Hydride Induced Degradation of Mechanical Properties of Zr-2.5Nb Alloy

Tubes made of Zr-2.5Nb alloy are used in pressurized heavy water reactors as pressure boundaries for hot coolant and are called pressure tubes, which are subjected to slow aqueous corrosion resulting in nascent hydrogen generation. Though most of the nascent hydrogen evolved recombines to form molecular hydrogen and is unable to enter into the pressure tubes, a part of the hydrogen is picked up by the pressure tube material. Once the solid solubility is exceeded brittle hydrides of plate-shaped morphology form that make the host matrix brittle. Two forms of hydride embrittlement have been recognized for Zr-alloys. These are gross and localized hydride embrittlement. For the manifestation of gross hydride embrittlement, a minimum volume fraction of hydride is required. The degree of gross hydride embrittlement is significantly affected by the hydride plate orientation. The localized form of hydride embrittlement occurs for hydrogen concentration below the solid solubility limit and is caused by hydrogen migration to a localized spot. Once the local solid solubility is exceeded hydride precipitation takes place. Various aspects of hydride embrittlement of Zr-alloys and length scales involved are discussed.