Circumferential creep properties of stress-relieved Zircaloy-4 and Zr-Nb-Sn-Fe cladding tubes

In this study, the circumferential creep behaviors of stress-relieved Zircaloy-4 and Zr-Nb-Sn-Fe alloys were investigated. The out-of reactor creep resistance of Zircaloy-4 was found to be better than that of the Zr-Nb-Sn-Fe alloy in the temperature and stress range of this study, suggesting that the solution strengthening effect of Sn is more effective in restraining dislocation motion than the precipitate strengthening effect of Nb. Zircaloy-4 and Zr-Nb-Sn-Fe have stress exponents between 6.5 and 7.5 in the lower stress region. The stress exponent decreased to about 3 for Zircaloy-4 and to about 4.2 for Zr-Nb-Sn-Fe in the high stress region. The activation energy increased from 240 to 270 kJ/mol for Zircaloy-4 and Zr-Nb-Sn-Fe as the applied stress increased from 80 to 150 MPa. The creep activation energy observed in this study is close to those of creep for Zircaloy-2 and Zr. The lower stress exponent in the higher stress region for Zircaloy-4 with a relatively higher Sn content compared to Zr-Nb-Sn-Fe alloy may be supported by the fact that the drag force of Sn atoms acting on dislocations increases with increase of Sn content. Larson-Miller Parameter (LMP) decreased with the increase of applied stress, and Zircaloy-4 has a higher LMP than the Zr-Nb-Sn-Fe alloy, compatible with the lower creep rate in Zircaloy-4.