Materials Science Forum Vols. 467-470

Abstract: High resolution Electron Backscatter Diffraction (EBSD) in a field emission gun scanning electron microscope (FEGSEM) was used to study substructural development during the deformation by cold rolling of a single-phase Al-0.1Mg alloy with initial grain sizes between ~3µm and 120µm. In the coarse-grained material, bands of elongated cells aligned at approximately 35º to the rolling direction were formed at low strains. However, as the grain size was reduced, fewer of these aligned microstructural features were formed, and at the smallest grain sizes, there was little evidence of significant substructure within the deformed grains. The alignment of low angle boundaries was analysed from EBSD data and shown to be a function of grain size, strain and boundary misorientation.

Abstract: Deformation characteristics of cold rolled grains and their contribution to the development of recrystallization texture in IF steel were investigated using EBSD. Quantitative evaluation of the intra-granular orientation spread in the deformed grains was attempted. Both of the GAM and SGS could represent the orientation spread but in different manner. The GAM value was higher in {111}//ND orientations than in other orientation groups implying steep orientation gradient within short ranges in {111}//ND oriented grains. This steep orientation gradient was generally originated from the existence of deformation heterogeneities. Particularly, {111}<112> orientation which was the dominant component in the recrystallization texture showed the maximum values of both GAM and SGS, which was caused by the frequent development of shear bands. SGS values of {001}//ND oriented soft grains were unexpectedly large in spite of their small GAM values. It could be confirmed that their uniform deformation behavior led to the gentle but monotonous orientation gradient in long ranges. The lowest values of both GAM and SGS were obtained in the {112}<110> orientation, probably, because of the highest orientation stability in that orientation during plain strain deformation condition. GAM was more closely related to the development of recrystallization texture and the significance of the steep orientation gradient in the recrystallization behavior should be re-addressed.

Abstract: It has been shown in literature that the mechanical properties of Fe3Al base alloys are strongly dependent on the heat treatment subsequent to warm rolling. Therefore, the recrystallization behavior of 3 different hot and warm rolled and annealed Fe3Al-based alloys has been investigated. Two of these alloys contain different forms and amounts of second phase particles, while a pure binary alloy was taken as reference. All alloys develop a-(<110>||RD) and g-(<111>||ND) fiber bcc-type rolling and annealing textures, however, the amount of a- and g-fibers vary in dependence of the alloy composition. The current work presents the investigations on the nucleation process during annealing that has been studied by means of high resolution backscatter electron diffraction (EBSD) in the SEM. In particular the occurrence of orientation gradients in the deformed structure and their crystallographic relationship to the formation of new grains was investigated. It was shown, that small particles favor the a-fiber component by hindrance of the growth of new grains. In contrast, large particles lead to particle stimulated nucleation. This weakens the overall texture but does not randomize it since the orientation gradients around particles keep a relationship with the matrix orientation.

Abstract: A new method for in-situ studies of nucleation in bulk metals based on high energy synchrotron radiation is presented. Copper samples cold rolled 20% are investigated. The crystallographic orientations near triple junctions are characterized using non-destructive 3DXRD microscopy before, during, and after annealing for 1 hour at 290°C. This method allows in-situ identification of new nuclei and the deformed material, which spawns the nuclei. Also, since data is acquired during annealing nucleation kinetics can be studied.

Abstract: A novel statistical mechanics approach to quantify the effects of hot rolling and deformation on the formation of dislocations in a single grain scenario is presented. The dislocations are dealt as equilibrium defects in the crystal structure, which is assumed to be deformed via the formation of dislocations or single atom displacements at the grain boundary, which involve breaking their bonds and are thus termed “bond breaking atoms”. The deformation process is applied to steels of a variety of grain size and dislocations densities. The model has the capacity to describe the grain energy increase as a function of crystallography, grain sizes, temperature and degree of deformation, providing thus an aid in predicting the conditions for dynamic recovery and recrystallization.

Abstract: This work is part of a PhD study which investigates mechanisms of nucleation during recrystallisation in steels. A specially built SEM with EBSD and heating stage allows us to observe static recrystallisation in situ. Changes in samples of Ti-stabilised ultra low carbon steel heated to 550°C are documented via still images, videos and automated EBSD maps. Recrystallisation begins rapidly but slows down within an hour. A temperature increase to 850°C forces complete recrystallisation. EBSD mapping shows that the ‘new’ grains appear first on the boundaries of deformed {111} grains and have high angle misorientations with the deformed grains, although the new grains are also of {111} type. {111} type deformed grains are recrystallised first due to their higher stored energy relative to {001} grains. Mapping of a section through the sample along the rolling direction reveals intragranular shear bands. Future work will look at these sites in more detail to determine their role as sites for nucleation.

Abstract: The orientation dependence of recrystallization nucleation in 95% cold-rolled polycrystalline copper was studied by means of electron microscopy and calorimetry. Local orientation characteristics of microstructure at the beginning of recrystallization process were analyzed. Combined calorimetric and microscopic investigations, including local orientations measurements, imply that recrystallization is a superposition of several local processes that develop in two steps. In the first step, discussed in this paper, recrystallization process develops in the areas of localized strain, including shear bands and regions of more or less distorted matrix subgrains. Orientations of nuclei reproduce all components of deformation texture. Further growth of nuclei is accompanied by selective generation of successive recrystallization twins. In fact, not single grains are growing, but entire colonies comprising the nuclei developed from fragments of deformed matrix and recrystallization twins [e. g. 1].

Abstract: The nucleation of recrystallization in deformed and annealed metals is reviewed. The main mechanisms are thought to involve the growth of subgrains by low angle boundary (LAGB) migration in an orientation gradient or the strain induced boundary migration (SIBM) of existing boundaries. Although these mechanisms are reasonably well understood, the details of the dislocation recovery mechanisms which are often required before migration can occur, particularly in metals in which recovery is slow, are poorly understood. Complete experimental investigation of the nucleation event requires a 3-d in-situ technique which will resolve dislocations, and this is not currently available. Although recrystallized grains of orientations not in the deformed structure have been reported, there is as yet no substantial evidence or theory to suggest the creation of new orientations by mechanisms other than annealing twinning. It is concluded that further understanding of the deformed state is required before adequate models of nucleation can be formulated and verified.

Abstract: Plane strain channel die compression (PSC) deformation has been carried out on Al-Mg alloys with Mg contents of 0.1 to 5 % at 350°C, and the deformed microstructures characterised by channelling contrast backscattered electron imaging (BSE), secondary electron imaging (SE) and high resolution electron backscatter diffraction (EBSD). Measurements of orientation spread and misorientation gradient obtained from EBSD maps have been used to quantify the microstructural inhomogeneity developing in the 4.98 % Mg alloy. The results are consistent with inhomogeneous plasticity with more deformation occurring in the grain boundary regions. In-situ FEGSEM hot deformation experiments on the Al-4.98 % Mg alloy have provided evidence of stress driven boundary migration at low strains.

Abstract: After secondary recrystallization, the Fe-3%Si alloys, grade Conventional Grain Oriented (C.G.O.), exhibit a Goss texture that is sought for minimizing watt losses in transformer cores. The mechanisms of Goss grain formation and their evolution during the processing route from hot rolling to decarburizing such as the early first steps of abnormal growth are not still well cleared up. This work deals with the influence of local microstructure and texture heterogeneities observed by X-ray diffraction (XRD) and Electron Back Scattered Diffraction (EBSD) at the hot rolling step. The present results complete those previously obtained by neutron diffraction [1]. Presence of Goss grain colonies at about the quarter of the hot rolled sheet is probably, as it has already been suggested, at the origin of the Goss grain presence at the primary recrystallized state.