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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.

An alternative procedure for the determination of the FLC0 value, the limit strain value corresponding to the plane strain mode of the Forming Limit Curves (FLC), a critical parameter in the sheet formability analysis, is suggested and compared with conventional Nakazima simulation tests. The procedure was tested using two different materials: interstitial-free quality steel (IF) and a spheroidized SAE 1050 steel. The intrinsic tensile test, in a near plane strain state, was performed using a small number of samples, with dimensions suggested by the literature. The results were checked against Nakazima test results using the same materials. The plane strain test was reliable in determining consistent FLC0 values and should be preferred since it is not affected by the geometric aspects and by friction, which do affect the Nakazima test. The reliability of the FLC0 values obtained by near plane strain was also corroborated through comparison with literature data.