Comparison of residual defect structures in explosive shock-hardened Inconel 600 and Chromel-A by transmission electron microscopy

Abstract

Sheet samples of Inconel 600 alloy (76% Ni, 16% Cr, 7% Fe) and Chromel-A alloy (78% Ni, 20% Cr) were simultaneously shock-loaded in sandwich assemblies at pressures of 80, 180, 240 and 460 kilobars. The residual defect structures in Inconel 600 and Chromel-A were characterised by planar dislocation arrays exhibiting a prominence of dislocation loops and dipoles. Deformation twins having an average width of 150Å were observed to nucleate in the shock-loaded Inconel 600 in the pressure range 200–240kb, while the critical twinning pressure for the Chromel-A was roughly 240kb. Twin volume densities at 240kb were observed to be 14 and 5 per cent for the Inconel and Chromel respectively, and at 460kb the corresponding volume density of twinned material was 24 and 18 per cent respectively. A detailed comparison of the microstructures revealed that prismatic dislocations in the Inconel and Chromel are the most likely sources of deformation microtwins for shock pressures above the critical twinning pressure. This evidence suggests that a pole mechanism for twin formation is increasingly prominent in shock deformed materials of high stacking-fault energy (> 25ergs/cm 2), and that twinning in shock-loaded nickel is probably completely dependent upon prismatic twin sources.