Optimization Design And Microstructure Control Of Nanostructured Oxide Dispersion-strengthed High-entropy Alloys

High entropy alloy（HEAs）has become a hot research topic in the field of metal materials because of its novel design concept,broad design space and unique properties.FCC high-entropy alloys have attracted the most attention in many developed high-entropy alloy systems,because these alloys generally have a series of excellent properties,such as good ductility,high fracture toughness,corrosion resistance,oxidation resistance and radiation resistance,etc.,in particular,it has great potential as a new radiation-resistant material for nuclear reaction systems.However,due to the limitation of phase structure,the strength of FCC structure high entropy alloy is usually low,which can not meet the requirement of safe bearing of structure material.Oxide dispersion strengthening（ODS）is an effective method to improve the strength of materials at room and high temperature,and it is also beneficial to enhance the ability of anti-irradiation damage.Therefore,it is of great significance to design oxide dispersion-enhanced FCC-HEAS（ODS-FCC-HEAs）which has both advantages of ODS and FCC.In this paper,a series of ODS high entropy alloys based on NiCoFeCuTi high entropy alloy and Y₂O₃ were prepared by mechanical alloying（MA）+Spark Plasma Sintering（SPS）.The effects of Y₂O₃ content and Y/Ti atom ratio on the precipitation behavior,phase composition,microstructure and mechanical properties of ODS-NiCoFeCuTi high entropy alloy were studied.The main findings are as follows:（1）After treated with MA,the composition is Ni₂₆Co₂₆Fe₂₅Cu₁₇Ti₆+x wt.%Y₂O₃（x=0,0.35,0.70,1.05,1.40 and 1.70,denoted as T6-x Y）and Ni_{26-（y-6）/4}Co_{26-（y-6）/4}Fe_{25-（y-6）/4}Cu_{17-（y-6）/4}-Ti_y(y=4,8,10 and 12,Ty-1.05Y alloy powders were composed of FCC and BCC solid solutions;After SPS sintering,the original powder BCC phase in the alloy disappeared,and then a small amount of oxide phase was formed in the bulk of the alloy;After SPS sintering,the original powder BCC phase in the alloy disappeared,and then a small amount of oxide phase was formed in the bulk of the alloy;In addition,Ni₃Ti intermetallic compounds were precipitated in Ty-1.05Y alloy when y≥8;（2）SEM-BSE analysis shows that there are two areas of different contrast in the microstructure of six kinds of T6-x Y and four kinds of Ty-1.05Y ODS bulk high entropy alloys:the light gray area corresponding to the FCC matrix phase and the dark gray area with a large number of fine precipitated particles dispersed;With the increase of the content of Y₂O₃（That’s an increase in the value of x）,the proportion of the light gray area(V_f-lg)in the microstructure of T6-x Y bulk alloy decreases first and then increases,in addition,the size of dispersed oxide particles in the dark gray area became thinner first and then became thicker;However,with the increase of Ti content（y value）in the composition,that is,the decrease of Y/Ti ratio,the ratio of the two regions in the microstructure of Ty-1.05Y alloy did not change significantly,however,the size of precipitated particles in the dark gray area gradually increased.（3）TEM analysis shows that the ODS bulk high-entropy alloys with T6-x Y and Ty-1.05Y all have a dual-scale grain structure,that is,there are both coarse and fine grain regions,this phenomenon corresponds to the two contrast regions observed in the SEM-BSE field of view;The content of Y₂O₃ increased from 0 to 1.75 wt.%When x=1.05,the proportion of fine grain area in T6-1.05Y alloy is the highest（～67.25%）,and the overall average grain size is also the smallest（～333nm）;（4）HRTEM analysis shows that the change of Y₂O₃ content has a significant effect on the typical characteristics of oxide particles precipitated in T6-x Y ODS high entropy alloy When Y₂O₃ is not added into the Ti-containing high-entropy alloy matrix,TiO oxides with size of 30 nm and discordant with the matrix are formed in T6-0Y alloy;With the addition of Y₂O₃,Y₂Ti₂O₇-type ternary oxides with average particle size of～8 nm and mainly semi-coherent with FCC matrix were precipitated in T6-1.05Y and T6-1.75Y alloys,and the highest precipitated ratio of Y₂Ti₂O₇ was obtained when x=1.05,therefore,the average size of dispersed particles in ODS high entropy alloy is the smallest（～17nm）.（5）According to the results of EBSD analysis of T8-1.05Y and T10-1.05Y,the proportion of fine grain region in two-scale structure is 32.4%and 42.4%,respectively;The content of fine grains in Ty-1.05Y ODS bulk alloy decreases with the increase of Ti,that is,the decrease of Y/Ti ratio,this may be related to the decrease of pinning effect on the matrix grains due to the increase of particle size such as oxides precipitated in the fine grain region.（6）The results of mechanical properties test show that the strength and plasticity of T6-x Y high entropy alloy decrease first and then increase with the increase of Y₂O₃ content under compression and tensile conditions at room temperature;When x=1.05,T6-1.05Y alloy shows the best comprehensive mechanical properties.The yield strength,tensile strength and elongation of T6-1.05Y alloy increase by 21%,27%and 157%respectively compared with the matrix high entropy alloy This special phenomenon may be related to more Y₂Ti₂O₇oxide particles precipitated in the alloy and the obvious strengthening and toughening effect;In the case of ODS bulk high entropy alloy with Ty-1.05Y composition,with the increase of Ti content,that is,the decrease of Y/Ti ratio,the strength of the alloy increases obviously,but the plasticity decreases obviously,this may be related to the formation of Ni₃Ti-type brittle intermetallics due to excessive Ti and the coarsening of the average size of oxide particles precipitated in the alloy.