Impact of hydrogen on plasticity and creep of unirradiated zircaloy-4 cladding tubes

This paper deals with the impact of hydrogen on plasticity and creep of unirradiated CWSR Zircaloy-4 cladding tubes. Specimens with low tin content (approximate to 1.3%) were charged with hydrogen between 100 and 1100 ppm using gaseous technique. Uniaxial and biaxial mechanical tests (tension and creep) were conducted on as-received and hydrided material at room and elevated temperatures (350 and 400 degrees C). Analysis of the results showed that both precipitated hydrides and hydrogen in solid solution play a role in the viscoplastic behavior of the Zircaloy-4 cladding. With respect to plasticity, hydrogen affects the hardening kinetics by decreasing the yield stress, increasing the linear hardening coefficient, and enhancing the dynamic recovery kinetics, especially in the hoop direction. It has no effect on the strain anisotropy. Hydrogen increases the creep resistance. The creep rate of the hydrided material decreases by a factor of 2 to 4 depending on the temperature, the hydrogen concentration, and the stress biaxiality. Potential mechanisms such as localized plasticity in the vicinity of hydrides are discussed, as well as the composite effect induced by hydrides acting like inclusions within the ductile metal matrix, and the respective role of hydrogen in solid solution and of hydrides on the dislocation mobility and recovery. Even if no firm conclusions regarding those mechanisms have been reached, the results of this study are already useful in predicting the mechanical behavior of hydrided Zircaloy-4 Cladding under handling and long-term interim dry storage conditions.