Microstructural investigation of the annealing behaviour of high-pressure torsion (HPT) deformed copper

Samples of polycrystalline 99.99% Cu were deformed by high-pressure torsion at room temperature and a hydrostatic pressure of about 8 GPa to a shear strain of about 100. After annealing at different temperatures the structural-elements sizes (grain/subgrain, scattering domain) were determined by scanning electron microscopy and multiple X-ray Bragg profile analysis. The evolution of the strength of the samples was monitored by microhardness measurements and its relation to the development of the structural-elements sizes has been investigated. The strength turns out to be correlated to the grain size: it decreases when the grain size starts to increase considerably upon annealing at homologous temperatures well above 30% of the melting temperature. Compared with conventional deformation, a very high dislocation density is present in the initial state and after annealing at 0.3Tm (Tm – melting temperature) the number of dislocations is reduced considerably. Thus, in the present case, the recovery of dislocations starts at lower homologous temperatures than in conventionally deformed materials.