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 Sigma 3 coincident site lattice boundaries was observed for the benefit of Sigma 1 low angle grain boundaries having misorientations lower than 15 degrees. Increasing the velocity to 70.6 m s(-1) led to a decrease of the dislocation density, in parallel with the regeneration of Sigma 3 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. (C) 2010 Elsevier B.V. All rights reserved.