Papers by Author: Dorte Juul Jensen

Abstract: Recrystallization is governed by the migration of high angle grain boundaries traveling through a deformed material driven by the excess energy located primarily in dislocation structures. A method for investigating the interaction between a migrating grain boundary and dislocation boundaries using molecular dynamics (MD) was recently developed. During simulations migrating high angle grain boundaries interact with dislocation boundaries, and individual dislocations from the dislocation boundaries are absorbed into the grain boundaries. Results obtained previously, using a simple Lennard-Jones (LJ) potential, showed surprisingly irregular grain boundary migration compared to simulations of grain boundary migration applying other types of driving forces. Inhomogeneous boundary-dislocation interactions were also observed in which the grain boundaries locally acquired significant cusps during dislocation absorption events. The study presented here makes comparisons between simulations performed using a LJ- and an embedded atom method (EAM) aluminum potential. The results show similarities which indicate that it is the crystallographic features rather than the atomic interactions that determine the details of the migration process.

Abstract: Grain structures in polycrystalline materials are typically three dimensional (3D) structures, but by far the most characterizations of grain structures are done by microscopy and are thus limited to 2D. In the present work 3D grain structures in a well-annealed cylindrical aluminium (AA1050) sample is characterized and analyzed. The characterization is done by 2 methods i) by non-destructive 3-dimensional x-ray diffraction (3DXRD) ii) by serial sectioning and subsequent EBSP mapping of entire circular 2D sample sections; 50 sections are mapped In total 333 grains are reconstructed. It is found that the 3D grain morphologies can be quite complex in particular for the larger grains, the number of neighbours varies significantly and values above 20 are not unusual. When the results from the 2 methods are compared, it is found that the crystallographic agreement is very good and within experimental uncertainties. Slightly more significant differences are found when the reconstructed grain morphologies are compared. Reasons for this are discussed.

Abstract: A partially recrystallised sample has been characterised by 3DXRD. A gauge volume of 200μm × 700μm × 300μm has been fully mapped. Deformed and recrystallised regions within the selected gauge volume are distinguished based on the sharpness of the diffraction spots. Information corresponding to a 5D (Φ1, Φ, Φ2, z, x · y) map is deducted from the 3DXRD data.

Abstract: The distribution of nucleation sites in 90% cold rolled aluminium (AA1050) annealed for 1h at 280°C has been determined by serial sectioning the sample and characterizing each section using the electron back scattering pattern (EBSP) method in the SEM. In total 1844 nuclei are identified. It was found that the nuclei are not randomly distributed but clustered preferentially along TD-RD planes in the sample. The results are presented and implications of clustered nucleation on recrystallization kinetics are discussed.

Abstract: Effects of the crystallographic misorientation across boundaries between recrystallising grains and the neighbouring deformed matrices are discussed and exemplified by recrystallisation investigations of fcc metals. Classic misorientation observations are reviewed in the introduction, whereas the main parts of the paper focuses on two special boundary migration phenomena observed by in-situ recrystallisation experiments; namely protrusions and facets.

Abstract: By 3 dimensional X-ray diffraction (3DXRD) using high energy X-rays from synchrotron sources it is possible to study in-situ the nucleation and growth during recrystallization. In this paper it is described and discussed how 3DXRD can supplement EBSP measurements of nucleation and growth. Three types of studies are considered: i) orientation relationships between nuclei and parent deformed matrix, ii) recrystallization kinetics of individual bulk grains and iii) filming of growing grains in deformed single crystals.

Abstract: TEM, Kikuchi diffraction analyses, EBSD, neutron diffraction and hardness measurements have been applied in a study of commercial purity aluminum (AA1200) cold rolled to strains 2 and 4 and afterwards recovered by a heat treatment for 2h at temperatures up to 220 °C. The deformation microstructure is a lamellar structure delineated by dislocation boundaries and high angle boundaries ( ) parallel to the rolling plane. The macrotexture is a typical rolling texture which is composed of individual texture components present as micrometer- and submicrometre-sized volumes. In the lamellar structure, correlations have been established between microstructural parameters and the local texture, showing for example that the density of high angle boundaries and the stored energy vary locally. The local variations affect the annealing behaviors in a way that some regions coarsen faster than others, leading to a recovered structure which is heterogeneous.

Abstract: Ni with columnar grain was cold rolled and then recrystallized. The orientations at triple junctions before and after annealing were characterised using the EBSD technique, and recrystallization nuclei at triple junctions were identified. A comparison was made between the orientations of the nuclei and the orientations of the grains in the same area before annealing. Both nuclei with orientations within the orientation spread of the deformed grains as well as away from that were found.

Abstract: A critical point in the understanding of recrystallization textures is the development of crystallographic orientations of the nuclei. Here an issue, which has been debated much recently [eg. 1], is if nuclei have orientations identical to those of the deformation microstructures from which they originate or not. Traditional nucleation mechanisms like strain induced boundary migration [2] and particle stimulated nucleation [3] operate with nuclei orientations identical to the “parent”deformation microstructure. This is also what is commonly incorporated in recrystallization modeling. However, a number of studies have found recrystallization nuclei in orientations that were not expected from measurements on deformed structures. Some of these results are reviewed and discussed in this paper, and new in-situ results obtained by the 3 dimensional X-ray diffraction (3DXR) method are presented.