Effects Of Minor Reactive Elements On Nano Precipitates In Al-alloyed High Cr-ODS Steels

Al-alloyed high-Cr ODS steel is one of the most promising candidate cladding materials for lead-cooled fast reactors and supercritical water-cooled reactors due to its superior high-temperature strength and excellent resistances to creep,radiation and corrosion etc.The outstanding comprehensive properties of ODS steel,especially the high temperature strength,mainly depend on the phase,dispersion morphology and heterogeneous interface structure of nano-precipitates dispersed in grains and along grain boundaries.It has been demonstrated that some minor reactive elements,e.g.,Hf and Zr,can affect the phases and the corresponding number fractions,the dispersion morphologies and,moreover,the structures of heterogeneous interfaces of the oxide nanoparticles in grains.In this paper,the effects of minor reactive elements on the phase,morphology and heterogeneous interface structure of the nano-precipitates in Al-alloyed high Cr-ODS steels have been extensively studied.（1）The nano-mesoscopic and microscopic structures of 4Al ODS and 4Al-0.62Hf ODS steels were characterized by using high resolution transmission electron microscopy（HRTEM）,diffraction contrast techniques（such as weak beam dark field（WBDF）imaging）and scanning transmission electron microscopy–energy dispersive X-ray spectroscopy（STEM-EDX）.It is found that relative to 4Al ODS steel,the grain size of 4Al-0.62Hf ODS steel is slightly smaller while the mean diameter and the number density of oxide nanoparticles are considerably smaller and much higher,respectively.The mean diameter,number density and inter-particle spacing of oxides in4Al ODS steel are 7.13 nm,1.38×10²² m^-3 and 101.24 nm,respectively.In sharp contrast,the mean diameter,number density and inter-particle spacing of oxides in4Al-0.62Hf ODS steel are 4.22 nm,5.53×10²² m^-3 and 67.46 nm,respectively.Most of the nanoparticles in 4Al ODS steel are composed of Y-Al complex oxides while the nanoparticles in 4Al-0.62Hf ODS steel are predominantly composed of coherent and incoherent Y₂Hf₂O₇ oxides.There are almost no precipitates along the grain boundaries of 4Al ODS steel,while there are a large number of nano-precipitates in the grain boundaries of 4Al-0.62Hf ODS steel.The mean diameter,number density and inter-particle spacing of the nano-precipitates are 26.04 nm,6.9×10¹³m^-3,and 30.15nm.The much improved high temperature mechanical properties of 4Al-0.62Hf ODS steel,relative to 4Al ODS steel,are attributed to the dispersion strengthening of oxide nanoparticles with smaller diameter and higher number density in grains and the pinning effect of carbides with good thermal stability such as Hf C,Ti C and oxide precipitates along grain boundaries.（2）The nano-mesoscopic and microscopic structures of 4Al ODS and 4Al-0.63Zr ODS steels were characterized by using HRTEM,WBDF and STEM-EDX.It is found that relative to 4Al ODS steel,the grain size of 4Al-0.63Zr ODS steel is just slightly smaller while the mean diameter and the number density of oxide nanoparticles are remarkably smaller and significantly higher,respectively.The mean diameter,number density and inter-particle spacing of oxides in 4Al-0.63Zr ODS steel are 4.83 nm,2.3×10²² m^-3 and 95.25 nm,respectively.Most of the oxide nanoparticles in 4Al-0.63Zr ODS steel are composed of coherent and semi-coherent Y₄Zr₃O₁₂,Y₂Zr₂O₇ and Y₂Ti O₅oxides.There are a lot of nano-precipitates on the grain boundaries of 4Al-0.63Zr ODS steel.The mean diameter,number density and inter-particle spacing of the nano-precipitates are 23.24 nm,4.1×10¹⁴ m^-3 and 12.35 nm,respectively.By adding Zr,the MX carbides and oxide particles in ODS steel are uniformly distributed along the grain boundary.Their low coarsening rate at high temperature makes them have excellent high temperature strength.In addition to the pinning dislocation mechanism of fine dispersed carbides and oxide particles with good thermal stability at grain boundary,Zr C in grain boundary stabilizes the deformation structure at high temperature,which is also the reason for improving the creep strength of ODS steel at high temperature.The number percent of the precipitates along grain boundaries that behave like carbides during the thermally activated detachment of dislocations from dispersoid particles in4Al-0.63Zr ODS is 71.4%.（3）The nano-mesoscopic and microscopic structures of 3.7Al-0.1Ti-0.6Zr ODS and 3.8Al-0.12Ti-0.5Zr ODS steels were characterized by HRTEM.In3.7Al-0.1Ti-0.6Zr ODS steel,about 87.4%of the particles are composed of Y-Zr complex oxides with the proportions of trigonal Y₄Zr₃O₁₂ and cubic/orthorhombic Y₂Zr₂O₇ of～61.1%and～26.3%,respectively;about 3.5%of the particles are composed of Y-Al complex oxides.However,in 3.8Al-0.12Ti-0.5Zr ODS steel,about 54.4%of the particles were found to be consistent with Y-Zr complex oxides with the proportions of trigonal Y₄Zr₃O₁₂ and orthorhombic/cubic Y₂Zr₂O₇ of～37.1%and～17.3%,respectively;about 5.2%of the particles are composed of Y-Al complex oxides.This indicates that minor addition of Zr inhibits the formation of Y-Al complex oxides remarkably while prompts the significant occurrence of Y-Zr complex oxides.While in 3.7Al-0.1Ti-0.6Zr ODS steel,about 5.7%of the particles are composed of Y-Ti complex oxides with the number fractions of orthorhombic Y₂Ti O₅ of～5.1%.However,in 3.8Al-0.12Ti-0.5Zr ODS steel,about 36.6%of the particles are composed of Y-Ti complex oxides with the number fractions of orthorhombic/hexagonal Y₂Ti O₅ of～29.6%.This indicates that it is effective for increasing the proportion of Y-Ti complex oxides by increasing the content of Ti from 0.1 wt.%to 0.12 wt.%with the content of Zr decreased from 0.6 wt.%to 0.5wt.%.