The Nitrogen-absorption Isotherm for Fe-21.5at.%Cr Alloy: Uptake of Excess Nitrogen in Dependence on the Precipitation Morphology

Santosh Hosmani (1) , Ralf Schacherl (2) , Lidia Lityńska-Dobrzyńska (3) , Eric Jan Mittemeijer (1)

1 Max Planck Institute for Metals Research
2 Institute of Physical Metallurgy
3 Institute of Metallurgy and Materials Science

Nitriding of Fe-21.5at.%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 °C. To assure a constant precipitation morphology the Fe-21.5at.%Cr specimen was first homogeneously pre-nitrided (at 580 ºC in an ammonia/hydrogen gas mixture of nitriding potential of 0.103 atm-1/2) and then de-nitrided (at 470 ºC 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.

Mots clés: Physical Sciences

Santosh Hosmani, Ralf Schacherl, Lidia Lityńska-Dobrzyńska, Eric Jan Mittemeijer. The Nitrogen-absorption Isotherm for Fe-21.5at.%Cr Alloy: Uptake of Excess Nitrogen in Dependence on the Precipitation Morphology. Philosophical Magazine, 2008, 88 (16), pp.2411-2426. ⟨10.1080/14786430802345660⟩. ⟨hal-00513944⟩