Effects Of Ti And/or Zr Addition On The Nano-oxides In ODS Ferritic Steels

Oxide dispersion strengthened?ODS?steel is one of the most promising candidate structural materials for the cladding of generation IV nuclear fission reactors and the blanket of future fusion reactors because it has excellent resistance to irradiation,creep,corrosion,high temperature strength and,moreover,good chemical thermal stability and strong capability for managing helium due to the ultrahigh density oxide nanoparticles.The types,proportions and structures of oxides formed by different alloy compositions in ODS steel are different,and the nano-oxides with different phase and crystal structure have different properties.It is of great significance to make statistical analysis of the types and quantities of nano-oxides in ODS steel.Hence,in this paper,FeCr-ODS steels?Fe–16Cr–0.1Ti–0.35Y2O3?and FeCrAl-ODS steels?Fe–15Cr–2W–0.1Ti–4Al–0.63Zr–0.35Y₂O₃?are studied.The nanoparticles in two kinds of steels were analyzed by high resolution transmission electron microscopy?HRTEM?.The number proportions of various types of nano oxides in the two steels are obtained by statistical analysis.Therefore,the effects of Ti and Zr addition on oxide particle composition and the corresponding proportions of two ODS steels were studied respectively.Then the influence of Ti and Zr elements on the irradiation tolerance,high temperature stability and mechanical property was analyzed respectively.The main conclusions are as follows:?1?About 59.5%?number proportion?of the small particles in FeCr-ODS steel are composed of Y–Ti–O oxides.Small amount of Ti addition,i.e.,0.1 wt.%into Fe–Cr ODS steel with 0.35 wt.%Y₂O₃ makes the low Ti/Y₂O₃ ratio,promotes the significant formation of orthorhombic and hexagonal Y₂TiO₅?45.6%?,instead of cubic Y₂Ti₂O₇?2.5%?.About 8.9%and 2.5%of the particles are composed of orthorhombic YTiO₃and orthorhombic YTi₂O₆,respectively.In addition,some Y–Cr–O complex oxides were detected,with the number fractions of tetragonal YCrO₄ and orthorhombic YCrO₃ of 7.6%and 5.1%,respectively.Simple oxides were also found,the proportions of TiO₂ oxides with monoclinic or orthorhombic structure and Y₂O₃ oxides having cubic or monoclinic structure are 17.7%and 10.1%,respectively.Orthorhombic YTi2O6 and tetragonal YCrO₄ oxide were firstly found in Fe–Cr–Ti–Y₂O₃ steel.The fine Y–Ti–O oxides in FeCr-ODS steel,especially the orthorhombic Y2TiO5,have excellent irradiation tolerance and unusual thermal stability,as well as good anti-irradiation swelling and anti-helium embrittlement ability.Most of the oxide particles in FeCr-ODS steel are tend to be coherent or semi-coherent with the bcc steel matrix.?2?Most?87.4%?of the small particles in FeCrAl-ODS steel are Y–Zr–O oxides,which indicates that the addition of 0.63 wt.%Zr promotes the formation of Y–Zr–O,with the number fractions of trigonal Y4Zr3O12 and orthorhombic or cubic Y2Zr2O7 of61.1%and 26.3%,respectively.Y4Zr3O12 and Y2Zr2O7 oxides both have excellent irradiation tolerance and thermal stability.Only a small amount of Y–Al–O oxides were detected in FeCrAl-ODS steel,with the number fractions of orthorhombic YAP and hexagonal YAH of 2.9%and 0.6%,respectively.The results show that the addition of Zr can significantly inhibit the formation of Y–Al–O oxides in FeCrAl-ODS steel.In addition,a small amount of Y–Ti–O oxides were found,with the number fractions of orthorhombic Y2TiO5 and orthorhombic YTiO3 of 5.1%and 0.6%,respectively.The large particles are mainly identified as ZrO₂ oxides,constituting 3.4%of the oxide particles.Most of the oxide particles in FeCrAl-ODS steel are tend to be coherent with the bcc steel matrix.?3?The formation mechanisms of various kinds of oxides in FeCr-ODS steel and FeCrAl-ODS steel and,moreover,the reasons of the excellent irradiation tolerance,thermal stability and mechanical properties of the ODS steel were discussed based on the results obtained here,which will be helpful to guide future research in alloy design and optimization.