Twin evolution and work-hardening phenomenon of a bulk ultrafine grained copper with high thermal stability and strength-ductility synergy

摘要

In this paper, Cu is severely cryorolled by employing low deformation strain per pass, large roller diameter, and moderate deformation speed. This process leads to the evolution of a unique homogenous ultrafine grained (UFG) microstructure enriched with deformation nanotwins and dislocation-rich structures. Excellent thermal stability combined with high strength-ductility synergy is observed during isochronous annealing of the UFG material. Accelerated recovery consumes the thermal energy imparted during low temperature (150-200 °C) annealing and averts premature grain and twin coarsening. Microstructural homogenization prevents disproportionate recrystallization and grain growth. Dense clusters of deformation nanotwins reduce the grain boundary mobility and arrest the grain growth. At 200 °C, twin complexes comprising a crisscross network of fine deformation and annealing twins are formed, further enhancing the thermal stability at higher annealing temperatures (200-250 °C). However, large-scale growth of these annealing twins at 300 °C devours the nanotwin complexes and reduces their stabilizing efficiency, thereby compromising the thermal stability. Ultimately at 350 °C, a complete transformation of the strained UFG microstructure into a fully recrystallized bimodal microstructure is observed. Large concentration of thermally stable, recovered nanometric grains and twins reduces the dislocation mean free path and promotes rapid work-hardening in UFG Cu. They help in retaining good strength-ductility synergy of the material up to a very high annealing temperature.