Effect of hydrogen and hydrides on the viscoplastic behaviour of the recrystallized zircaloy-4

Zircaloy-4 is the main material of PWR fuel assemblies. In service as during the storage, the integrity of these compounds has to be guaranteed in spite of the presence of hydrogen (in solution in the zirconium matrix) and of hydrides (which precipitate when the amount of hydrogen is higher than the solubility limit). The aim of this work is to characterize the hydrogen and hydrides effect on the viscoplastic behaviour of the non irradiated recrystallized zircaloy-4. The presence of hydrogen in solid solution induces a decrease of the mechanical properties: the creep kinetics are then increased and the tensile stresses decreased. This decrease is particularly visible in conditions of oxygen/dislocations dynamic interactions (revealed on the material without hydrogen). The advanced hypothesis, strengthened by the atomic simulation results, is that the hydrogen facilitates the dislocations movement, in diminishing the effects of anchoring by the interstitials, and/or in increasing the intrinsic mobility of dislocations. The hydrides effect induces a hardening of the material (decrease of the creep kinetics, increase of the tensile stresses and of the relaxed stresses) compensating the decrease by hydrogen. The hardening mechanism is due to an increase of the internal constraints, determined by load-unload tests. For the very weak plastic deformations, the hydrides are an obstacle to the dislocations gliding. They are then passed (that corresponds to a saturation of the internal constraint). The TEM observations as well as the results obtained on the titanium indicate that the precipitates are then submitted to a deformation mechanism. (O.M.)