Measuring the strain rate sensitivity by instrumented indentation. Application to an ultrafine grain (equal channel angular–pressed) eutectic Sn–Bi alloy

The ability of using nanoindentation raw data for characterizing the strain rate sensitivity of the flow stress of elastic–plastic materials has been assessed by finite element calculations. Correction factors for deducing the true strain rate sensitivity from the hardness sensitivity to the hardness penetration rate have been obtained for situations where the elastic contribution to the penetration is not negligible. The results of the analysis are applied to the experimental measurement by nanoindentation of the strain rate sensitivity of an ultrafine grain Sn–Bi eutectic processed by equal channel angular pressing. The flow stress of this material displays bilinear power-law strain-rate dependence. It is also shown that at very low indentation depths, discontinuities of the hardness–penetration curve of the fine-grain Sn–Bi can be correlated with local grain or interphase boundary slip events near the indentation.