The role of Fe on the solubility of Nb in α-Zr

Abstract

The solid solubility of Nb in α-Zr is an important parameter that has a potential impact on the corrosion properties of Zr–Nb alloys at reactor operating temperatures, i.e. below the monotectoid temperature. Work on dilute Zr–Nb alloys has shown that Fe is a common impurity that confounds the assessment of the solid solubility limit for Nb in Zr. This is because Fe has a very low solubility limit and it forms precipitates with both Nb and Zr. To assess the effect of Fe on the phases formed in the binary Zr–Nb alloy system, alloys containing 0.1–0.7 wt% Nb and <11 to 470 wt ppm Fe were heat-treated at temperatures between 575 °C and 600 °C and examined by transmission electron microscopy. Results indicate that, even at a concentration ⩽ 24 ppm, Fe readily combines with Nb to form precipitates in the alloys with Nb contents in the range of 0.20 to 0.29 wt%. However, β-Nb particles were not observed for these same alloys and were only seen when the Nb content was ⩾ 0.49 wt%. Because β-Nb particles were not found in the 0.29 wt% Nb alloy and the precipitation was estimated to have a negligible effect on the amount of Nb remaining in solution (reduced by <0.001 wt%), it is proposed that the solubility limit of Nb in a true binary Zr–Nb alloy would be between 0.29 and 0.49 wt%.