Orientation Relationships between Gamma (f.c.c.) and Sigma Phases in an Iron-Chromium-Nickel Alloy

Abstract

Single crystals of an Fe-25% Gr-20 % Ni alloy as grown were of gamma phase (f. c. c. structure). When they were annealed for about 200hours at 815•Ž, a considerable amount of sigma phase precipitated in the f. c. c. matrix. By means of X-ray diffraction the orientation relationships between gamma and sigma phases were determined to be as follows: A or B The difference between these two orientation relationships A and B is less than 1•‹. Based upon the orientation relationships established here, the process of the sigma formation from the gamma matrix was discussed. It is suggested that the nucleation of the sigma phase will occur easier at lattice defects such as stacking faults or twins.