Microstructure Control and Mechanical Properties Optimization of Dual Phase Medium Entropy Alloy by Microalloying
The non-equiatomic ratio multiphase second-generation high entropy alloy developed from the first generation of high entro-py alloy with equal atomic ratio.The number of atoms of each element is mixed in different proportions.The design freedom of the alloy is greater and the"cocktail"effect of performance is brought into full play.Due to the increase of principal component number and mix-ing entropy,high entropy effect is produced to inhibit the formation of intermetallic compounds and other ordered phases.The different sizes and binding forces between elements lead to lattice distortion and slow diffusion effect.Higher lattice distortion promotes the me-dium entropy alloy to overcome greater lattice resistance when dislocations move,resulting in better fine grain strengthening effect.To explore new type of high-performance medium and high entropy alloy with high strength and plastic toughness has attracted more and more attention in the field of metastable metal materials.Among them,face-centered cubic(fcc)medium entropy alloy is easy to slip along(111)plane,with large crystal plane spacing,weak atomic binding force,small hindrance to dislocation,and there are many slip systems on(111)plane,small solid solubility and weak solid solution strengthening effect.Improving the strength of fcc high en-tropy alloy has become an urgent problem to be solved.Microalloying is one of the most effective methods to improve the comprehen-sive mechanical properties of alloys.Microalloying elements can refine grains and improve the mechanical properties of medium entro-py alloys.On the one hand,the dual phase fcc structure produces hard hexagonal closepacked(hcp)structure phase through martens-itic transformation,and transition induced plasticity(TRIP)effect increases the strength and plasticity of the alloy.On the other hand,fcc structure without transformation is also particularly important to improve the mechanical properties of medium entropy alloys.In this paper,three alloy system,(Fe63.3Mn14Si9.1Cr9.8C3.8)100-xCux,(Fe63.3Mn14Si9.1Cr9.8C3.8)100-xAgx and(Fe63.3Mn14Si9.1Cr9.8C3.8)100-2xCuxAgx(x=0,0.5,1,1.5,2;atom fraction)were fabricated by the water-cooled copper crucible magnetic levitation melting and copper mold negative pressure suction casting.By adding the positive mixing enthalpy elements Cu and Ag to fcc medium entropy alloy matrix,and obtained a novel biphasic fcc medium entropy alloy system.And the stacking fault energy of entropy alloy in fcc was calculated by Ol-son Cohen thermodynamic model.After the addition of Cu,Ag and their mixture,the atomic size difference,electronegativity and va-lence electron concentration increased,which promoted the formation of solid solution structure.The stacking fault energy of entropy alloys in the three series decreased with the addition of Cu/Ag.The stacking fault energy of Fe63.3Mn14Si9.1Cr9.8C3.8 alloy was 9.69 mJ·m-2,while(Fe63.3Mn14Si9.1Cr9.8C3.8)98Cu2,(Fe63.3Mn14Si9.1Cr9.8C3.8)98Ag2 and(Fe63.3Mn14Si9.1Cr9.8C3.8)96Cu2Ag2 alloys were 7.72,7.63 and 7.23 mJ·m-2,respectively.For(Fe63.3Mn14Si9.1Cr9.8C3.8)100-xCux(x=0,0.5,1,1.5,2;atom fraction),when x=0,Fe63.3Mn14Si9.1Cr9.8C3.8 alloy showed the single fcc phase;when x＜2,Cu could be dissolved in fcc lattice of the matrix,and the microstructure retains the single fcc solid solution.The mixing enthalpy between Cu,Ag and most other elements in the medium entropy alloy was positive and large,and relevant studies showed that the negative mixing enthalpy between elements led to the formation of intermetallic compounds,while the large positive mixing enthalpy between elements led to the phenomenon of phase separation.When x=2,there were two different fcc phases in the alloy,fcc1 was the poor Cu/Ag matrix phase,and fcc2 was the rich Cu/Ag second phase.The lattice constants of fcc1 and fcc2 were 0.36173 and 0.36468 nm in(Fe63.3Mn14Si9.1Cr9.8C3.8)98Cu2,0.36283 and 0.36598 nm in(Fe63.3Mn14Si9.1Cr9.8C3.8)98Ag2,and 0.36251 and 0.36574 nm in(Fe63.3Mn14Si9.1Cr9.8C3.8)96Cu2Ag2.After the compression fracture,X-ray diffraction(XRD)of the three al-loys systems showed that when x＜2,the diffraction peaks of the medium entropy alloy corresponded to fcc phase and hcp phase.When x=2,the diffraction peaks of the alloy correspond to fcc1+fcc2+hcp phase.Some of the poor Cu/Ag matrix phases underwent martensit-ic transformation under stress induction to induce the hard hcp phase.The strength of the alloy was further improved,and Cu/Ag rich fcc2 phase did not undergo transformation and remains in the post fracture samples.The compressive mechanical properties of medium entropy alloys first increased and then decreased with the addition of Cu/Ag elements.When x=1.5,the comprehensive mechanical properties of entropy alloys in the three series were the best.The compressive strength and plastic strain were 2539.26 MPa and 25.56％of(Fe63.3Mn14Si9.1Cr9.8C3.8)98.5Cu1.5,2468.61 MPa and 23.45％of(Fe63.3Mn14Si9.1Cr9.8C3.8)98.5Ag1.5,and 2604.32 MPa and 25.91％of(Fe63.3Mn14Si9.1Cr9.8C3.8)97Cu1.5Ag1.5,respectively.The reason for the strength and plasticity enhancement of medium entropy alloy lied in the strengthening effect of microalloying on the alloy and the hard hcp phase produced by martensitic transformation.The design of dual phase medium entropy alloy and the improvement of its comprehensive mechanical properties through microalloying could make it a wide range of potential applications as structural materials.
medium entropy alloymicroalloyingmechanical propertiesphase separation
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