Normal and excess nitrogen uptake by iron-based Fe-Cr-Al alloys; the role of the Cr/Al atomic ratio

Kyung Sub Jung (1) , Ralf Schacherl (2) , Ewald Bischoff (1) , Eric Jan Mittemeijer (1)

1 Max Planck Institute for Metals Research
2 Universität Stuttgart

Upon nitriding ferritic iron-based Fe-Cr-Al alloys, containing a total of 1.50at.% (Cr + Al) alloying elements, with varying Cr/Al atomic ratio (0.21 - 2.00), excess nitrogen uptake occurred, i.e. more nitrogen was incorporated in the specimens than compatible with only inner nitride formation and equilibrium nitrogen solubility of the unstrained ferrite matrix. The amount of excess nitrogen increased with decreasing Cr/Al atomic ratio. The microstructure of the nitrided zone was investigated by X-ray diffraction (XRD), electron probe microanalysis (EPMA), transmission electron microscopy (TEM) and electron energy loss spectroscopy (EELS). Metastable, fine platelet-type, mixed Cr_1-xAl_xN nitride precipitates developed in the nitrided zone for all of the investigated specimens. The degree of coherency of the nitride precipitates with the surrounding ferrite matrix was discussed in view of the anisotropy of the misfit. Analysis of nitrogen-absorption isotherms, recorded after subsequent pre- and de-nitriding treatments allowed to differentiate quantitatively different types of nitrogen taken up. The amounts of the different types of excess nitrogen as function of the Cr/Al atomic ratio were discussed in terms of the nitride/matrix misfit and the different chemical affinities of Cr and Al for N. The strikingly different nitriding behaviors of Fe-Cr-Al and Fe-Cr-Ti alloys could be explained on this basis.

Mots clés: Physical Sciences

Domaines: Matière Condensée [cond-mat]

Kyung Sub Jung, Ralf Schacherl, Ewald Bischoff, Eric Jan Mittemeijer. Normal and excess nitrogen uptake by iron-based Fe-Cr-Al alloys; the role of the Cr/Al atomic ratio. Philosophical Magazine, 2011, pp.1. ⟨10.1080/14786435.2011.563760⟩. ⟨hal-00681620⟩