Microstructure-informed modeling of hydrogen diffusion in zirconium polycrystals
Abstract
Zirconium alloys are widely used in the core of various types of nuclear reactors. During service, the hot water coolant reacts with zirconium and releases hydrogen atoms that ingress into the lattice of the metal alloy. With time, hydrogen concentration exceeds its terminal solid solubility limit in zirconium, and a brittle phase known as zirconium hydride forms. This phase severely deteriorates the mechanical properties of zirconium alloys, leading to safety concerns regarding the integrity of nuclear pressure tubes. This thesis uses a crystal plasticity finite element model coupled with diffusion equations to study the effects of localized deformation at the grain scale on the hydrogen diffusion and hydride precipitation in zirconium. Attention is given to the effects of crystals elastic and plastic anisotropy, texture, microstructure, and plastic deformation by slip as well as twinning.
By considering the effects of the transformation strain associated with the formation of hydrides, it is shown that, hydrides tend to grow from their tips and are more probable to grow within mechanically “harder” grains. It is further shown that parallel hydrides are more probable to interlink than perpendicular ones. The effects of hydride shapes are also investigated where the results indicate that hydrides have an optimum width of 1 µm.
A three-point bending set-up is used to deconvolute the contributions of texture, microstructure, and external strains to the diffusion of hydrogen atoms towards micro-scale notches. While it is shown that grain-grain interactions significantly affect the distribution of hydrogen atoms, it is revealed that as the acuity of the notch increases, the effect of texture on hydrogen transport towards the notch root becomes less significant.
Lastly, the contribution of dislocations to hydrogen diffusion is studied using a novel non-local crystal plasticity finite element model coupled with diffusion equations. It is shown that slip bands and heavily deformed regions could greatly contribute to the hydrogen embrittlement of metals.