Analysis of intergranular impurity concentration and the effects on the ductility of copper-shaped charge jets
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6/30/04
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Analysis of Intergranular Impurity Concentration and the Effects on the Ductility of Copper-Shaped Charge Jets ADAM J. SCHWARTZ, MUKUL KUMAR, and DAVID H. LASSILA 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 breakup time and the transition to complete monolayer coverage of such boundaries.
I. INTRODUCTION
SHAPED charge liners (SCLs) are commonly fabricated from a thin-walled metallic cone that is inserted apex first into a charge of high explosive and used for penetration purposes in the defense, mining, and oil drilling industries. When the high explosive is detonated, the shock wave first passes through the metal, rapidly increasing the dislocation density. Afterward, the “push” from the high explosive compresses the material in the region of the apex and pushes, or extrudes, the liner material out in the form of a solid “jet” or an extending rod.[1] The liner material undergoes strain rates between 104 and 107 s1, and a tip velocity of several kilometers per second, as it stretches beyond the initial “push and extrusion” process. This extreme deformation places a premium on the metallurgical aspects of the liner material, which directly impacts the time to breakup for the expanding jet, or the implied ductility. There has been significant discussion in the literature regarding the importance of grain size and impurity content on the
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