Depth dependence and strain rate sensitivity of indentation stress of 6061 aluminium alloy

Indentation tests were performed on samples of 6061 aluminium alloy in the annealed, T4 and T6 temper conditions. The tests were performed over a range of loading rates to study the effect of indentation strain rate on the indentation depth dependence of the average indentation stress σ_ind. While changes by several orders of magnitude during the constant loading rate nano-/microscale indentation tests, we observed that the strain rate sensitivity of σ_ind increases with decreasing indentation depth for all the samples tested. By applying an obstacle limited dislocation glide description of the deformation process, we were able to demonstrate that the apparent activation energy of the obstacles to dislocation glide increases with decreasing indentation depth and is also dependent upon the heat treatment condition of the 6061 test material. This suggests that, based upon the assumption of the operative deformation mechanism chosen, the strength of the dislocation–obstacle interactions that limit the rate of deformation is significantly increased in indentations of depth <∼4 μm.