Research On Microstructure And Mechanical Property Of 15Cr-ODS Ferritic Alloys

Nano-structured oxide dispersion strengthened(ODS)ferritic alloys are the leading candidate structural materials for fuel cladding application in Generation IV fission reactors because of their excellent improved high temperature strength and irradiation tolerance.These superior properties come mainly from high-density nanoscale oxides,which depend strongly on alloy composition and processing parameter.However,the formation mechanism of nanoscale oxides is not yet to be solved comprehensively.In this study,mechanical alloying(MA)and hot isostatic pressing(HIP)were used for preparing ODS alloys.The influences of annealing temperature and the additions of alloying elements(Ti and Zr)on microstructure and mechanical properties of 15Cr-ODS alloys were studied systematically.The following conclusions were obtained:(1)With the increasing milling time,the average particle size of MA powders increases firstly and reaches up to the largest(35.2 μm)at 1 h,then the average particle size decreases.After 50 h MA,the average particle size does not change obviously.The grain size of MA powders decreases with the increasing milling time and then trends to reach a saturation value at 50 h.The lattice strain and hardness increase with the increasing milling time.After 50 h,the grain size,lattice strain and hardness do not change obviously.(2)The MA powders suffer severe plastic deformation during MA.Ultrafine and nano grains are found in 15Cr-ODS alloy powders after MA and annealing at 900,1000 and 1100 ℃.The average grain size increases and the hardness decreases with the increasing annealing temperature.High-density of nanoscale oxides are formed in the powders annealed at the temperature from 900 to 1200 ℃.The size of oxides increases and the density decreases with the increasing annealing temperature.The chemical composition and structure of nanoscale oxides are also influenced by annealing temperature.At 900 ℃,Y-Ti-Zr-rich oxides are observed.At 1000 ℃,core/shell structure oxides with Y-Zr-O-rich cores and Ti-rich shells are found for the first time in ODS ferritic alloys with Ti and Zr adiitions.At 1100 ℃ or above,the Ti-rich shell disappears and only Y-Zr-rich oxides are remained The formation of mechanism of core/shell structure oxides mainly lie in the following two aspects:1)the alloy powders with trace N element undergoes bcc to bcc+fcc transformation during heating,2)the diffusion ability of Ti in oxides is enhanced with the increasing annealing temperature.The Ti atoms in oxides may be repelled by Y atoms due to higher energy of Y-Zr-O oxides and trend to diffuse the interface between oxides and γ-Fe matrix due to low diffusion ability in y-Fe.(3)In Al-free 15Cr-ODS alloys,the additions of Zr,Ti and Ti-Zr reduce the average grain size,while the proportion of ultrafine grains(100-500 nm)and low angle grain boundaries(2～15°)gradually increase in ODS alloys.The additions of Zr,Ti and Ti-Zr sufficiently reduce the average size of nanoscale oxides and increase the number density of oxides.When Ti and Zr are added together,the average size of oxides is the smallest,and the number density is the highest.The additions of Zr,Ti and Ti-Zr sufficiently affect the crystal structure and composition of oxides.In 15Y alloy,the oxides are mainly Y2O3.In 15YZr alloy,the oxides are mainly Y4Zr3O12.In 15YTi alloy,the oxides are mainly Y2Ti2O7.In 15YTiZr alloy,nearly all the oxides are Y42r3O12.Meanwhile Cr-(Ti)-O phase with a diameter of 20-300 nm is observed in four kinds of Al-free ODS ferritic alloys.The size of Cr-(Ti)-O phases is gradually refined with the addition of Zr,Ti and Ti-Zr.(4)In Al-containing 15Cr-ODS alloys,the additions of Zr,Ti and Ti-Zr reduce the average grain size,while the proportion of ultrafine grains(100～500 nm)and low angle grain boundaries(2-15°)gradually increase in ODS alloys.The additions of Zr,Ti and Ti-Zr sufficiently reduce the average size of oxides and increase the number density of oxides.When Ti is added together with Zr,the average size of oxides is the smallest,and number density is the highest.In 15YA1 alloy,most of the oxides are Y3Al5O12(YAG).In 15YAlZr alloy,the oxides are mainly Y4Zr3O12.In 15YAlTi alloy,the oxides are mainly YAlO3(YAH).In 15YAlTiZr alloy,nearly all the oxides are Y4Zr3O12.Meanwhile large Al2O3 with the diameter 20-70 nm and nanoscale core/shell structure oxides with Al2O3 cores and Y-rich amorphous shells with the diameter 11-20 nm are observed in four kinds of ODS alloys with Al addition.(5)In Al-fiee and Al-containing 15Cr-ODS alloys,the addition of Zr,Ti and Ti-Zr gradually enhance the tensile strength,total enlongation and fatigue strength.When Ti is added together with Zr,the ODS alloy exhibits the best mechanical properties.The plasticity of Al-containing 15Cr-ODS alloys is superior to Al-free ODS alloys.The fatigue cracks usually initiate from rich Cr phase in Al-free 15Cr-ODS alloys.The fatigue cracks usually initiate from rich Al phase in Al-containing 15Cr-ODS alloys.