Kinetics of high-temperature deformation of polycrystalline OFHC copper and the role of dislocation core diffusion

Abstract

The mechanisms of the high-temperature deformation of oxygen-free high-conductivity (OFHC) copper have been evaluated over a wide temperature (300–950°C) and strain rate (0.001–100 s⁻¹) regime. The stress–strain behaviour in hot compression is typical of the occurrence of dynamic recrystallization with an initial peak in the flow stress followed by a steady state, preceded by oscillations at lower strain rates and higher temperatures. The results are analysed using the kinetic rate equation involving a hyperbolic sine relation of the steady-state flow stress with the strain rate. In the temperature and strain rate range covering 500–950°C and 0.001–10 s⁻¹, a stress exponent of 5 and an apparent activation energy of 145 kJ/mol were evaluated from this analysis. The power law relationship also yielded similar values (5.18 and 152 kJ/mol, respectively). On the basis of these parameters, the rate-controlling mechanism is suggested to be dislocation core diffusion. The flow stress for the OFHC copper data reported by earlier investigators for different oxygen contents is consistent with the above analysis and revealed that an oxygen content of less than about 40 ppm does not have any significant effect on the core diffusion since it is too low to ‘clog’ the dislocation pipes. At strain rates greater than 10 s⁻¹ and in the temperature range 750–950°C, the stress exponent is about 3.5 and the apparent activation energy is 78 kJ/mol, which suggests that the plastic flow is controlled by grain boundary diffusion.

Acknowledgement

The work described in this paper was fully supported by a grant from the Research Grants Council of the Hong Kong Special Administrative Region, China (project ref No. CityU/1004/01E).