Investigated On Composition Control Mechanism And Properties Of Fe-Cr-Al Cladding Materials

In order to further improve the comprehensive benefits of nuclear energy systems,countries around the world launched the fourth-generation nuclear energy system research and development plan in 2000,requiring that the constituent materials will have higher economy,safety and sustainability,and be able to overcome more stringent tasks.surroundings.After the Fukushima nuclear accident in Japan in 2011,the concept of accident-tolerant fuel（ATF）was proposed,requiring new cladding materials to have better oxidation resistance,high temperature stability,and radiation resistance.Under the water accident,it can still operate safely for a long enough time,and the nuclear fuel cladding materials currently in use can no longer meet the requirements of the fourth-generation nuclear energy system.Therefore,it is necessary to develop a new type of nuclear fuel cladding material with good mechanical properties,stable structure and excellent radiation resistance.By means of X-ray diffraction（XRD）,scanning electron microscope（SEM-EDS）,transmission electron microscope（TEM）,universal mechanical testing machine,microhardness tester and other characterization and testing methods,the experimental study on the component control of the mechanical properties and the resistance to high-temperature water vapor oxidation of Fe-Cr-Al cladding materials was systematically carried out,and the effect of component content on the microstructure,mechanical properties and high-temperature water vapor oxidation behavior of Fe-Cr-Al cladding materials was established.On the basis of experiments,the molecular dynamics simulation deduction reveals the influence mechanism of Al content and Cr content on the composition of Fe-Cr-Al cladding materials at the micro scale,which provides a theoretical reference for the composition optimization of Fe-Cr-Al cladding materials.Through the observation and analysis of the microstructure of Fe-Cr-Al cladding material,it is found that the increase of Cr content will increase the recrystallization rate and grain growth rate to a certain extent,while Al content has no obvious effect on grain growth.Through the experimental study and analysis of mechanical properties,it is found that under the same process conditions,Cr can affect the mechanical properties of Fe-Cr-Al cladding materials by affecting the recrystallization degree and grain size.The increase of Al content will obviously improve the mechanical strength of Fe-Cr-Al cladding material at room temperature,but will reduce the plasticity of Fe-Cr-Al cladding material at room temperature,and make its fracture mechanism at room temperature change from ductile fracture to brittle fracture;At the temperature of 380℃,the increase of Al content will significantly improve the mechanical strength and plastic toughness of Fe-Cr-Al cladding material.In the optimized composition range（Al content is 6～7 wt%,Cr content is 12～14 wt%）,the tensile strength of Fe-Cr-Al cladding material at 380℃is 530～570 MPa,the yield strength is 270～280 MPa,and the elongation at break is 27～30%.Based on molecular dynamics simulation,the tensile evolution model of Fe-Cr-Al cladding material with different Cr and Al contents was established,and the influence of alloy element composition on crystal structure,slip surface,dislocation and deformation propagation behavior was analyzed.Through the analysis of distortionless tensile evolution,it is found that the deformation mechanism of Fe-Cr-Al cladding material in the distortion-free region is twinning deformation,and the starting energy of the slip system is high.The energy required for the starting of the slip system can be reduced with the increase of temperature and Cr/Al content.By comparing the tensile behavior of the deformed polycrystalline Fe-Cr-Al cladding material with that of the distortionless Fe-Cr-Al cladding material,it can be found that the grain boundary and dislocation distortion region greatly reduce the energy required to start the slip system within its influence range,and the main deformation mechanism is slip.Through the analysis of the simulated tensile evolution process of the deformed polycrystalline Fe-Cr-Al cladding material,it is found that the energy required for the starting of the slip system of the Fe-Cr-Al cladding material decreases with the increase of temperature,and the increase of Al content also further reduces the starting energy of the slip system.When the effect of the starting of the slip system on the plasticity is greater than the effect of the plastic reduction caused by the obstruction on the slip surface,the plastic ability increases,Therefore,the increase of Al content makes the strength of Fe-Cr-Al cladding material increase and toughness decrease at room temperature,while the Fe-Cr-Al cladding material strengthen and toughen at high temperature.At the same time,it is found that the increase of Cr content will increase the resistance to slip deformation,but will increase the grain size and reduce the intra-grain distortion to a certain extent.When the effect of the distortion degree on the strength reduction is greater than that of the resistance on the slip surface,the strength will decrease.Therefore,the increase of Cr content will make the strength of Fe-Cr-Al cladding material increase first and then decrease.Through the experimental study of high temperature steam oxidation,it is found that the weight gain of Fe-Cr-Al cladding material with optimized composition range after 4 h of high temperature steam oxidation at 1200℃is 0.12～0.45 mg/cm².The weight gain after 20 h of high temperature steam oxidation at 200℃is 0.69～3.34 mg/cm².Combined with experimental observation,molecular dynamics simulation and oxidation thermodynamics analysis,it was found that the high temperature steam oxidation process of Fe-Cr-Al cladding material was mainly caused by the diffusion of O atoms from the oxides of Fe and the low valence oxides of Cr to the matrix of Fe-Cr-Al cladding material.The increase of Cr content can promote the growth of Al₂O₃ layer,and the increase of Al content can improve the high-temperature water vapor resistance of Fe-Cr-Al cladding material.The oxide layer is mainly bonded to the substrate through an amorphous transition layer.Fe-Cr-Al cladding material with Al content higher than 4 wt%will form an oxide layer dominated by Al₂O₃,which is dense and stable,and can effectively protect against high-temperature water vapor.