Multi-scale Microstructural Characterization and Precipitation Mechanism of Hydrides in Zr-2.5Nb Alloy Pressure Tube

The integrity and performance of Zr-2.5Nb alloy pressure tubes in nuclear reactors are significantly influenced by the behavior of hydrides within the material. A comprehensive understanding of the hydride distribution, orientation relationship, and precipitation mechanism is crucial for predicting and mitigating potential degradation in these critical components. This study presents a multi-scale characterization approach, integrating scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy, to investigate the mesoscale, microscale, and atomic-scale features of hydrides in Zr-2.5Nb alloy pressure tubes. The results reveal that hydrides predominantly form along α/α grain boundaries and α/β phase boundaries, with minimal intragranular presence. Distinct crystallographic orientation relationships between interfacial and intragranular hydrides and the α-Zr matrix are identified. Interfacial hydrides (γ-ZrH, δ-ZrH_1.66, and ε-ZrH₂) exhibit a strong hereditary orientation relationship with the α-Zr matrix, characterized by <112¯0>_α//<110>_γ//<110>_δ//<111>_ε and {0001}_α//{111}_γ//{111}_δ//{101}_ε. Intragranular hydrides maintain the relationship of <112¯0>_α//<110>_γ//<110>_δ//<110>_ε and {0001}_α//{200}_γ//{200}_δ//{200}_ε. High-resolution transmission electron microscopy observations uncovered a continuous slip of Shockley partial dislocations within the α-matrix, originating from 60° mixed-type <a> perfect dislocations on each basal plane. This slip, coupled with an atomic shuffle mechanism, facilitates the B-type phase transition, leading to the precipitation of δ-ZrH_1.66 with a face-centered cubic structure.