Michelle A. Othon

Misorientation Mapping for Visualization of Plastic Deformation via Electron Back-Scattered Diffraction

The ability to map plastic deformation around high strain gradient microstructural features is central in studying phenomena such as fatigue and stress corrosion cracking. A method for the visualization of plastic deformation in electron back-scattered diffraction (EBSD) data has been developed and is described in this article. This technique is based on mapping the intragrain misorientation in polycrystalline metals. The algorithm maps the scalar misorientation between a local minimum misorientation reference pixel and every other pixel within an individual grain. A map around the corner of a Vickers indentation in 304 stainless steel was used as a test case. Several algorithms for EBSD mapping were then applied to the deformation distributions around air fatigue and stress corrosion cracks in 304 stainless steel. Using this technique, clear visualization of a deformation zone around high strain gradient microstructural features (crack tips, indentations, etc.) is possible with standard EBSD data.

Electron Back-Scattered Diffraction Misorientation Mapping Applied to Stress Corrosion Cracking of Stainless Steels

Stress corrosion cracking (SCC) is a persistent problem that limits metallic component life in a wide variety of environments. Because SCC is largely influenced by the crack tip oxide rupture rate, life prediction models require characterization of the crack tip strain rate for SCC cracks. Crack tip strain rate may be mathematically derived from crack tip strain; however, crack tip strain is difficult to characterize due to the required small scale of the measurement and its highly localized nature [1]. Measuring the spatial gradient in crack tip strain requires both sensitivity to plastic strain and high spatial resolution. EBSD and related techniques, such as electron channeling patterns, have been used with some success to measure plastic strain around cracks in Fe-3Si single crystals [2], Cu single crystals [3], and Ni-based superalloys [4], among others. In this paper, we will discuss the use of misorientation mapping as a means for visualizing plastic deformation around crack tips in air fatigue and stress corrosion cracks.

Assessing Residual Strains in Nuclear Power Reactor Internal Components Weld Mockups of Nickel Alloys Using EBSD

The most common failure mode of commercial nuclear reactor internal components is environmentally assisted cracking (EAC). Environmentally cracking is promoted by the simultaneous effects of three affecting variables, namely (1) presence of tensile stresses and strains, (2) and aggressive environment and (3) a susceptible microstructure. If one or more of these affecting variables are removed, cracking will not occur or propagate. For example, EAC susceptibility and growth rate will be minimized if the residual tensile stresses or strains are controlled. Similarly, EAC may be minimized by controlling the chemistry of the weld metal, for example by increasing the content of chromium in the weld metal. The objective of the current research was to complete laboratory weldments of 76 mm thick plates of Alloy 600 using weld metal with three different compositions. Electron backscatter diffraction (EBSD) results of cross sections of the weld showed that approximately 10 to 15% of residual plastic strain existed at near the root of the weld and in the middle section (at approximately 38 mm); however, lower residual plastic strain existed near the top of the weld (last passes).Copyright