Originally Posted by
arsprod
Understood - isn't stainless more brittle?
Impossible to say in general -- you'd have to compare specific alloys & treatments. Most bicycle stainless bolts are Austenitic and not particularly brittle. Many black steel allen bolts are through-hardened for use in tooling, etc. and could experience brittle fracture under extreme impact (you would see the fracture plane rotated ~45 degrees). Instead, it appears that you are dealing with fatigue failure possibly accelerated by corrosion.