FeCr sigma phase

The prototype FeCr sigma phase is of particular interest because the free atoms have very nearly the same size (ratio = 1.01), but they condense into a rather intricate structure. In the pure metals, the diameter of Cr is 2.50 A, while that of Fe is 2.48 A. (a difference of less than one percent), and both are bcc. Therefore, the existence of the sigma phase is determined by spd-hybridization of the electron orbitals. It is sometimes called a size-effect phase, but this is not really descriptive. 

Type 315-This has a composition that provides a similar oxidation resistance to type 309 but has less liability to embrittlement due to sigma formation if used for long periods in the range of 425 to 815°C. (Sigma phase is the hard and brittle intermetallic compound FeCr formed in chromium rich alloys when used for long periods in the temperature range of 650 to 850°.)... 

In addition to carbide precipitates, other intermetallic phases (Sigma-phase FeCr with up to 50% Cr, Chi-phase Fe35Cr]2Moio and Laves-phase Fe2Mo) can also cause intergranular corrosion. In ferritic Cr steel Cr2N must be mentioned. There are three reasons for the reduced corrosion resistance of the grain boundaries ... 

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