The effect of grain size on strain rate sensitivity and activation volume -: from nano to ufg nickel

The strain rate sensitivity of nanocrystalline nickel was studied at different temperatures in tensile tests and with a nanoindenter in order to examine the effect of grain size on the different deformation mechanisms of nanocrystalline materials. The experiments yielded, depending on temperature and strain rate, the strain rate sensitivity, the activation volume and the creep exponents as a function of stress and grain size. From the creep experiments the transition between grain boundary sliding and dislocation climb as a function of temperature was obtained. The strain rate jump tests gave extremely small activation volumes, nearly a factor of 100 smaller than in conventional nickel as a function of grain size. To help in understanding this behaviour the strain rate sensitivity of single grains was tested with a nanoindenter. The results clearly showed that the primary interaction of dislocations with grain boundaries is the reason for the strong rate effects and small activation volumes observed.