Texture and Residual Strain Measurements on Polished Alloy 600 Coupons Containing Two Scratches Using Synchrotron Micro-LAUE Diffraction

Abstract

Mechanical strain fields near two mechanical scratches (40 and 2 µm deep respectively) on polished Alloy 600 surface have been studied using micro-Laue diffraction technique using beamline 34-ID at the Advanced Photon Source (APS), Argonne, Illinois. Three-dimensional resolution of the diffraction patterns is enabled by differential aperture X-ray microscopy (DAXM). The DAXM technique is capable of producing depth resolved local grain orientation and strain distribution maps. The material under the deep scratch was found to be severely deformed (nanocrystalline) in the entire sampling depth; i.e., only a diffuse Laue pattern was observed. Grain structure was detected at region away from the scratch to depths of 60 to 100 µm into the metal. In contrast, a three layered structure was observed for the material under the shallow scratch: a nanocrystalline layer to the depth of 20 µm (diffuse Laue pattern), followed by a plastically deformed region (streaked Laue pattern) of approximately 10 µm in depth, and then a slightly deformed grain structure (indexed Laue patterns). The residual strain distribution maps indicate that the strain under the shallow scratch region appears to be triaxial. This is the first major study where three-dimensional grain orientation and strain mapping has been applied to the study of nickel-based steam generator tubing materials. The ability to produce micro-strain distribution laterally and depth wise across several grains would be very useful in enhancing the understanding the stress corrosion cracking behaviour of these alloys. Laboratory simulations tend to suggest that under certain operating conditions, SCC could occur at locations that have sustained mechanical damage such as fretting, dings and dent marks during manufacturing and/or operation. If micro-strain distribution at deformed regions can be determined under these conditions, then it is potentially possible to determine the critical strain level(s) that could result in crack initiation.