Preparation,Microstructure And Mechanical Properties Of ODS-AlCrFeNi High Entropy Alloy

High entropy alloy(HEA)has high strength,high hardness,excellent corrosion resistance and radiation resistance.Oxide dispersion strengthening is a widely used strengthening method for conventional alloys,and oxide dispersion strengthening is also beneficial to improve the radiation damage tolerance of materials.Oxide dispersion strengthened high-entropy alloy(ODS-HEA)is a novel high-entropy alloy that combines the advantages of high-entropy alloy and dispersion strengthening,which is an important candidate for new anti-irradiation structural materials for advanced nuclear reactor application.In this article,AlCrFeNi high-entropy alloy and ODS-AlCrFeNi high-entropy alloy were prepared by mechanical alloying(MA)and spark plasma sintering(SPS).The effects of MA process parameters,SPS sintering temperature and different Y2O3 and Ti contents on the microstructure and mechanical properties of AlCrFeNi and ODS-AlCrFeNi high-entropy alloy were were investigated.Considering the size,shape and solid solution degree of the alloy powder after ball milling,the optimum ball milling process is found as follows:ball ratio 10:1,speed 250 rpm,ball milling time 45 h,then add alcohol wet grinding,speed 150 rpm,ball mill time 5 h.AlCrFeNi and 0.3 Y(an ODS-AlCrFeNi alloy,0.3 wt%Y2O3 and 0.3 wt%Ti)alloys were preparated at different SPS sintering temperatures(1050℃,1100℃,1150℃,1200℃,1250℃).It was found that the relative density of the alloy gradually increases with the increase of the sintering temperature and tends to be stable.When the sintering temperature is between 1050℃ and 1150℃,AlCrFeNi is a mixed structure of BCC[Fe,Cr]phase and B2(NiAl)phase.When the sintering temperature reaches 1200℃ and above,a small amount of FCC phase appears.The 0.3Y alloy sintered at a temperature of 1200℃ and below is a mixed structure of BCC[Fe,Cr]phase and B2(NiAl)phase,and a small amount of FCC phase appears after sintering at 1250℃.EBSD analysis showed that the average grain size of 0.3Y alloy increases with the increasing sintering temperature.The hardness of the alloy increases first and then decreases with increasing temperature,and the peak value appeares at 1100℃,which is 636 HV.Therefore,it is determined that the optimum SPS sintering process is 1100℃,the pressure is 40 MPa,and the holding time is 5 min.The microstructure and mechanical properties of ODS-AlCrFeNi with different Y2O3 and Ti contents prepared by the above MA-SPS process were analyzed.It was found that the hardness of ODS-AlCrFeNi high-entropy alloy is not significantly improved when Y2O3 content is changed when Ti content is fixed at 0.3wt%.However,when the Y2O3 and Ti contents are increased with the same ratio,the hardness value of the ODS-AlCrFeNi high-entropy alloy is remarkably improved.A systematic study on the microstructure and mechanical properties of AlCrFeNi and four ODS-AlCrFeNi alloys(0.3YT,0.6YT,3.0YT and 5.0YT)was carried out,and the following conclusions were obtained:(1)The phase structure of AlCrFeNi and four ODS-AlCrFeNi alloys after ball milling is a single-phase BCC solid solution.After spark plasma sintering,a mixed structure of BCC[Fe-Cr]phase and B2(NiAl)phase is formed.(2)EBSD analysis showed that with the same proportion of Y2O3 and Ti content increasing(0.3～3 wt%),the average grain size of ODS-AlCrFeNi is gradually refined.But when it reaches 5 wt%,the average grain size increases.The average grain size of the 3YT alloy is the smallest,which is 240 nm.(3)With increasing the content of Y2O3 and Ti,the average size of nanoscale oxides in ODS-AlCrFeNi decreases and the number density increases.But the excessive addition of Y2O3 and Ti(5 wt%)will result in the average size of oxides increasing and the number density decreasing.The average size of nanoscale oxides in 3 YT alloy is the smallest(8.7 nm),and the numer density is the highest(3.3×1022 m-3).High resolution transmission electron microscopy(HRTEM)analysis showed that the nanoscale oxides in 3 YT alloy includes AlYO3,Al2Y4O9,Y2TiO5,YTiO3,Al2TiO5 and Al2O3.(4)The hardness of ODS-AlCrFeNi alloy increases firstly and then decreases with the increasing of the same proportion of addition of Y2O3 and Ti.The hardness of 3 YT alloy is the highest,which is 751 HV.The high hardness of the ODS-AlCrFeNi alloy is mainly attributed to the contribution of fine grain strengthening and precipitation strengthening.