Modeling of spontaneous transformation of nitrides to Z phase accounting for stress relaxation by diffusion and interface activity

To date the dominating high-temperature structural materials in power plants are ferritic/martensitic 9% Cr steels, which are strengthened by nitrides, VN or NbN. Higher Cr contents would be desirable to improve the oxidation resistance. However, higher Cr contents favor the transformation of the fine nitrides into a small number of coarse Z-phase particles, so that the strength of the material drops drastically long before the required lifetime. A model based on the Thermodynamic Extremum Principle is developed which treats the growth of nitrides and the transformation of some of them to Z phase in 9-12% Cr steels. The model includes the misfit stress and its relaxation by vacancy annihilation at the interface. Numerical solutions of the kinetic model are presented for several systems with different chemical composition at different temperatures. The ability of the particle/matrix interfaces to absorb vacancies has a great influence on the relaxation of the misfit stresses, and consequently on the whole particle growth and transformation process. Moreover, it is shown that the Z phase loses its energetic advantage compared to the nitrides, if the Nb content is moderately lowered. The simulations based on the model can motivate the development of new ferritic/martensitic Cr steels with improved long-term creep and oxidation properties.