Supervisors: Professor Dorte Juul Jensen, Senior Researcher Yubin Zhang, Associale Professor Guilin Wu
External supervisors: Senior Researcher Xiaoxu Huang, DTU Wind Energy, Associale Scientist Rimain Quey, Gwonges Griedel Lab, Ecole des Mines de Saint-Etienne, Professor Hongwang Zhang, Yanshan University
Abstract: Nucleation of Recrystallization at selected sites in deformed fcc metals
The nucleation process strongly affects the development of recrystallization texture and microstructure development of metals and alloys, and is thus of significant importance for the final properties of these materials. In the present study, nucleation of recrystallization at selected sites in deformed fcc metals is explored.
In the cold rolled columnar grained nickel samples, the preference of TJs and 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 of 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 aluminium samples, it is found that hardness indentations lead to large orientation rotations near indentations tips. In initial grains of different crystallographic orientations, the grains with higher 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.
In the 4D investigations, nucleation is directly related 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, but the boundaries are not those conventionally considered, namely original grain boundaries, but are strain induced dislocation boundaries.