This thesis presents the results of a study aimed at investigating the evolution of dislocation structures in individual grains in copper polycrystals following a strain path change or a change in temperature. Copper samples were pre-deformed in tension to a strain of 5% at room temperature or to a strain of 7% at a temperature of -196 ○C, and the samples were characterized by electron microscopy and mechanical tests. Transmission electron microscopy showed that the pre-deformation produced a characteristic dislocation cell structure consisting of regions with relatively high dislocation density, called dislocation walls, enclosing regions with very low dislocation density, called subgrains. The mechanical tests showed that a tension-tension strain path change leads to an increase in the yield stress if the change in strain path is sufficiently severe, and to a transient phase with a reduced work hardening rate. The main part of the study consisted of a number of X-ray diffraction experiments in which the pre-deformed samples were further deformed in tension in situ at the APS synchrotron (the Advanced Photon Source at Argonne National Laboratory), for some samples along an axis different from the pre-deformation axis. In the X-ray diffraction experiments a technique was employed with which it is possible to obtain high-resolution reciprocal space maps from individual bulk grains. The high-resolution reciprocal space maps contain features related to the dislocation structure in the grains: A spread-out ‘cloud’ of low intensity caused by diffraction from the dislocation walls and a number of sharp peaks of high intensity caused by diffraction from the individual subgrains. By acquiring reciprocal space maps at a number of different strain levels the evolution of the dislocation structures can be studied, and by analyzing the sharp peaks information about the strain in the individual subgrains and about the intra-granular stresses can be obtained. For the analysis of the reciprocal space maps a mathematical method was developed to partition the intensity distribution into two components corresponding to the contributions from the subgrains and the walls. The analysis showed that the morphology of the dislocation structures is almost unchanged during the micro-plastic range of the in situ deformation, and during the macroplastic range the evolution occurs in a gradual manner without any sudden major changes and with no indications that intermittent dynamics plays a major role in the evolution of the dislocation structures. An analysis of the position of the radial profiles from the individual subgrains revealed a substantial variation in the elastic back-strain in the subgrains and showed that the distribution of the elastic back-strain in the subgrains can be well approximated by a Gauss distribution. Furthermore, it was found that on average the elastic back-strain is larger in the larger subgrains than in the smaller subgrains. The analysis also showed that, following a strain path change, the intra-granular stresses are substantially redistributed during the micro-plastic range. In a few individual subgrains it was possible to follow the evolution of the elastic back-strain from the tensile to the compressive case. Following an increase in temperature from -196 ○C to room temperature, both the average intra-granular stress and the variation in the intra-granular stresses go through an initial phase of decrease or stagnation. An analysis of the width of the radial profiles from the individual subgrains showed that the dislocation density in the subgrains remains constant at a low level during the deformation. Finally, an analysis of the radial profiles from the individual grains indicated that a change in loading conditions leads to a less ordered dislocation structure in the walls.
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Poulsen, Henning Friis, Pantleon, Wolfgang
Risø National Laboratory for Sustainable Energy, 2011