Effects of oxygen and hydrogen on the microstructure and the mechanical behavior of zirconium alloys after treatment at high temperature

During hypothetical Loss-of-Coolant-Accident (LOCA) scenarios in pressurized water reactors, zirconium-based fuel claddings can be exposed to high temperatures (up to 1200 C) and, under certain conditions, absorb locally a significant amount of hydrogen (up to 3000 wppm) and of oxygen (up to 1 wt.%). This work aims to study the isolated and combined effects, which have been little investigated hitherto, of oxygen and hydrogen in high contents, on the metallurgical evolutions and the mechanical behavior of two industrial zirconium alloys (Zircaloy-4 and M5Framatome) during and after cooling/quenching from the βZr temperature domain (≥ 700 C). The first part of this work consisted of producing 'model' materials, from cladding tube sections and plates, homogeneously charged with oxygen, up to 1 wt.%, and with hydrogen, up to 7000 wppm. The phase transformations occurring on cooling from the βZr domain in the materials charged with hydrogen and the changes in chemical composition and lattice parameters of the phases were then quantified using several techniques such as calorimetry, in situ neutron diffraction during cooling from 700 C, neutron and X-ray diffraction at room temperature, electron microprobe, μ-ERDA and EBSD. The experimental results were compared with thermodynamic predictions, taking into account all of the chemical elements in the materials. In addition to the stable phases expected at equilibrium, the presence of metastable phases such as γZrH hydrides, and βZr phase enriched in H and Nb in the case of M5Framatome, as well as of a significant amount of hydrogen remaining in solid solution within the αZr, was pointed out at room temperature at the end of cooling. The mechanical properties of the (prior-)βZr phase were characterized by performing uniaxial tensile tests at temperature between 700 and 30 C on cooling from the βZr domain, on materials charged with hydrogen and/or oxygen. The results showed that the mechanical behavior and the failure mode strongly depend on the testing temperature and on the hydrogen and oxygen contents. Empirical correlations and a phenomenological model have been proposed to describe the macroscopic ductile-brittle transition temperature, the evolutions of the mechanical characteristics and the plastic behavior of the material (in the case of ductile macroscopic failure), as a function of temperature and contents of oxygen and hydrogen. Observation of the fracture surfaces, μ-ERDA and electron microprobe analyses and a tensile test performed in situ under SEM highlighted the heterogeneity of the deformation and the failure mode at the local scale, due to the effects of the partitioning of chemical elements, especially of hydrogen and oxygen, during the phase transformations. (author)