Sigma-phase in alloys of iron: crystal and microstructure

Sigma-phase is an intermetallic compound. It takes many forms: FeCr, FeMo, FeW, (Fe,Ni)(W,Mo) etc. with the ratio of iron to the other solute not exactly stoichiometric. It is very difficult to deform and so embrittles the steel when present in a coarse form. However, as fine particles, it does not embrittle, rather, it strengthens the alloy, especially when the service temperature is above ambient. Sigma-phase often nucleates at austenite-ferrite boundaries in duplex stainless steels.
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