The effect of small deformation on abnormal grain growth in bulk Cu

The effect of small deformation below the level (about 8 pct) required for primary recrystallization on abnormal grain growth (secondary recrystallization) has been investigated in bulk polycrystalline Cu. The starting microstructure, without any texture and with a nearly uniform grain size of 168 mum, has been obtained by compressing a cylindrical Cu specimen and recrystallizing at 800 degreesC. The fully recrystallized specimen shows distinct abnormal grain growth (AGG) after heat treatment at 800 degreesC for 12 hours. Most of the grain boundaries are faceted when observed under transmission electron microscopy (TEM), and most of the faceted segments are expected to be singular. A singular grain boundary free of defects will migrate by two-dimensional nucleation of new layers, with its velocity varying nonlinearly with the driving force arising from the grain-size difference. Such a growth mechanism is analogous to the well known process for the growth of crystals with singular surfaces from liquid or vapor. The grains slightly larger than the average size will hardly grow, because the driving force for their growth is not sufficient for nucleation of new crystal layers at the boundaries. Those grains larger than a certain critical size will, however, grow at ever-increasing rates with their increasing size, because of the sufficient driving force for two-dimensional nucleation. Such a selective accelerated growth of large grains results in overall AGG behavior. The specimen deformed to 2 pct shows AGG after heat treatment for only 5 minutes at 800 degreesC, and after I hour, large impinged grains are obtained. The grain boundaries show many extrinsic dislocations even after the heat treatments. As proposed earlier by Gleiter, Balluffi, Smith, and their colleagues, the extrinsic grain-boundary dislocations increase the grain-boundary mobilities even at low driving forces, and, hence, even those grains slightly larger than the average size can rapidly grow at the early stages of the heat treatment, in agreement with the observation. In the specimens deformed to 4 to 8 pct, below the level for primary recrystallization, all grains grow steadily without producing distinct AGG. With high densities of extrinsic dislocations at the grain boundaries even after long heat treatments, all grains can readily grow, resulting in overall growth patterns resembling the normal growth. When deformed to 20 and 50 pct, primary recrystallization occurs, and the subsequent AGG behavior depends on the grain size obtained at the completion of the primary recrystallization. Similar small-deformation effects are observed with heat treatment at 600 degreesC.