The Effect of Iron on Dislocation Evolution in Model and Commercial Zirconium Alloys

Although the evolution of irradiation-induced dislocation loops has been well correlated with irradiation-induced growth phenomena, the effect of alloying elements on this evolution remains elusive, especially at low fluences. To develop a more mechanistic understanding of the role iron has on loop formation, we used state-of-the-art techniques to study a proton-irradiated Zr-0.1Fe alloy and proton- and neutron-irradiated Zircaloy-2. The two alloys were irradiated with 2-MeV protons up to 7 dpa at 350°C and Zircaloy-2 up to 14.7 × 10²⁵ n · m⁻², approximately 24 dpa, in a boiling water reactor at approximately 300°C. Baseline transmission electron microscopy showed that the Zr₃Fe secondary-phase particles in the binary system were larger and fewer in number than the Zr(Fe,Cr)₂ and Zr₂(Fe,Ni) particles in Zircaloy-2. An analysis of the irradiated binary alloy revealed only limited dissolution of Ze₃Fe, suggesting little dispersion of iron into the matrix, while at the same time a higher <a>-loop density was observed compared with Zircaloy-2 at equivalent proton dose levels. We also found that the redistribution of iron during irradiation led to the formation of iron nanoclusters. A delay in the onset of <c>-loop nucleation in proton-irradiated Zircaloy-2 compared with the binary alloy was observed. The effect of iron redistributed from secondary-phase particles because of dissolution on the density and morphology of <a> and <c> loops is described. The implication this may have on irradiation-induced growth of zirconium fuel cladding is also discussed.