Figure 1: Example of a typical crack tip in the 316SS samples: (a) bright-field image with matrix dislocation structure in contrast, (b) bright-field image with matrix structure out of contrast to elucidate crack tip, and (c) FeO darkfield image highlighting epitaxial oxide on wall and at tip. Crack-Tip Characterization in 316SS The 316SS samples that will be discussed were removed from a service component after many years in a 250-290°C, deoxygenated water environment [6,7]. Cracked samples from several locations have been examined and many crack tips characterized. In nearly all cases, wide (>~0.5 µm) cracks were filled with a multi-layer corrosion product oxide, consisting of a Cr-rich FeO and spinel inner layer plus large-grained magnetite (Fe3O4) filling the center of the crack. This multi-layer film is similar to that reported at stainless steel surfaces [8,9] with the exception of the FeO-structure oxide. The focus here is on the crack tips as illustrated in Figure 1. A somewhat “classic” appearance of a SCC crack tip in a passive alloy is seen with a non-porous, epitaxial FeO-structure oxide film on the walls. The crack narrows to ~10 nm at the tip comparable to the oxide film thickness ahead of the tip and on the adjacent crack walls. Evidence of significant deformation (high dislocation density) is present in the metal surrounding the tip as illustrated in Figure 1(a), but localized slip bands have not been observed intersecting the crack. However, twins are seen at or near primary or secondary crack tips as demonstrated in Figure 2. Scanning electron microscopy indicates that most twins are present before the propagation of SCC cracks and that many cracks end at intersections of twins with grain boundaries [10]. The examples presented in Figure 2 show deformation twins immediately ahead of a crack tip in (a) and a crack that has propagated through several twins in (b). Oxide films at these crack tips were again found to be non-porous, but were predominately a nanocrystalline FeCr spinel versus the epitaxial FeO-structure oxide seen at other tips. Low levels of solution impurities (e.g., Na) were discovered in the oxides indicating that a caustic environment may have been in cracks. 2 4 6 8 10 12 14 -10-8-6-4-2 0 2 4 6 810 8 10 12 14 16 18 20 22 Cr or Ni Concentration, wt% Mo Concentration, wt% Distance from Grain Boundary, nm Mill-Annealed 316SS Mo Cr Ni (c) Figure 2: Crack-tip interactions with matrix twins in a 316SS sample with twin present immediately ahead of the crack tip in (a) and immediately behind the tip in (b) where the crack is clearly displaced off the grain boundary plane. The composition profile in (c) was taken across the grain boundary in a non-attacked base metal region well ahead of the crack tip.
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