The effect of grain boundaries on the deformation mechanisms of nickel aluminide during laser-induced shock loading

A fundamental question in the behavior of polycrystalline materials under shock loading regards the effect of local defects and anisotropy on their large scale deformation and failure. This dissertation addresses some of the basic aspects of this issue through study of the material response to the passage of a pressure pulse over a region containing a grain boundary. To this end, direct drive laser shocks have been applied to Nickel Aluminide (NiAl) single crystals and bicrystals 100 to 300 mum thick and roughly 5 mm in diameter. Single crystals were shocked along [001], [110], and [111] directions to peak pressures at one surface between 9 and 18 GPa. Bicrystals having <001> and <227> loading directions along with bicrystals having <123> and <335> loading directions were grown using a modified Czochralski technique and subjected to high energy laser pulses producing peak pressures between 5 and 10 GPa at nanosecond timescales to elucidate the effect of property mismatch across a grain boundary due to material anisotropy. Plan view transmission electron microscopy (TEM) showed that shocked [001] single crystals underwent localized plastic deformation in the bulk of the samples, whereas [110] and [111] samples had a more homogeneous distribution of dislocations. Cross-section TEM (XTEM) of a [001] single crystal indicated that a hydrodynamic layer approximately 1 mum thick near the energy deposition surface existed where large densities of defects and large lattice rotations were induced by the applied pressure. Observations of the free surface showed that cracking typically occurred near the grain boundary pointing to a clear grain boundary affected zone. It was found that the position of these cracks could be explained almost entirely by the theory of elastic wave propagation using the slowness method. Slowness analysis along with finite element simulations showed that a clear relationship exists between the amplitude of the refracted quasi-longitudinal (QL) wave and the ratio of the incident to refracted QL waves. The relationship shows that a decrease in amplitude will take place if a fast to slow sound speed transition is made across the boundary whereas the amplitude will increase for a slow to fast transition.