Experimental Investigation and Thermodynamic Modeling of the Ni-Rich Part of the Ni-N Phase Diagram

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NITRIDING is a thermochemical heat treatment process where N is incorporated into the surface region of a metallic workpiece. Different microstructural features can be generated by nitriding, which result in improved mechanical and/or corrosive properties.[1] Nitriding is usually applied to ferritic steels, but also to Ni-based alloys, or superalloys, containing nitride forming elements such as Ti, Cr, V, Mn,… In particular nitriding of Ni-based alloys is employed to obtain high surface hardness, to increase the wear resistance, and/or to improve fatigue life by forming so-called expanded austenite or S-phase.[2–7] Occasionally (high-temperature) nitriding of Ni-based alloys can lead to unfavorable properties, e.g., by development of alloying element nitrides in the austenitic matrix instead of expanded austenite[2]; such a process can thus be regarded as a corrosion process.[8,9] Nitriding of Ni-based alloys is predominantly performed by plasma nitriding, also known as ion nitriding MATEJ FONOVIC´, Ph.D. Student, and ANDREAS LEINEWEBER, Research Scientist, are with the Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), Heisenbergstraße 3, 70569 Stuttgart, Germany. Contact e-mail: [email protected] ERIC J. MITTEMEIJER, Director, is with the Max Planck Institute for Intelligent Systems (formerly Max Planck Institute for Metals Research), and Professor, at the Institute for Materials Science, University of Stuttgart, Heisenbergstraße 3, 70569 Stuttgart, Germany. Manuscript submitted January 30, 2014. Article published online July 15, 2014 METALLURGICAL AND MATERIALS TRANSACTIONS A

or glow-discharge nitriding[3,10]; only occasionally gaseous nitriding in NH3-H2 gas atmospheres has been applied.[5,11] In spite of the interest in the nitriding of Ni-based alloys, there is little information about the thermodynamics of nitriding of Ni-based alloys or even pure nickel. Indeed, already thermodynamic knowledge on the pure Ni-N system is scarce. A phase diagram from 1985 summarizes the limited amounts of experimental (phase boundary) data.[12] A thermodynamic description based on the CALPHAD approach was reported later,[13] where, however, considerable parts of the thermodynamic functions were roughly estimated. Thus, it is no surprise that the resulting phase diagrams of References 12 and 13 shown in Figure 1, strongly disagree with respect to the course of the phase boundaries. The available knowledge on the Ni-N system can be summarized as follows: existence of a Ni[N] solid solution containing up to about 0.29 at. pct N, next to Ni3N, was reported for Ni powders nitrided using NH3 gas of 1 atm at temperatures around 673 K (400 C).[14] The solubility of N in nickel (apparently containing some Mn, the presence of which, was, however, not considered) was also determined at temperatures from 1173 K to 1473 K (900 C to 1200 C) in an atmosphere containing 99 vol pct N2 and 1 vol pct H2 at 1 atm[15]; thereby N contents up to 0.0004 at. pct N were observed in solid sol

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