Microstructure And Mechanical Behavior Of NiMnCu And NiMnV Multi-principal Element Alloy

The concept of multi-principal element alloy was first put forward in 2004.It has attracted much attention because it is significantly better than traditional alloys in hardness,strength,plasticity and thermal stability.At present,the reported ternary alloy CoCrNi not only shows excellent mechanical properties at room temperature,but also has better strength and toughness at low temperature than at room temperature,which is contrary to the low temperature brittleness of traditional alloys.Although a series of multi-principal element alloys with excellent mechanical properties have been reported,there are still some unsolved problems in the field of multi-principal element alloys.First of all,there are not many kinds of multiprincipal element alloys with high strength and toughness,and the alloy components are mainly concentrated in Ni,Co,Cr,Al,Ti,Mo and other elements.There is an urgent need to further expand the selection of alloy elements and develop more multi-principal element alloy with excellent mechanical properties.Secondly,the multi-principal element alloy developed at present still does not solve the problem of alloy strength and plasticity inversion,and the comprehensive mechanical properties are difficult to meet the needs of social development,so we should further develop the coexistence of multi-mechanism,multi-structure and cross-scale strengthening manners to realize the high strength and toughness of the alloy.In this paper,the composition of the alloy is screened by first-principle calculation,and the multi-principal element alloy systems of Ni50Mn50-xCux and Ni50Mn50-xVx(x=12.5,17.5 and 25)were prepared by non-consumable vacuum arc melting and suction casting system.The changes of microstructure and mechanical properties were studied systematically by X-ray diffractometer(XRD),scanning electron microscope(SEM),electron backscatter diffraction(EBSD),transmission electron microscope(TEM),electron universal testing machine and so on.The main conclusions are as follows:The first-principles calculation results show that Ni50Mn50-xCux multi-principal element alloy(x=5,12.5,15,17.5 and 25)is the most stable under Fcc structure,and with the decrease of mole fraction of Cu,the stacking fault energy decreases gradually,and the stacking fault energy is 64.96,73.41,74.04,72.14 and 74.53 mJ/m2,respectively.The experimental results show that the Ni50Mn50-xCux multi-principal element alloy has a single Fcc structure.With the decrease of the mole fraction of Cu,the strength and plasticity of Ni50Mn50-xCux multi-principal element alloy increase obviously.Ni50Mn37.5Cu12.5 shows the best comprehensive mechanical properties.When fully recrystallized Ni50Mn7.5Cu12.5 is deformed at room temperature,the average grain size is 9.79 μm,showing the yield strength of 294 MPa,the tensile strength of 658 MPa and the uniform elongation of 64.4%;When partially recrystallized Ni50Mn37.5Cu12.5 is deformed at room temperature,it shows the yield strength of 396 MPa,the tensile strength of 747 MPa and the uniform elongation of 47.3%.With the decrease of the mole fraction of Cu,the strength and plasticity of Ni50Mn50-xCux multi-principal element alloy increase at the same time.This is attributed to the enhancement of chemical bond energy and the decrease of stacking fault energy in alloy crystals with the decrease of Cu mole fraction.The first-principles calculation results show that the energy of Ni50Mn50-xVx multiprincipal element alloy(x=5,12.5,15,17.5 and 25)is the lowest in Fcc structure,but with the increase of mole fraction of V,the stability of Fcc structure becomes worse,and with the increase of mole fraction of V,the stacking fault energy decreases gradually,and the stacking fault energy is 54.48,44.23,40.84,37.12 and 28.22 mJ/m2,respectively.V can effectively reduce the stacking fault energy of Ni50Mn50-xVx multi-principal element alloy.The experimental results show that nano-sized precipitates are gradually formed in the grains of Ni50Mn25V25 multi-principal element alloy with the increase of mole fraction of V.With the increase of V mole fraction,the strength of Ni50Mn50-xVx multi-principal element alloy increases obviously,but the plasticity decreases.Ni50Mn25V25 has excellent comprehensive mechanical properties.When fully recrystallized Ni50Mn25V25 is deformed at room temperature,the average grain size is 3.23 μm,showing the yield strength of 607 MPa,the tensile strength of 992 MPa and the uniform elongation of 40.51%.When partially recrystallized Ni50Mn25V25 is deformed at room temperature,it shows the yield strength of 897 MPa,the tensile strength of 1134 MPa and the uniform elongation of 18.63%.This is attributed to the fact that with the increase of V mole fraction,the grain size of Ni50Mn50-xVx multi-principal element alloy decreases gradually and nano-sized precipitates are formed in the Ni50Mn25V25 grains.Due to the combined action of fine grain strengthening and precipitation strengthening,with the increase of V mole fraction,the strength of Ni50Mn50-xVx multi-principal element alloy increases obviously,but the plasticity decreases.