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

Abstract

While the evolution of irradiation-induced dislocation loops is 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 of Fe on loop formation, state-of-the-art techniques have been used to study a proton-irradiated Zr-0.1Fe alloy and proton-and neutronirradiated Zircaloy-2. The two alloys have been irradiated with 2 MeV protons up to 7 dpa at 350 °C and Zircaloy-2 up to 14.7 x10 25 n m-2 , ~24 dpa, in a BWR at ~300 °C. Baseline TEM characterisation showed that the Zr 3 Fe secondary phase particles in the binary system are larger and fewer in number than the Zr(Fe, Cr) 2 and Zr 2 (Fe, Ni) particles in Zircaloy-2. Analysis of the irradiated binary alloy revealed only limited dissolution of Ze 3 Fe suggesting little dispersion of Fe into the matrix while at the same time a higher a-loop density is observed in comparison to that in Zircaloy-2 at equivalent proton dose levels. It was also found that the redistribution of Fe during irradiation leads to the formation of Fe nanoclusters. A delay in the onset of c-loop nucleation in proton-irradiated Zircaloy-2 compared to the binary alloy was observed. The effect of Fe redistributed from secondary phase particles, due to dissolution, on the density and morphology of a-and c-loops is described. The implication this may have on irradiation-induced growth of Zr fuel cladding is also discussed.

Investigating the thermal stability of irradiation-induced damage in a zirconium alloy with novel in situ techniques

“…The binary 1.5 dpa Zr-0.1Fe alloy used in this work has been investigated in detail as part of another study, looking into the effect of redistributed Fe from SPPs on dislocation densities [20]. It was found that this alloy contains ~1-2 μm coarse Zr 3 Fe secondary phase particles (SPPs) unevenly distributed throughout the sample.…”

“…Close to these SPPs, which have undergone some irradiation-induced dissolution, Fe has been observed to segregate to pyramidal loops with a likely Burgers vector of ! " 1123 [20].…”

“…Specially fabricated Zr-Fe binary alloys have been used in this study to eliminate some of the complexity inherent in commercial alloy systems, whilst retaining key aspects of material evolution under irradiation, allowing the effect of specific microstructural features to be evaluated. Proton irradiation has previously been shown to be a good surrogate for neutron-irradiation in Zr alloys, providing similar damage structures and chemical evolution [10,[20][21][22][23]. Proton irradiation allows for the design of systematic studies, achieving comparable damage levels in short timescales with low residual activity compared to neutron-irradiated samples.…”

Primary damage production in the presence of extended defects and growth of vacancy-type dislocation loops in hcp zirconium

“…Fe segregation can stabilize a-type loops as evidenced by the results above, which may impede the transition from the aligned a-type loops to a coarse c-component loop. This may explain why c-component loop nucleation is delayed in Zr-Fe alloys, in comparison to pure Zr, as reported by Topping et al[83]. In addition, the enhanced stabilization of atype loops by the presence of Fe could prevent the absorption of a-type loops by the GBs.…”

“…The collision cascade induced a rotation and gradual glide towards the GB, which led to eventual absorption by the GB as illustrated in Figure 8 (e). The importance of Fe alloying on the formation of -component loops was discussed by Carlan et al [82], and a recent study by Topping et al [83] suggested that an increase in Fe content in a Zr alloy delays -loop nucleation. Here, the effect of Fe on the loop stability is explored.…”