Laser shock compression of copper and copper-aluminum alloys: The slip to twinning transition in high-strain-rate deformation

Laser shock experiments are a relatively new approach to achieving extreme strain rates and pressures at time durations of nanoseconds. Copper and copper-aluminum alloys (2 and 6 weight percent) were subjected to laser pulses with durations of nanoseconds and energies between 70 J to 300 J. These conditions resulted in strain rates from 107--109 s -1 and pressures between 10--60 GPa. A quantitative, predictive understanding of plastic deformation (slip and twinning) and failure (void nucleation and growth) under these extreme regimes was developed through experiments, characterization and analysis. The mechanistic understanding this provided successfully incorporates effects of pressure, crystal orientation and stacking fault energy.; There were four thrusts to this research: (1) Two orientations, [001] and [1¯34] were examined under various experimental conditions providing quantitative insight into the deformation behavior as a function of orientation, stacking-fault energy, and distance from impacted surface. Advanced characterization techniques were used to examine the lattice behavior and defects that form as a result of shock compression. The experimentally observed slip-twinning transition was quantified experimentally. (2) The formation of defects during the movement of the shock front through these specimens was analytically studied. Loop formation is explained in terms of thermal activation in shock loading. The dynamic yield strength at the shock front was determined by using data obtained by dynamic x-ray diffraction. Dislocation densities were calculated as a function of shock pressure and compared to experimentally obtained values. (3) The slip-twinning transition pressure was calculated using a modified Mechanical Threshold Stress (NITS) constitutive description and the Swegle-Grady equation relating pressure to strain rate. These analytical results were compared to the experimental results. Orientation, stacking fault energy, grain size, and temperature effects were examined in terms of their influence on the threshold pressure. This methodology can be applied to other transitions such as martensitic transformations in steel, where increasing carbon content results in a change from lath (slipped) martensite to plate (twinned) martensite as described in Appendix A. (4) The spall behavior of these crystals was characterized and described analytically using a mechanistic analysis based on dislocations. Because shock compression generates plastic deformation, new insights into void growth mechanisms are enabled. Diffusion-based mechanisms are reviewed in terms of high-strain-rate phenomena and shown to be unfeasible at the time scales of these experiments.