Phase transformations in iron-nitride compound layers upon low-temperature annealing: Diffusion kinetics of nitrogen in
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I. INTRODUCTION
THE nitriding of iron and steel is of considerable technological importance, because it can make a pronounced improvement in the fatigue, the wear, and the corrosion resistance of these materials. One of the most important nitriding methods is the gas nitriding method,[1,2] which allows control of the chemical potential of nitrogen at the surface via the composition of the gas (NH3-based) phase. Thus, the controlled formation of iron-nitride phases as compound layers on the surface of the (in most cases, ferritic) solid substrate is possible. In such layers, usually local equilibrium is assumed at the phase interfaces, i.e., at the gassolid interface, according to the chemical potential of the nitrogen in the gas atmosphere, and at the solid-solid interfaces, in accordance with the phase diagram (for pure Fe-N, refer to Figure 1).[3,4]* *Single-phase iron-nitride specimens in equilibrium with a nitriding atmosphere can be generated by “through nitridin,” starting from iron powder or thin iron foils.
By quenching to ambient temperature after nitriding, the nitrogen concentration-depth profile at the nitriding temperature can be retained. By slow cooling after nitriding, a method often employed in industry, the nitrogen concentration-depth profile may change, in association with changes in the (equilibrium) phase-boundary concentrations. These changes may lead to phase transformations in the compound layer[5] and, thus, to alterations in the microstructure of the
T. LIAPINA, Postdoctor, formerly with the Max Planck Institute for Metals Research, Heisenbergstraße 3, D-70569 Stuttgart, Germany, is with the Max Planck Institute for Steel Research GmbH, D-40237 Düsseldorf, Germany. A. LEINEWEBER is with the Max Planck Institute for Metals Research. Contact e-mail: [email protected] E.J. MITTEMEIJER, Director, Max Planck Institute for Metals Research, is Professor, Institute for Physical Metallurgy, University of Stuttgart, D-70569 Stuttgart, Germany. Manuscript submitted April 13, 2005. METALLURGICAL AND MATERIALS TRANSACTIONS A
compound layer. Information about processes occurring in the compound layer at temperatures below the nitriding temperature is scarce. The present work reports a study of the annealing-induced redistribution of N in /-iron nitride compound layers grown by gas nitriding on the surface of a thin plate of -iron that had become saturated with nitrogen during the (sufficiently long) nitriding process. Annealing was performed in the temperature range of 613 to 693 K, which is well below the nitriding temperature of 823 K (Figure 1). The first results of such experiments performed by the present authors at an annealing temperature of 633 K revealed a “backward growth” of the -sublayer at the cost of the -sublayer.[6] This effect can be qualitatively explained by the shift in the equilibrium N concentration in the -phase in contact with the -phase to a higher value, upon lowering the temperature (i.e., from the nitriding to the annealing temperature, as shown in Figure
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