Quantitative analysis of crystal scale deformation heterogeneity during cyclic plasticity using high-energy X-ray diffraction and finite-element simulation

Modern high-energy X-ray diffraction (HEXD) experiments coupled with a crystal-based finite-element model employing forward projection of virtual X-rays through each element is applied to study cyclic plasticity. An Okegawa mold copper specimen was cyclically deformed in situ at the Advanced Photon Source. The strain amplitudes of the cyclic experiments reached well into the plastic regime and diffraction images were generated at several points in the loading history using a HEXD methodology. Four grains within the bulk of a polycrystalline sample were tracked and interrogated with X-rays. Diffraction peak data were reduced to center of mass (COM) and full width at half maximum (FWHM) values in the detector coordinates 2θθ (radial) and ηη (azimuthal). The peaks evolved with cycles and changed significantly when the plastic strain amplitude was increased. Large changes in the peaks (especially the azimuthal FWHM values) were also observed during the course of one loading cycle; larger ηη FWHM values were seen at the compressive end of the cycles. This trend was reversed when the sample was initially loaded in compression. Diffracted intensity distributions were also seen to change significantly from one grain to the next. Using a virtual diffractometer model, COM and FWHM values were computed from the modeling results by projecting virtual X-rays through the finite-element mesh and compared to the experimental data. The finite-element polycrystal model serves as the final step in the data reduction process, revealing significant spatial heterogeneity of orientation, stress and plastic strain rate distributions. Studying these distributions collectively will be necessary to fully understand the detailed elastic–plastic deformation behavior within each grain and to explore problems such as microcrack initiation hypotheses in polycrystalline materials.