Microstructure And Mechanical Properties Of Ni_43.9Co_22.4Fe_8.8Al_10.7Ti_11.7B_2.5 High Entropy Alloys

Traditional alloys optimize their properties by adding a small number of other elements to a single main element.Although they are used in engineering,it is difficult for them to achieve both strength and plasticity.Therefore,it is extremely urgent to improve the strength and plasticity of alloys at the same time.High entropy alloy is a new kind of material composed of multiple principal elements.It is of great significance for the development and promotion of high entropy alloy to change the microstructure and optimize the mechanical properties by controlling the directional solidification parameters and heat treatment process.In this work,Ni_43.9Co_22.4Fe_8.8Al_10.7Ti_11.7B_2.5 high entropy alloy was prepared by vacuum induction suspension melting furnace.By using directional solidification technology,experiments were carried out at different withdrawal rates（3μm/s,10μm/s,50μm/s,100μm/s,200μm/s）to study the effect of withdrawal rates on its microstructure,phase structure and mechanical properties,and to explore the growth mechanism and alloy strengthening mechanism.Meanwhile,the as-cast and directionally solidified high entropy alloy were heat treated to study the effect of precipitation on the phase structure and mechanical properties of the alloy.The main research results are as follows.The Ni_43.9Co_22.4Fe_8.8Al_10.7Ti_11.7B_2.5 high entropy alloy prepared by directional solidification are all FCC single phase structure.Ni and Al are positive element segregation and enriched in dendrite region.Fe,Ti,Co and B are negative element segregation,which are enriched in the interdendritic region.With the increase of withdrawal rate,the solid-liquid interface changes from cellular dendrite to dendrite,and the grain size decreases gradually.When the drawing rate is 200μm/s,the tensile strength and elongation of Ni_43.9Co_22.4Fe_8.8Al_10.7Ti_11.7B_2.5 high-entropy alloy are the highest,which are 1083.01 MPa and11.8%respectively.The compressive strength and hardness showed maximum values of2467.84 MPa and 548.65 HV_0.2,respectively.After solution treatment,with the increase of solution temperature and time,the grain size of as-cast alloy increases.The precipitated phase is substantially dissolved into the FCC matrix to form a supersaturated solid solution.After aging treatment,the alloy produced nano-sized L1₂ precipitates,and the size of L1₂ phase gradually increases with the increase of aging temperature.The tensile strength and elongation after solution at 1050℃for 4h and aging at800℃for 4h are the highest,being 928.31 MPa and 35.48%,respectively.The microstructure of directionally solidified alloy remains columnar crystals after solution aging at withdrawal rate of 10μm/s.Compared with the alloy without solution aging,the precipitation of L1₂ phase results in an increase in the elongation and basically the same tensile strength of the alloy.The results confirm that heat treatment can significantly optimize the organization and mechanical properties of Ni_43.9Co_22.4Fe_8.8Al_10.7Ti_11.7B_2.5 high-entropy alloys in the as-cast and directionally solidified states.