The Preparation Technology,Microstructure And Properties Of FeCrAl Alloy For Nuclear Fuel Cladding

In the Fukushima nuclear accident in 2011,zirconium alloy fuel cladding reacted with water vapor to release a large amount of hydrogen under high temperature accident conditions,resulting in reactor explosion and huge losses.Since then,scholars at home and abroad have been trying to find alternative materials for zirconium alloy,among which FeCrAl alloy has good high temperature oxidation resistance,high temperature steam corrosion resistance and short commercialization cycle,which is considered as one of the best candidate materials to replace Zr alloys for utilization on nuclear fuel cladding.Adding proper content of Nb in FeCrAl alloys would result in the formation of fine and dispersed Fe2Nb Laves phase during annealing,which could hinder the grain growth and effectively improve the strength of the alloys at high temperature.However,the irrational adjustment of the size and shape of the hard Laves phase and grains will worsen the processing properties of the alloy and affect its high temperature mechanical properties.In this work,Fe-13Cr-4.5Al-2Mo-1.2Nb-0.05Y alloy was selected as the investigated materials on the influence of rolling and heat treatment parameters on the microstructure evolution,formability,and high temperature strength of the FeCrAl alloy.The aims of this work are to investigate the relationship between process,microstructure,and properties.Therefore,the formability and high temperature strength of the alloy can be controlled,the the corresponding controlled mechanisms could be clarified.The main conclusions are reached as follows:(1)The effects of rolling and annealing parameters on the microstructure,precipitation behavior and mechanical properties of the alloy were researched.With the increasing rolling temperature,the degree of dynamic recovery and recrystallization both increased,decreasing the strength of the hot-rolled sheets while the elongation could be enhanced.Specifically,when the rolling temperature increases from 400? to 1000? the strength of the alloy decreases from 1088 MPa to 664 MPa while the elongation increases from 2.1%to 20.8%;the Nb and Mo elements mainly solid soluted into the(Fe,Cr)2(Nb,Mo)-Laves phase when the rolling temperature was higher than 700?.According to the results,the size of Laves phase increased and their content would decrease with the increasing rolling temperature.For the samples rolled at 700?,the minimum size and the maximum amount of Laves phase were?65 nm and?7.1%,respectively.Moreover,more lower rolling temperature(e.g.400?-600?)could enhance the distorted storage energy,thus promoting the formation of the more uniform structure in the samples annealed at higher temperature(1000?).(2)The influences of annealing temperatures(800?-1100?)on precipitation,recrystallization behaviors and mechanical properties of hot rolled sheets were studied.According to the results,The recrystallization rate of matrix and their grain size both increased to some extent,while the size of Laves phase decreased with the increasing temperature.The tensile strength and the fracture elongation of the investigated alloy were firstly increased and then decreased with the increasing annealing temperature.The highest amount of Laves phase was?7.8%and the tensile strength reached to 727 MPa under the condition of annealing at 800? for 1h.When the annealing temperature increased to 1100?,the Laves phase was completely dissolved and the matrix grains abnormally grew up to?160 ?m,resulting in the relatively low fracture elongation of 5.6%.The relatively optimum comprehensive mechanical properties of the hot rolled alloy sheet could be obtained after annealing at 1050? for 1h.The fracture elongation and tensile strength could reach to 28.2%and 614 MPa,respectively.(3)The influences of microstructures of hot-rolled sheets after annealing on the microstructural evolution,precipitation behavior and mechanical properties of cold-rolled and the following as-annealed sheets were investigated.The amount of Laves phase in the sheets after cold-rolling and annealing shows an inversely change comparing with the hot-rolled-annealed ones.As for the sample annealed at 1200? for 2min,Laves phase was completely dissolved into the matrix.However,the amount of Laves phase increase to about 7%after cold rolling and annealed at 800?.The alloy exhibited the better ductility after cold rolling and annealing for the hot-rolled annealed samples with the mixed microstructure of the equiaxed gains and shear bands.For the hot rolled sample annealed at 1050? for 1h,the elongation could reach to 24.5%after cold rolling and annealed at 900? for lh.By contrast,the alloy with uniform equiaxed grain structure showed higher strength of 769 MPa after cold rolling and annealed at 800? for 1 h.The balance of strength and ductility in the final sheets could be matched in a certain range by adjusting the hot rolling and annealing parameters.(4)The effects of rolling and annealing parameters on the mechanical properties of the alloy at elevated temperatures were studied.Results showed that the high-temperature strength mainly depended on the size and quantity of Laves precipitates in the cold-rolled samples after annealing.With the increase of annealing temperature,the precipitates of laves were coarsened while the amount of precipitates was reduced,resulting in the deteriorated high-temperature strength for the samples.The Laves phase content is 4.6%of hot-rolled and annealed at 1200? for 2 min alloy and its high temperature strength is 494MPa.This study is of great scientific significance to enrich and develop the theory of microstructure control and precipitate control of FeCrAl alloy for nuclear fuel cladding.It has certain practical significance for the development of FeCrAl alloys for nuclear power with independent intellectual property rights.