Evolution of dislocation and stacking-fault densities for a Cu-0.7Cr-0.07Zr alloy during cryogenic tensile test: An in-situ synchrotron X-ray diffraction analysis

Processing at cryogenic temperatures is one of the ways of improving mechanical properties of copper alloys. However, such processing routes may result in unstable microstructures, which make it difficult to understand the deformation mechanisms controlling microstructure evolution during cryogenic deformation. In this study, the microstructure evolution of a Cu-0.7Cr-0.07Zr alloy was analyzed as a function of the flow behavior during tensile tests performed at 123 K and 298 K, through in-situ X-ray diffraction (XRD) experiments in a synchrotron source. The tensile behavior was analyzed in terms of the dynamic recovery rate using the Kocks-Mecking model, while the evolution of stacking-fault density, dislocation density and crystallite size during the tests was estimated by the modified Williamson-Hall method. Solution-treated and peak-aged conditions were tested to assess the influence of particles distribution. At 298 K, the strain-hardening rate was controlled by dislocation glide and the flow stress was affected mainly by particle strengthening. At 123 K, a simultaneous increase in strength and ductility was attributed to the twinning induced plasticity (TWIP) effect, promoted by precipitate interfaces acting as nuclei for stacking-faults, thus delaying necking and increasing the plasticity. Based on the evolution of dislocation and stacking-fault densities with increasing plastic strain, it was concluded that, at 123 K, mechanical twinning predominated at strain levels below 0.15, mainly for aged conditions, while dislocation glide between twin boundaries, and its accumulation, started after TWIP mechanism reached its saturation.