Characterization of SPTS-processed ultrafine grained Cu

Coarse-grained copper was subject to severe plastic torsional straining (SPTS) and thermal treatment to study the effects of increasing amounts of deformation and subsequent annealing on the evolution of microstructure and microhardness. Cellular subgrains with low-angle grain boundaries were first formed at low strain. Some of the low-angle grain boundaries transformed to high-angle grain boundaries at higher strains, refining the average grain size from 200 μm to 150 nm. High internal stress and nonequilibrium grain boundaries were observed in unannealed samples. Annealing previously deformed samples at temperatures as low as 50 °C decreased the microhardness, indicating a very low thermal stability of the deformation induced microstructures. Differential scanning calorimetry (DSC) revealed an exothermic peak between 180 and 280 °C. Calculation of the energy released suggests that the peak was caused by both recrystallization and recovery processes. Annealing twins were also formed during recrystallization.