Phase transformation and microstructural changes during the hydriding and aging processes in dual phase Zr alloy

Abstract

The Zr-2.5 wt%Nb alloy was studied in order to elucidate phase transformations and the distribution/redistribution of hydrogen. The study focuses on understanding the stability of the ZrH system; surface hydride formation and its transformation during subsequent ageing was inspected in detail, using SEM imaging, EBSD, X-ray diffraction and AFM. During the hydriding process, the H in solution → γ → δ transformation occurred, which resulted in the formation of δ-hydrides on the sample surface. As hydride precipitation occurs, the system enters a metastable state before it reaches a new equilibrium state. The ZrH system approach to the new equilibrium state was monitored during room temperature ageing over the course of several weeks. The total surface hydride fraction decreased with ageing time. Along with a δ → γ phase transformation, partial hydrogen dissolution occurred followed by diffusion out of the surface X-ray diffracting volume into the depth of the sample.