| High entropy alloys(HEAs)consist multi-principal elements as a new kinds of alloys,which break the design framework of traditional alloys.The coordination of multi-principal elements in HEAs lead to alloys have many excellent properties,such as high strength/hardness,excellent wear resistance,good corrosion resistance,high low temperature toughness and so on.Therefore,the HEAs possess broad prospects of application.However,most of HEAs are solid solution alloys with poor casting liquidity and macro/micro composition segregation restricting the large-scale industrial application of HEAs.Furthermore,eutectic high entropy alloys(EHEAs)combining the advantages of both HEAs and eutectic alloys exhibit excellent castability,so it has the important economy value and theoretical significance.In this paper,equal proportion mixture method for designing the eutectic composition in HEAs based on binary phase diagrams is proposed,and CoFeNi(Cr)-M(M = Nb,Ta,Zr,Hf,Al)alloys are obtained.Present paper is mainly studied the composition,microstructures and mechanical properties of above alloys by a variety of test and analysis methods.The main conclusions are as follows:(1)A new method is proposed to design EHEA compositions.Firstly,eutectic compositions are obtained by binary alloy phase diagram,and then the binary eutectic compositions are mixed in equal proportion to obtain the eutectic alloy with different components.This method is suitable for FeNiNb0.35 ternary eutectic alloy,CoFeNiNb0.5 quaternary eutectic alloy and(CoCrFeNi)Mx(M = Nb,Ta,Zr,Hf)quinary eutectic alloys.This simple and effective method provides a new way for the composition design of multi principal EHEAs.(2)The binary eutectic simple mixture strategy is applied to a series of(CoCrFeNi)Mx(M = Nb,Ta,Zr,Hf)HEAs,leading to the successful designed of new EHEAs,namely,CoCrFeNiNb0.45,CoCrFeNiTa0.4,CoCrFeNiZr0.55 and CoCrFeNiHf0 4.These new EHEAs are composed of FCC and Laves two phases with fine lamellar morphology.Wherein,the CoCrFeNiNb0.45 and CoCrFeNiTa0.4 alloys exhibit the best comprehensive mechanical properties.The mainly strengthening mechanism of CoCrFeNiTa0.4 EHEA is the Laves second phase strengthening and Hall-Petch strengthening.(3)The bulk CoCrFeNiNbo.45 EHEA with ultrafine microstructure(lamellae width about 200 nm)is designed and prepared by direct solidification.The alloy shows outstanding high temperature softening resistance,inwhich,the Vickers hardness of the alloy decreases only 7%(to 479 HV)after 1100 ? annealing comparing the as-cast sample with hardness of 515 HV.Moreover,at 700 ?,the alloy still shows better mechanical properties,the maximum strength of 1082 MPa,yield strength of 843 MPa,and fracture strain of 28%.(4)HEA coatings based on the CoCrFeNiNbx(x = 0.455 0.5,0.75,1.0)compositions with excellent wear resistance were synthesized on 304 stainless steel substrate by laser cladding.The microstructure of samples varied from hypoeutectic structure(x = 0.45,0.5)to hypereutectic structure(x = 0.75,1.0).The hardness value of CoCrFeNiNb1.0 is 590 HV,which is 2.8 times of substrate hardness.The CoCrFeNiNb1.0 HEA coating shows the best wear resistance under the same wear condition.(5)CoFeNi2Al0.9 EHEA with FCC and B2 dual-phases regular lamellar eutectic structure exhibits a better mechanical properties:room temperature tensile yield strength of 559 MPa,fracture strength of 1005 MPa,elongation of 6.2%;600 ? high temperature tensile fracture strength of 706 MPa,elogation of 26.4%.In addition,the bulk CoFeNi2Al0.9 EHEA shows a potential industrial application.(6)TEM is used to analysis the dislocation morphology of CoFeNi2Al0.9 EHEA,results indicate that in FCC phase the dislocation changes from plane dislocations to bending dislocation,high density dislocation,dislocation network,then to dislocation wall,Taylor lattice.In B2 phase the evolution of dislocation is from a very small number of short dislocations to a number of planar dislocations. |
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