Zefeng Yu

We have investigated proton irradiation induced Nb redistribution in Zr-xNb alloy (x = 0.4, 0.5, 1.0) by using scanning transmission electron microscopy (STEM) and atom probe tomography (APT). We have found by STEM that 2MeV proton irradiation at 350°C induces precipitation of Nb-rich needle-like particles in the Zr matrix. Initially without irradiation effect, the Zr matrix only contains βNb and Laves phase native precipitates. After irradiation, in addition to the needle-like particles, we have also found by APT that Fe- and Nb-rich nanoclusters (less than 20 nm diamter) are present in the Zr matrix for Zr-0.5Nb, 1000°C annealed Zr-0.5Nb and Zr-1.0Nb. Despite different irradiation dose level, the total Nb content in the entire APT tip for all the samples ranges from 0.24 – 0.40 at. %, which is below the maximum solubility limit of 0.6 at. % Nb in Zr solid solution. After cluster removal from the Zr matrix of the irradiated samples, Nb concentration in the Zr solid solution is shown to significantly decrease with irradiation dose, which is suspected to be responsible for the improved corrosion resistance of ZrNb alloy in the reactor environment at high burnup.

Proton irradiation induced Nb redistribution in Zr-xNb alloys (x ¼ 0.4, 0.5, 1.0 wt%) has been investigated using scanning transmission electron microscopy/energy dispersive X-ray spectroscopy (STEM/EDS). ZrxNb alloys are mainly composed of Zr matrix, native ZreNbeFe phases, and b-Nb precipitates. After 2 MeV proton irradiation at 350 C, a decrease of Nb content in native precipitates, as well as irradiationinduced precipitation of Nb-rich platelets (135 ± 69 nm long and 27 ± 12 nm wide) were found. Nb-rich platelets and Zr matrix form the Burgers orientation relationship, [111]//[2110] and (011)//(0002). The platelets were found to be mostly coherent with the matrix with a few dislocations near the ends of the precipitate. The coherent strain field has been measured in the matrix and platelets by the 4D-STEM technique. The growth of Nb-rich platelets is mainly driven by coherency and dislocation-induced strain fields. Irradiation may both enhance the diffusion and induce segregation of interstitial Nb to the ends of the irradiation induced platelets, further facilitating their growth.

Due to the absence of significant in-reactor corrosion acceleration, commercial ZrNb alloys such as ZIRLO, M5, and E110 have replaced Zircaloy-4 as the fuel cladding in Light Water Reactors. Although the enhanced corrosion resistance of ZrNb alloys has allowed significant increase in fuel burnup over the recent decades, the coupling effect of irradiation and corrosion has yet to be mechanistically understood. Recently, a zirconium alloy corrosion kinetic model, called Couple-Current-Charge-Compensation (C4) model, has predicted that less Nb content in the Zr solid solution leads to a decrease in oxidation kinetics. Therefore, to understand the effect of irradiation induced Nb redistribution on corrosion kinetics, this study focuses on using atom probe tomography (APT) and transmission electron microscopy (TEM) to precisely characterize microstructure and microchemistry evolution as irradiation dose. In order to simulate in-reactor irradiation, Zr-1.0Nb have been irradiated using 2-MeV protons up to 1 dpa at 350 °C. (S)TEM and EDS have confirmed the formation of irradiation induced Nb-rich precipitates. APT experiments have been performed on both unirradiated and irradiated Zr-xNb alloys. 3D reconstruction of unirradiated needles show homogeneous Nb distribution, whereas all irradiated samples shows nano-scaled Nb rich clusters. Careful cluster analysis on irradiated sample demonstrates a significant reduction of Nb% in solid solution upon irradiation, which correlates with the C4 model’s prediction. Thus, this study represents the first attempt to measure Nb redistribution upon irradiation at the atomic scale and results are critical to fully understand the coupling effect of irradiation and corrosion on ZrNb alloys.