Effect of cryogenic rolling on the microstructural evolution and mechanical properties of pure copper sheet

The pure copper sheets were rolled with a small strain in one pass at cryogenic and room temperature, respectively. The total thickness was reduced by 55% through three rolling passes. The microstructural evolution and mechanical properties of Cu sheets were investigated by metallographic characterizations, tensile tests and fracture analysis. It is found that several nanoscale mechanical twins are generated in the cryogenically rolled Cu sample. The restricted atomic motion at ultralow temperature inhibits the dislocation slip and then induces the occurrence of grain kinking and mechanical twining. The yield strength and ultimate tensile strength of cryorolled Cu sample are enhanced by the remarkable accumulation of dislocations and mechanical twins. During annealing treatment, the recrystallization behavior is accelerated by the dramatic strain energy and microstructural defects in cryogenically rolled Cu samples. Compared with the recrystallization concentrated on the surface of room temperature rolled Cu sample, a few of recrystallized grains are formed in the central position of cryogenically rolled Cu sample at 180 degrees C annealing. The rapid progress of thermal recovery during annealing treatment improves the tensile ductility and strain-hardening capacity of cryo-rolled Cu sample. Several pores are formed on the tensile fracture due to the incompatible deformation between the inhomogeneous grains. The results demonstrate that combining the cryogenic rolling and 200 degrees C annealing is able to improve the mechanical properties of pure copper sheet.