Composition Design And Properties Of FeCoNi Based Eutectic High Entropy Alloys

Eutectic high-entropy alloys(EHEAs)have the advantages inherited from both HEAs and eutectic alloys,i.e.,broad composition space for the former while good casting fluidity and low microsegregation for the latter.Thus,EHEAs can meet the requirements of numerous precision instruments and complex large casting parts,opening up new directions for practical application of HEAs.However,design principles for the composition selection of multiprincipal component eEHEAs are currently not available,which greatly limits their development and application.As such,this study aims to estable a method for quickly locating the composition of multicomponent EHEAs based on the involved binary eutectic compositions in the equilibrium phase diagrams,coupling with the mixing enthalpy between various atomic pairs,and to extend this method from quaternary to high-order EHEA systems.Subsequently,microstructure,phase composition and elemental distribution of the designed alloy were characterized,and the thermal stability and compressive mechanical properties were also systematically studied.A preliminary relationshipof the composition-structure-property was established,providing a guidance for the efficient development and practical application of EHEAs.First,taking the A(A is either Nb,Ta,Hf or Zr)-FeCoNi(hearafter referred to as B-type components)alloy system as an example,a design method based on A-Bj(Bj=Fe,Co,Ni)binary eutectic point,coupling with the mixing enthalpy between A-Bj atomic pairs,was proposed to quickly determine the composition of a quaternary EHEA.Wherein,A-Bj(Bj=Fe,Co,Ni)has binary deep eutectic points,and the mixing enthalpy between A-Bj atomic pairs is largely negative while B-type elements are miscible.As a result,formation of A-Bj cluster is thermodynamically favorable,whilst from a kinetic point,chemical clusters also tend to form among Fe,Co and Ni atoms due to their high concentration at the liquid state.Upopn solidification,solid solution and intermetallic compound will crystallize simultaneously to form eutectic structure.Specifically,using the binary eutectic composition of Nb10.6Fe89.4,Nb13.9Co86.1,and Nb15.5Ni84.5,and the mixing ratio of these eutectic components which was determined based on the mixing enthalpies between these atomic pairs-16 kJ/mol-1,-25 kJ/mol-1,and-30 kJ/mol-1,a quaternary EHEA of Nb0.62Fe1.98Co1.22Ni1.00 was quickly obtained.The experimental results indicate that the developed composition possesses a nanoscale layered eutectic structure consisting of fcc(body-centered-cubic)and Laves phases.Similarly,according to this design method,the quaternary eutectic composition of A-type element of Ta,Hf,and Zr,was also designed,which is Ta0.64Fe2.07Co1.29Ni1.00,Hf0.47Fe2.11Co1.22Ni1.00,and Zr0.53Fe1.94Co1.19Ni1.00,respectively.These results demonstrate that the range of EHEA composition can be quickly located by selecting the binary eutectic points,coupled with the mixing enthalpy between AB atomic pairs.Second,the above design methodwas extended to the high-order EHEA systems.Specifically,by changing the type and number of A-type elements in the A-FeCoNi alloy system,based on the binary eutectic point composition of Ai-Bj(Ai=Nb,Ta,Hf,Zr or Ti;Bj=Fe,Co,Ni)and combined with the mixing enthalpy between Ai-Bj atomic pairs,the EHEA composition for high-order alloy systems was quickly located.A total of 9 high-order EHEAs including five to eight main elements were designed.The experimental results show that the calculated compostions are near or close to the eutecitic points,and an increase in the number of principal contituents affected the distribution coefficient and atomic diffusion between the liquid and solid phases during eutectic solidification,thereby affecting the interlayer spacing and morphology of the resultant eutectic structure.In addition,effects of heat treatments on microstructure stability of all 13 EHEAs designed in this work were studied.The experimental results show that after heat treatment at 600℃,the microstructure of the EHEAs is similar to that of the as-cast counterparts,and there is no extra precipitation in the quaternary EHEA.However,for the high-orderEHEAs,especially for the Hf0.16Zr0.13Ti0.36Fe2.04Co1.18Ni1.00,Nb0.21 Hf0.11Zr0.11Ti0.25Fe1.98Co1.19Ni1.00 and Ta0.11Nb0.18Hf0.08Zr0.10Ti0.19Fe1.96Co1.22Ni1.00 alloys,a large amount of 10～60 nm granular or layered Laves phase were precipitated in the bcc(body-centered-cubic)matrix.After heat treatment at 900℃,theEHEA structure underwent severe spheroidization,but t no significant change in the phase composition and elemental distribution.This analysis shows that all microstructural changes after the heat treatments are closedly related the dominant effect of temperature and entropy of mixing on the Gibbs free energy.Finally,the room-temperature compressive mechanical properties of the designed EHEAs wereinvestigated.Due to the solid solution strengthening effect,mechanical properties of FeCoNi based EHEAs were superior to those of the binary eutectic alloys.The relationship between eutectic structure with different interlayer spacing and yield strength can be well described by σy=σ0+L/k,where σ0 and k are constants,σ0 is related to the yield strength when the constituent phases of the eutectic structure are single crystals;k is related to the influence of interlayer spacing and grain boundaries on strength;L represents the interlayer spacing.For the Nb0.62Fe1.98Co1.22Ni1.00 quaternary EHEA,σ0 and k were determeined to be 1.24Gpa and 22.70 GPa/nm,respectively.It is worth noting that when the number of primary constituents increases to six or more,the phase interfaces in the microstructure becomes coarse,leading to a decrease in the compressive plasticity.To sum up,this study proposes a method for rapidly locating multicomponent EHEAs based on the involved binary eutectic points,coupled with the mixing enthalpy between the main atomic paris.Based on a systematic study of the microstructure,phase composition,elemental distribution,phase stability and compressive mechanical properties of the developed EHEAs,a relationship of the composition-structure-property has been established,providing a guidance for the efficient development c of EHEAs.