Fabrication And Properties Of Nano ZrC Dispersion Strengthened FeCrAl Based Alloys

FeCrAl alloy has become one of the important candidate materials for advanced nuclear accident tolerant fuel(ATF)cladding because of its excellent high temperature steam corrosion resistance,stable thermophysical properties and good radiation resistance.However,as an advanced light water reactor core cladding material,low mechanical strength and high neutron absorption cross section(about 10 times of Zr alloy)are still the important bottlenecks to limit the application of FeCrAl alloy.In order to reduce the economic loss caused by the high neutron absorption cross section,it is necessary to further reduce the thickness of FeCrAl alloy cladding tube.At the same time,it is also necessary to improve the mechanical properties of FeCrAl alloy to make up for the reliability loss caused by reducing the structural quality.For this reason,this article combines the principle of nano second phase carbide dispersion strengthening,using mechanical milling and spark plasma sintering technology to prepare nano ZrC particles dispersion strengthened FeCrAl based alloy(ZrC-FeCrAl).By means of SEM,oxygen content analysis,particle size analysis,XRD,EBSD,TEM,density test,hardness analysis and tensile property test,the effects of milling process,SPS sintering process and nano ZrC addition on the microstructure and properties of FeCrAl alloy were systematically studied.The main achievements are as follows:(1)The effects of ball milling process on the morphology,particle size distribution,oxygen content,lattice parameters of FeCrAl powder,as well as the microstructure and mechanical properties of the alloy were systematically studied.The results show that the prolongation of milling time is beneficial to fine the powder particles,but the mechanical properties of the sintered materials become worse due to the high oxygen content.When the milling time is 30 h,the powder is coarsened due to cold welding effect,with an average particle size of 72.88?m and oxygen content of 0.14 wt.%.The results of XRD refinement show that the lattice constant of powder increases with the increase of milling time.When the milling time is 30 h,the alloy has the best mechanical properties,ultimate tensile strength,elongation and Vickers hardness are 1046 MPa,12.1% and 349.9 HV,respectively.The results show that the average particle size increases first,then decreases and then increases with the increase of milling speed.When the milling speed is 400 rpm,the particle size increases,but the oxygen content is the lowest.The results of XRD refinement show that the increase of milling speed can cause lattice distortion of powder due to the increase of lattice constant.EBSD results show that the grain can be refined effectively by high-energy ball milling.When the milling speed is 400 rpm,the average grain size of the sample is refined to 0.86 ?m,and the density increases to 99.5%.TEM results show that increasing milling speed can make nano ZrC more evenly dispersed in the matrix,which can more effectively hinder the movement of dislocations and grain boundaries.Under the synergistic effect of dispersion strengthening and fine grain strengthening,Vickers hardness,tensile strength and elongation of FeCrAl alloy increase with the increase of milling speed.The tensile strength of ZrC-FeCrAl alloy reached the maximum value of 113 MPa after milling at 400 rpm at 800 ?,and the elongation of ZrC-FeCrAl alloy decreased gradually due to the deformation of grain boundary at high temperature.(2)The influence of SPS sintering process on microstructure,phase,density and mechanical properties of ZrC-FeCrAl alloy was studied.The results show that the powder particles can not be bonded completely when sintering temperature is too low,and there are many pores in the sample,and the relative density of the alloy is low and the mechanical properties are poor.The increase of sintering temperature can accelerate the material transfer speed.By promoting the formation and growth of sintering neck,the powder particles will gradually bond completely,and then the pores shrink,and finally obtain high density sintered materials,and the samples show good mechanical properties.When sintering temperature is1050 ?,the comprehensive properties of the sample are the best.With the increase of sintering pressure,the accumulation distance of powder particles is shortened and the contact area increases,thus the densification process of powder is accelerated,the pore number is reduced and the densification of sintered materials is improved.The density,Vickers hardness,ultimate tensile strength and elongation of the samples were increased with the increase of sintering pressure.When the sintering pressure was 50 MPa,the results showed that the density,Vickers hardness,ultimate tensile strength and elongation of the samples were increased to 99.9%,370.4 HV,1080 MPa and 13.4%,respectively.(3)The effect of nano ZrC addition on Vickers hardness and tensile properties of FeCrAl alloy was studied.The results show that with the increase of the content of nano ZrC particles,the higher the density of nano ZrC particles in the matrix,the greater the hindrance to dislocation slip and grain boundary movement.Therefore,the deformation resistance of FeCrAl alloy can be effectively improved.When the content of nano ZrC increases from 0.1 wt.% to1.0 wt.%,the Vickers hardness and tensile strength of FeCrAl alloy increase gradually.However,when the content of nano ZrC is more than 1.0 wt.%,the uneven dispersion of nano ZrC will lead to the segregation at the grain boundary and the stress concentration,which will eventually lead to the brittle fracture of the sample.In addition,the elongation of the alloy decreases with the increase of ZrC content.