Hydrogen Content, Preoxidation, and Cooling Scenario Effects on Post-Quench Microstructure and Mechanical Properties of Zircaloy-4 and M5® Alloys in LOCA Conditions

Abstract

Previous papers pointed out the influence of long-term service exposures on the thermal-mechanical behavior of Zr alloys in LOCA conditions and, especially, the impact of in-service hydrogen pick-up on post-quench mechanical properties. Moreover, the oxide layer grown under in-service conditions was occasionally expected to have a protective effect against high temperature oxidation. Finally, the oxygen and hydrogen distributions within the prior-β layer appear as a key parameter with regard to the residual ductility of the alloy, especially as a function of the cooling scenario. The objective of the study presented here was to further investigate the influence of these parameters on the post-quench mechanical properties. Unirradiated Zircaloy-4 and M5® cladding tubes were consequently hydrided up to different concentration levels, then oxidized at high temperature (1000–1200°C) up to at least 10 % measured equivalent cladding reacted (ECR) and directly quenched to room temperature (RT). Ring compression tests (RCT), 3-point bending tests (3PBT) at RT and 135°C, as well as impact tests at RT were then performed to determine the evolution of the post-quench mechanical properties of Zircaloy-4 and M5® alloys with H content. Similarly, specimens preoxidized out-of-pile were also submitted to high temperature oxidation and direct quench, as well as to post-quench ring compression tests. Along with calculations of oxygen diffusion in the metal, results from those tests allowed us to estimate the assumed protective effect of the pretransient oxide layer. Finally, using specimens in the as-received condition or hydrided to typical end-of-life H contents, the effect of temperature history after oxidation at 1200°C was studied, i.e., at the end of the high temperature isothermal oxidation, samples were either submitted to direct quenching to RT or to slow cooling to different final quenching temperatures. It was thus demonstrated that the cooling scenario has a significant impact on the post-quench mechanical properties. All test samples were investigated by means of fractographic examinations to assess the type of failure mode. Moreover, a deep metallurgical analysis has been performed: SEM and image analysis were used for accurate phase thickness measurements, nuclear and electron microprobes for quantitative mapping of hydrogen and oxygen. It proved that the oxygen and hydrogen contents and their distribution in the prior-β layer have a first-order influence on the residual ductility. From all the results obtained on as-received and hydrided samples directly quenched from the oxidation temperature, it was then possible to derive a relationship between structural parameters, i.e., oxygen and hydrogen contents and thickness of the prior-β layer, and the post-quench impact properties at RT.