Transformation of ordered face-centered tetragonal θ-MnNi phase to face-centered cubic austenite during isothermal aging of an Fe–Mn–Ni alloy

The transformation of the grain boundary fct θ-MnNi precipitates to thermodynamically stable austenite was investigated in an Fe–Mn–Ni alloy using transmission electron microscopy (TEM). During aging of Fe–Mn–Ni alloys, fine bands began to develop in the θ-MnNi intermetallic particles. The HAADF images revealed these thin bands to be stacking faults and twins that were formed by the glide of 16〈112〉-type Shockley partial dislocations on {1 1 1} planes. The presence of iron in the twin bands was detected by electron energy-loss spectroscopy (EELS) analysis. These observations supported the conclusion that the crystal structure of the twin bands was transformed to that of face-centered cubic (fcc) austenite by iron diffusion into the twin bands. A further transformation of the θ-MnNi particle to austenite was proceeded by the development of new austenite bands rather than by the broadening of the existing bands. The mechanism of this transformation was discussed based on the crystal structure of both phases.