Fine-grained nickel deformed by direct impact at different velocities: Microstructure and mechanical properties

High purity electrolytic nickel (99.99%) samples deformed dynamically in compression using a direct impact Hopkison pressure bar test at the velocities of 10.9, 28.2 and 70.6 m s−1 were investigated. The dislocation density increased with increasing the impact velocity up to 28.2 m s−1 resulting in an increase of nanohardness and quasi-static compressive flow stress. At the same time, a decrease of the fraction of Σ3 coincident site lattice boundaries was observed for the benefit of Σ1 low angle grain boundaries having misorientations lower than 15°. Increasing the velocity to 70.6 m s−1 led to a decrease of the dislocation density, in parallel with the regeneration of Σ3 boundaries. As a consequence, the nanohardness decreased to a similar value as in the initial state. These observations suggest possible dynamic recovery/recrystallization that might have occurred at the highest impact velocity.