Study On Nucleation Of Recrystallization In Deformed FCC Metals By 3D And In-situ Techniques

The objective of this study is to explore nucleation of recrystallization at selected sites in deformed face-centered-cubic(FCC)metals,including columnar-grained nickel deformed by cold rolling and high purity aluminium deformed by cold rolling followed by indenting.Various techniques,including Vickers hardness tests,optical microscope,electron backscattered diffraction(EBSD),electron channeling contrast(ECC)and differential-aperture X-ray microscopy(DAXM),were used to characterize the microstructures on different dimensions to explore nucleation mechanisms and the correlation of nucleation to local microstructures.In cold rolled nickel samples,the prevalence of triple junctions(TJs)and grain boundaries(GBs)as nucleation sites is observed after annealing.The majorities of the nuclei have the same orientations as the surround matrix or are twin related to a surrounding deformed grain.Only a few of nuclei formed with new orientations.Microhardness measurements at TJs in the deformed sample indicate a weak correlation between the difference in hardness of the grains at the TJs and the potentials of the junction to form nuclei: the higher the difference,the more likely is nucleation.In 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 stored energy in the rolled microstructures have higher average hardness values and higher nucleation probabilities.In general,indentations with higher hardness values have higher nucleation potentials.Furthermore,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 cover the orientations of the nuclei observed in the annealed state.Whereas,the above results are obtained by the EBSD technique,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 related to the local deformation microstructures in the bulk of the sample.It is found that nuclei evolve from embryonic volumes at local sites of high stored energy below the surface and develop because of the advantage of fast migrating boundaries surrounding the initial embryonic volumes.All nuclei have crystallographic orientations from 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.