Understanding the microscopic deformation mechanism and macroscopic mechanical behavior of nanocrystalline Ni by the long-term stress relaxation test

The long-term stress relaxation tests with a relaxation time of about 7 h are performed on the bulk dense nanocrystalline Ni (with a mean grain size of 27 nm) pre-deformed at strain rate from 4.17 s-1- 4.17 × 10-6 s-1, where a phenomenon that the initial relaxation behavior of nc Ni depends on itself deformation history. That is, the nc Ni s pre-deformed at higher strain rate (not less than 4.17 × 10-3 s-1) exhibit a three-staged relaxation process from the initial near linear rapidly stress delayed (LRSD) stage and the subsequent lumber nonlinear stress delayed (LNSD) one and the final near linear slowly stress delayed (LSSD) one while only the later two stages are observed for the nc Ni s pre-deformed at low strain rate. The three-stage relaxation behavior is attributed to the transition from the initial dislocation-dominated plasticity to the mixture of dislocation motion and diffusion-based GB activity and finally to the entire diffusion-based GB activity including GB sliding or grain rotation in the rate-controlling deformation mechanism, which was illuminated by the attained three-staged strain rate sensitivity and activation volume and the exhaustion of mobile density of deformed nc Ni in the first two stages of relaxation. Such rate-controlling deformation mechanism well interpreted the macroscopic tensile mechanical behavior of nc Ni and simultaneously an optimizing strategy in improving the ductility of nc Ni is also mentioned.