Phase-field study of diffusion controlled phenomena: reorientation of zirconium hydrides and oxidation of metals

Abstract

Materials under extreme conditions, such as under high temperatures and/or corrosion environments, undergo different microstructural changes that can significantly affect their properties and performance during operations. In this Ph.D. research, by means of computational modeling, we aim to understand the re-orientation mechanisms of zirconium hydrides in nuclear fuel claddings and oxidation induced stresses in steels, which are among two critical problems in materials used in water-cooled nuclear reactors. For this purpose, we developed micro-scale computational frameworks based on the phase-field method.We investigated the shape evolution and reorientation of zirconium hydrides under applied external loads, resulting in embrittlement of zirconium alloys. Two-seed and multi-seed hydride simulations revealed the significant influence of neighboring hydrides on the required load for reorientation. For the first time, we could explain that hydride reorientation happens more easily when clusters of hydrides are present, due to the strain fields around the hydrides, and the required external load for reorientation is within the elastic limit of the cladding material, similar to reported experiments. In addition, we studied the interaction between induced stresses and the formation of oxides for both internal and intergranular cases. The evolution of oxide layer occurs to reduce the localized stress concentration, and increasing the temperature may alter the stress levels by changing the contribution of eigen strains on the elastic energy. By considering different diffusivities at the grain boundaries, depending on their structures, the simulations captured intergranular oxidation and oxidation resistance of certain low grain boundaries, as observed in experiments.