Strain-rate sensitivity of strength in macro-to micro-to nano crystalline nickel
The strain-rate sensitivity of strength is a key parameter to evaluate the deformation mechanism in crystalline materials. It is widely reported that many metals strengthen with increasing strain rate, wherein an increase in the strain-rate exponent occurs as the grain size decreases. The strain-rate sensitivity exponent as evaluated from a power-law relationship between yield strength and strain rate is thought to increase when the deformation mechanisms change. As increase in the strain-rate occurs, strengthening is attributable to alloy content, then to dislocation activity, and finally to an increase in effective mass – also known as the phonon drag regime. We will evaluate the behavior of nickel over eight-orders of magnitude change in strain rate to see if the change in strain-rate exponent is affected by the scale of the grain size from the macro- to micro- to nano- scale range as the strain rate increases. In this study, tensile testing is used to measure the strain-rate dependence of the tensile strength on the grain size in crystalline nickel foils. Similarly, micro-scratch testing is used to determine the strain-rate dependence of the scratch hardness variation with scratch velocity. Results for these two tests methods are compiled for strain rates that range from 0.00001 to 1000 sec-1. It is found that these mechanical test results can be directly compared, and the increase in strain rate sensitivity exponent with increasing strain rate is slower for nanocrystalline than for microcrystalline nickel.