The nitrogen-absorption isotherm for Fe-21.5 at. % Cr alloy: dependence of excess nitrogen uptake on precipitation morphology

Nitriding of Fe-21.5 at. % Cr alloy leads to a discontinuously coarsened, chromium-nitride/ferrite lamellar precipitation morphology in the nitrided zone. The nitrogen-absorption isotherm for this alloy with this precipitation morphology was determined at 560 degrees C. To assure a constant precipitation morphology the Fe-21.5 at. % Cr specimen was first homogeneously pre-nitrided (at 580 degrees C in an ammonia/hydrogen gas mixture of nitriding potential 0.103 atm-1/2) and then de-nitrided (at 470 degrees C in hydrogen gas atmosphere). The amount of nitrogen remaining in the de-nitrided specimen indicated that the composition of the nitride precipitates is CrN and not (Fe, Cr)N. The measured nitrogen-absorption isotherm revealed the presence of excess nitrogen in the nitrided specimen, which is a surprise in view of the coarse, lamellar precipitation morphology. The occurrence of this excess nitrogen could be ascribed to an unexpected, minor fraction of the total chromium content in the alloy present as coherent, tiny nitride platelets within the ferrite lamellae of the discontinuously coarsened lamellar precipitation morphology, as evidenced by transmission electron microscopy. A possible kinetic background for this unusual phenomenon was discussed.