Application of Nano-Scale Characterization Techniques in Failure Analysis of Stress Corrosion Cracking in an Alloy C-276 Pull Rod
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A failure analysis was performed on an alloy C-276 pull rod which underwent unexpected brittle, fracture after exposure to nuclear-relevant, high temperature aqueous environments: Pb-contaminated alkaline (8.5 < pH300C < 9.5), and sulfate-containing acidic solutions (3.0 < pH300C < 5.0). The component was characterized using advanced electron microscopy methods to demonstrate the benefits of these techniques for determining the nanoscale chemical and material factors contributing to stress corrosion cracking (SCC), and to provide insight into the mechanism of failure. Site-specific transmission electron microscopy samples containing crack tips were prepared using a focused ion beam. Nanoscale chemical characterization methods revealed that Pb was present in some oxidized regions of cracks, suggesting that the element may be inhibiting or impairing the passivity of the Cr-rich oxide. Complementary nanoscale microstructural analysis was performed. At an intergranular to transgranular cracking mode transition, it was observed that the transgranular crack (and corrosion process) propagated along the (110) crystallographic plane. Also, the cracking mode was highly dependent on the tensile stress direction relative to grain boundary orientation, the crystallographic orientation of grains, and slip systems. A comparison of results with proposed mechanisms for SCC of Ni alloys in similar environments are discussed; the highly directional nature of cracking is consistent with a slot-tunnel corrosion mechanism.
URI for this recordhttp://hdl.handle.net/1974/28022
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