Tensile Deformation Microstructures and Deformation Behaviours in Electrodeposited Nanocrystalline Ni with Broad Grain Size Distribution

The electrodeposited nanocrystalline (nc) Ni with an average grain size of 27.2 nm and with a broad grain size distribution (BGSD) ranging from 5 to 120 nm was prepared. The tensile strain rate ((epsilon) over dot) jump test was performed at room temperature to measure the strain rate sensitivity (m). The result shows that m increases with decreasing of (epsilon) over dot. In particular, when (epsilon) over dot is less than 2 x 10(-5) s(-1), in increases rapidly which reaches 0.054 at (epsilon) over dot = 5 x 10(-6) s(-1), indicating that the deformation mechanisms involved in grain boundary diffusion and sliding are possibly activated. The room temperature loading-unloading tensile test was carried out. The results show that the capability of storing dislocations in BGSD nc Ni is very limited and the dislocation density is saturated when the stress reaches 1052 MPa with a strain of 7.8%. From the observation of TEM microstructures in the vicinity of tensile fracture, it is confirmed that there is significant intragranular dislocation sliding similar to that in coarse-grained materials in the process of plastic deformation of BGSD nc Ni.