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International audience; The oxidation by oxygen of a zirconium based alloy, M5TM (which is a ZrNbO alloy, containing 1% of Nb) has been studied. The M5TM alloy, like many zirconium alloys, undergoes a kinetic transition. The aim of the present work is to achieve a better understanding of the oxidation in the pre-transition stage, and to clearly identify the differences between the pre- and post transition stages from the kinetic point of view. The oxidation of M5TM was followed by isothermal gravimetry at 490°C, under a controlled partial pressure of oxygen (in the range 7 to 200hPa). First, we have verified the steady state assumption, by coupling thermogravimetry and differential scanning calorimetry (DSC) : it is shown that the system is in a steady state from the beginning of the oxidation, in the pre- and post-transition stages. Then, the existence of a rate-limiting step was verified in the pre-transition stage using an experimental method based on temperature or pressure jumps; this assumption is no more verified in the post-transition stage, which means that the oxidation does not proceed in the same way as in the pre-transition stage. Finally, we have obtained the variations of zirconia growth reactivity  with the oxygen pressure, in the pre-transition stage (using the pressure jump method). The oxygen pressure has a slightly accelerating effect, which cannot be interpreted by the diffusion of oxygen vacancies through a dense oxide layer (in that case no effect of the oxygen pressure would be observed). In the post-transition stage, the oxygen effect is more important.

12th International Conference on Metal Forming, Date: SEP 21-24, 2008 Cracow POLAND; International audience; A study of the microstructural evolution of Zircaloy-4 (Zy-4) during hot extrusion has been performed on a small scale device mounted on a standard NITS machine. Before extrusion the billets present a Wind-manstatten lamellar microstructure. Additional data have been obtained by hot torsion tests. After hot deformation some samples have been heat treated in various conditions. Thus the fragmentated volume fraction has been quantified with optical microscopy for various strain, strain rate and temperature histories, focusing on post dynamic effects (activation and kinetics of grain growth after deformation). For describing the microstructural evolution, the model separates dynamic and post dynamic effects. The dynamic part uses a Gourdet-Montheillet approach to describe the fragmentation. The growth is described by an explicit differential Avrami approach which can model the non isothermal evolution during cooling after extrusion. The first calculations with this model provide a good prediction of the recrystallized volume fraction for several thermal and mechanical conditions

International audience; The kinetic curves of oxidation of Zircaloy-4 exhibit a transition, which is a sharp increase in the oxidation rate when the oxide thickness reaches a critical value. The pre-transition stage is controlled by the diffusion of oxygen vacancies in the oxide layer. In the post-transition stage, oxygen or water vapour have an accelerating effect on the oxidation (whereas they have no influence during the pre-transition) and the oxide layer is damaged, with large cracks parallel to the metal/oxide interface and connected to the gaseous atmosphere by pores. Consequently, it is clear that the post-transition stage cannot be accounted for by the same mechanism as in pre-transition. In this paper, we propose a geometrical modelling allowing to describe the progressive transformation of the oxide layer during the transition. This model is based on a random appearance of pores (connected to the external surface) which leads to the transformation, from a pre-transition stage to the post-transition stage, of small sections s0 of the oxide layer (analogy with the models of thermal transformations of powders, involving the processes of nucleation and growth of a new phase). The model allows to describe the kinetic curves obtained for the oxidation by water vapour of Zircaloy-4.

International audience; The oxidation of Zircaloy-4 by water vapour has been studied between 500 and 550 °C, the water vapour partial pressure ranging in 13-80 hPa, using isothermal and isobaric thermogravimetry, and calorimetry. During gravimetry experiments, sudden changes in temperature or water vapour pressure have also been performed. It results that the approximations of steady state and rate-limiting step are only valid before the kinetic transition. In the post-transition region, a significant influence of water vapour and hydrogen partial pressures has been found, contrarily to the kinetic behaviour before the transition (which is in this last case, in good agreement with a rate-limiting step of diffusion of oxygen vacancies). It comes out that the post-transition kinetic behaviour is definitely not the same as before the transition.

International audience; The oxidation of a zirconium based alloy (recrystallised Zircaloy-4) has been studied at 500°C, by thermogravimetry under controlled partial pressures of water vapour and hydrogen. The study is restricted to the pre-transition state of oxidation, the aim being to identify the rate-limiting step of the oxide growth. It has been verified experimentally that the system is in a steady state during the pre-transition period. Moreover, using a method based on the isolation method, it has been shown that the oxidation could be described by a single rate-limiting step only when a weight gain of 7.5 mg.dm-2 has been reached. Since the kinetic curves are not parabolic, a diffusion model has been proposed, which takes into account the effect of barriers on the diffusion (such as pores, blisters, cracks... in the oxide layer) and which is in good agreement with the experimental curves. Then, numerical fitting has shown that the beginning of the oxidation could be interpreted by a mixed reaction-diffusion steady solution, which tends towards a single diffusion rate-limiting step when the oxide layer is thick enough (about 0.5 m).