Static autoclave tests in steam at 673 K and tensile tests at room temperature were conducted to investigate the effects of alloying elements, such as tin (0.2 to 1.5 wt%), niobium (0.05 to 1.0), iron (0.2 to 0.6), chromium (0.1 to 0.4), molybdenum (0.05 to 0.5), and vanadium (0.1 to 0.5), and heat treatments on the corrosion resistance and mechanical properties of niobium-containing zirconium-base alloys, Moreover, the characterization of second-phase particles was carried out by scanning transmission electron microscopy (STEM).
The corrosion resistance of zirconium-base alloys compared with that of Zircaloy-4 (nominal composition: Zr-1.5Sn-0.2Fe-0.1Cr) was significantly improved by the reduction of the Sn content by 0.5 wt% and by a small addition of Nb (about 0.05 to 0.2 wt%). However, the decrease in solute Sn atoms degraded mechanical properties such as ultimate tensile strength and 0.2% yield strength.
The increase of the total content of Fe and Cr from 0.3 to 0.7 wt% improved the mechanical properties without affecting the corrosion resistance. Small additions of Mo and/or V resulted in a further improvement of mechanical properties. Thus it is possible to design alloys with equivalent mechanical properties and superior corrosion resistance to Zircaloy-4.
The corrosion resistance increased and the average size of precipitates decreased from 0.1 to 0.05 μm with the decrease of the Fe/Cr ratio from 6.0 to 0.5 in the niobium-containing zirconium-base alloys.
Detailed investigations on the effects of annealing temperatures on the corrosion behavior of the zirconium-base alloys were also performed. Results showed that increasing the Nb content of the alloys lowered the most suitable annealing temperature for corrosion resistance. These experiments led to the development of niobium-containing zirconium-base alloys with high corrosion resistance.