Annealing behavior of gradient structured copper and its effect on mechanical properties

In this study, hot-extruded copper rods were subjected to free-end-torsion and subsequent annealing. The microstructures and mechanical properties of the deformed samples and annealed samples were investigated. The results show that torsion can introduce gradient lamellar dislocation substructures (LDS) into the copper rods, suggesting a gradient distribution of stored energy from core to surface. Different degrees of recrystallization were formed in the same torsional rod after recrystallization annealing due to such gradient stored energy. Annealing can change the microstructure inversely from deformed gradient into annealed gradient distribution via fully recrystallization in the surface layer and incomplete recrystallization in the middle layer. Therefore, gradient structured copper rods with soft core and hard shell or hard core and soft shell could be obtained by torsion or the combination of torsion and subsequent annealing, respectively. The tensile yield strength of such gradient structured rods has a linear relationship with the volume fraction of hard and soft regions no matter where the hard region is located, core or surface. The incomplete recrystallization mechanisms of gradient structured copper rods induced by torsion were also discussed.