Correlation Among Valence Electron Concentration And Microstructure And Properties For High Entropy Alloys

High entropy alloys(HEAs),also named multi-principal elements alloys(contain five or more than five principal elements generally and the content of each element is between 5%and 35%).HEAs have attracted the attention of metallurgists for the alloys have simple crystal structure(FCC,BCC,HCP)and better room temperature,low temperature and high temperature mechanical properties.HEAs contain many principal elements,it brings more possibilities for alloy composition design.Which also lead HEAs have a great development prospect and also bring a great challenge for the alloy composition design.For conventional alloys,the method of adding alloying element in matrix is an effective way to improve the microstructure and mechanical properties.In the field of HEAs,some metallurgists have studied the effect of alloying element on microstructure and mechanical properties by alloying method in the form of pseudo binary alloy(MY_x).However,how to select an alloy elment and adjust the phase composition by an effective method to improve the mechanical properties of HEAs,is the key problem need to be solved in the field of HEAs.Recent reports show that valance electron concentration(VEC)is a very important physical parameter for effecting the phase composition of HEAs.An alloy design idea is that VEC was changed by adjusting the alloy composition,aim to adjust the phase composition and then improve the mechanical properties of HEAs.If this alloy design idea is verified,it would have practical significance for the design of HEAs.To the above problem,this thesis studied the effect of the lower VEC elements(Ti,Mo,W and Nb)on the microstructure,phase composition,and mechanical properties of the HEAs(CoCrFeMnNi)with FCC structure;studied the effect of the high VEC elements(Co,Ni)and the nonmetallic element(C)on the microstructure,phase composition,and mechanical properties of the HEA(AlCoCrFeNi)with BCC structure.The results show that the strength of the matrix can be improved by selecting an element with a VEC(valence electron concentration)lower than the average VEC of the matrix,while ductility can be improved by choosing another element with a VEC higher than the average VEC for the matrix.This is a very simple way to guide composition design for HEAs.In this thesis,a five-element-equimolar and a five-element-non-equimolar ratio HEAs(CoCrCuMnNi HEA and Co₉Cr₇Cu₃₆Mn₂₅Ni₂₃ HEA)with excellent mechanical properties strengthened by nanoprecipitation were designed and prepared.The phase stability of this HEA system was studied.The results show that many nanometer precipitates(5-50 nm in size)are found in the two HEAs.The nanometer precipitates play a positive role in improving the HEAs's mechanical properties.By analyzing the solidification processing and the component distribution of the nanometer precipitates,we found that some atoms were separated from the supersaturated solid solution and precipitated because of their solid solubility was decreased with decreasing the temperature.The precipitates grew to a size of a few nanometers and further growth was hindered by the sluggish diffusion effect.Therefore,the nanoprecipitates were formed.The tensile test results show that the two HEAs possess better comprehensive mechanical properties than most reported HEAs.The yield strength,tensile strength and fracture strain of CoCrCuMnNi and Co₉Cr₇Cu₃₆Mn₂₅Ni₂₃ HEAs are 458MPa,742MPa,40%and 401MPa,700MPa,36%respectively at casting state.The results of heat treatment(The heat treatment condition are 200,400,600,800,1000?for 2 h)on Co₉Cr₇Cu₃₆Mn₂₅Ni₂₃HEA show that some sigma phases riched in Co and Cr are formed in Co₉Cr₇Cu₃₆Mn₂₅Ni₂₃HEA when the heat treatment condition is 800?for 2 h.The tensile test results show that the formed sigma phase is deleterious for the mechanical propertie of Co₉Cr₇Cu₃₆Mn₂₅Ni₂₃HEA.The experimentally observed thermal stability and phases are compared to the calculated phase diagram(CALPHAD)and rationalized by recourse to thermodynamics and kinetics.The investigation reveals the formation mechanism of the sigma phase.Only when a suitable temperature is selected and make the thermodynamic and kinetic conditions are satisfied,can sigma phase be formed in the alloy,which is the equilibrium mechanism of kinetic and thermodynamic for the sigma phase formation.The total energy of composition phase of Co₉Cr₇Cu₃₆Mn₂₅Ni₂₃ high entropy alloy was calculated by the method of first principles(EMTO-CPA).The reason of forming two FCC phase instead of one FCC phase was explained from the perspective of energy.