Effect of fast neutron fluence on the creep anisotropy of Zr–2.5Nb tubes

The in-reactor behaviour of internally pressurised capsules of Zr–2.5Nb tubes is analysed in detail to separate the stress dependent component of deformation (creep). It is found by a rigorous statistical analysis that the creep rate varies with fast neutron fluence. At 555 K the axial creep rate increases while the transverse creep rate decreases with fluence. At 588 K the creep rate in both the axial and transverse directions increases with fluence. It is also shown that the creep anisotropy ratio R, i.e., the ratio of axial to transverse creep rate for a pressurised tube, varies with fluence, stress and irradiation temperature. These findings are discussed in terms of the irradiation-induced evolution in microstructure. The possible impact of the evolution of the dislocation substructure is discussed with reference to a self-consistent polycrystalline model that takes into account the crystallographic texture and the grain interaction strains present in zirconium alloys. The lower temperature creep behaviour is consistent with an increase with fast fluence of the single crystal creep compliance related to prismatic dislocation climb and glide, or a decrease in the single crystal creep compliances relating to basal and pyramidal slip. The creep behaviour at the higher irradiation temperature is more complicated, and there may be an influence of phase changes as well as dislocation structure. It appears that all three eigenvalues describing the single crystal creep behaviour depend on fast fluence.