Lisa M. Young

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.

Crack Tip Microsampling and Growth Rate Measurements in Low-Alloy Steel in High-Temperature Water

Abstract The importance of the dissolution of manganese sulfide (MnS) inclusions in environmentally assisted cracking (EAC) of pressure vessel steels in high-temperature water is well recognized. However, no direct measurement of the crack tip chemistry that develops during EAC has been performed, and only estimates exist for the dissolution rate of MnS and the resulting sulfur (S) levels in the crack. In the present work, microsampling of the crack tip solution in ASTM A533(B) low-alloy steel (0.013% S) exposed to constant and cyclic loading in 288°C water under various test conditions was used to measure the crack tip S concentration directly, while simultaneously monitoring crack length. A reversing direct current (DC) potential drop method was used to monitor crack length continuously. Ion chromatography (IC) and inductively coupled plasma (ICP) were used to measure the dissolved S species in the microsampled solutions. Most experiments involved varying the corrosion potential by changing the dissolve...

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.