Study On Microstructure Evolution And Properties Of CoCrFeNi-based Eutectic High-entropy Alloy

High-entropy alloys(HEAs)are currently a major research hotspot in the field of metal materials.They have excellent comprehensive properties,such as high strength,high hardness,good corrosion resistance,high temperature creep resistance and radiation resistance,bringing great potential for applications.The CoCrFeNi-based HEAs are the most extensively studied.Among them,eutectic high-entropy alloys(EHEAs)have become a hot research topic due to their good casting fluidity and good combined plastic toughness.Researches have demonstrated that by adjusting the content of alloying elements of the CoCrFeNi-based HEA,the eutectic high-entropy alloy with excellent properties can be obtained.The Al_xCoCrFeNi_2.1and CoCrFeNi Mo_x HEAs were prepared by changing the content of Al and Mo in the alloy,and the crystal structure,microstructure and mechanical properties of the alloy were systematically studied.Concluded as follow:(1)With the increase of Al element,the crystal structure of Al_xCoCrFeNi_2.1 HEAs changes from single-phase FCC solid solution to FCC+small amount of BCC phase,then FCC and B2 phases are formed,and finally FCC phase is completely transformed into B2phase.With the increase of Al content,the volume fraction of the hard BCC/B2 phase increases,the hardness,tensile strength and yield strength of the alloy increase,while the elongation continuly decrease.The fracture mode changed from ductile fracture to cleavage fracture.It is worth mentioning that the unique"steel frame"eutectic structure of Al CoCrFeNi_2.1 alloy is composed of FCC phase,BCC phase and L1₂ phase.The unique eutectic microstructure and semi-coherent interface between L1₂/FCC and B2 in Al CoCrFeNi_2.1 alloy can prevent crack propagation and hinder dislocation movement.The alloy shows excellent mechanical properties and significant notch strengthening effect.The corrosion resistance of Al_xCoCrFeNi_2.1 HEA in 3.5wt.%Na Cl solution decreases with the increase of Al content.The corrosion resistance of Al_0.9CoCrFeNi_2.1 and Al CoCrFeNi_2.1alloys with eutectic structure is slightly improved,and the corrosion resistance of Al_1.1CoCrFeNi_2.1 alloy is also slightly improved due to the transformation of the matrix phase.(2)With the increase of Mo content,the crystal structure of CoCrFeNi Mo_x HEAs changes from single-phase FCC solid solution to FCC phase and?phase,and the volume fraction of?phase increases with the increase of Mo element content.With the increase of Mo content,the difference in atomic radius and the volume fraction of hard?phase in the alloy increase.The hardness and strength of the alloy increase while the elongation decreases.The fracture mode of the alloy also changed from ductile fracture to cleavage fracture.CoCrFeNi Mo_0.6 alloy achieves a perfect combination of high strength and high ductility due to its hypoeutectic structure,and its strength and ductility are 2051MPa and 23%,respectively.CoCrFeNi Mo_x alloys have obvious passivation in Na Cl solution and has good corrosion resistance.The addition of Mo element can significantly improve the pitting corrosion resistance of the alloy,but excessive addition of Mo element will cause a large amount of precipitation of?phase,leading to the preferential dissolution of the lean(Cr,Mo)FCC phase,and the corrosion resistance of the alloy is weakened.