微合金化对双相中熵合金组织与力学性能影响研究

Microstructure Control and Mechanical Properties Optimization of Dual Phase Medium Entropy Alloy by Microalloying

采用水冷铜坩埚磁悬浮熔炼和铜模负压吸铸法制备了(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(100-x)Cu_(x),(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(100-x)Ag_(x)以及(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(100-2x)CuxAgx(x=0,0.5,1,1.5和2;原子分数)3种中熵合金体系,探究了通过添加正的混合焓元素Cu,Ag合金化于面心立方(fcc)结构的中熵合金基体,调控组织制备了双相fcc的新型中熵合金,并采用Olson-Cohen热力学模型计算fcc中熵合金的层错能时忽略间隙原子偏聚对fcc中熵合金的影响。结果表明,分别添加Cu,Ag以及Cu和Ag元素混合添加后,原子尺寸差电负性增大、价电子浓度减小,促进固溶体结构形成。中熵合金系为fcc相结构,随着Cu,Ag元素含量的增加,层错能降低同时细化晶粒,合金的综合力学性能提高。当Cu,Ag含量均为2%时,合金中出现两种不同的fcc结构,其中fcc1为贫Cu/Ag的基体相,fcc2富含Cu/Ag,且基体相在形变过程中诱发相变产生密排六方(hcp)相。((Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8)))_(97)Cu_(1.5)Ag_(1.5)中熵合金的综合力学性能较为优异,其屈服强度、断裂强度以及塑性应变分别为1106.73 MPa,2604.32 MPa和25.91%。

The non-equiatomic ratio multiphase second-generation high entropy alloy developed from the first generation of high entropy 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 mixing 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 medium 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 entropy alloy has become an urgent problem to be solved.Microalloying is one of the most effective methods to improve the comprehensive mechanical properties of alloys.Microalloying elements can refine grains and improve the mechanical properties of medium entropy alloys.On the one hand,the dual phase fcc structure produces hard hexagonal closepacked(hcp)structure phase through martensitic 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,(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(100-x)Cu_(x),(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(100-x)Ag_(x) and(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(100-2x)Cu_(x)Ag_(x)(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 Olson Cohen thermodynamic model.After the addition of Cu,Ag and their mixture,the atomic size difference,electronegativity and valence 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 Fe_(63.3)Mn_(14)S_(9.1)Cr_(9.8)C_(3.8)alloy was 9.69 mJ·m^(-2),while(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(98)Cu_(2),(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(98)Ag_(2) and(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(96)Cu_(2)Ag_(2) alloys were 7.72,7.63 and 7.23mJ·m^(-2),respectively.For(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(100-x)Cu_(x)(x=0,0.5,1,1.5,2;atom fraction),when x=0,Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8)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 fcc1and fcc2 were 0.36173 and 0.36468 nm in(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(98)Cu_(2),0.36283 and 0.36598 nm in(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(98)Ag_2,and0.36251 and 0.36574 nm in(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(96)Cu_(2)Ag_2.After the compression fracture,X-ray diffraction(XRD)of the three alloys 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 martensitic 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 and25.56%of(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(98.5)Cu_(1.5),2468.61 MPa and 23.45%of(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(98.5)Ag_(1.5),and 2604.32 MPa and 25.91%of(Fe_(63.3)Mn_(14)Si_(9.1)Cr_(9.8)C_(3.8))_(97)Cu_(1.5)Ag_(1.5),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.