Grain boundary evolution during low-strain grain boundary engineering achieved by strain-induced boundary migration in pure copper

Grain boundary engineering (GBE) approaches involving small deformation and annealing to modify grain boundary networks have been widely used to improve grain boundary-related properties of polycrystalline materials. However, most GBE approaches are designed by trial-and-error method since the mechanism of GBE is unclear. An important issue still under debate is whether GBE is achieved by strain induced boundary migration (SIBM) or recrystallization. Also, the evolution of grain boundary structure during GBE process is unclear. a series of strains and annealing treatments covering SIBM and recrystallization were applied to pure copper in the current study. The result indicates that SIBM is more effective in the optimization of grain boundary character distribution compared with recrystallization. Then a quasi in-situ heating electron backscatter diffraction method was employed to the 10% compression/500 degrees C annealing copper to study the microstructural evolution during SIBM. SIBM was observed to be activated consecutively at high residual stress regions and then sweep into surrounding deformed areas until almost the whole material was covered by SIBM. The major procedure of SIBM involves the formation of numerous new Sigma 3 boundaries behind the migrating grain boundary front to enhance the fraction of special boundaries and the introduction of low energy segments to interrupt the connectivity of random high-angle boundary networks. A schematic model is proposed to understand the SIBM controlled GBE process. Our results provide the underlying insights needed to guide the design of GBE routes and parameters.