Hydride growth mechanism in Zircaloy-4: investigation of the partitioning of alloying elements

TL;DR

This study investigates how alloying elements and microstructural features influence hydride growth in Zircaloy-4, revealing compositional changes and segregation phenomena that affect the alloy's behavior during nuclear reactor operation.

Contribution

It provides detailed insights into the microstructural and compositional evolution of Zircaloy-4 during hydrogen ingress and hydride formation, using advanced microscopy techniques.

Findings

Deuterium remains in the matrix after cooling and hydride growth.

Sn segregates at hydride-matrix interfaces and faults, potentially aiding hydride growth.

Microstructural features influence the alloy's properties during service.

Abstract

The long-term safety of water-based nuclear reactors relies in part on the reliability of zirconium-based nuclear fuel. Yet the progressive ingress of hydrogen during service makes zirconium alloys subject to delayed hydride cracking. Here, we use a combination of electron back-scattered diffraction and atom probe tomography to investigate specific microstructural features from the as-received sample and in the blocky-alpha microstructure, before and after electrochemical charging with hydrogen or deuterium followed by a low temperature heat treatment at 400C for 5 hours followed by furnace cooling at a rate of 0. 5C per min. Specimens for atom probe were prepared at cryogenic temperature to avoid the formation of spurious hydrides. We report on the compositional evolution of grains and grain boundaries over the course of the sample's thermal history, as well as the ways the growth of the hydrides modifies locally the composition and the structure of the alloy. We observe a significant amount of deuterium left in the matrix, even after the slow cooling and growth of the hydrides. Stacking faults form ahead of the growth front and Sn segregates at the hydride-matrix interface and on these faults. We propose that this segregation may facilitate further growth of the hydride. Our systematic investigation enables us discuss how the solute distribution affects the evolution of the alloy's properties during its service lifetime.