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

The solid solubility of Nb in alpha-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 degrees C and 600 degrees 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 0.20-0.29 wt%. However, beta-Nb particles were not observed for these same alloys and were only seen when the Nb content was >= 0.49 wt%. Because beta-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%. (C) 2008 Elsevier B.V. All rights reserved.