Unusual Iron Nitride Formation Upon Nitriding Fe-Si Alloy
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THE life and serviceability of engineering components can be enhanced by dedicated surface treatments. Nitriding is possibly the most versatile thermochemical surface engineering method among those employed in practice as, by tuned application, it can drastically improve the fatigue, wear and corrosion resistances of engineering components.[1–6] Although this surface treatment method has been widely employed in the industry for the past 100 years,[7] fundamental understanding of the underlying (micro)mechanisms is not complete yet. Therefore, nitriding studies have been performed on relatively simple iron-based binary alloys, especially in recent years. On the one hand, most work has been done on the alloying element nitride precipitation in ferritic substrates (responsible for enhancing the fatigue resistance), Fe-Al,[8–12] Fe-Cr,[13,14] Fe-Si[15–20] and Fe-V,[21,22] and ternary alloys, such as Fe-Cr-Al,[24] Fe-Cr-Ti[25] and Fe-Cr-C,[12,23]
S.R. MEKA is with the Metallurgical and Materials Engineering Department, Indian Institute of Technology Roorkee, Roorkee, 247667, India, and Max Planck Institute for Intelligent Systems, Stuttgart, Germany. Contact e-mail: [email protected] A. SCHUBERT is with the Institute for Materials Science, University of Stuttgart, Stuttgart, Germany. E. BISCHOFF is with the Max Planck Institute for Intelligent Systems. E.J. MITTEMEIJER is with the Max Planck Institute for Intelligent Systems and also with the Institute for Materials Science, University of Stuttgart. Manuscript submitted October 30, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Fe-Me-Si.[26] On the other hand, the effect of alloying elements on the formation of the surface iron nitride (c¢-Fe4N1x and e-Fe2N1y) compound layer, usually indicated as the ‘‘white layer’’ (responsible for improved wear and corrosion resistances), has only recently been investigated.[27] Whereas in the presence of strong nitride-forming alloying elements, such as Cr, V and Ti, the usual layer-type growth of iron nitride is observed,[28–30] weak nitride-forming alloying elements, such as Al and Mo, induce an unusual (e.g., plate-like) morphology of the formed iron nitride.[31–33] Usually steel contains Si: it is an important de-oxidizer for steel melts and an important alloying element in transformation-induced plasticity (TRIP), bainitic and electrical steels.[34,35] After the first demonstration that silicon nitride precipitates in ferrite in a highly unusual amorphous state,[36] only recently have the kinetics of this amorphous silicon nitride precipitation been investigated more extensively.[18–20,26] It was shown that the precipitation of amorphous silicon nitride in the ferrite matrix takes place only very slowly because of the large volume misfit, which has to be accommodated upon precipitating silicon nitride in the ferrite matrix. Thus, Si is effectively a weak nitride former,[26] although, from a chemical point of view, Si is a strong nitride-forming element.[37,38] A thermodynamic background for such different nitriding be
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