Abnormal grain growth in bulk Cu—The dependence on initial grain size and annealing temperature

Abstract

The dependence of abnormal grain growth (AGG), also termed secondary recrystallization, on annealing temperature in the range between 600 ЊC and 1050 ЊC has been observed in pure bulk Cu specimens compressed to various levels between 5 and 75 pct. There is no grain texture after annealing. The average grain size after primary recrystallization, which represents the initial grain size for secondary recrystallization during further annealing, decreases with increasing deformation and is nearly independent of the annealing temperature, in agreement with previous observations. The incubation time for AGG decreases and the number density of abnormally large grains increases with increasing deformation (hence, a decreasing initial grain size) and increasing annealing temperature. At low temperatures, most of the grain boundaries are faceted, with some facet planes probably of singular structures corresponding to cusps in the polar plots of the grain-boundary energy vs the grain-boundary normal. With increasing temperature, the grain boundaries become defaceted and, hence, atomically rough. The observed grain-growth behavior appears to be qualitatively consistent with the movement of faceted grain boundaries by two-dimensional nucleation of boundary steps. The temperature dependence appears to be consistent with roughening of grain boundaries. Before the onset of AGG, stagnant growth of the grains occurs at low rates, probably limited by slow two-dimensional nucleation of boundary steps, and, at low deformations and low annealing temperatures, the stagnant growth persists for 100 hours. The specimens with relatively small initial grain sizes (because of high deformation) show double AGG when annealed at high temperatures.

Temperature Dependence of Grain Boundary Structure and Grain Growth in Bulk Silicon-Iron.

“…Normal grain growth occurred at these temperatures. This correlation between the grain boundary structure and grain growth is similar to those found in other metals [13][14][15][16][17][18] and oxides. 19,20) The results of this study thus confirm that AGG can arise from the singular grain boundary structure without any precipitates, surface energy anisotropy, or texture.…”

“…As pointed out earlier, [13][14][15][16][17][18][19][20]31,32) some flat segments of the h&v grain boundaries are likely to be singular corresponding to the cusps in the grain boundary Wulff plot. 33) The kinked shapes are the limiting cases of the h&v shapes, because the h&v shapes can evolve to the kinked shapes by a coarsening process.…”

“…[13][14][15][16][17][18] The transition temperature of approximately 1 000°C (about 0.73 T m ) also falls into the range of 0.6-0.9 T m observed in other metals, [13][14][15][16][17][18][27][28][29] although the transition temperatures vary among the grain boundaries and depend also on additives and impurities. In 316L stainless steel, the grain boundary defaceting transitions were observed at temperatures between 1150 and 1200°C.…”

“…It was recently observed that AGG and normal grain growth (NGG) in bulk polycrystalline metals [13][14][15][16][17][18] and oxides [19][20] were related to grain boundary structures. At low temperatures or in the presence of additives, grain boundaries had hill-and-valley (h&v) shapes with singular segments and AGG occurred.…”

“…The previous observations in pure and single phase metals show that grain boundaries can undergo roughening transitions in the temperature range of 0.6-0.9 T m , where T m is the melting point in absolute scale. [13][14][15][16][17][18][27][28][29] For Si-iron, this will be about 780-1 300°C. Previous evidences for possible grain boundary roughening transition in pure Si-iron can be found in the grain boundary migration experiments with bicrystals by Tsurekawa et al 30) …”