Analysis of Orientation Relations Between Deformed Grains and Recrystallization Nuclei
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I.
INTRODUCTION
WHEN a metal deformed to medium and high strains is annealed, recrystallization typically takes place. During this process, nuclei that are small and almost strain-free crystals develop locally in the deformed structure and grow to become grains, driven by the stored energy in the deformed material. When all the deformed material has been consumed by strain-free grains, recrystallization is complete. Typical sites for nucleation of recrystallization are (1) grain boundaries, (2) triple junctions and corners, (3) second phase particles, and (4) shear and transition bands,[1,2] as all these sites are prone to build up locally a higher stored energy during deformation. Although these sites are well accepted as likely nucleation sites, it is still not known which will give nucleation or where along them it will occur. Understanding the mechanisms of nucleation is vitally important for making realistic models of recrystallization. As generally relatively few nuclei are present, the study of nucleation has been compared with the search for a ‘‘needle in a haystack.’’ The search also is complicated by the fact that, usually, the whole threedimensional structure cannot be observed, but only the surface plane of polish can be studied. Thus, when a nucleation event takes place in the bulk of the material, the nucleus consumes the deformed material, making it STINE S. WEST, Ph.D., is with the Center for Fundamental Research, Metal Structures in Four Dimensions, Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark, DK-4000 Roskilde, Denmark. GRETHE WINTHER, Senior Scientist, and DORTE JUUL JENSEN, Head of Materials Research Division, are with the Danish-Chinese Center for Nanometals, Materials Research Division, Risø National Laboratory for Sustainable Energy, Technical University of Denmark. Contact e-mail: [email protected] Manuscript submitted April 28, 2009. Article published online November 20, 2010 1400—VOLUME 42A, MAY 2011
impossible to quantify what was present before at the nucleation site. This has been referred to as the problem of ‘‘lost evidence.’’[3] The crystallographic orientation of the nuclei also cannot be fully predicted. Accepted nucleation theories propose that the orientations of the nuclei must already be present in the deformed state before annealing. The nuclei could form by mechanisms such as strain-induced boundary migration (SIBM),[4] subgrain coarsening,[5] or subgrain coalescence.[6] SIBM takes place at highangle grain boundaries, where the bulging of the boundary creates a small strain-free region capable of growing into the neighboring grain. Subgrain coarsening and coalescence involve the merging of multiple subgrains of similar orientations either by rotation (subgrain coalescence) or by the migration of the low-angle grain boundary (subgrain coarsening). Indeed, the nucleation of grains with orientations within the spread of the deformed matrix is commonly observed (e.g., Reference 7). However, several studies have s
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