A study of annealing stages in commercial pure Cu using mechanical measurements and positron annihilation lifetime technique

Abstract

Mechanical property measurements, positron annihilation lifetime (PAL) measurements and metallographic observations, have been performed to study the isochronal annealing of commercial pure Cu in the temperature range from 25 up to 850 °C. A positive correlation has been found between positron lifetime ( τ) and both the tensile strain (Δ L/ L 0) and Vicker's microhardness ( H v). This correlation shows the presence of three annealing stages in commercial pure Cu which are attributed to recovery, recrystallization, and grain growth. These different stages were studied by both pure tensile and combined torsion–tension deformation for samples pre-annealed at the different annealing stages. Plastic instability behavior is observed in the case of combined torsion–tension deformation. It is observed that the onset and disappearance of this instability depend on some parameters such as mode of deformation, applied axial tensile stress and pre-annealing temperature. The activation energy is found to be 0.5 eV for the recovery stage which is attributed to the energy for dislocation annihilation by glide or cross-slip. The recrystallization stage is a multi-energy stage (1.35, 1.6, and 1.71 eV) which is attributed to lattice diffusion or boundary diffusion.