Study On Aging Precipitation Behavior And Mechanical Properties Of Ni₅₆Cr_19.5Fe_19.5X₅(X=Ta,Ti,Mo)High Entropy Alloy

High-entropy alloy(HEA)is a new type of metal material made up of a variety of elements in equal or near equal atomic ratio.It exhibits many excellent mechanical,physical and chemical properties,it has the potential as a new type of structural material.Single-phase FCC structure HEAs usuaally exhibit outstanding ductility but low strength,single-phase BCC structure HEAs usually has high strength but poor ductility.So its comprehensive mechanical properties need to be improved.In recent years,the second phase strengthening of high-entropy alloys has attracted widespread attention from scholars at home and abroad.Research indicates,introducing a nano,dispersed,and coherent second phase into the FCC structure can improve the strength of the alloy while maintaining better plasticity.In this Paper,three elements of Ta,Ti,and Mo were added respectively to the FCC matrix composed of Ni Cr Fe,and three alloys:Ni₅₆Cr_19.5Fe_19.5X₅(x=Ta?Ti?Mo)were designed and pre Pared.XRD,SEM and TEM were used to study the aging precipitation behavior and its influence on mechanical properties,and the precipitation strengthening effect and mechanism of the precipitates were discussed.The main conclusions are as follows:(1)After the Ni₅₆Cr_19.5Fe_19.5Ta₅ alloy is aged at 700?,the FCC matrix of the alloy can precipitate the^??-(Ni Cr Fe)₃Ta ordered precipitation strengthening phase with the D0₂₂ structure in a coherent relationship with the matrix.After proper aging treatment(700°C,100h),the strength of the alloy was significantly improved,the yield strength and tensile strength were 1094 MPa and 1283 MPa,respectively,while maintaining 26%elongation after fracture.The improvement of alloy strength is mainly due to the orderly strengthening and coherent strengthening of the nano-sized^??phase dispersed in the crystal.At the same time,the coherent relationship between the^??phase and the matrix increases the strength of the alloy while maintaining good plasticity.(2)The strength of Ni₅₆Cr_19.5Fe_19.5Ta₅ alloy is significantly improved after aging treatment at 700?,and the strength of the alloy further increases with the increase of aging time.The yield strength and tensile strength of the alloy aged at 700?/20 h are866 MPa and 1080 MPa,respectively.compared with the strength of the unaged solid solution alloy(480 MPa,and 848 MPa),they are increased by 80%and 27%,respectively while retaining 21%elongation.The yield strength of the alloy aged at 700?/150 h reached 1152 MPa,but the elongation rate was 12%.(3)After the Ni₅₆Cr_19.5Fe_19.5Ti₅ alloy is aged at 650?/20 h,the nano-precipitation strengthening phase of the L1₂ structure can be precipitated in the alloy FCC matrix in a coherent relationship with the matrix.and the L1₂ phase is dispersed in the crystal and the size is less than 5nm.The precipitation of the L1₂ nanophase significantly increases the strength of the alloy after aging at 650?/20 h.The yield strength and tensile strength of alloy are 411 MPa and 656 MPa,respectively,which are increased by 290%and 228%respectively compared with the solid solution alloy without aging treatment.(4)After the Ni₅₆Cr_19.5Fe_19.5Ti₅alloy is aged at 650?/50-150 h,another phase is precipitated at the grain boundary of the alloy.Combined with the TTT curve,this phase is?phase.This new precipitated phase destroys the continuity of the alloy and reduces the yield strength and tensile strength after aging at 650?/50-150 h.(5)After the Ni₅₆Cr_19.5Fe_19.5Mo₅alloy is annealed at 700?/20-150 h,there is no precipitation phase in the FCC matrix,but annealing twins are observed in the annealed alloy.In addition,the work hardening ability of 700?annealed alloys has been significantly improved.The increase in work hardening ability of high-entropy alloys after annealing is relatively rare,and there are few reports in the literature,which are worthy of further study.