USE OF RIETVELD REFINEMENT FOR ELASTIC MACROSTRAIN DETERMINATION AND FOR EVALUATION OF PLASTIC STRAIN HISTORY FROM DIFFRACTION SPECTRA

Macrostrain variations in engineering components are frequently examin ed using neutron diffraction, at both reactors and pulsed sources. It is desirable to minimize the sampling volume in order to maximize the spatial resolution, although this increases the required measurement t ime. At reactors, macrostrain behavior is inferred from a single latti ce reflection (deemed to be representative of the bulk response). At a pulsed source, a complete diffraction pattern is recorded and accordi ngly it is natural to fit the entire diffraction spectra using a Rietv eld [J. Appl. Cryst. 2, 65 (1969)] refinement. This means that an idea lized crystal structure is fit to the measured distorted crystal struc ture, which includes deviation of the measured lattice reflections fro m the ideal due to elastoplastic strain anisotropies, which are depend ent on the particular lattice reflection (hk1) considered. We show tha t elastic macrostrains calculated from lattice parameter changes in Ri etveld refinements (without accounting for hk1 dependent anisotropies) are almost identical to the bulk elastic response and are comparable to the response obtained from a single lattice reflection typically us ed by practitioners at a steady state source. Moreover good refinement s on the complete pattern are obtained with short measurement times co mpared to what is required for good statistics for single reflections. By incorporating a description of the elastic strain anisotropy expec ted in cubic materials into the Rietveld code, an empirical prediction of plastic strain history is possible. The validity of these argument s is demonstrated by analysis of a uniaxial tensile load test and a re analysis of previously reported data taken on a deformed stainless ste el ring. The plastic strain predictions compare favorably with a finit e element model. (C) 1997 American Institute of Physics.