Anomalous softening and severe aging deterioration in cryorolled Cu-Ni-Si alloys at extreme strains driven by twin-induced DDRX

Cu-Ni-Si alloys are widely used for electronic contacts due to their high strength and electrical conductivity. Cryogenic rolling (CR) effectively enhances strength with only a minor reduction in conductivity by suppressing dynamic recovery and introducing deformation twins. However, the limits of microstructural stability under severe cryogenic strain remain unclear. In particular, whether the strain hardening behavior established during room-temperature rolling (RTR) can be extended to cryogenic deformation at extreme reductions is still uncertain. To elucidate the mechanisms responsible for severe-strain-induced performance deterioration, a commercial C70250 Cu-Ni-Si alloy was subjected to CR and RTR up to 90% thickness reduction. Microstructural evolution and precipitation behavior were characterized by high-energy synchrotron X-ray diffraction (HEXRD), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). The 80% cryorolled and aged sample achieves optimal properties, with a tensile strength of 831 MPa, an elongation of 11.1%, and an electrical conductivity of 42.6% International Annealed Copper Standard (IACS). When the reduction is increased to 90%, the CRed sample exhibits anomalous softening. This degradation is driven by twin-induced discontinuous dynamic recrystallization (DDRX), which reduces stored energy and weakens defect-assisted precipitation. As a result, coarse precipitates form during aging, leading to a marked strength decrease to 668 MPa. In contrast, the RTRed sample is dominated by dynamic recovery (DRV) and continuous dynamic recrystallization (CDRX), resulting in a more stable structure and more uniform precipitation. These results reveal a severe-strain-induced softening mechanism and highlight the need to balance recrystallization and precipitation during cryogenic processing.