Investigation Of Irradiation Behavior And Corrosion Properties Of Nuclear Grade FeCrAl Alloys

As one of the accident-tolerant fuel cladding(ATF)materials,FeCrAl alloy is regarded as an excellent candidate for traditional zirconium alloy fuel cladding materials because of its excellent radiation resistance and high temperature corrosion resistance.In this thesis,400keV Fe+ion and H+ion irradiation experiments at 400 ?and then high-temperature and high-pressure corrosion experiments for three kinds of FeCrAl alloys with different chemical compositions were carried out.Microstructure,chemical composition,phases,and mechanical properties of the corroded,non-irradiated and irradiated alloys were observed,tested and analysed by transmission electron microscopy(TEM),scanning electron microscopy(SEM),slow positron annihilation technique(PAT),grazing small angle X-ray diffractometer(GIXRD)and nanoindenter,so that we can get deeper understanding about the ion irradiation behavior of FeCrAl alloys and corrosion mechanism.There are differences in the type,distribution of precipitats and microstructure for these three kinds of FeCrAl alloys due to the difference in the types and contents of trace alloy elements.Fe13Cr4AlNb alloy mainly contains about 200nm Laves phase with hcp structure,namely(Fe,Cr)2(Nb,Mo,Ta,Si).Fel6Cr4.5Al alloy contains a large number of finely dispersed Y3Al5O12 precipitates that are mainly distributed in the grain boundary and a small amount of Laves precipitates with hcp structure,namely(Fe,Cr)2(W,Ti,V.,Ni).There are mainly fine Y4Al2O9 prcipitates with orthogonal structure in Fe19.5Cr5AlY alloy.A small amount of Y-Al-Ti-O precipitates with ellipse shape distributes in Fe16Cr4.5AlY alloy and Fe19.5Cr5AlY alloy.Statistical analysis results of the size of precipitates and nanohardness value show that Fe19.5Cr5AlY alloy has the highest nanohardness.Although the density of precipitates of Fe13Cr4AlNb alloy is the largest in three kinds of FeCrAl alloys,the dispersion of the precipitate and the dislocation density are the smallest.Due to Fe+ ion irradiation,dislocations disappear,meantime phase boundary and grain boundary become vague.Among them,the situation of Fe13Cr4AlNb alloy is the most serious.Irradiation induces the instability of Laves phase,which causes the amorphization transformation of Laves precipitates.However,Y-Al-O precipitates always keep stable.With the increasing irradiation dose,the size and density of the dislocation loops increase continuously,but their growth rate slows down.And their density approachs a certain value.According to the statistical results of the size of dislocation loop,Fel9.5Cr5AlY alloy has the smallest size and density,and its radiation hardening value is also lower than that of other alloys,while the size of dislocation loop,density and radiation hardening value of Fe13Cr4AlNb alloy are the largest among them.There is a power function relationship between the radiation hardening value and the displacement damage.Irradiation softening of Fe16Cr4.5AlY alloy and Fel9.5Cr5AlY alloy appears under H+ion irradiation.Within the depth range of 200-800nm from the surface of the alloy,the vacancy defect concentration and nanohardness of Fe16Cr4.5AIY alloy decrease with the increase of irradiation dose,and the dislocation lines decrease.At the 0.5dpa,the vacancy concentration and the nanohardness of Fe19.5Cr5AIY alloy are the lowest.When the irradiation dose is increased continuously,the concentration of vacancy in the alloy and the nanohardness begin to increase.The reason for the difference of the irradiation soften phenomenon for the two alloys is mainly due to the difference of the original microstructure.The corrosion test results of FeCrAl alloy show that the irradiation has effects on the corrosion of FeCrAl alloy.In the early 50 hours of corrosion,due to the cleaning effect of irradiation on the surface of the alloy,corrosion product on the surface of the unirradiated alloy is a layer of oxide particles and oxide particle clusters.There are only a large number of oxide particle clusters on the surface of the irradiated alloy.The loose a-Cr203 layer is formed on the sample surface.The segregation of Fe element and the appearance of radiation defects near surface promote the outward diffusion of alloying elements for alloy matrix,which improves the corrosion rate.During the corrosion process,a-Cr203 oxide layer tends to grow along the grain boundary,and oxide particle clusters on the surface grow by aggregation.On the surface of the FeCrAl alloy,it mainly forms the oxide layer structure:the outermost layer composed of cluster-like a-Fe203 oxide and the outer layer composed of the thinner ?-Cr2O3 oxide layer.With the increase of irradiation dose,the corrosion rate of alloy increases.Fe19.5Cr5AlY alloy has better corrosion resistance than Fe16Cr4.5AlY alloy,which is mainly related to the content of Cr element and Al element.