Abstract
Kinetics and thermodynamics of the process of internal nitridation of various nickelโbase alloys have been investigated in oxygenโfree nitrogen atmospheres. Furthermore, the influence of formation and spalling of a protective oxide scale on the internal nitridation behavior of the alloys was studied by isothermal and cyclic oxidation tests in air. In general, nitridation kinetics of model nickelโbase alloys of the system NiโCrโTi was found to obey a parabolic rate law indicating that the nitridation process is diffusionโcontrolled. The temperature dependence of the nitridation rate constants is well described by an equation of the Arrhenius type. A thermodynamic calculation of the NiโCrโTiโAlโN system was used to determine the nitrogen solubility in respective alloys as a function of temperature and alloy composition. The results show that a higher chromium content gives rise to an increase in the nitrogen solubility of NiโCrโTi alloys leading to an increased nitridation rate in accordance with the experimental observations. From the calculated values for the nitrogen solubility, the diffusion coefficients of nitrogen were assessed using Wagner's classical theory of internal oxidation. A computer model of internal nitridation was developed that combines a commercial thermodynamic software (ChemApp) with a finiteโdifference diffusion calculation. It was found that this model describes the internal nitridation process in reasonable agreement with the experimental results and allows to treat the case of simultaneous formation of different nitrides. The dependence of internal nitridation behavior on spalling and cracking of the oxide was incorporated into the simulation on the basis of simple assumptions showing that this calculation method successfully applies also to complex internal corrosion processes.