Study On Phase Transformation And High Temperature Oxidation Failure Behavior Of Cr-coated Zirconium Alloy

Cr-coated zirconium alloy has excellent properties such as high temperature and pressure resistance,high thermal conductivity and low thermal neutron absorption cross section,so it has a wide range of application prospects in the manufacture of accident tolerant fuel（ATF）cladding.The Loss of Coolant Accident（LOCA）is one of the most important benchmark accidents for nuclear reactor design.In the LOCA,the surface of the cladding material will undergo high-temperature steam oxidation,quenching brittleness,and other processes,which in turn will cause the leakage of nuclear fuel.Therefore,studying the interfacial structure transformation and oxidation mechanical failure performance of Cr-coated zirconium alloy under high-temperature oxidation conditions will greatly promote the application of Cr-coated zirconium alloy materials in the field of core cladding materials.This paper takes Cr-coated zirconium alloy core cladding tube as the research object,using backscattered electron（BSE）,electron backscatter diffraction（EBSD）,focused ion beam（FIB）,bright field image（BF）,energy spectrum（EDS）,selected area electron diffraction（SAED）and other analytical techniques,systematically studied the effects of quenching temperature（700℃,900℃,1100℃,1300℃）and cooling methods（water cooling,air cooling,furnace cooling）on the microstructure of Cr-coated zirconium alloys in both vacuum(10^-3Pa)and air heating environments.The ring compression test of each cladding tube after quenching process is carried out to analyze the influence of the structure on the mechanical failure behavior,and the following conclusions are mainly obtained:（1）Under vacuum heating conditions,the rate of accelerating cooling drops,substrate phase change:Martensite→Basketweave→Lenticular;900-1100℃FCC sum HCP two types of structure Zr（Cr,Fe）₂ Laves structure;Zr presence of Cr layers Medium diffusion rate low In Cr presence of Zr layers.（2）Under the condition of air heating,the oxidation of Cr-coated zirconium alloy conforms to the exponential law,which is divided into two stages:the slow oxidation in the early stage and the diffusion and oxidation controlled by the diffusion of Cr,Zr and oxygen in the later stage.The thickness of the oxide film increases with the increase of the oxidation temperature and time.The oxide film cracks and peels at 1100℃,which causes oxygen to diffuse into the zirconium matrix.（3）The mechanical failure behavior of Cr-coated zirconium alloy under different quenching conditions is analyzed.Under the condition of not being oxidized,the yield stress generally increases with the increase of temperature and quenching rate,but at1300℃,due to Zr/Cr undergoes a eutectic reaction,resulting in a significant decrease in performance.In the case of oxidation,the failure mode of 700-1100℃is ductile fracture,the strength decreases with the increase of temperature,and the ductility increases with the decrease of cooling rate.