Internal Nitriding of Fe-Cr-Mo Alloys— Precipitation of Mixed Nitrides and Role of the Cr/Mo-Ratio

Controlled gaseous nitriding of ternary Fe-Cr-Mo alloys leads to the development of ternary, mixed nitrides in the ferrite matrix, which show complex chemical, structural, and morphological transformations as a function of nitriding time: initially continuous precipitation of fine, coherent, cubic, NaCl-type (Cr,Mo)N_x nitride platelets develop, which later transform to a novel, hexagonal CrMoN₂ nitride by a discontinuous precipitation reaction. Some relatively coarse cubic nitrides also occur in the ferrite lamellae. The Fe-Cr-Mo alloys with varying Cr/Mo ratio, but all containing a total alloying element (Cr+Mo) content of 2 at.%, showed similar kinetics of continuous precipitation of cubic (Cr,Mo)N_x nitride. The kinetics of the discontinuous precipitation reaction is faster for the alloys with lower Cr/Mo ratio.