Methodology of Synchrotron Edxrd Strain Profiling

We report the application of energy dispersive X-ray diffraction (EDXRD) to residual strains depth profiling. Extremely tight incident/diffracted b!"#$ %&''(#")(&*$ +,-.$ /#0$ is used to define a fixed micro-scattering-volume, through which the specimen is scanned to produce the profiling. Exhaustive theoretical modeling of the experiment provides outstanding agreement with the observed results, emphasizing our solid understanding of the as-implemented experimental technique. The contributions of the size/geometry of the diffraction volume to variations of the effective sample scattering region and diffraction angle near surfaces are discussed. The relations needed to extract the lattice parameter from the data including all instrumental factors are discussed. High precision procedures for a rigorous energy calibration, precise scattering angle determinations are presented. We discuss the EDXRD spectral fitting, using flexible shape and background functions, along with lattice parameter determination based on a statistical optimum set of diffraction peaks. This methodology allows lattice parameter precisions of ~ 0.0001  as needed for strain profiling. INTRODUCTION The detailed spatial distribution of residual stress/strain (RS/S) fields in structural materials, arising either in a components manufacture/processing or duty cycle, are frequently decisive in the failure of load bearing component [1-5, see footnote 6 regarding stress vs. strain]. Conventional X-ray and neutron scattering have traditionally been the only direct, nondestructive methods for strain profiling as a function of depth into a material [3-5]. The substantial limitations imposed by the small penetration depth of the former, and the large scattering volume required by the latter, have motivated investigations high energy synchrotron X-ray scattering for RS/S depth profiling [7-10]. Energy dispersive X-ray diffraction (EDXRD) has been the front-runner in these rather new and evolving efforts to develop RS/S profiling techniques. In this paper we discuss the EDXRD techniques we have been developing at the Brookhaven National Synchrotron Light Source (NSLS). In addition detailed modeling results to provide a firm theoretical underpinning to our methods will be presented. EXPERIMENTAL A schematic of our implementation of EDXRD is shown in Figure 1. The wiggler beamline, X17B1 at NSLS [7] provides high intensity, high-energy (up to ~200 keV) X-rays. The data collection geometry is extremely stable with stationary incident/diffracted beam paths and with the profiling being accomplished by sample micro positioning. Extremely tight incident/diffracted beam collimation (with 2mm thick Ta metal slits-apertured to ,-.$ /#$ (*$ height, d) are used. The small slit size, large spacing detween the diffraction collimation slits (LDS~1 m in the figure), and close placement of the first slit to the sample (i.e. LS~0.1 m) Copyright(C)JCPDS-International Centre for Diffraction Data 2003, Advances in X-ray Analysis, Vol.46 98