Unusual microstructural development upon gaseous nitriding of Fe-4.65at% Al alloy

Abstract

Employing NHM3/H2 gas mixtures, Fe-4.65at% Al alloy specimens were nitrided to assess how the presence of Al, originally dissolved in the ferrite matrix, influences the development of γ′-Fe 4N1-x phase in the surface adjacent region. The nitrided specimens were characterized by light microscopy, X-ray diffraction, Electron Backscatter Diffraction and Electron Probe Micro Analysis. Surprisingly, formation of ε-Fe2N1-x was observed, although, for the applied nitriding parameters (nitriding potential and temperature), only the formation of γ′-Fe4N1-x would be expected in case of nitriding pure ferrite. An unusual plate-type morphology of γ′-Fe4N1-x was observed, contrasting with the usual continuous layer-type growth observed upon nitriding iron, Fe-Cr and Fe-V alloys. These unexpected phenomena may be explained as consequences of the need to realize a very high nitrogen supersaturation in the ferrite matrix in order to initiate the precipitation of AlN. © (2010) Trans Tech Publications.