Thermodynamic Database Of Zirconium Alloys And Its Application

Zr alloys are widely used in nuclear power station as cladding materials. Besides their good corrosion resistance and acceptable mechanical performances, the main reason of this application is the low neutron capture cross section which is strictly demanded to the cladding materials. The performance of the current commercial Zr alloys such as M5, Zr-4and ZIRLO can not satisfy the urgent demand of high burn-up of nuclear reactors with high performances.. Scientists all over the world are dedicating in development of new Zr alloys for nuclear industry.Development of new-type Zr alloys with our own intellectual properties is of critical importance in China in view that currently Zr alloys used in nuclear industry of our country are totally depending on importation. This is a very hard task due to the rigor performance requests and the wide coverage of existing patents of this alloy. A thermodynamic database is developed in this work for construct a material design platform for Zr alloys, which sims at realizing the correlation of material structure with its composition and process conditions. Optimal alloy composition and process conditions can be searched under help of this material design platform with less experimental work. Relationship between structures and performance can be studied under help of this material design platform with more valuable information.The thermodynamic Zr alloys database established in this work including13elements, i.e. Nb, Fe, Sn, Cr, Ni, O, Cu, Mg, Mn, Si, V and Al. Besides the models collected from other databases of our group and literatures, thermodynamic models for binary V-Zr and O-Zr, ternary Fe-Nb-Zr and Nb-O-Zr are established in this work by thermodynamical optimization. The lattice stabilities used in sub-lattice model are estimated by first-principles calculations and Debye theory. Key experiments are performed for getting necessary information concerning structures and phase relationships requested by above theoretical calculations and thermodynamic optimizations. The database are checked by comparison of calculated phase equilibria with available experimental results."Process Phase Diagrams" of two common Zr alloys, i.e. M5and ZIRLO, are predicted which would be valuable for optimizing related thermal processes.Disagreements persisted in literatures concerning compositions and structures of the ternary precipitates in the Fe-Nb-Zr system. Structures of these phases are hard to identify with the commercial Zr alloys due to their very little amount. This work focuses on synthesizing metal compounds of possible composition of the with high concentrations so that their structures can be closely observed by XRD, TEM&EDS. A controversial ternary precipitates named Fe4NbZr5is clarified. It is neither Fe4NbZr5nor Zr2M phases, but also a mixture of two phases Zr2(FeNb) and Zr (FeNb)2. The phase equilibrium relations measured by the experiments are directly applied to the thermodynamic optimization of the Zr-Fe-Nb ternary system.A Zr-Fe-Nb sample of single phase with Laves_C15structure is successfully prepared. Crystal structural parameters of this phase are obtained using TEM&SAD, EDS, XRD and Rietveld refinement method. TEM&SAD results show that this sample is with the compositions ratioes of xzr/(xFe+xNb)≈1/2, and xNb/(xFe+xNb)≈12-17%. The formula for the compound can be expressed as Zr1+δ(Fe1-xNbx)2, where-0.16<δ<0.05and0.12<x<0.17. The results of Rietveld refinement also show that Nb replaces Fe and enters into16d site.Microstructure of oxide film produces at the initial step of corrosion is a crucial factor of the further corrosion process of Zr alloys. The calculation method of dislocation energy and strain energy between Metal/Oxide interface is established in this work, and then the microstructure of oxide film is simulated using Shen-Dann growth modle. Simulated results show that the thickness of the transition layer is about12atoms. This calculation results is consistent with the experimental observation of high-resolution electron microscopy. Effects of other elements, if they enter into the oxide film, are also estimated base on similar calculations. The thickness of the transition layer would increase, and the dislocation density would be reduce, if Nb enters into the crystal sites of oxide film. In other words, the lattice would become denser. It is favor to prevent from further corrosion and consistent with experimentals results which shows that Nb content in the oxide film could improve the corrosion resistance of Zr alloys.In summary, A thermodynamics database for zirconium alloys is developed for predicting their phase structures and thermodynamic properties. Some important precipitations are examined by experimental techniques, and a controversial ternary precipitates named Fe4NbZr5is clarified. The calculation method of the structure of oxide film is established using first-principles calculations, and effects of other elements, if they enter into the oxide film, are also discussed.