Strain-rate sensitivity of strength in macro-to-micro-to-nano crystalline nickel

The mechanical behavior of nickel is evaluated over eight-orders of magnitude change in strain rate to see if the change in strain-rate sensitivity exponent is affected by the grain size decreasing from the micron to nanoscale range. Tensile testing is used to measure the strain-rate dependence of the tensile strength on the grain size in crystalline nickel foils. Micro-scratch testing is used to determine the strain-rate dependence of the scratch hardness variation with scratch velocity. It is found that the mechanical test results can be directly compared. The increase in the strain-rate sensitivity exponent that is found with increasing strain rate is slower for nanocrystalline than for microcrystalline nickel foils where strength values begin to converge at the highest strain rates. ► Grain refinement enhances the strength of nickel from the micro-to-nano scale. ► The strain-rate sensitivity of strength is measured using tensile and micro-scratch testing. ► Nano-crystalline nickel is more strain-rate sensitive at lower strain rates. ► Micro-crystalline nickel is more strain-rate sensitive in the phonon drag regime.