Study On Design,strengthening Mechanism And Properties Of Ni-Co-Fe-Based High-entropy Alloys

In recent years,the performance requirements for alloy materials are increasing with the rapid development of the scientific and technological society.The alloy properties obtained through traditional alloy design concepts have almost reached the bottleneck,and it is difficult to achieve the mechanical performance indicators required for metal structural materials to serve in extreme environments.High-entropy alloys are composed of a variety of elements in an equiatomic ratio and/or near-equiatomic ratio,and obtain better properties than traditional alloys with a new design concept and unique microstructure.Precipitation strengthening is one of the most simple and effective methods to improve the strength of high-entropy alloys.In recent years,many studies have found that the introduction ofγ?precipitates into FCC alloys has a significant strengthening effect relative to high-entropy alloys.Therefore,in this study,theγ?-strengthened（NiCoFe）₈₈Ti₆Al₆ and(NiCo_1.5Fe)₉₀Ti₆Al₄ high-entropy alloys were designed and prepared by combining thermodynamic calculations with experimental studies.On this basis,the refractory element W,Mo,V,Nb,Ta and Hf was added to the alloys.Using X-ray diffraction,scanning electron microscope,high-resolution transmission electron microscopy,electronic universal testing machine,microhardness tester,and friction and wear testing machine technology and equipment,such as system to study theγ?precipitates in the alloy precipitation and refractory elements on alloy microstructure,phase structure,hardness,tensile properties and tribological properties of the influence law.The main conclusions of this study are as follows:（1）In this study,γ?-strengthened Ni-Co-Fe-based FCC high-entropy alloys were designed by the CALPHAD method combined with commercial Ni-based and Fe-based alloy thermodynamic databases,and（NiCoFe）₈₈Ti₆Al₆ and(NiCo_1.5Fe)₉₀Ti₆Al₄ high-entropy alloys with the FCC+L1₂ structure were obtained.The experimental research and calculation results are in a good agreement.（2）For（NiCoFe）₈₈Ti₆Al₆ high-entropy alloys,the impurities Fe₂X（X=Nb,Hf and Ta）laves were detected in the Nb-,Hf-and Ta-doping alloys while the other three present a perfect（γ+γ’）dual-phase microstructures.And the W-,Mo-and V-doping HEAs possess a much better strength-plasticity combination than the Ti6Al6 alloy at room-and high-temperature.The 1.5W alloy has a tensile strength of～1300 MPa and a good plasticity（>30%）at room temperature.In the meantime,it also performances much good at 800 ^oC（～11%elongation and the tensile strength of～750 MPa）.The addition of W,Mo and V elements causes the main peak of the HEAs to shift to the left to varying degrees,among which the biggest shift is 1.5V HEA,follow by 1.5Mo HEA,the smaller is the 1.5W HEA.At the same time,theγ’precipitates phase of the alloy have a highly coherent relationship withγmatrix phase,which ensures that the strength of the alloy is increased without sacrificing its plasticity.The study of（NiCoFe）_88-xTi₆Al₆R_x（R=Nb,Ta,Hf,W,Mo,and V）high-entropy alloys can provide a new alloy design strategy for solving the medium-temperature brittleness problem of high-entropy alloys.（3）The research on the aging process of(NiCo_1.5Fe)₉₀Ti₆Al₄ high-entropy alloy found that the alloy has better phase stability,and the alloys heat-treated under different aging times can maintain a single FCC solid solution structure.The structure of the alloy consists of aγmatrix and aγ?precipitates.Theγ?precipitates of the alloy after 6h aging treatment is irregularly elliptical until the aging time is increased to 12h,and its shape is transformed into a regular cubic shape.Continue to increase,and the shape of theγ?precipitates will no longer change.The transition ofγ?precipitates from elliptic to cubic results in the decrease of lattice misfit,and the coarsing ofγ?precipitates result in the transition of dislocation from cutting through to Orowan bypass mechanism throughγ?precipitates during deformation.The results of friction and wear show that the alloy has excellent tribological properties.The wear mechanism of the alloy is mainly plough wear and peeling wear,in addition,there is oxidation wear,and the degree of peeling wear becomes serious with the increase of aging time.（4）The study of(NiCo_1.5Fe)_88.5Ti₆Al₄R_1.5（R=Nb,Ta,Hf,W,Mo and V）high-entropy alloys shows that the strength of Nb and Mo alloys increases by 46.7%and 33.3%,respectively,but most of the elongation is sacrificed（they can only maintain about～10%）.The addition of Hf,W,Ta and V can not only improve the strength of the alloy,but also slightly improve the elongation of the alloy or almost do not sacrifice the elongation.Among them,the addition of Ta has the best improvement effect on the comprehensive mechanical properties of the alloy.The yield strength of the alloy is increased by 26.7%（from 750MPa to 950MPa）,and the elongation of the alloy is increased by～20%,which is slightly higher than that of the matrix alloy.In addition,the alloy has multistage strain hardening behavior,the greater the fluctuation of strain hardening curve of the alloy,the higher the strength of the alloy.(NiCo_1.5Fe)_88.5Ti₆Al₄R_1.5 alloys with multistage strain hardening behavior can improve the strength-plasticity trade-off relationship of the alloys.