Study of secondary intermetallic phase precipitation/dissolution in Zr alloys by High Temperature High Sensitivity Calorimetry.
Résumé
It is now well established that in Zr alloys, secondary precipitate phases (SPP) have a major influence on the materials properties (corrosion and mechanical behaviour). Thus, it is of great importance to know the SPP formation and dissolution temperatures and their possible metastability as a function of the alloy chemical composition, the thermal treatments and the SPP characteristics (crystallographic structure and chemical composition).The aim of the present paper is to give an overview of the studies performed recently on different alloy families, that is Zr-Fe-V, Zr-Fe-Mo, Zr-Nb-Fe and Zircaloy (Zr-Sn-Fe-Cr et Zr-Sn-Cr-Ni) types. High Temperature High Sensitivity Calorimetry (Setaram HF-DSC) has been extensively applied on these alloys. From the experimental thermograms, we have derived the SPP fraction evolution as a function of the temperature, and also the enthalpy associated with their dissolution upon heating and precipitation upon cooling. It has thus been possible to study the reversibility of the reactions involved. Depending upon the thermal diffusivities of the alloying elements, we have shown that the alloys studied can be classified into two sub-groups (1)alloys with fast-diffusing elements, such as Cr, Fe, V, showing reversible phase transformations upon heating and cooling ;(2)alloys with slow-diffusing elements, such as Nb or Mo, showing metastable phase transformations, that are characterised by a partial Zr-Beta decomposition and metastable SPP precipitation upon cooling.In addition, the sensitivity of calorimetric measurements to low fractions of SPP (less than 1percent) is demonstrated.Thermodynamic calculations have been performed using Thermocalc software with an updated version of the Zircobase thermodynamic database developed a few years ago through a collaboration between CEA, Paris XI university and INPG ( ). This new version allows to predict the existence of ternary intermetallic phases such as Zr(Fe,Cr)2 showing a good agreement with the experimental results.