| Since Professor Ye Junwei put forward the concept of "high entropy alloy",high entropy alloy has been widely studied and greatly concerned by scholars at domestic and international.The high entropy alloy has broken through the design concept of the traditional alloy,and has the excellent characteristics that the traditional alloy does not have,such as high strength,high hardness,good corrosion resistance,excellent wear resistance,etc.At present,high entropy alloy has become a research hotspot in the field of materials,so the development of high entropy alloy with both high strength and high plasticity has very important theoretical value and engineering significance.In this paper,the high entropy alloy ingots of(Co25Fe25Ni25Mn25)100-xCx(x=4,5,6,7,8),CoFeNiMn(TixC100-x)0.25(x=20,30,40,60,70,80)were prepared by vacuum arc melting.The phase structure,microstructure and mechanical properties of the alloys were analyzed by X-ray diffraction(XRD),scanning electron microscope(SEM),transmission electron microscope(TEM)and universal compression testing machine.Then the spray casting experiments of the two systems were carried out to investigate the microstructure and mechanical properties of the alloy s under the condition of rapid cooling.The main conclusions of this paper are as follows:(1)The as-cast(Co25Fe25Ni25Mn25)100-xCx(x=4,5,6,7,8)high entropy alloys were prepared by adding element C.When x≤5,the alloys are single FCC phase solid solution structures.When x>5,the alloy phase structures change to FCC+M23C6 two-phase structures.The lattice constant(aFCC)of FCC phase in the alloy increases first and then decreases with the increase of C content.When x=6,the lattice constant of the FCC phase reaches the maximum value of 3.630 A.When the carbon content in the alloy is low(x=4,5),the alloy structure is composed of equiaxed crystals.When the content of carbon in the alloy reaches 6 at.%,the microstructure of the alloy is completely transformed into dendrite,and the interdendrite is rich in carbides.When the carbon content in the alloy is high(x≥7),the interdendritic structure is transformed into eutectic structure composed of FCC phase and M23C6 carbide.With the increase of C content,the strength and hardness of the alloy increase,while the ductility decreases.The yield strength,compressive strength,fracture strain and hardness of(Co25Fe25Ni25Mn25)92C8 alloy are 1078 MPa,2773 MPa,46.7%and 413 HV0.3,respectively.(2)The in-situ TiC particle reinforced CoFeNiMn(TixC100-x)0.25(x=20,30,40,60,70,80)high entropy alloys were prepared.With the increase of x value,the alloy changes from a single FCC phase to FCC+TiC two-phase structure,and the αFCC decreases firstly and then increases with the increase of x value.The microstructure analysis shows that regular micron-size TiC particles are formed in the alloy,and more than 80%of the TiC particles are smaller than 3 μm in size.The yield strength and hardness of the alloy decrease firstly and then increase with the increase of x value,while the alloy still maintains excellent plasticity,and the fracture strain is greater than 70%.(3)For the spray-cast(Co25Fe25Ni25Mn25)100-xCx high entropy alloy,when x≤7,the alloys are only composed of single FCC phase solid solution;When x>7,the alloy phases ae composed of two phase structure of FCC+M23C6.The precipitation of M23C6 carbide phase can be effectively inhibited under the condition of fast cooling.The aFCC increases firstly and then decreases with the increase of C content,and the maximum aFCC(3.626 A)is obtained at x=7.When the carbon content in the alloy is low(x=4,5),the microstructure of the alloy is composed of grains.When the carbon content in the alloy is increased to 6 at.%,the microstructure of the alloy is completely transformed into dendrite,and there are carbides in the interdendrite.The mechanical properties show that the yield strength and hardness of the alloy increase with the increase of C content.(Co25Fe25Ni25Mn25)92C8 alloy has excellent mechanical properties,and its yield strength,compressive strength,fracture strain and hardness are 166 MPa,544 MPa,37%and 398 HV0.3,respectively.(4)With the increase of x value in the alloy,the structure of the spray cast CoFeNiMn(TixC100-x)0.25 high entropy alloy changes from a single FCC phase to FCC+TiC two-phase structure,and the lattice constant of the FCC phase decreases firstly and then increases.Scanning electron microscope analysis shows that the matrix of the alloy is a uniform solid solution structure,and TiC particles are uniformly distributed in the microstructure.The yield strength of the alloy decreases continuously with the increase of x value.The alloy has good plasticity and the fracture strain is more than 60%.The hardness of CoFeNiMn(Ti60C40)0.25 alloy decreases firstly and then increases with the increase of x value,and the hardness of CoFeNiMn(Ti60C40)0.25 alloy reaches the minimum value of 195 HV0.3. |
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