Effect of Nb on hydride embrittlement of Zr–xNb alloys

The effects of Nb on hydride Zr alloys were investigated. Various Zr alloys with different Nb content up to 2.0% were prepared in a sheet shape and charged with hydrogen up to 850 ppm. It was found that the fraction of recrystallized grains was reduced with increasing Nb content during the heat treatment. Intergrain and intragrain hydrides were tangled in unalloyed Zr, which has fully recrystallized grains. On the other hand, Nb containing Zr alloys that have partially cold-worked grains had only intergrain hydrides precipitated along the rolling direction. In tensile tests, elongation was decreased significantly with increasing hydrogen content for unalloyed Zr. However, the reduction of elongation was less significant for Nb-containing alloys. The softening of the Zr matrix was caused by hydrogen, which was arrested in cold-worked grain. And β-Nb precipitates, which have high solubility of hydrogen, retarded the precipitation of hydrides, thereby enhancing the resistance of hydride embrittlement. Fracture surface of the tensile tested specimen was observed by SEM. From the SEM observation, secondary cracks caused by hydrides were found on the fracture surface of the hydrogen-charged specimens. Cleavage facets were observed on the fracture surface of unalloyed Zr; however, a mixture of dimple and cleavage facets was observed on the surface of the Nb-containing alloys, which indicates that Nb increased the resistance to hydride embrittlement of Zr alloys.