Effect of initial microstructure on high velocity and hypervelocity impact cratering and crater-related microstructures in thick copper targets: Part I Soda-lime glass projectiles

Abstract

Three uniquely different initial microstructure regimes were created in 2.5 cm thick copper targets: an as-received 98 μm grain size containing ∼1010 dislocations/cm2 (Vickers hardness of 0.89 GPa); an annealed 124 μm grain size containing ∼109 dislocations/cm2 (Vickers hardness of 0.69 GPa); and a 763 μm grain size containing ∼109 dislocations/cm2 (Vickers hardness of 0.67 GPa). Each of these target plates was impacted by 3.18 mm diameter soda-lime glass spheres at nominal impact velocities of 2, 4 and 6 km s-1. Grain size was observed to have only a very small or negligible contribution to cratering, while the dislocation density had a controlling influence on both the target hardness and the cratering process. Residual crater hardness profiles were correlated with specific microstructure zones extending from the crater wall into the target, and both hardness profiles and residual microstructures differed for each specific target, and for each different impact velocity. Microbands coincident with traces of {1 1 1} planes were associated with a zone of residual target hardening and increased with increasing grain size and impact velocity. No significant melt-related phenomena were observed, and crater-related target flow occurs by solid-state plastic flow through dynamic recrystallization, forming a narrow, softened zone at the crater wall.