In binary Zr-Nb alloys (Zr—1% Nb and Zr-2.5% Nb), the microstructure is usually in a metastable state due to the thermomechanical processing in the upper a range or in the a + p domain. Indeed, at this relatively low temperature (around 580 °C), the atomic mobility is low and the equilibrium state cannot be reached in reasonable time. After cooling, the matrix is therefore supersaturated in Nb and the composition of secondary phases (Nb rich) still corresponds to the high-temperature chemical com­position. It is indeed shown by Toffolon-Masclet eta/.85 that a Zr—1% Nb-O alloy that has undergone a final heat treatment at 580 °C for a few hours can still evolve toward its thermodynamic equilibrium after 10000 h of heat treatment at 400 °C.

Under irradiation, it is observed that the micro­structure of Zr-Nb alloys is not stable and very fine Nb-rich precipitates, with diameter of a few nanometers, are observed in very high density (Figure 11). This precipitation of Nb from the super­saturated matrix is observed in any type of binary alloys: in Zr—1% Nb such as M5™(86) and E110(12,87) as well as Zr—2.5% Nb.88 This needle-like precipita­tion has been studied mainly by TEM, and also by small angle neutron scattering (SANS) analyses.86

Simultaneously, a noticeable decrease of Nb content in the matrix occurs.89

This precipitation is due to an enhanced mobility of Nb atoms under irradiation due to the very high vacancy concentration created by irradiation. This enhances the Nb mobility and allows the rapid evolu­tion of the microstructure toward its thermodynamic equilibrium, leading to precipitation of very fine Nb — rich precipitates in Zr—Nb binary alloys.

In Zr—Nb alloys, the Nb-rich phases also undergo chemical changes under irradiation. Indeed, it is shown that the o phase, obtained in Zr—2.5Nb by transformation of the p-Nb after extrusion, disap­pears and transforms into p-Nb.60 For the p-Nb phase and in the case of M5™ alloys, an evolution of the chemical composition under irradiation has also been observed, but the p-Nb precipitates still remain fully crystalline even after six PWR cycles of irradiation (70 GWd t_ ). Only a decrease in Nb content with a small increase in the size of the precipitates has been noticed86 (Figure 11). The same has been obtained for E110 and E635 Russians alloys, where p-Nb precipitates are altered in com­position to reduce the Nb content from 85—90% to 50%.12

Moreover, for the Zr(Nb, Fe)2 Laves phases with hcp structure found in E635 and E110 alloys, it seems that a release of iron atoms into the matrix from the

precipitates has occurred after irradiation, leading to the transformation into p-Nb particles with bcc

12,89

structure.