Microstructure development during hot deformation of aluminum to large strains
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11/7/03
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Microstructure Development during Hot Deformation of Aluminum to Large Strains TANJA PETTERSEN, BJØRN HOLMEDAL, and ERIK NES During deformation, the original grains change their shape and the surface area per unit volume increases with strain until a certain critical strain has been reached. The structure of high-angle boundaries has been monitored at increasing strains with the aim of finding the effect of grain breakup and strain-induced boundary migration. It has been found that the distance between the high-angle boundaries does not depend only on geometrical considerations. At high Zener–Hollomon parameters, the distance between the high-angle boundaries was found to be smaller than predicted from geometry, indicating that high-angle boundaries are formed during deformation. In the case of deformation at very low Zener–Hollomon parameters, the distance between the high-angle boundaries was found to be larger than predicted from geometry, which indicates migration of the original grain boundaries in a direction opposite to the one imposed by the deformation. The evolution in grain-boundary structure during deformation has been successfully modeled on the basis of expressions for the grain breakup and restoration reactions.
I. INTRODUCTION
DURING the last few decades, investigations have demonstrated that during deformation of aluminum, a structure consisting primarily of high-angle boundaries can form without recrystallization reactions involving nucleation and growth. Several authors[1–5] observed the development of a structure of equiaxed grains of the same order of magnitude as the subgrains formed in the material. It is commonly observed that the original grains elongate at increasing deformation and that the original grain boundaries develop serrations of the same order of magnitude as the subgrain size. At increasing strain, the distance between the original grain boundaries will eventually be of the order of magnitude of the subgrain size, and a concept termed geometric dynamic recrystallization (GDRX) was suggested by McQueen and co-workers[2,4,5] to account for the further development in the microstructure. The basic idea of GDRX is that when the strain reaches a level where the grain thickness equals twice the subgrain diameter, the concave serrations of the original grain boundary will come into contact and perforate the original grain. The model hence predicts that when this critical strain is reached (the strain where the grains are pinched off), further straining of the material will not lead to an increasing fraction of high-angle boundaries. From this simple picture, the critical strain where the grains are pinched off, and the development of the grain morphology (here, represented by the shortest distance between the original grain boundaries), can easily be predicted solely by using geometrical considerations. Knustad et al.[6] and Humphreys and Hatherly[7] reported that the critical strain for pinch off to occur indeed was consistent with this simple idea. Duri
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