Microstructure changes in HPT-processed copper occurring at room temperature

In the present work, the long-term stability of ultrafine-grained (UFG) copper at room temperature was investigated. The pure copper specimen was processed by 10 revolutions of high-pressure torsion (HPT) at room temperature. This procedure imposes an equivalent strain of about 300 to the material sample. In the region of these large strains, a saturation in grain size refinement occurs. UFG copper, deformed up to the region of microstructure saturation, was subsequently annealed at room temperature for 6 years. Microstructure changes of HPT-processed copper were investigated by means of 2D and 3D electron back scatter diffraction (EBSD) and also by transmission electron microscopy. It was found that the UFG microstructure of copper with saturated HPT-grain sizes coarsens significantly during long-term storage at room temperature. The analysis of grain volumes showed that the boundaries of coarse grains often contain flat segments with the coincidence site lattices (CSL) Sigma 3 and Sigma 9. The misorientation distributions revealed that most boundaries in the annealed microstructure are low energy grain boundaries of these kinds. However, groups of fine grains that are surrounded by random boundaries can also be found in the microstructure. Furthermore, 3D EBSD data were analysed in order to obtain a statistical microstructural information. The microstructure contains a high number of fine grains, but they form only a minority of the investigated volume. Quantitative geometrical characteristics of grain boundaries including CSL were described and interpreted.