Kinetics of Static Grain Growth in Dynamically Recrystallized Copper Polycrystals with Different Purities

摘要

Effect of purity on static softening and grain growth taking place after dynamic recrystallization (DRX) was studied by means of interrupted compression tests and metallographic observations of polycrystalline copper specimens with three different purities of 99.9 (3N), 99.99 (4N) and 99.9999 (6N) mass percent. The fractional softening-annealing time (X-t) curves after dynamic recovery consist of two stages, followed by complete softening. In contrast, the softening curves for DRX matrices consist of three distinct stages accompanied with three plateaus, followed by incomplete softening (stages I, II and III). The X-t curves for DRX matrices of specimens 3N and 4NCu are almost the same, but clearly different from that for specimen 6NCu. The static softening for a DRX matrix of specimen 6NCu takes place one order of magnitude faster than that for specimen 3N or 4NCu and finally approaches X= 1 after a long period of annealing time in stage III. The average rate of grain growth in stage III is always smaller than that for normal grain growth, although both the rates are controlled by grain boundary diffusion irrespective of purity of copper. This results from the stable existence of metadynamically recovered grains which have many-sided irregular shapes as well as high density dislocations. As these grains are metastable in thermodynamics, however, DRX matrices only for specimen 6NCu finally replace fully recrystallized ones, followed by the occurrence of normal grain growth.