Recovery and recrystallization of polycrystalline copper after hot working

Abstract

The restoration processes that take place during the isothermal annealing of hot worked copper, in the temperature range 450–540°C, have been studied in detail. The annealing response was monitored by means of an interrupted compression technique and by quantitative metallography. The study firmly establishes the existence of three distinct mechanisms of static restoration, namely recovery, classical recrystallization and metadynamic recrystallization. The presence of the third process requires the occurrence of dynamic recrystallization during the prior hot working strain. The relative importance of the contribution of each of the three mechanisms to static annealing is shown to be highly sensitive to the degree of prestrain. There exists a critical strain ( ~ 12%) below which no recrystallization occurs during the anneal, and recovery is the only restoration process. At higher strains, both recovery and recrystallization take place, and operate sequentially, because the latter mechanism requires a high degree of substructural rearrangement before it can be initiated. Once the material is worked beyond the critical strain for dynamic recrystallization ( ~ 22%) static restoration proceeds by the operation of metadynamic recrystallization as well as recovery and classical recrystallization. The former process is shown to commence immediately on the cessation of straining, as it requires no induction period for the formation of new grain nuclei. Instead, the nuclei are formed during the prior deformation so that recrystallization proceeds by growth from these centers, until all such sites are exhausted. For all conditions considered in the present work, it was found that the grains produced by both types of static recrystallization were smaller than those produced by dynamic recrystallization. It is argued that potential nuclei that are destroyed by work hardening on continued deformation, become critical when straining ceases, thereby increasing the nucleus density and producing a finer grain size.