Void nucleation and growth at grain boundaries in copper bicrystals with surface perturbations

Material failure on the microstructural level is important in determining macroscale behavior. When a material is subjected to dynamic (shock) loading conditions, damage and deformation patterns due to spall failure can provide a basis for connecting micro- to macroscale behavior. By analyzing deformation patterns at and around interfaces and boundaries that are representative of those found in engineering materials at high strain rates, we can develop stronger structures that can withstand impact collisions and rapid crack propagation. The addition of surface perturbations to one side of the samples provides insight on how strain localization occurs during the shock loading process and how the rippled release wave interacts with the boundary. Copper bicrystal samples were grown from two single crystal seeds using the vertical Bridgeman technique. A photolithography process was developed to create periodic surface perturbations on one side of the samples. The square wave ripples had a 150 μm wavelength and 5 μm amplitude. The bicrystals were shocked using laser ablation on the perturbation side at the Trident laser at Los Alamos National Laboratory and monitored using a VISAR (velocity interferometer systems for any reflector) and TIDI (transient imaging displacement interferometry) system. Shock pressures used were around 8 - 10 GPa. Targets measured 5 mm in diameter and 100 microns thick. The orientations of the grains were [001] and [111] along the shock direction with a 50° misorientation angle for the boundary, which was aligned parallel to the shock direction. Samples were soft recovered and cross-sectioned to perform quantitative characterization of damage using electron backscattering diffraction (EBSD) and Scanning Electron Microscopy (SEM) to gather information on the characteristics of the grain boundary and its surroundings, with emphasis on how the rippled surfaces and material anisotropy affected strain localization and spallation, initial results show that damage indeed localized at the grain boundary and that surface perturbations led to heterogeneity of spall damage distribution in the grain bulks.