation depth, fall on the same linear trend of apparent activation work ΔW'versus ΔG0; however, the annealed material displayed a distinctly different linear trend than the 80% cold-worked material. This suggests that the operative dislocation-obstacle mechanism that occurs during the indentation is essentially independent of indentation depth but depends upon the prior dislocation density of the indented material.