Thermal stability behaviors of ultrafine-grained Cu-Cr-Zr alloy processed by friction stir processing and rolling methods

Abstract

In order to investigate the effect of different microstructures on thermal stability, the thermal stability behaviors of ultrafine-grained Cu-Cr-Zr alloy processed by friction stir processing (FSP) and cold rolling (CR) methods was contrastively investigated by comparing with pure copper in this study. In addition to the precipitate strengthening, the FSP and CR samples were mainly strengthened by high angle grain boundaries (HAGBs) and dislocations, respectively. When there were no precipitates in Cu matrix, the onset temperature of softening of CR sample was 50 °C higher than that of the FSP sample at the same strength increment. But the precipitates in the FSP and CR Cu-Cr-Zr samples stabilized the deformed microstructures and precipitate coarsening occurred before the recrystallization of deformed microstructures. Therefore, the thermal stability differences disappeared in the FSP and CR Cu-Cr-Zr samples with the same onset temperature of softening (400 °C). In Cu-Cr-Zr alloys, although the stored energy of the FSP sample (39.1 J/mol) was 17.1% higher than that of the CR sample, the activation energy (∼175 kJ/mol) for grain growth of FSP sample was the same as that for recrystallization of CR sample. The different decrease degrees of thermal stability caused by different stored energy are completely masked by the kinetic constraint of precipitates.