Blog

The role of niobium in zirconium alloys

The stable oxide of niobium is Nb2O5, and the valence of niobium ion is +5. The niobium ion is solid-dissolved in Zr with the valence of +5 and replaces the zirconium ion with the valence of +4, and the oxygen ion vacancy is reduced. The reduction of oxygen vacancies can stabilize the tetragonal ZrO2 and hinder the migration of oxygen ions.

Nb has a relatively high strengthening effect on zirconium. A stable β phase is produced during quenching. The increase in the strength of the zirconium-niobium alloy depends on the transformation of β solid solution to martensite (α' phase). As the Nb content increases, more β phases are fixed, and the strength decreases.

In the Zr-Nb phase diagram, Nb and β-Zr are continuous solid solutions. At the single precipitation temperature (590-610℃), the Nb content in α-Zr is 1.1%, while the Nb content in α-Zr at low temperature The solubility is very small, so in the Zr-Nb alloy, the alloy can be strengthened by β-α phase transformation and α-Zr aging treatment.

Zr-1Nb Russia is used as a PR cladding material.

Zr-2.5Nb was used as a pressure tube material for heavy water reactors in the former Soviet Union. It can be strengthened by quenching aging and cold working.

After Zr-2.5Nb quenching and aging, β-Nb precipitates from the supersaturated α-Zr, and the fine and uniform distribution of β-Nb is beneficial to improve the corrosion resistance.

However, after welding, the uniform distribution of the precipitated phases will be destroyed and the corrosion resistance performance will be impaired. This shortcoming can be recrystallized and annealed at 500°C to make the structure better.

Nb can eliminate the harm of C, Ti, Al and other magazines on the corrosion resistance of zirconium, and reduce the amount of hydrogen absorption.

The addition of Cu to Zr-2.5Nb can improve its high-temperature corrosion resistance, so the spacer ring material of the pressure pipe of the heavy water reactor is made of Zr-2.5Nb-0.5Cu alloy.