Thermal evolution of hydrides and subsequent mechanical response in hydrogen-charged zirconium alloys

Abstract

C7 zirconium alloy hydrogen-charged to 800 ppm was annealed at 470 °C, 490 °C and 510 °C for 3 h to evaluate the effects of annealing on hydrogen-induced damage and mechanical performance. The effects of annealing temperature on hydride morphology, microstructural evolution and tensile properties were systematically investigated. Hydride morphology and matrix microstructure were characterized by SEM-BSE and EBSD, respectively, and circumferential tensile tests were conducted at room temperature and 350 °C. Fine plate-like and strip-like hydrides were observed after annealing at 470 °C, with a banded but non-segregated distribution. Hydrides coarsened and aggregated markedly at 490 °C, forming continuous or semi-continuous layers along grain boundaries and interconnected networks within grains, coinciding with a lower recrystallized fraction. The hydride distribution became more discontinuous and homogeneous at 510 °C, with acicular hydrides observed after cooling, consistent with possible partial dissolution during annealing followed by re-precipitation during cooling. Room temperature tensile tests showed that among the annealed conditions, 510 °C produced the highest elongation to failure of 32.42% and ductile-dominated mixed fracture. During tensile testing at 350 °C, the ultimate tensile strength was only slightly affected by annealing temperature, whereas the highest elongation of 41.31% was obtained after annealing at 490 °C, likely reflecting the combined effects of the retained substructured matrix, hydride distribution, and enhanced plastic deformation at 350 °C. Overall, among the annealing conditions examined, 510 °C for 3 h provided the most favorable balance of room temperature ductility, microstructural homogeneity, and strength in hydrogen-charged C7 zirconium alloy.