Electron Microscope Study of Sigma Phase Precipitation in an Iron-Chromium-Nickel Alloy

An Fe-25% Cr-20% Ni alloy was cold-rolled up to about 20% and then annealed at 700°C for various periods of time up to 20 hr in order to precipitate the sigma phase. Electron microscope observation was made on thin foils prepared by electropolishing. Deformation twins and stacking faults were found in the cold-rolled specimens. The sigma phase precipitates preferentially at grain boundaries, stacking faults and twins, and possibly at dislocation crowded regions. Precipitates of the sigma phase were observed more often at both ends of the stacking fault than at the intermediate position and the sigma phase is thought to be nucleated on the stacking fault. The nucleation of the sigma phase also seems to occur on the twin interface. But when the sigma phase precipitates at the intersection of two twins of different orientations, the sigma phase seems to be nucleated on neither of the intersecting twin planes, but on the other {111} type plane. In most cases, the orientation relationship between the two phases, the matrix and the sigma phase, was the same as that determined by X-ray diffraction, though there is some indication of possible deviation.