The Kinetics of the Nitriding of Ternary Fe-2 at pct Cr-2 at pct Ti Alloy
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NITRIDING is a thermochemical surface treatment that has been applied successfully in industry to improve properties of iron-based steel components, such as the resistances against fatigue, wear, and corrosion.[1–7] It would be beneficial to describe the kinetics of the nitriding process quantitatively to tune the resulting microstructure and thus to optimize the properties of the components. Unfortunately, current knowledge does not allow for such model-based nitriding. So-called nitriding steels, as used in technology, usually contain more than one alloying element exhibiting a large affinity for nitrogen. As compared with a vast body of literature on the nitriding behavior of binary iron-based alloys,[8–30] only recently fundamental research on the nitriding behavior of ternary iron-based Fe-Me1-Me2 alloys has been initiated. Questions to be addressed include the following: Do Me1 and Me2 precipitate as separate nitrides (simultaneously or sequentially) or as mixed nitride (jointly)? Recent research on Fe-Cr-Al alloys has demonstrated that mixed nitrides can develop, metastably, a crystal structure (even) incompatible with the K.S. JUNG, PhD Student, and E. BISCHOFF, Senior Scientist, are with the Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), Heisenbergstraße 3, 70569 Stuttgart, Germany. R.E. SCHACHERL, Senior Scientist, is with the Institute for Materials Science, University of Stuttgart, Heisenbergstraße 3, 70569 Stuttgart, Germany. Contact e-mail: [email protected] E.J. MITTEMEIJER, Professor, is with the Institute for Materials Science, University of Stuttgart, and is also a Director, with the Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research). Manuscript submitted March 14, 2011. Article published online September 20, 2011 METALLURGICAL AND MATERIALS TRANSACTIONS A
equilibrium crystal structure of the separate nitrides.[31] It was also shown that the ternary alloys can absorb amounts of nitrogen considerably larger than necessary for precipitation of all alloying elements as nitride and equilibrium saturation of the unstrained ferrite matrix—excess nitrogen.[32,33] Against this background, it is deemed necessary to investigate if the kinetics of the nitriding process for such alloys (1) sensitively reflects the operating mechanism of nitriding and (2) allows for the determination of physically meaningful parameters that provide a quantitative description of the nitride precipitation process. Thus not only is nitride precipitation understood but also the avenue opens up for model-based optimization of the nitrided microstructure of a multicomponent. A quantitative description of the nitriding kinetics, as exhibited by the evolution of the nitrogen concentration depth profile, should express the roles of the diffusion of nitrogen through the matrix and the rate of precipitation of the alloying element nitrides. The shape of the nitrogen concentration depth profile depends on the strength of the interaction of the
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