Mechanisms of high temperature deformation in electrolytic copper in extended ranges of temperature and strain rate

Abstract

The hot deformation behavior of electrolytic copper in isothermal compression has been studied in the temperature range 300–950°C and strain rate range 0.001-100 s⁻¹ with a view to evaluate the rate controlling mechanisms relevant to different ranges. The flow curves observed are typical of the occurrence of dynamic recrystallization (DRX) and have exhibited single or multiple peak in the flow stress before reaching steady state, depending on the temperature and strain rate. Three different windows of temperature and strain rate have been identified in which the kinetic rate equation is obeyed by the temperature and strain rate dependence of flow stress. Both the power-law and hyperbolic sine relation yielded similar results. In the strain rate range 0.001-1.0 s⁻¹ and temperature range 400-600°C, the stress exponent is 7.8 and the apparent activation energy is 159 kJ/mole, and these indicate that dislocation core diffusion is the rate controlling mechanism. In the strain rate range 0.001-1.0 s⁻¹ and temperature range 700–950°C, the stress exponent is 7.1 and the apparent activation energy is 198 kJ/mole which matches well with that for lattice self diffusion in copper. The switching of the rate controlling mechanisms at temperatures higher than 700°C has been attributed to the "clogging" of the dislocation pipes due to the rapid increase in the solid solubility of oxygen at higher temperatures. In the high strain rate range 30–100 s⁻¹ and temperature range 700-950°C, the stress exponent is 3.5 and the apparent activation energy is 91 kJ/mole which matches with that for grain boundary diffusion in copper. The apparent activation energy in electrolytic copper in all the above temperature and strain rate windows increases with increasing oxygen content and this is attributed to the increase in the back stress due to the presence of copper oxide particles in the matrix.