Influences Of Alloy Components And Spark Plasma Sintering Processing Parameters On Microstructure And Mechanical Properties Of ODS Alloys

Nanostructured oxide dispersion strengthened(ODS)alloys are considered as one of the most promising structural materials for future advanced fourth-generation and fusion reactors because of excellent high temperature strength and irradiation telorance.The excellent performance is mainly from high-density of nano size oxides.These very fine oxides could effectively impede the movement of grain boundaries and dislocations within the materials,and capture the defects and helium produced when the materials are irradiated.Thereby they improve the high temperature performance and irradiation telorance.The formation of nano-oxide is closely related to the processing process and alloy composition.In processing respect,14Cr-ODS ferritic alloys were prepared by mechanical alloying(MA)and spark plasma sintering(SPS)in this work.The micro structures of the alloys were characterized by using electron backscattered diffraction(EBSD),transmission electron microscopy(TEM)and synchrotron radiation small angle X-ray scattering(SAXS).Vickers microhardness and tensile were used to evaluate the mechanical properties of 14Cr-ODS alloys.The results showed that ultrafine grains are formed in the ODS steels sintered at low temperature,and a mixed microstructure of fine grains and coarse graitns is observed in the steels sintered at high temperature.With the increasing of sintering temperature from 950? to 1025?,the average grain sizes increase.High-density nanoscale Y-Ti-rich oxides,Y2Ti2O7,and some large Cr-Ti-rich oxides TiCr2O4 are observed by high resolution transmission electron microscopy(HRTEM).The number density of Y-rich oxides decreases but the average size increases with the increasing sintering temperature.The tensile strength of SPS950 sample is higher than that of the sample sintered at higher temperature.Therefore,950? is the optimum sintering temperature.After determining the optimum sintering temperature,the thermal stability of microstructure of the samples sintered at 950? was studied.The results showed that,after the 1200? for 5h heat treatment,the grain structure and the nano-oxide of the 950? sintered sample occur coarse.The average grain size increases to?635 nm,and the average nano-oxides size increases to 9.6 nm with the number density decreasing to 2.3×1022 m-3,which showed excellent high temperature stability.The tensile strength of the sample heat-treated at 600? for 5 h is 1460 MPa,which is the similar to that of the non-heat-treated sample(1494 MPa).Although the tensile strength value of the sample heat-treated at 1200? for 5 h decreases to 1353 MPa which is lower than the original sample,it maintained a high tensile strength.In alloy composition respect,La2O3 and CeO25 instead of Y2O3,are added to the 14Cr ferritic matrix powders,respectively,followed by MA and SPS.The results showed that ultra-fine grains with random crystallographic orientations were formed in the SPSed ODS alloys.The average grain sizes of 14Cr-ODS alloy with La2O3(14L)and 14Cr-ODS alloy with CeO2(14C)are 0.56 ?m and 0.82 ?m,respectively.The average oxides size of 14L and 14C samples are 7.6 nm and 11.4 nm respectively.And the number density of nanoscale oxides of 14L and 14C samples are 3.4×1022 M-3 and 1.2×1022m-3.High density nanoscale La2Ti2O7 oxides in 14L and Ce2Ti2O7 oxides in 14C are observed,respectively.The yield strengths of 14L and 14C samples are 1433 MPa and 1405 MPa,respectively.It indicated that the ODS alloys which contain La2O3 and CeO2 could be maintained at high yield strength.The results showed that the La2O3 and CeO2 of which content were higher in common rare earth based ores had the prospect of replacing Y2O3.Energy dispersive spectrometer(EDS)mapping and X-ray absorption finestructure(XAFS)were used to study the formation and structural of the Y-rich oxides with the addition of Ti and Zr in ODS powders in this work.The results showed that the additions of Ti,Zr and Ti-Zr reduce the average nano-oxides size,while the number density of nano-oxides increase in ODS powders.In only Y2O3-containing sample,the nano-oxides are mainly Y2O3 with a small amount of YCrO3.Y2Ti2O7 oxides and Y4Zr3O12 oxides are found in Ti-containing powders and Zr-containing powders,respectively.For the powders with the Ti and Zr additions,the linear combination fits of the normalized X-ray absorption near edge struction(XANES)results indicated the content of Y-Zr is much higher than that of Y-Ti(84%:16%),and the density is the highest and the size of oxides is the smallest by SAXS.