Hydride stacking structure and phase transformation of hydride dissolution induced by thermal transient in zirconium alloys

Zirconium hydrides that significantly degrade mechanical properties of nuclear fuel claddings are commonly expected to consist of individual nano-hydride platelets. However, their internal microstructure and the associated platelet stacking remain ambiguous. This work reports a backbone structure characteristic of δ-hydrides identified during their partial dissolution induced by the thermal transient of electrical discharge machining. The backbone structure is characterized as acicular hydrides with discrete separating distances, between which the nano α-Zr subgrains in the original hydride domain present a distinct crystallographic orientation with that of the α-parent grain, indicating the δ→α transformation process of hydride dissolution. The subgrain orientation variation can be attributed to the phase transformation accomplished on the inconsistent δ-{111} planes with hydride precipitation. The presented results are expected to provide solid experimental evidences for hydride stacking structure and new insights into hydride dissolution/precipitation behavior in zirconium alloys.