Strain rate dependence of tensile strength and ductility of nano and ultrafine grained coppers

An ultrafine grained Cu with a wide distribution of grain size and an average grain size of d= 110 nm was prepared by electric brush-plating technique. Tensile properties, microstructure and deformation surface morphologies of this ultrafine grained Cu were compared with two results on an electric brush-plated nanocrystalline Cu (d= 59 nm) and an electrodeposited ultrafine grained Cu (d= 200 nm) previously prepared by our group. Based on these comparisons, the strain rate dependences of strength and ductility of these three materials and underlying mechanisms were studied. It was revealed that the pronounced strain rate sensitivity (larger m value) of these three materials is a result of the competition between the dislocation and grain boundary mediated deformations. The grain boundary mediated deformation plays a very important role in controlling the tensile ductility of these three materials, which affects the deformation accommodation by promoting different modes of strain localization and hence leads to the completely different strain rate dependences of tensile ductility (the elongations decrease, remains unchangeable and increase with increasing strain rate).