Synthesis High Strength And Ductility Of CoCrFeNi Based High-entropy Alloy By High Gravity Combustion

High gravity combustion synthesis technology is a new method to prepare materials by combining high gravity field with combustion synthesis technology.The purpose of this thesis is to expand the new application of high entropy combustion synthesis technology in the field of high entropy alloy preparation.High entropy alloy was prepared by high gravity combustion synthesis technology,and its phase composition,microstructure,mechanical and thermodynamic properties were systematically characterized.The influence of Ni3Al content and Ti/Ta elements on the microstructure and mechanical properties of high entropy alloy were studied.The specific work includes.The Cr0.9FeNi2.5V0.2Al0.5,CoCrFeNi(Ni3Al)x and CoCrFeNi3Al0.75(Ti/Ta)high entropy alloys were successfully prepared from low-cost thermite powder by the combination of high gravity combustion synthesis technology and post-treatment.In this process,the high entropy alloy melt and ceramic melt are formed rapidly through the high-temperature combustion synthesis reaction.They are completely separated in the high gravity field,and the parent alloy is obtained after cooling.Then,the mechanical properties of the alloy were improved by the conventional vacuum arc remelting,homogenization,cold rolling and annealing.Cr0.9FeNi2.5V0.2Al0.5 was used as the alloy system.The high entropy alloy with excellent tensile properties was successfully prepared by high gravity combustion synthesis and vacuum arc remelting.The tensile strength reached 1100 MPa and plastic deformation exceeded 3%.Through the calculation of the phase separation kinetics of high-temperature melt,it is found that a higher gravity coefficient is expected to directly prepare a high-performance final alloy across the intermediate alloy.In CoCrFeNi(Ni3Al)x system,a new strategy of high entropy alloy is presented,high strength and promising ductility are realized by precipitation strengthening mechanism.that is,adding Ni3Al to CoCrFeNi to form a structure of dispersed nano-L12 precipitates on FCC high entropy alloy matrix.By optimizing the amount of Ni3Al,the mechanical properties of the alloy can be effectively controlled.CoCrFeNi(Ni3Al)0.75 alloy has the best comprehensive strength and toughness,the tensile fracture strength reaches 1200 MPa,the yield strength reaches 910 MPa,and the elongation reaches 14%.Compared with the CoCrFeNi high entropy alloy without precipitates,the yield strength is increased by three times.TEM analysis and theoretical calculation show that the strengthening mechanism of the precipitation is the main factor to improve the properties of the alloy.On the basis of CoCrFeNi(Ni3Al)0.75,the properties of the alloy were improved by adding Ti and Ta microelements.The tensile fracture strength,yield strength,and elongation of the alloy were 1450 MPa,1150 MPa and 15%,respectively.When Ti and Ta were added at the same time,the tensile properties of the alloy reached the original target,the tensile breaking strength was 1550 MPa,the yield strength was 1220 MPa,and the elongation was 6%.The main strengthening mechanism of the two alloys is the precipitation strengthening.