Plasticity effects in dynamically loaded nickel aluminide bicrystals

This work is concerned with achieving an improved understanding of the role of material strength effects during interactions of shock waves with grain boundaries. To this end, experiments have been performed using nanosecond laser shocks of nickel aluminide bicrystals at tens of GPa. Velocity histories were measured along a line on the free surface of the bicrystals and used to characterize the material behavior. Unstable plastic flow in 〈 1 0 0 〉 grains was seen to occur when loaded above 700 m s^-1 free surface velocity. Observations of the grain boundary region showed that a smooth transition occurred between the elastic precursors in both grains as well as the plastic waves (when plastic flow is evident). Futhermore, the length of the transition zone across the boundary was observed to be larger than the projection of the boundary, indicating that refraction of the incident wave resulted in the initiation of transmitted and reflected waves. A model is developed to explain the experimentally observed behavior. The model describes the scattering of elastic-plastic waves off of grain boundaries in crystalline materials where slip is the active deformation mechanism. Reflected elastic release waves and transmitted plastic waves scattering at large angles (i.e. 56° for the transmitted wave) from the boundary are predicted for the case of an incident plastic wave propagating along 〈 1 1 1 〉 and impacting a 45° inclined boundary, which appears to be consistent with the experimental VISAR records. The case of an incident 〈 1 0 0 〉 plastic wave could not be treated since the incident polarization vector is either parallel or perpendicular to the operative slip vectors in NiAl.