Microstructural Effects on Damage Nucleation in Shock-Loaded Polycrystalline Copper

Polycrystalline copper samples with varying thermomechanical histories were shock loaded to induce spall via laser-driven plate impacts at low shock stress (<6 GPa). Electron backscattering diffraction was used to obtain statistics on grain boundary (GB) misorientations within the spall plane and at all GBs that contained damage. Specimens with pre-existing plastic deformation showed dominant intergranular damage at boundaries in the 25 to 50 deg misorientation range, while heat-treated samples had mixed trans- and intergranular damage with a lessened misorientation influence at damaged GBs. 3-D X-ray tomography data were used to analyze global volume statistics and qualitatively inspect the shape of voids present in samples of varying thermomechanical histories. It was found that annealed samples had a mixed mode of spherical- and sheet-like voids, indicative of trans- and intergranular damage, respectively, and the microstructure with the highest number of Σ3 twin boundaries had the highest concentration of spherical voids. Data from a plastically pre-strained sample showed a dominance of needle- and sheet-like voids, indicating primarily intergranular damage due to the higher strength of the bulk material forcing the damage to nucleate at weaker defects, in this case GBs.