Influence of oxygen on rate-controlling mechanisms in hot deformation of polycrystalline copper : Oxygen-free versus electrolytic grades

The kinetics of hot deformation in oxygen-free and electrolytic grades of polycrystalline copper has been studied in a wide temperature range (300-950 °C) and strain rate range (0.001-100 s^-1) using isothermal hot compression tests. The flow curves are typical of the process of dynamic recrystallization with initial peak in the flow stress followed by flow softening. At lower strain rates and higher temperatures, multiple stress peaks have been observed. In the lower temperature range (400-600 °C) and strain rate range (0.001-1 s^-1), both grades of copper yielded an apparent activation energy of 159 kJ/mol, suggesting that dislocation core self-diffusion is the rate-controlling mechanism. However, in the higher temperature range (700-950 °C), the electrolytic copper exhibited higher apparent activation energy (198 kJ/mol), indicating that the mechanism has switched to that controlled by lattice self-diffusion. This switching of mechanism in electrolytic copper is explained in terms of "clogging" of dislocation pipes by the interstitial oxygen atoms at higher temperatures due to a rapid increase in the solid solubility of oxygen in copper beyond 700 °C. In the higher strain rate range (3-100 s^-1) and high temperature range (700-950 °C), the apparent activation was much lower (about 91 kJ/mol) irrespective of the oxygen grade and supports a mechanism involving grain boundary self-diffusion in copper. © 2004 Elsevier B.V. All rights reserved.