Investigations of the Microstructure and Mechanical Properties of Prior-β Structure as a Function of the Oxygen Content in Two Zirconium Alloys

Abstract

It is now well acknowledged that, after a prototypical loss of coolant accident (LOCA) transient, the resultant mechanical properties of fuel cladding tubes depend strongly on the oxygen content of the residual prior-β layer, as this phase is the only metallic part of the high-temperature oxidized cladding that may show some residual ductility. The aim of this study is to obtain relevant information on the evolution of the mechanical properties, on the one hand, of the prior-β structure as a function of the oxygen content, assuming that there is a critical oxygen content that leads to a ductile-to-brittle failure mode transition at low testing temperatures (20–135°C); and on the other hand, of the α(O) structure as a function of the oxygen content. Sheets of Zircaloy-4, 1 to 3 mm thick, and M5®M5® is a registered trademark of AREVA-NP. advanced alloys from AREVA NP have been studied. To obtain different oxygen contents, they were oxidized at high temperature and then annealed under vacuum in order to reduce the oxide layer. Systematic post-treatment measurements of the oxygen concentration and of its homogeneity within the sheet thickness were performed. The different prior-β and α(O) structures thus obtained have homogeneous oxygen content between ∼0.14 wt. % and 0.9 wt. % and ∼2 wt. % and 7 wt. %, respectively. Such oxygen concentration ranges cover the solubility values that are expected in the β phase and in the α(O) phase at high temperatures typical of LOCA transients. Detailed microstructure investigations were subsequently performed on the prior-β structures since it is considered to be the most important layer when regarding the post-quench mechanical behavior of the material. Continuous cooling temperature (CCT) phase diagrams as a function of the oxygen content were established to correctly interpret the results. Electron backscattered diffraction (EBSD) analysis has then allowed the crystallographic orientations and the morphology of prior-β phase sub-grains to be determined. For each considered prior-β grain, it was possible to interpret the data by taking into account the “Bürgers” crystallographic relationship between the parent β phase and the resultant α phase. Complementary electron probe microanalysis (EPMA) was also used. These last experiments have shown a spatial fluctuation of the oxygen content within the microstructure that depends both on the nominal oxygen content and on the cooling rate. Nanohardness measurements were also performed and correlated with this oxygen spatial partition. These measurements proved to be useful for the understanding of the tensile macroscopic mechanical behavior. Finally, on the one hand, tensile tests were performed on prior-β phase at testing temperatures ranging from −100°C up to 260°C. The ductile-to-brittle temperature transition and the mechanical constitutive laws as a function of the oxygen content were then described. These tests show the existence of a ductile-to-brittle failure mode transition at 20°C for a critical oxygen concentration of ∼0.5 wt. %. A detailed fractographic analysis was performed to assess the failure mechanism. On the other hand, four-point bending tests were conducted on α(O) phase at 25°C and 135°C in order to obtain behavior laws. Preliminary finite element calculations were performed to simulate ring compression tests carried out on multi-layered high-temperature oxidized cladding tubes.

Microstructural influence on high temperature creep flow of Zr–1%NbO alloy in near-α, (α+β), and β temperature ranges in a high vacuum environment

“…Oxygen is known to markedly partition between a and b phases, even for cooling rates that are much higher than those used in the present study [15], so that no significant difference in the oxygen content of the a phase is expected between the various near-a microstructures. Although the role of partitioning of niobium remains to be clarified, such a strong effect of microstructure cannot be explained only by a change in chemistry of a and b phases, respectively.…”

Influence of the cooling rate on the texture and the microstructure of Zircaloy-4 studied by means of a Jominy end-quench test

“…Autissier [3] drew continuous cooling transformation curves which show that the temperatures of the start and the finish of the biphasic aþb domain do not depend much on the cooling rate _ T when _ T o1000 1C=s. These temperatures are, nevertheless, sensitive to the quantity of oxygen in the alloy [4], since oxygen favors the a phase.…”

Study of Oxidation Kinetics in Air of Zircaloy-4 by <i>in Situ</i> X-Ray Diffraction

“…To follow the oxidation kinetics, we used, in-situ x-ray diffraction for different temperatures (25˚C, 350˚C, 500˚C, 830˚C and 1000˚C), in-situ XRD for temperatures of 350˚C and different times of ageing: 00 min, 10 min, 20 min, 40 min and 60 min. During a postulated loss-of-coolant-accident (LOCA), the cladding tube is submitted in steam environment to a fast increase in the temperature (up to 800˚C -1200˚C), which induces an accelerated oxidation phenomenon until quenching [1]. To simulate this case, we have done X-ray diffraction with Zry-4 samples water quenched at 1050˚C with different times at this temperature (05 sec, 10 sec, 30 sec, 01 min, 02 min, 05 min, 10 min and 20 min).…”

“…The use of zirconium alloys for the cladding fuel rod of the pressurized water of the nuclear reactors (P.W.R.) is related to their very weak absorption of the thermal neutrons, good water corrosion resistance, good mechanical properties at high temperature [1], compatibility with the fuel and sufficient resistance to the irradiation effects (embrittlement, swelling …) (Figure 1). This material is submitted to the extreme conditions of the use, and various phenomena which degrade their characteristics.…”