Unveiling the characteristics of the residual point defects of collision cascade in Zr-xNb binary system: A molecular dynamics study

Zirconium-based alloys are commonly used as fuel cladding materials for light-water reactors. As an alloying element, niobium can further improve the performance of Zr alloys. However, the mechanism of the Nb effect on the irradiation damage (e.g., the irradiation-induced precipitation of Nb-rich clusters) of the alloy is still unclear and should resort to atomic-scale modeling. In this work, we unveiled the characteristic of the residual point defects produced at the end of the collision cascade caused by primary knock-on atoms (PKAs) in the Zr-xNb binary alloy through many molecular dynamics simulations. The effects of PKA directions (<0001> and <1010>), PKA energies (2 keV, 5 keV, 10 keV, 20 keV, 30 keV, and 50 keV), temperatures (300 K, 600 K, and 800 K), and the initial Nb contents in the alloy (cNb = 0 at.%, 0.5 at.%, 1 at.%, and 2 at.%) on the remaining Frenkel pairs, the displaced atoms, the composition of these point defects, the size and number of clusters, and the composition of the clusters with different-size have been studied. Several concepts were clarified from these simulation results. A special focus of this work is on the Nb proportion in the point defects and their clusters. It is worth emphasizing that the interstitial-Nb proportion in the interstitials and their clusters is far greater than the initial Nb content in the alloy, which can be the nucleate center of Nb-rich precipitation particles. This distinct feature may be solving the experimental puzzle of radiation-induced Nb precipitates/clusters of Nb-containing Zr alloys.