Deformation mechanisms of pure polycrystalline iron shocked in excess of one megabar pressures

Abstract

High purity polycrystalline iron was shocked at pressures of 1 and 2 Mbar (100 and 200 GPa) via plasma driven compression waves created by ablating plastics using 1 nanosecond long square-shaped 70 and 150 Joule laser pulses, respectively. The experiments were performed on the Omega laser at the University of Rochester Laboratory for Laser Energetics. The iron targets were recovered post-shock. A combination of nanoindentation, microscopy, and diffraction techniques were used to characterize the properties and microstructures of the targets post-mortem as a means to understand the deformation mechanisms of iron at these extreme pressures and strain rates. The sample shocked at 1 Mbar exhibited nearly identical hardness, stiffness, and microstructure to the un-shocked material, except for a sizeable spall region where the shock broke out of the target. This result indicates that the temperature and pressure of the material as the shock released was sufficient for recovery of defect structures that may have been formed by deformation mechanisms that accommodated the shock. Thus, the post-mortem attempt to ascertain the mechanics of shock accommodation of this iron sample was inconclusive. Contrarily, the sample recovered from 2 Mbar shock showed remarkable grain refinement from ~ 500 microns in the unshocked state to less than 50 microns post-shock. The recrystallized grains were equiaxed and twins were not found. Instead, dramatic microbanded structures were observed to be sheared about {2 2 3} planes, which are ~ 9° misaligned from {1 1 2} planes. This result suggests that the Ferrite to Hexaferrum phase transformation known to occur in iron above 0.013 Mbar did not occur, but instead adiabatic heating due to shock release recrystallized the heavily deformed Ferrite that accommodated the shock via plastic flow. It is possible that the short time scale of the deformation and heating prevented phase transformation during loading, as well as grain growth or recovery of defect structures after recrystallization.