Microstructure and strength of nickel subjected to large plastic deformation at very high strain rate

A polycrystalline coarse-grained Ni with 98.4 wt% purity was deformed plastically to the strain of 1.2 at the strain rate of ~104 s−1 using a Dynamic Hopkinson Plastic Deformation technique. The microstructure investigations indicate the ability of this loading method to process very fine microstructure with high dislocation density and large fraction of low-angle grain boundaries. It turned out that there is a transition in the mechanisms which determine the flow stress during the impact test. In the beginning of deformation viscous dislocation drag dominates, while at the end of the impact test the velocity of the individual dislocations is low due to the high dislocation density, therefore the obstacle hardening determines the flow stress. Accordingly, the flow stress measured in-situ during impact test is in good agreement with the values determined ex-situ after deformation. The main contribution to the flow stress is given by dislocations.