Dynamic Recrystallization in Copper and Mild Steel during High Temperature Torsional Deformation

Abstract

The high temperature deformation of copper and mild steel has been studied at the homologous temperatures ranging from 0.5Tm (407°C) to 0.9Tm (948°C) for copper and from 0.7Tm (989°C) to 0.9Tm (1350°C) for mild steel under the constant true strain rate from about 10-3/sec to 30/sec in torsion. From the determination of their stress-strain behaviors and optical microstructural observations, the rate controlling mechanisms of the dynamic restoration process responsible for the occurrence of steady state deformation were investigated.The flow curves of the both materials exhibited the maxima, after which the flow stress decreased to a steady state value. It is clear from the above observations that dynamic recrystallization occurs at strains greater than a critical value and the recrystallized grain size depends only on the flow stress.The maximum flow stress data for both materials were plotted against the temperature compensated strain rate, using the activation energy for self diffusion. The form of strain rate dependence of the maximum flow stress in copper was similar to those found in a number of materials, studied over a wide range of strain rate: below a critical stress, that is approximately 10kg/mm2 in this case, a power law was observed with a stress exponent of 6.5, and above this critical stress a steeper stress dependence occurred. The stress exponent for mild steel was about 5.5 in the temperature range studied, as a straight line was obtained when the temperature-compensated strain rate was plotted against the maximum flow stress. The similar dependence was observed for the steady state flow stress when this condition was achieved.The dynamically recrystallized grain size of a copper specimen which deformed in each testing condition was measured by using the line intercept technique, from which the average recrystallized grain size (d) was determined. When the reciprocal grain size (1/d) was plotted against Zener-Hollomom parameter, a straight line was obtained. Therefore, it is confirmed that the operative restoration process during the high temperature deformation of copper is the dynamic recrystallization, since there is a constant relationship between the recrystallized grain size and deformation parameters.From the similarity in the stress-strain behaviors of copper and mild steel and the metallographic observations, it is reasonable to consider that the restoration process of mild steel in γ-range is dynamic recrystallization.