Nucleation of recrystallization at selected sites in deformed fcc metals

Chaoling Xu

Research output: Book/ReportPh.D. thesis

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The objective of this thesis is to explore nucleation of recrystallization at selected sites in selected face-centered-cubic (FCC) metals, namely cold rolled columnar-grained nickel and high purity aluminum further deformed by indenting. Various techniques, including, optical microscopy, electron backscattered diffraction (EBSD), electron channeling contrast (ECC) and synchrotron X-ray technique, differential-aperture X-ray microscopy (DAXM), were used to characterize the microstructures, to explore nucleation sites, orientation relationships between nuclei and deformed microstructures, and nucleation mechanisms. In the cold rolled nickel samples, the preference of triple junctions (TJs) and grain boundaries (GBs) as nucleation sites is observed. The majorities of the nuclei have the same orientations as the surrounding matrix or are twin-related to a surrounding deformed grain. Only a few nuclei are observed with orientations different from the surrounding matrix. Hardness measurements at TJs in the deformed sample indicate a weak correlation between the difference in hardness among the three grains at the TJs and the potentials of the junctions to form nuclei: the higher the difference, the more likely is nucleation. In the weakly rolled and indented aluminum samples, it is found that hardness indentations lead to large orientation rotations near indentation tips. In initial grains of different crystallographic orientations, the grains with higher stored energy (SE) in the rolled microstructures have higher average hardness values and higher nucleation probabilities. In general, indentations with higher hardness values have higher nucleation potentials. The orientations of the nuclei from different indentations in a given grain are observed not to be randomly distributed, but clustered in limited orientation spaces. The orientation spread observed near the indentation tips in the deformed state covers the orientations of the nuclei observed in the annealed state. Whereas the above results are obtained by the EBSD technique and thus are 2D observation, the nucleation at hardness indentations is also investigated non-destructively by the DAXM technique. By first characterizing the deformation microstructure within a selected gauge volume near a hardness indentation, then annealing the sample and measuring the same volume again, nucleation is directly correlated to the deformation microstructures in the bulk of the sample. It is found that the nuclei evolve from embryonic volumes at areas of high SE below the surface and develop because of an advantage of fast migrating boundaries surrounding the initial embryonic volumes. All nuclei have crystallographic orientations as those present within the embryonic volumes in the deformed state. It is further suggested that boundaries between nuclei and the deformed matrix of less than 5° hinder subsequent growth of the nuclei. For all the observed cases, it is suggested that the nucleation mechanism may be strain induced boundary migration (SIBM), but the boundaries are not those conventionally considered, namely original grain boundaries, but are strain induced dislocation boundaries.
Original languageEnglish
Place of PublicationKgs. Lyngby
PublisherDTU Wind Energy
Number of pages128
Publication statusPublished - 2016
SeriesDTU Wind Energy PhD


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