Irradiation reduces the anisotropy of strength in zirconium

Abstract

Zirconium alloys, widely used as fuel cladding and pressure tubes in nuclear reactors, exhibit strong mechanical anisotropy due to their hexagonal close-packed (HCP) structure and manufacturing-induced textures. While irradiation hardening in zirconium has been well studied, irradiation’s impact on mechanical anisotropy, especially in polycrystals, remains unclear. This work develops a mechanism-informed, bottom-up model to investigate how irradiation weakens strength anisotropy by decoupling the effects of interactions between dislocation and irradiation-loops, slip system hardening, and texture. First, the individual dislocation-loop interaction mechanisms in Zr have been systematically studied using discrete dislocation dynamics (DDD) simulations, which show good agreement with molecular dynamics simulations. Through large-scale DDD simulations of dislocation-loop ensembles, we quantify slip system hardening and reveal that the higher occurrence of helical turns in the prismatic slip system results in stronger irradiation hardening compared to its basal counterpart. A theoretical model is then developed, accurately predicting the reduction in strength anisotropy for both single-crystal and polycrystalline zirconium. The predicted ratio of maximum to minimum yield stress under different crystallographic orientations decreases from $\sim$3 to $\sim$1.5 for single crystals, while the ratios for yield stress along axial (AD), tangential (TD), and radial (RD) directions of ${\sigma }_{\mathrm{TD}}/{\sigma }_{\mathrm{AD}}$ and ${\sigma }_{\mathrm{TD}}/{\sigma }_{\mathrm{RD}}$ decrease from 1.70 and 1.28 to nearly 1.0 for polycrystalline pressure tubes at low irradiation doses (<1 dpa). Furthermore, the model is applied to investigate the statistical distribution of dislocation channels under loading along AD, TD, and RD in irradiated cladding materials, showing good agreement with TEM observations. This work offers critical insights into irradiation hardening in zirconium, guiding alloy design and texture optimization for improving safety and performance in nuclear reactors.