Analysis of intergranular impurity concentration and the effects on the ductility of copper-shaped charge jets

Abstract

Shaped charge liners (SCLs) are thin-walled metallic cones that are explosively driven to high pressures and strain rates. From a metallurgical point of view, the SCLs provide an excellent experimental test bed to evaluate the influence of microstructure on high strain rate deformation and failure. In this work, a geometrical analysis based on an assumed tetrakaidecahedron grain shape is applied to determine the relationship between grain size, overall impurity content, and ductility of the liners. The measured parameter for ductility in this case is the break-up time for sulfur-doped, oxygen-free electronic (ofe) copper SCLs after they are explosively driven. The calculations determine the number of impurity atoms as a function of grain size, the number of available sites at the intercrystalline defects, and the intercrystalline impurity concentration. Recent experiments have shown that larger grain size liners with low impurity contents exhibit better ductility than smaller grain size liners with higher impurity concentrations, which is contrary to conventional wisdom that the liner ductility scales directly with grain size or impurity content alone. Within the range of grain sizes and bulk impurity contents in this study, the analysis suggests that the quadruple nodes and triple lines are saturated with impurities. Over this same range of impurities and grain sizes, only a fraction of a monolayer of impurities exists at the grain boundaries if all boundaries are assumed to be equally susceptible to sulfur segregation. Modification of the analysis by assuming partitioning of the sulfur only to crystallographically random boundaries, however, suggests that there is a correlation between the jet break-up time and the transition to complete monolayer coverage of such boundaries.