Nitriding of ferritic Fe–Cr–Al alloys

Upon nitriding ferritic iron-based Fe–Cr–Al alloys (containing 0.25–1.25 at.% Cr and Al) a very hard surface layer develops containing nano-precipitates of alloying element nitrides. The microstructure of the nitrided zone was investigated by X-ray diffraction (XRD) and transmission electron microscopy (TEM). Coherency of the nano-nitrides with the ferrite matrix, as revealed by TEM, especially for the chromium-rich alloys, is compatible with the absence of separate nitride reflections in the X-ray diffractograms; the large coherency misfit strains induce extreme and particularly shaped broadening of the diffraction lines. The h k l dependence of the diffraction-line broadening is indicative of the specific orientation relationship of the nitride nano-precipitates and the ferrite matrix, as determined by TEM. For increasing Cr/Al atomic ratio, the microhardness and the diffraction-line broadening increase, whereas the amount of excess nitrogen decreases.