Microstructural evolution, microhardness and thermal stability of HPT-processed Cu

Coarse-grained copper was subject to high-pressure torsion (HPT) 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 subgrain boundaries transformed to high-angle grain boundaries at higher strains, refining the average grain size from 200 mu m to 150 nm. X-ray diffraction patterns showed the formation of crystallographic texture. Microhardness increased monotonically with increasing torsional strain. High internal stress and nonequilibrium grain boundaries were observed in unannealed samples. Annealing as-deformed samples at temperatures as low as 50 degrees C decreased the microhardness, indicating a very low thermal stability of the deformation induced microstructures. Differential scanning calorimetry (DSC) revealed an exothermal peak between 180 and 280 degrees C, caused by recrystallization. Annealing twins were also formed during recrystallization. (C) 2000 Elsevier Science S.A. All rights reserved.