Effect Of B On Microstructure And Mechanical Properties Of CoCrFeNi And CrFeNi₂ Multi-principal Element Alloys

As a new alloy system different from traditional alloys,high-entropy alloys have broken through the pattern of traditional alloy based on one or two elements,and have won domestic and foreign scientific researchers'favor due to its simple structure,high strength,high hardness and other excellent properties.Based on previous studies,this paper starts with the alloying of single non-metallic elements and the substitution of Ni elements for Co elements,designs and prepares as-cast(Co Cr Fe Ni)B_xand Cr Fe Ni₂B_x(x=0,0.2,0.4,0.6,0.8,and 1.0)high-entropy alloys.The purpose of this paper is to observe the microscopic structures and mechanical properties of high-entropy alloys and find the alloys with higher yield strength and better plasticity,using X-ray diffractometer(XRD),scanning electron microscope(SEM),etc.combined with energy spectrum analyzer(EDS)and other auxiliary instruments to characterize the microstructures of as-cast(Co Cr Fe Ni)B_xand Cr Fe Ni₂B_xhigh-entropy alloys.In addition,conduct microhardness experiments and room temperature compression experiments to test all aspects of the alloys'mechanical properties,and systematically study the relationships between their compositions,organizations and properties.Research indicates:(1)With the change of the B element content,the microstructure of the(Co Cr Fe Ni)B_xhigh-entropy alloy also changes.Only a single FCC1 phase appears in the Co Cr Fe Ni alloy,and there is boride phase between the dendrites of the(Co Cr Fe Ni)B_0.2alloy.Compound phase is formed,the alloy becomes FCC1 phase+boride phase mixed structure.The boride phase will increase with the increase of the B element content,and after adding more B element(Co Cr Fe Ni)B_0.8alloy,a new FCC2 phase will be formed,after which the alloy has always been a dual-phase FCC+boride phase mixed structure.(2)With the addition of B element,the(Co Cr Fe Ni)B_xhigh-entropy alloy produces lattice distortion and the formation of hard borides.With the increase of B element,the borides increase,and the yield strength of the alloy also increases.However,the borides in the(Co Cr Fe Ni)B_0.6alloy begin to precipitate first and aggregate into agglomerates,making the yield strength of the alloy begin to decrease instead,and the increase in borides also causes the compressive elongation to continue to decrease.The Vickers hardness of the(Co Cr Fe Ni)B_xhigh-entropy alloy increases with the increase of the B element content,which is caused by the combination of lattice distortion and the formation of hard borides.The(Co Cr Fe Ni)B_xhigh-entropy alloy has the best comprehensive mechanical properties in the(Co Cr Fe Ni)B_0.4alloy.The hardness of the alloy is 305.1 HV,the yield strength and fracture strength are 954.3 MPa and 1013.5 MPa,respectively,and the compressive elongation is42.0%.(3)With the change of the B element content,the microstructure of the Cr Fe Ni₂B_xalloy also changes.The Cr Fe Ni₂alloy has a single-phase FCC1 structure.There is a boride phase between the dendrites of the Cr Fe Ni₂B_0.2alloy,and the alloy becomes a mixed structure of FCC1 phase+boride phase.The boride phase will increase with the increase of the B element content.After adding more B element,a new FCC2 phase will be formed in the Cr Fe Ni₂B_0.6alloy.It has always been a two-phase FCC+boride phase mixed structure after that.(4)With the addition of element B,the Cr Fe Ni₂B_xalloy produces lattice distortion and the formation of hard borides.As the element B increases,the yield strength of the alloy also rises.At the same time,the yield strength of the alloy increases.The increase also leads to a continuous decrease in the compressive elongation.The Vickers hardness of the Cr Fe Ni₂B_xalloy increases with the increase of the B element content,which is caused by the combination of lattice distortion and the formation of hard borides.The Cr Fe Ni₂B_xalloy with the best comprehensive mechanical properties is the Cr Fe Ni₂B_0.4alloy.The alloy's hardness is248.4 HV,the yield strength and fracture strength are 926.2 MPa and 1082.3 MPa respectively,and the compressive elongation is 32.2%.The study found that B element can affect the microstructure of Co Cr Fe Ni and Cr Fe Ni₂alloys,and enhance the comprehensive mechanical properties of Co Cr Fe Ni and Cr Fe Ni₂alloys.It is found that replacing the Co element with Ni element can indeed increase the strength of the alloy,but will reduce the hardness and ductility of the alloy.