Sigma Phase

Definition - What does Sigma Phase mean?

A sigma phase refers to a non-magnetic stage composed of predominantly iron and calcium in ferritic and austenitic stainless steels. It is the intermolecular stage that causes metals to lose ductility, toughness, stability and corrosion resistance.

Corrosionpedia explains Sigma Phase

The sigma phase occurs during metallic exposure at 560ºC to 980ºC (1,050ºF to 1,800ºF). It is generally strain intolerant at temperatures under 120ºC to 150ºC (250ºF to 300ºF).

The sigma phase forms at ferrite/austenite interfaces. However, its formation is delayed with lower energy ferrite or austenite surfaces that can be formed during welding processes. An X-ray diffraction highlights the various mechanisms of sigma phase formation between 700ºC and 850ºC (1,292ºF and 1,562ºF). These mechanisms show changes from discontinuous precipitation to the growth of the existing sigma after nucleation site saturation.

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