Microstructures in a nitrided zone of an Fe-18 mass percent Cr binary alloy plasma-nitrided at temperatures between 743 and 943 K were investigated by means of electron microscopy. The morphology of the nitrided zone changed drastically depending on the nitriding temperatures. On nitriding at 943 K, disk- and rod-shaped CrN and rod-shaped Cr_2N were observed in the original ferrite grains. Examination of orientation relationships between the ferrite matrix and the rod-shaped Cr nitrides suggested that a transition from Cr_2N to CrN occurs during nitriding at 943 K. On nitriding at 843 K, elongated columnar-shaped ferrite grains containing regularly spaced lamellar CrN precipitates developed from the specimen surface and grew inwards. Most of the columnar ferrite grains held OMEGA 9 coincidence site lattice relationships with respect to the original ferrite grains. The columnar ferrite tends to form at a lower nitriding temperature or a higher Cr content.
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