Effects Of Zr On Microstructures And Properties Of FeCrAl Alloy

With the development of nuclear power technology in China,the development of safer and more reliable high-performance fuel cladding materials has become one of the focuses of nuclear material research.FeCrAl alloy does not react with water vapor to produce hydrogen gas in high-temperature environments in the loss of coolant accidents,thus meeting the safety requirements of fuel cladding materials.Based on ensuring safety and reliability,optimizing the composition design scheme of FeCrAl alloy and adjusting the processing technology to improve the structural stability and comprehensive mechanical properties of the material in high-temperature environments to meet the engineering application requirements of fuel cladding materials is currently a key research direction related to FeCrAl alloy.In the research work mentioned above,it is of great significance to elucidate the influence mechanism of alloying elements on the microstructure evolution and mechanical properties of FeCrAl alloy at room and high temperature environments.In response to the above issues,this paper takes FeCrAl alloy as the research object.Firstly,through phase diagram analysis,composition design,and comparison of microstructure and mechanical properties,the alloy element Zr is selected to improve the mechanical properties of FeCrAl alloy.Subsequently,the relationship between the microstructure and mechanical properties of alloys with different Zr contents is established,and specific process parameters for rolling and subsequent heat treatment are determined.Characterized the microstructure of FeCrAl alloy,analyzed the influence of Zr element on the microstructure of FeCrAl alloy,and elucidated the promotion mechanism of precipitation relative to grain nucleation and the inhibition mechanism of grain growth;Based on the investigation of the influence of Zr on the distribution of precipitates in FeCrAl alloy,the mechanical properties of FeCrAl alloy at room temperature and service temperature of 300℃were analyzed and compared by calculating the strengthening contribution values of various mechanisms;Finally,the element distribution and phase composition of the oxide layer were analyzed,and the influence of Zr on the formation of the oxide layer of FeCrAl alloy under high temperature conditions and the anti stripping performance of the oxide layer during high-temperature exposure were systematically studied.The influence of alloying elements on the microstructure and mechanical properties of FeCrAl alloy was studied.Research has found that Y and Ce have a certain refining effect on the microstructure of FeCrAl alloy,and the content needs to be strictly controlled.The alloying element Nb can effectively inhibit the recrystallization of FeCrAl alloy and improve its yield strength.The alloying element Zr can refine grains and stabilize FeCrAl alloys with grain sizes ranging from 20 to 30μm level,and increase the yield strength by about 50MPa,while maintaining the elongation after fracture at over 40%.The influence of thermal mechanical treatment process on the microstructure and mechanical properties of FeCrAl alloy was studied.Choose 750℃single-αphase zone rolling temperature and 90%of total rolling deformation can be used for rolling treatment to obtain FeCrAl alloy with a microstructure mainly composed of fine and uniform ferrite,and the alloy can achieve excellent comprehensive properties of high yield strength and good plasticity.With the increase of annealing temperature,the microstructure of FeCrAl alloy changes from rolled strip structure to annealed equiaxed grain structure,and the mechanical properties are characterized by a decrease in alloy strength and an increase in plasticity.As the Zr content increases,the rate of grain growth in FeCrAl alloy slows down and the grain size distribution becomes more uniform.The effect of Zr on the microstructure stability of FeCrAl alloy was studied.Annealing at temperatures below 750℃,the dislocation movement in the stress concentration area at the interface between Fe₂Zr precipitates and the matrix provides a driving force for recrystallization nucleation,promoting recrystallization nucleation.Annealing at temperatures above 750℃,the Fe₂Zr precipitates inside the alloy increase the pinning force of Fe₂Zr precipitation relative to grain boundaries,making it greater than the driving force of grain boundary migration,thereby inhibiting grain growth.Under the action of two mechanisms,the microstructure stability of FeCrAl alloy is improved.When the annealing temperature is above 750℃,the microstructure of FeCrAl alloy is equiaxed grains with uniform size.Based on the thermodynamic and kinetic calculations within two temperature ranges,the size of the precipitated phase is controlled between 0.3 and 2.9μm,thus the stability of the microstructure of FeCrAl alloy can be effectively improved.The effect of Zr on the mechanical properties of FeCrAl alloy at room temperature and service temperature of 300℃was studied.The contribution of Zr to the improvement of yield strength of FeCrAl alloy（increased by about 50MPa）is attributed to the fine grain strengthening and Orowan strengthening caused by the stable microstructure of precipitates.At 300℃,with the increase of Zr content,the strength and plasticity of the alloy have significantly improved（with a strength increase of over 100MPa and an elongation after fracture of>30%）.The Fe₂Zr precipitates at the micron level exhibit obvious stacking faults inside,and the dense stacking mode transitions from C15 to short range quasi-C14 type.This transformation is caused by the interaction between grain boundary migration and the dragging of precipitates,resulting in shear stress changing the dense stacking method.The appearance of precipitation phase stacking fault is helpful to improve the work hardening effect of FeCrAl alloy.The effect of Zr on the anti stripping performance of FeCrAl alloy oxide layer was studied.There are two mechanisms by which Zr acts on the surface oxide layer of FeCrAl alloy.Firstly,by adding Zr,the grain refinement effect is achieved,providing more grain boundaries for the matrix and increasing the diffusion pathway of Al towards the interface to improve the synergistic effect of oxide formation;Secondly,during the high-temperature oxidation process,the oxidation reaction of Zr replaces the initial formation of Fe and Cr oxides and occurs simultaneously with the formation process of Al₂O₃,improving the stability of the reaction process.After the introduction of Zr element,Zr O₂ is rapidly produced and its chemical state is stable.The way Zr regulates the oxidation reaction sequence promotes the synergistic formation of Al₂O₃ and Zr O₂ in the oxidation reaction,resulting in the preparation of 1μm stable and dense oxide layer.The strain energy release rate of the regulated oxide layer was reduced from 387.9J/m² to 11.8J/m²,and the weight change index measured in the constant temperature oxidation weighing test was transformed from the weight loss without Zr O₂ to the stable and slow weight increase with Zr O₂.