Overload fracture resistance of δ-hydrides grown at notch-tip in a zirconium alloy evaluated using synchrotron X-ray diffraction

• Zr-2.5Nb and δ-hydride present different strain build-up behaviour during overload • A higher hydride formation stress leads to lower overload fracture resistance • Strain relaxation at room temperature results from hydride fracture • Strain relaxation at 200 °C arises from localized matrix deformation at the notch tip • Localized Zr-2.5Nb deformation slows hydride strain build-up in overload at 200 °C Stress-oriented hydride precipitation in flaw tips presents a deleterious effect on the fracture resistance of Zr2.5Nb pressure tubes in CANDU® reactors, as well as in fuel cladding applications. The Canadian fitness-for-service guidelines determine that during pressure tube overload scenarios, where the stress applied is greater than the hydride formation stress, the stress-oriented hydrides should not crack, so as to prevent delayed hydride cracking (DHC) initiation. In this work, in situ Synchrotron X-ray diffraction was used to measure the strain of Zr-2.5Nb and stress-reoriented δ-hydrides grown at a notch during subsequent loading at room temperature or 200 °C. The effect of hydride formation stress was also discussed in terms of overload fracture resistance (OFR). The results indicated that increasing the hydride formation stress affected the stress-oriented hydride density and decreased the OFR. At room temperature, a 2×10 -3 lattice strain difference was seen between the hydrides and the Zr-2.5Nb matrix, wherein the hydride acted as the stiffer phase. A drastic change was seen for a test at 200 °C, where the matrix strain was 1×10 -3 higher than the hydride strain. Increasing the temperature to 200 °C also caused some dissolution of stress-oriented hydrides and encouraged plastic deformation of the Zr-2.5Nb matrix prior to hydride fracture, improving the OFR.