Creep Anisotropy in Nb-Bearing Zirconium Alloys: A Comparative Study of ZIRLO® and HANA-4

Zirconium-based alloys remain essential for nuclear fuel cladding due to their excellent mechanical properties and radiation resistance. Among them, ZIRLO® and HANA-4—both Nb-containing alloys—exhibit distinct microstructures that significantly affect their biaxial creep behavior under reactor-relevant conditions. This study presents a comparative analysis of their anisotropic creep response in both cold-worked stress-relieved (CWSR) and recrystallized (Rx) states, with a focus on the role of crystallographic texture and grain morphology in governing deformation mechanisms.

Biaxial creep tests were performed on internally pressurized cladding tubes subjected to axial loading, with hoop-to-axial stress ratios (σθ/σz) ranging from 0 to 2. Anisotropy was quantified using parameters derived from the modified Hill criterion, along with the formability parameter (B), to characterize material behavior under multiaxial stress states. Creep loci constructed at constant energy dissipation levels revealed clear deviations from isotropic behavior, dependent on both alloy type and processing condition.

Electron backscatter diffraction (EBSD) was employed to assess crystallographic orientation distribution functions (CODFs) and grain aspect ratios, enabling direct correlation between grain shape anisotropy and directional creep performance. The influence of temperature and applied stress on creep kinetics was also examined to elucidate rate-controlling mechanisms.

The results highlight substantial differences in anisotropic creep behavior between ZIRLO® and HANA-4, underscoring the critical impact of microstructural evolution and processing history. These findings offer valuable guidance for tailoring zirconium alloys to improve dimensional stability and enhance the long-term performance of nuclear fuel cladding.