Beta phase controls hydride precipitation within alpha phase in dual-phase Zr-2.5Nb

Abstract

Formation of a new phase usually follows a fixed orientation relationship with matrix in metallic materials, i.e., hydrides prefer to form on the basal plane in hexagonal zirconium (Zr) under thermal equilibrium conditions. Here, we identify that hydride precipitation within α-Zr in dual-phase Zr-2.5Nb deviates from the traditional manner, being governed by β-Zr. Two types of orientation relationships between γ-ZrH and β-Zr are identified, namely Kurdjumov-Sachs and Pitsch. The habit plane of hydrides consistently aligns with {112}_β. Hydrides can cross various colony boundaries but are obstructed by the β-Zr grain boundary. The high solubility and the fast diffusion rate of hydrogen inside body-centered cubic β-Zr are the origin of the unique hydride phase transformation in dual-phase Zircaloy. Our findings may provide some new understanding for solid-state phase transformation in dual-phase materials.