Zirconium-niobium alloys, and especially the Framatome Zr-1%Nb-0 (M5^™) alloy [1], are now considered to be the reference materials for application to the cladding tubes and structural components of PWR fuel assembly. Low contents of iron are present as impurity or intentionally introduced in such alloys, and, consequently, the precipitation of Zr-Nb-Fe intermetallic phases occur due to the very low solubility of iron in the α-Zr matrix (less than ∼100 ppm). Taking into account that these intermetallic precipitates may influence some of the alloy properties (corrosion resistance, irradiation growth …) and that the crystallographic structure, the chemical stoichiometry, and the temperature of precipitation/dissolution of these phases are not well established, it appeared necessary to get a better insight of the Zr-Nb-Fe-(O) phase diagram. For that reason, both thermodynamic calculations and associated experimental works were performed.

Different alloys with a niobium content ranging from 0.5 to 2 wt% and a Fe content from 0.03 to 0.75 wt% are considered. Two ternary intermetallic phases are present in these alloys: one has a hexagonal crystallographic structure (hc: a ∼ 0.54 nm, c ∼ 0.86 nm) and the other one has a face-centered cubic structure (fcc: a ∼ 1.21 nm). The chemical stoichiometry of both phases is determined by energy and wave dispersive spectrometries (EDS and WDS). The phase existence temperature range is determined by transmission electron microscopy (TEM) performed after heat treatments at different temperatures, by calorimetry, and by specific neutron diffraction experiments under heating/cooling. The results obtained using these experimental techniques show a good agreement. Moreover, the crystallographic space groups of the two intermetallic phases are determined using electron, X-ray, and neutron diffraction facilities in order to obtain a thermodynamic description of the Zr-Nb-Fe system. To assess the results obtained on dilute alloys (Zr > 97%), two Nb and Fe-enriched alloys were fabricated by CEZUS, with an intermetallic volume fraction higher than 80%.

Finally, a first thermodynamic calculation of the Zr-Nb-Fe(O) system is proposed, using all the experimental data obtained. Calculated and experimental data are then compared at different temperatures.