Deformation Substructures and Their Transitions in Laser Shock–Compressed Copper-Aluminum Alloys

Abstract

It is shown that the short pulse durations (0.1 to 10 ns) in laser shock compression ensure a rapid decay of the pulse and quenching of the shocked sample in times that are orders of magnitude lower than in conventional explosively driven plate impact experiments. Thus, laser compression, by virtue of a much more rapid cooling, enables the retention of a deformation structure closer to the one existing during shock. The smaller pulse length also decreases the propensity for localization. Copper and copper aluminum (2 and 6 wt pct Al) with orientations [001] and $[\ifmmode\expandafter\bar\else\expandafter\=\fi{1}34]$ were subjected to high intensity laser pulses with energy levels of 70 to 300 J delivered in an initial pulse duration of approximately 3 ns. The [001] and $[\ifmmode\expandafter\bar\else\expandafter\=\fi{1}34]$ orientations were chosen, because they respectively maximize and minimize the number of slip systems with highest resolved shear stresses. Systematic differences of the defect substructure were observed as a function of pressure, stacking-fault energy, and crystalline orientation. The changes in the mechanical properties for each condition were compared using micro- and nanohardness measurements and correlated well with observations of the defect substructure. Three regimes of plastic deformation were identified and their transitions modeled: dislocation cells, stacking faults, and twins. An existing constitutive description of the slip to twinning transition, based on the critical shear stress, was expanded to incorporate the effect of stacking-fault energy. A new physically based criterion accounting for stacking-fault energy was developed that describes the transition from perfect loop to partial loop homogeneous nucleation, and consequently from cells to stacking faults. These calculations predict transitions that are in qualitative agreement with the effect of SFE.