High resolution X-ray diffraction profile analysis of a cold-rolled polycrystalline aluminium

Abstract

High density dislocations lead to a broadening of Bragg diffraction profiles (third-order internal stresses). On the other hand, first-order internal stresses induce a shift of Bragg diffraction profiles. Between these two extremes, one can define a mesoscopic scale (second order) where the strain field comes from different arrangements of dislocations arrays, and gives both effects (broadening and shifting of Bragg diffraction profiles). According to Wilkens model [1], the asymmetry of diffraction profiles can be observed in the case of a cellular dislocation arrangement. This asymmetry depends on the sign and quantity of the deformation and on the type and orientation of Bragg reflection; it is in effect the consequence of the heterogeneity of the dislocation distribution and interpreted in terms of the elasto-plastic response of a two-component composite material: dislocation cell walls and cell interiors. During and after plastic deformation, the internal stresses at mesoscopic scale are induced by the strain incompatibilities between those two parts of the material. In other words, by geometric necessity, the dislocations are created at the interfaces between cell walls and cell interiors. These internal stresses induce shifting of sub-Bragg diffraction profiles, each corresponding either to the cell walls or to the cell interiors. The sum of these mutually shifted sub-Bragg diffraction profiles represent the observable asymmetric peak profiles. This kind of asymmetric profile analysis is studied essentially on deformed monocrystalline materials [2, 3]. Recent works on polycrystalline materials have also shown an asymmetry of the diffraction profile [4, 5], but the asymmetry effect is very small because of the resolution limitation of laboratory diffraction equipment in the case of polycrystalline materials. The present study is realized on a cold-rolled polycrystalline aluminum using high resolution Xray diffraction (synchrotron). With the great plastic deformation of the studied aluminum during coldrolling, a cellular dislocation microstructure was formed and the second internal stresses due to the interaction between cell interior and cell wall lead to a broadening and shift of the diffraction peak. The analysis of diffraction profiles obtained under laboratory conditions does not permit good separation