X-Ray Microbeam Laue Pattern Studies of the Spreading of Orientation in OFHC Copper at Large Strains

This paper assesses, via X-ray microbeam diffraction, the effects of development of dislocation substructure on the distribution of sub-grain misorientations in annealed OFHC copper deformed to large strains for compression, for shear, and for sequences of compression followed by shear. Polychromatic synchrotron x-radiation was used to study samples from four strain histories: virgin specimens, 50% effective strain in compression, 100% effective strain in torsion, and 50% compressive strain followed by 50% torsion. A very narrow beam illuminated an approximately 15 μm diameter column through the sample, and the microstructure of the specimens was mapped by translating the sample along two orthogonal axes perpendicular to the beam by increments of 10 μm. The beam diameter was considerably smaller than the average grain size in the virgin material. Both the degree of substructure formation and the nature of the distributed microstructure were quantified from the resulting Laue diffraction patterns. The polychromatic diffraction patterns of the polycrystalline samples consisted of well-defined streaks, and the azimuthal angular width of the streaks increased with plastic strain in a manner consistent with the scaling of the misorientation distribution of high angle boundaries for sub-grains reported recently using electron microscopy techniques limited to thin foils or thin surface layers. A lattice spin correction is introduced based on this scaling law in a simple extended Taylor scheme of polycrystal plasticity to achieve a retardation of texture development that is consistent with experimental results.