Creep properties of annealed Zr–Nb–O and stress-relieved Zr–Nb–Sn–Fe cladding tubes and their performance comparison

Creep properties of annealed Zr–Nb–O and stress-relieved Zr–Nb–Sn–Fe cladding tubes were studied and compared. The creep rates of the annealed Zr–Nb–O alloy were found to be greater than those of the stress-relieved Zr–Nb–Sn–Fe alloy. Zr–Nb–O alloy was found to have stress exponents of 5–7 independent of stress level whereas Zr–Nb–Sn–Fe alloy exhibited the transition of the stress exponent from 6.5 to 7.5 in the lower stress region to ∼4.2 in the higher stress region. The reduction of stress exponent at high stresses in Zr–Nb–Sn–Fe can be explained in terms of the dynamic solute–dislocation effect caused by Sn atoms. The constancy of stress exponent without the transition was observed in Zr–Nb–O alloy, supporting that the decrease of the stress exponent with increasing stress in Zr–Nb–Sn–Fe is associated with Sn atoms. The difference of creep life between annealed Zr–Nb–O and stress-relieved Zr–Nb–Sn–Fe is not large considering the large difference of strength level between annealed Zr–Nb–O and annealed stress-relieved Zr–Nb–Sn–Fe. The better-than-expected creep life of annealed Zr–Nb–O alloy can be attributable to the combined effects of creep ductility enhancement associated with softening and the decreased contribution of grain boundary diffusion due to the increased grain size.