Microstructure And Mechanical Properties Of Fe₆₀Co₁₀Cr₁₀Ni₁₀Mo₅V₅ Medium-entropy Alloy

Medium entropy alloys(MEAs)is a novelty material emerging in recent years.Its unique structure and properties have attracted extensive attention of many scholars.The traditional HEA with equal atomic ratio Fe Co Cr Ni and Fe Co Cr Ni Mn is a solid solution with FCC structure,which has good plasticity and toughness,but low strength,so it is difficult to meet the application requirements of engineering structural materials;In addition,use a large number of expensive alloy elements such as Co and Ni greatly increases the alloy preparation cost and further restricts its industrial application.Therefore,according to the results reported in the literature and the criterion of phase formation of HEA,a non-equiatomic ratio Fe₆₀Co₁₀Cr₁₀Ni₁₀Mo₅V₅ MEA was designed on the basis of Fe Co Cr Ni HEA,and the microstructure and properties of the alloy were studied.On the basis,through the carbon micro-alloying/deformation and annealing treatment,a large number of dispersed second phase were obtained.The effects of carbon micro-alloying/deformation and annealing treatment on the microstructure and mechanical properties of the MEA were systematically studied.The main research results are as follows:(1)The microstructure of Fe₆₀Co₁₀Cr₁₀Ni₁₀Mo₅V₅ MEA is a single FCC solid solution.After homogenization,the yield strength is 256 MPa,the tensile strength is 588 MPa,and the elongation is 42%.Compared with the traditional equal-atomic ratio Fe Co Cr Ni and Fe Co Cr Ni Mn HEA the mechanical properties are improved.The reason is that Mo and V atoms entering the matrix can produce solid solution strengthening effect.(2)By introducing 1 wt.%C,a large amount of networked carbides(about 0.66?m in thickness and 34%in volume fraction)were produced at the grain boundary of the MEA.After solution treatment at 1180?/1 h and aging treatment at 800?/4 h,a large number of VC precipitates with a size of about 46 nm were produced in the alloy,which had a cube to cubic orientation relationship with the FCC matrix.With the size of 46 nm and the volume fraction of5.6%,the alloy plays the role of diffusion strengthening and precipitation strengthening,which makes the yield strength and tensile strength of the alloy increase to 349 MPa and 626 MPa,respectively.However,the plasticity of the alloy is severely reduced by the large amount of reticulated carbides produced at grain boundaries.(3)After cold-roll and annealing at 850?1050?,a large number of dispersed?phase precipitates in the matrix of the MEA.The average size of?phase in the alloy annealed at 850? is 230 nm and the volume fraction is 16.3%,which can hinder the growth of recrystallized grains,thus obtaining fine grain structure with average grain size of 2.3?m.With the increase of annealing temperature,the?-phase size increases to 830nm,the volume fraction decreases to10.4%,and the grain size increases to 4.8?m.The MEA annealed at 850?/1 h has a tensile strength of 930 MPa and an elongation of 21.3%,which is higher than that of similar alloys reported in most literatures.The results of strengthening mechanism analysis showed that the strengthening effect mainly came from fine grain strengthening and precipitation strengthening,which reached 301 MPa and 177 MPa,respectively.The better ductility of the alloy can be attributed to the more uniform deformation caused by the fine grain strengthening and the better dislocation accommodating capacity of the annealing twin,which improves the plastic deformation capacity of the alloy.