Strain-rate dependence of mechanical behavior and deformation mechanisms in bimodal nanostructured Ni under micro-scratch testing

Bimodal nanostructure is a strategy to obtain a good combination of strength and uniform ductility for nanostructured (NS) metals. To inspect strain-rate dependence of mechanical behavior and deformation mechanisms in NS Ni with bimodal grain size distribution, the micro-scratch testing technique was selected to assess the average strain-rate sensitivity exponent over a wide range of strain rate based on continuous data of scratch hardness and plastic strain rate, the relationship between scratch plastic deformation mechanisms and microstructure of the bimodal NS Ni under different strain rate range were discussed. First, the scratch behaviors of the bimodal NS Ni were characterized in details, some critical parameters under the scratch test were investigated, such as scratch ditch depths and widths, scratch strain-rate, scratch hardness. Then, the scratch hardness varying with plastic strain rate under different scratch speeds was confirmed, and the average strain-rate sensitivity exponents of the bimodal NS Ni were calculated through the linear fitting. The results suggested that the values of strain-rate sensitivity of the NS Ni are much greater than the value of coarse-grained counterparts, and increase with increasing scratch speeds. Finally, the deformation mechanisms of scratch plastic deformation in bimodal NS Ni was inspected according to the strain-rate sensitivity exponents and TEM morphologies at end of scratch ditches under different scratch speeds. From the results, the deformation mechanism of the bimodal NS Ni sample changes gradually from the dislocation gliding and diffusion to grain boundary dislocation pile-up with increasing strain rate under micro-scratch testing. Meanwhile, we thought that the enhancement in ductility of the bimodal NS metals is dependent with the strain rate hardening behavior induced by the dislocation activity.