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I would agree that if the coating is intact then SCC does seem unlikely but to create tensile failures of the type being shown would need stress concentration factors in the region of 3-4 and the small surface defects that seem to present under the coating wouldn't generate this level of stress concentration.
Alloys that are similar to Dilavar seem to have a tensile elongation that is typically between 15and 20%. This would indicate that the material is relatively tough and reasonably notch insensitive.
Looking at the failures this doesn't seem to be the case and it is clear that the material is quite notch sensitive. This seems to hint that the material may have been poorly heat treated and that it is more brittle than it should be for this application.
The operating temperatures involved in the 911 engine is unlikley to affect the material properties of the stud and it is unlikely that the ductility has changed with time and the next time I have a spare stud I will carry out a simple tensile test and see what happens.
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