Thermodynamics and kinetics of internal reactions: Nitridation of Ni-Cr alloys

The implementation of Wa,oner's theory of internal precipitation for evaluating nitriding kinetics has been discussed. Nitriding experiments on Ni-Cr alloys were carried out in the range of 1-6000 bar of hydrostatic N-2-pressure and under a flowing NH3+H-2 gas mixture at 873-1398 K. When a moderately stable phase is precipitated out during an internal reaction, the behaviour of the penetrating atoms within the diffusion zone can be interpreted based on thermodynamic considerations. The thermodynamic assessment of the Ni-Cr-N system was performed using the Thermo-Gale databank and the results wen verified experimentally. However, "up-hill" diffusion of the penetrating species through the matrix towards the precipitation front can be predicted in certain cases and then Wagner's treatment cannot be utilised. It was demonstrated that under suitable experimental conditions of solute content, temperature and external activity of the penetrating element, Wagner's analysis can be applied to internal precipitation process in any material system.