The relationship between micro-structural evolution and deformation mechanisms for nanocrystalline Ni under high strain rate

To inspect deformation mechanisms of nanocrystalline materials under high strain rate, bulk NC Ni samples are prepared by plasma evaporation combined with hot pressure sintering, and dynamic impact mechanical properties of the samples are tested under different high strain rates on Split Hopkinson Pressure Bar (SHPB). After impact testing, the micro-structural parameters of the samples, such as grain size and micro-strain, are investigated by X-ray diffraction (XRD). The mean strain rate sensitivity of the samples under high strain rate is calculated according to the impact mechanical properties. In addition, the microscopic grain morphologies after the impact testing are investigated through transmission electron microscope (TEM). The results show that grain sizes of the impacted samples decrease compared with those of sintered samples, while the micro-strain of the grains increases. Meanwhile, the grains of the impacted bulk sample are elongated, and impact stress-strain responses of the bulk samples are rate-dependent. The mean strain rate sensitivity of the bulk samples is far bigger than that of NC metals under quasi-static strain rate. Finally, the special deformation mechanisms of the bulk NC sample under impact loading are described based on the experimental results.