Microstructural Stability Of Zr/Ta-alloying Fe-Cr-Al-M Ferritic Stainless Steel At High Temperature

With the continuous development of nuclear power technology,structural materials have become increasingly important in the nuclear power industry.Especially because of the Fukushima nuclear accident in 2011,the current Zr alloy cladding material reacted violently with high temperature water vapor and resulted in serious fracture.Therefore,from the perspective of improving the safety of nuclear power,it is urgent to find a material with high stability that can replace the Zr alloy cladding.At present,Fe-Cr-Al ferritic stainless steel is expected to be a substitute for Zr alloy because of its good resistance to high temperature steam oxidation.However,significant grain coarsening occurs in conventional Fe-Cr-Al stainless steels at high temperatures,which results in a significant decrease in the mechanical properties of the alloys at high temperatures.Therefore,it is of great importance to study the microstructure stability of Fe-Cr-Al alloys at high temperatures so as to develop new fault tolerant Fe-Cr-Al stainless steel cladding materials.In this work,the effect of alloying element Zr/Ta on the microstructure stability and mechanical properties of Fe-Cr-Al-M(Mo,Nb)alloy at high temperature was studied,and the internal relationship between alloy composition,microstructure and properties was established.And the mechanism of microstructure evolution at high temperature was revealed.The main research contents and conclusions are as follows:(1)Based on the Fe-Cr-Al-M alloy,two kinds of stainless steel components of Fe-Cr-Al series were designed by the addition of Zr/Ta,using the method of cluster composition design,respectively S1(Fe-13.5Cr-4.73Al-2.07Mo-0.5Nb-0.98Ta(wt.%))and S2(Fe-13.5Cr-4.73Al-2.07Mo-0.4Nb-0.78Ta-0.2Zr(wt.%).The present work investigated the synergetic effect of Zr/Ta co-alloying on the microstructural stability of precipitates in the body-centered-cubic(BCC)matrix at high temperatures in Fe-Cr-Al-M alloy.Alloy plates were fabricated by vacuum arc melting and multi-pass hot rolling,then aged at 800?for 24 h,and finally re-treated at different temperatures(1000?1200?)for 1 h to study the re-dissolution of precipitates.A large amount of Fe₂M(M=Mo,Nb,Zr,Ta)Laves phase particles with a high-entropy effect were dispersed on BCC matrix with a volume fraction of f?4.6%in 800?-aged alloy.With the increase of resolution temperature,the volume fraction of precipitated phase decreases to f?1.4%at 1050?;After 1 h treatment at 1200?,the degree of precipitated particles is more obvious,but the volume fraction of precipitated phase(f?0.7%)in S2 alloy is about 10 times higher than that in S1 alloy(f?0.05%),in which these particles are still distributed both in inner grains and on grain boundaries.The higher microstructural stability at high temperatures is ascribed to the co-precipitation of Fe₂M,Fe₂₃Zr₆,and core(Fe₂₃Zr₆)-shell(Fe₂M)-structured particles,rather than Laves phase alone.The mechanical property of this alloy at both room and high temperatures was also studied.The aging alloy exhibits excellent mechanical properties at room temperature(?_YS=506 MPa,El=4.5%)and high temperatures(600??_YS=358 MPa,800??_YS=74 MPa).(2)In order to study the effect of different heat treatment processes on the Laves phase precipitation in S2 alloy,the hot rolled S2 alloy was treated with solid solution and segemental aging treatment at 800?.The results show that only a small amount of Zr-rich precipitates are left in the ferritic matrix in the solid solution state.After aging treatment,the needle-like Laves phase on the ferritic matrix were gradually coarsen with the prolonging of aging time.The alloy exhibits the highest hardness value(?260 HV)at 5 min,and then the hardness gradually decreases to 234 HV at 24 h with the prolonging of aging time.The microstructure after solution aging is obviously different from that of spherical Laves phase particles after hot rolling aging.Comparing the mechanical properties of the alloys with the two different processes,it is found that the mechanical properties of the alloy with spherical precipitation are better than that of the alloy with needle precipitation,and the strengthening effect of the spherical particles(?_circ=110 MPa)is significantly greater than that of the alloy with needle precipitation(?_rod=47 MPa).In addition,the transformation kinetics curves of S2 alloy were obtained by using the transformation kinetics equation.The precipitation rate of Laves phase increased firstly and then decreased,and reached the maximum at 5-10 min of aging...