Surface layers produced by ion nitriding of austenitic Fe-Mn-Al alloys and the effects on hardness and corrosion resistance

Austenitic (fcc) Fe-Mn-Al alloys were nitrided in the temperature range 723 to 973 K by a glow discharge method using H₂-N₂ gas mixtures, and the microstructures and compositions of the nitrided layers were investigated in detail. A thin surface layer and a thick nitrogen-diffused layer (internal nitrided layer) were produced; the former consisted of both manganese nitrides and a ferritic (bec) phase, while the internal nitrided layer had an austenitic structure with the incorporated excess nitrogen atoms. X-ray photoelectron spectra revealed that the incorporated nitrogen atoms have a strong interaction with aluminium lattice atoms. By nitriding, hardness values of the Fe-Mn-Al alloys were increased fromH _v = 350 to ca. 1000, and the corrosion resistance to sulphuric acid solution was enhanced by a factor of 30. These improvements are associated with a uniform distribution of the incorporated nitrogen and also with the presence of nitride-like interactions between the nitrogen and aluminium atoms.