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THE nickel-based alloy 600 (A600) used in steam generator tubing in nuclear power stations is prone to stress corrosion cracking (SCC). Cold work is believed to be one possible stress component leading to SCC of A600 steam generator tubing. The use of indents as a model for cold work may provide insight into the elastic and plastic strain conditions leading to SCC. Indentation is used routinely to study the mechanical properties of materials. Numerous studies employing techniques[1–6] such as electron backscattered diffraction (EBSD) and transmission electron microscopy (TEM) investigated the plastic zones around indents. While EBSD yields plastic deformation information in the form of local lattice misorientation, it is not capable of providing three-dimensional (3-D) structural information without serial sectioning, nor is it capable of giving insight into slip systems. Elastic strain also remains elusive by EBSD.

MARINA SUOMINEN FULLER is former Research Scientist, Chemistry Department, The University of Western Ontario. Contact e-mail: [email protected] ROZALIYA BARABASH, Research Staff Member, is with the Materials Science and Technology Division, Oak Ridge National Laboratory, Oak Ridge, TN 37831. STEWART MCINTYRE, Professor (Emeritus), is with the Chemistry Department, University of Western Ontario, London, ON N6A 5B7, Canada. SRIDHAR RAMAMURTHY, Research Scientist, is with the Surface Science Western, University of Western Ontario. WENJUN LIU, Beamline Scientist, is with Advanced Photon Source, Argonne National Laboratory, Argonne, IL 60439. Manuscript submitted December 19, 2011. Article published online July 7, 2012 METALLURGICAL AND MATERIALS TRANSACTIONS A

The development of focused ion beam milling has made it possible to obtain cross sections of indents for TEM analysis, thereby providing plastic strain depth information. However, the technique is destructive in that thin slices of material must be cut and removed from the desired area, and the bulk elastic strain state is relaxed once the thin specimen has been cut out. A further limitation is that TEM is a ‘‘postmortem’’ technique. The development of Polychromatic X-ray Microdiffraction (PXM) as a nondestructive method to determine elastic and plastic strains, lattice rotations, or geometrically necessary dislocation (GND) density tensors has been ongoing for the past decade.[7–9] Furthermore, a 3-D technique based on PXM and referred to as differential-aperture X-ray microscopy (DAXM) is capable of probing local crystal structure, grain orientation, and strain tensors with submicrometer spatial resolution. In this study, a DAXM line scan was performed starting at the edge of a Brinell indent and extending outward from the indent, in order to investigate the grain structure after the extreme plastic deformation induced by indentation.

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EXPERIMENTAL

A. Sample Preparation Alloy 600 coupons were obtained from Rolled Alloys and prepared in the as-received condition. One face on the alloy 600 coupon was polished using a series of grade