Overview of the Grain Size Effects on the Mechanical and Deformation Behaviour of Electrodeposited Nanocrystalline Nickel − From Nanoindentation to High Pressure Torsion

Abstract

A summary of experimental results from nanoindentation, strain rate-controlled tension, in-situ bending and high pressure torsion on bulk electrodeposited nanocrystalline nickel, focusing on the effects of grain size on the mechanical behaviour and deformation mechanisms is presented. The interaction between dislocations and grain boundaries was locally examined by studying the dependence of nanohardness on grain size and indentation size; this is done by always performing nanoindents in the center of individual grains and by varying the grain size and indentation depth systematically. The grain size effects on the different deformation mechanisms of nanocrystalline nickel were revealed by strain rate-controlled tension and nanoindentation experiments, which show that with decreasing grain size the strain rate sensitivity increases and the activation volume decreases, indicating increased grain boundary mediated deformation processes in nanocrystalline nickel. Creep experiments at room temperature revealed that in nanocrystalline nickel grain boundary sliding or diffusion along the interface may dominate at lower stress levels, but with increasing stresses the deformation process is mainly controlled by dislocation creep. In-situ bending experiments in an atomic force microscope revealed directly that grain boundary mediated deformation processes play a significant role in nanocrystalline nickel, which is also supported by the observation of grain coarsening and softening of nanocrystalline nickel caused by high pressure torsion.