Study On Microstructure Control And Oxidation Behavior Of Fe-Al-Cr Alloys

Fe-Al-Cr alloy has become a hot candidate material for various nuclear reactor systems due to the formation of self renewing oxide film on the surface of the alloy,the good high temperature oxidation resistance brought by its self-cleaning ability and the good neutron radiation resistance.However,the large grain size of Fe-Al-Cr alloy hinders its large-scale application.Generally,the grain diameter of Fe-Al-Cr alloy is more than 500?m.Based on computational materials,solid state physics and metal science,the mechanism of grain coarsening and the method of restraining grain coarsening are studied,and the mechanical properties of the alloy and the effect of heat treatment on mechanical properties and the second phase are explored.By means of thermokinetic analysis and electron microscopy,combined with the first principle calculation and experiment,the oxidation resistance and the formation mechanism of surface oxide of Fe-Al-Cr alloy were also studied.It is found by thermodynamic calculation that when the nickel content in Fe-Al-Cr alloy is less than 8 wt.%,the austenitizing of the alloy is beneficial to the precipitation of TCP phase,and the relationship between the Ni/Cr equivalent ratio and the appearance of TCP phase basically conforms to the empirical law of Schaeffler diagram.On this basis,nine kinds of Fe-Al-Cr alloys were designed and melted:Fe-8Al-13Cr-Ni?Fe-12Al-10Cr-C-Ni,Fe-9Al-11Cr-C,Fe-8Al-9Cr-Cu-Nb,Fe-10Al-8Cr-V,Fe-5Al-9Cr-Si?Fe-11Al-11Cr-Ti,Fe-9Al-9Cr-Ti,Fe-5Al-8Cr.The formation of topologically dense phase(TCP)-Fe Cr(?)phase and Laves phase in the Fe-Al-Cr alloy was controlled by controlling the equivalence ratio of the alloy elements Ni/Cr The grain size of the alloy is controlled within 100?m,and the FeAlCrNi four element high entropy alloy is prepared by the high entropy of the alloy.The grain size of the alloy can be controlled within 5?m.X-ray diffraction(XRD),scanning electron microscopy(SEM),energy dispersive X-ray spectroscopy(EDS)and differential scanning calorimetry(DSC)were used to analyze the microstructure and phase distribution of the samples,as well as the energy change during the temperature change of the alloy.It is found that the solute atoms(Al)in the Fe-Al-Cr alloy move along the grain boundaries of the solvent atoms(Fe and Cr atoms in ferrite).Under the stress caused by the temperature change,the chemical potential of some atoms in the crystal is increased,the chemical potential of some atoms in the adjacent crystal is reduced,and the atoms flow from the region with higher chemical potential to the region with lower chemical potential,which makes the atoms diffuse,Driven by chemical potential difference,atoms are transported across grain boundaries.Therefore,in the process of temperature rise,the diffusion of aluminum atoms across the grain boundary leads to the release of atoms on the surface steps of retreated grains,which leads to the formation of vacancies,the loosening of grain boundary structure,the increase of mobility and grain boundary;in contrast,in the process of temperature fall,the occurrence of grain boundary migration leads to the reduction of the number of grain boundaries and the coarseness of grains.The mechanical properties of the alloy at room temperature were tested,including tensile test and microhardness test.The stress-strain curve of the alloy was drawn by tensile test.The tensile strength of large grain Fe-Al-Cr alloy in as cast condition is less than 10MPa,while that of Fe-Al-Cr Ni alloy(grain diameter less than 80?m)in as cast condition can reach 260 Mpa,and that of FeAlCrNi high entropy alloy can reach540Mpa.However,TCP phase will also become the channel of crack conduction under the stress of the alloy,which will cause the intergranular ductile fracture of the alloy.This is because the second phase Fe Cr(?)phase and fe2alcr have a semi eutectic orientation relationship.The experiment explored the orientation relationship between the second phase and the alloy matrix phase in the Fe-Al-Cr alloy through XRD,SEM,EDS and other means.Through a series of heat treatment systems of Austempering solution aging,the effect of heat treatment on the second phase of the alloy was explored.The oxidation behavior of Fe-Al-Cr alloy surface was studied by experimental means and the first principle calculation method:Al₂O₃ is preferentially generated at the grain boundary;Al₂O₃/(Al_0.9Cr_0.1)₂O₃ composite oxide will be generated in Fe-Al-Cr alloy under high temperature oxidation condition;this is due to the diffusion of Cr atom in Fe/Al₂O₃ interface layer.