Deformation and fracture in laser-shocked NiAl single crystals and bicrystals

Oriented single crystals and a [3 4 55]/[5 7 17] random bicrystal were used to study dynamic behavior in NiAl due to laser-driven shocks at moderate pressures (3 to 20 GPa). Disks 5 mm in diameter and 100- to 400-μm thick were tested at the TRIDENT facility at Los Alamos National Laboratory (LANL). Particle velocities were measured using laser velocimetry, which showed that shock-speed variations with orientation in monocrystals were consistent with anisotropic elasticity predictions, whereas the bicrystal showed spatial and temporal variations in the velocity field due to the grain boundary. The shocks displayed strong elastic precursors at the free surface, which agrees with transmission electron microscopy observations of a low dislocation density in <100> and <111> monocrystals and in the [5 7 17] grain of the bicrystal. The latter developed a damage zone in the [3 4 55] grain, with cracking and slip present close to the boundary. Orientation-imaging microscopy showed that the boundary produced in-plane misorientation gradients in the bicrystal and that all specimens developed through-thickness lattice rotations, which were more pronounced for the <111> and <110> loading axes. High rotations occurred within 20 μ of the shocked surface and decreased toward the bulk, indicating a fast decay of the plastic shock wave, which explains the strong elastic precursors observed.