Abstract
Remarkably little is known about the physical phenomena leading to nucleation of new perfect crystals within deformed metals during annealing, in particular how and where volumes with nearly perfect lattices evolve from structures filled with dislocations, and how local variations at the micrometer length scale affect this nucleation process. We present here the first experimental measurements that relate directly nucleation of recrystallization to the local deformation microstructure in the bulk of a sample of cold rolled aluminum, further deformed locally by a hardness indentation. White beam differential aperture X-ray microscopy is used for the measurements, allowing us to map a selected gauge volume in the bulk of the sample in the deformed state, then anneal the sample and map the exact same gauge volume in the annealed state. It is found that nuclei develop at sites of high stored energy and they have crystallographic orientations from those present in the deformed state. Accordingly we suggest that for each nucleus the embryonic volume arises from a structural element contained within the voxels identified with the same orientation. Possible nucleation mechanisms are discussed and the growth potentials of the nuclei are also analyzed and discussed.
Highlights
Little is known about the physical phenomena leading to nucleation of new perfect crystals within deformed metals during annealing, in particular how and where volumes with nearly perfect lattices evolve from structures filled with dislocations, and how local variations at the micrometer length scale affect this nucleation process
It is well accepted that original grain boundaries and triple junction lines are preferential nucleation sites[4], at which the nucleation mechanism may be strain induced boundary migration[5,6], it is not known where, along the many kilometers of original grain boundaries and triple junction lines that are present in typical samples, nuclei will form
It can be seen that all 9 nuclei are near the indentation tip, and along the symmetry lines in the indentation zone formed by the ridges of the diamond shaped indenter
Summary
Little is known about the physical phenomena leading to nucleation of new perfect crystals within deformed metals during annealing, in particular how and where volumes with nearly perfect lattices evolve from structures filled with dislocations, and how local variations at the micrometer length scale affect this nucleation process. Considering that annealing and recrystallization is used in almost every metal forming process, it is surprising that very little is known about actual nucleation mechanisms and nucleation sites. It is well accepted that original grain boundaries and triple junction lines (where 3 grains meet) are preferential nucleation sites[4], at which the nucleation mechanism may be strain induced boundary migration[5,6], it is not known where, along the many kilometers of original grain boundaries and triple junction lines that are present in typical samples, nuclei will form. Nuclei are few and small, which severely complicates any characterization They form in a so-far unpredictable way at sites in the bulk away from sample surfaces, and their formation can not be observed directly in-situ by light or electron microscopy. As such it could not be established in these studies which microstructural elements lead to nucleation
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