Microstructural characterization of U-7.5Nb-2.5Zr alloy after ageing and constrained fatigue

U-7.5Nb-2.5Zr cladded in Zircaloy-4 is one of the most studied fuel prototypes. The Nb and Zr are added to the U to stabilize the body-centered cubic (BCC) gamma phase and grant mechanical and swelling resistance. The U-7.5Nb-2.5Zr undergoes the gamma to α^′′ phase transformation, generating compressive stresses due to the volume reduction. The α^′′ phase also can transform to a combination of α + γ₂ phase (equilibrium phases), which are known to be hard and brittle. This work had the objective to test the effect of ageing the gamma to α^′′ phase in thermal cycling of a U-7.5Nb-2.5Zr cladded in Zircaloy-4 part. A co-laminated specimen was aged and thermally cycled in a dilatometry experiment. The samples were characterized through X-ray diffraction and metallography. The results show that the gamma to α^′′ phase transformations occur at the begin- ning of the initial ageing, followed by stress relief. During the cycling, the sample demonstrated non-equal thermal strains and presented fractures along with the U matrix.

Keywords:
U-7.5Nb-2.5Zr; thermal fatigue; phase transformation; fuel element