The purpose of this discussion is to assess the implications of fifteen years of studies of stress corrosion cracking (SCC) using the Analytical Electron Transmission Microscope (ATEM). This work has considered mainly Fe-Cr-Ni alloys of types used in water cooled nuclear power plants in the range of 250-350°C with hydrogenated and oxygenated water and with some contaminated environments. The objectives of this work have been mainly to determine causes of failures usually with respect to impurities in the water, faulty materials, geometric effects, irradiation, and from other sources. While the work, nominally, has not been directed toward mechanistic studies, the workers in this field have conducted some very good science enabling important insights into critical processes that affect predicting the course of SCC. Among the most important of the scientific advances has been showing that tips of cracks are in the range of 1-5 nm wide and are not in the range of large crack tip opening displacements (CTODs) that are calculated to be in the range of 2500-5000 nm wide. This finding of such narrow crack tips represents a "paradigm shift" in considering mechanisms of SCC. These very thin or "tight cracks", are so narrow that they must be treated on a molecular basis and not by a continuum basis. In view of this geometry, these crack tips are now referred to as "tight cracks" or as "molecular cracks." Second, these ATEM studies have shown that metal at the crack tip is often enriched in the noble component, Ni. This finding seems more likely in aborted cracks and less likely in propagating cracks although there is disagreement among researchers. Ni enrichment is not so common in high nickel alloys. This finding is a major advance and raises important questions about the resistance of all alloys now being used from the Fe-Cr-Ni alloy system. Also, this finding provides an avenue for conducting useful mechanistic research directed toward predicting long lifetimes. There is evidence suggesting that the advance of SCC is a brittle process and is not associated with breaking of passive films. Studies of the mature SCC after cracking at the crack tip, show that the oxides formed do not come from the crack tip but rather from in situ oxidation. From an engineering point of view the ATEM work has provided important answers to effects of contaminants and water chemistry. Recommendations for future scientific work with the ATEM include a thorough definition of dislocation arrays at the crack tip, detailed study of the metal composition ahead of the crack tip, and detailed characterization of the physical and chemical features of the tips of the tight cracks.
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