Effect of Grain size, dislocation cell size and deformation twin spacing on the residual strengthening of shock-loaded nickel

Abstract

The effects of grain size, dislocation cell size and intertwin spacing in shock-loaded nickel have been investigated. Three grain sizes (25, 50 and 105 μm) were shock loaded at 150, 250, 350, 450 and 550 kbar peak pressure at a constant pulse duration of 2 μs in order to produce varying dislocation cell sizes without altering the grain sizes as a subgrain refinement. In addition to the formation of cells, deformation twins were observed to occur at a pressure of 350 kbar and higher. The twins occurred preferentially in those grains oriented with [001] parallel to the direction of shock wave propagation. The preponderance of twins increased with increasing shock pressure and decreased in orientations other than (001), especially with decreasing grain size. Residual hardness, yield stress and ultimate tensile stress were observed to increase with decreasing grain size, and 0.1% offset yield stress followed to some extent a Hall-Petch relation when considering dislocation cell size d, intertwin spacing Δ and grain size D. It was known that dislocation cells and deformation twins or twin faults contribute to subgrain refinement and residual strengthening of shock-loaded nickel in a pressure range of 0 – 300 kbar by ρ=ρ o+ KD −1 2 + K′d − m and in a pressure range of 300 – 500 kbar by ρ=ρ o+ KD −1 2 + K″d − m + K′″δ −1 2 where σ is the yield stress, σ 0 is a frictional stress, K, K′, K″ and K‴ are constants which may be interrelated and m varies between 0.5 and 1.