First-principles Study Of The Phase Stability And Elastic Properties Of Face-centered Cubic High Entropy Alloys

High entropy alloys(HEAs)are new multi-principal-component alloys developed recently.Compared with traditional metals and alloys,HEAs possess unique physical properties and mechanical properties and have important research value as well as broad prospect for application.However,due to the natural complex elemental composition and the wide range of composition control of HEAs,the composition design and performance prediction of HEAs are still limited by the traditional "trial and error" method,and lack of reliable theoretical support.Therefore,it is helpful to design HEAs with ideal properties to evaluate the phase stability of HEAs reasonably and to explore the internal relationship between the properties of component elements and the mechanical properties of the alloys.In this thesis,by mainly using first-principles density functional theory(DFT)calculations and the special quasi-disordered structure method(SQS),we have systematically studied phase stability,structural property,elastic properties and mechanical properties of two types of facecentered cubie structure(FCC)HEAs,have discussed the magnetic properties' influence on thermodynamic stability,structure and the elastic properties of Cantor alloy,have designed noble metal HEAs which has ideal performance,have summarized a linear model of the yield strength of FCC alloy,have revealed constituent elements' impact on the mechanical behavior of noble metal HEAs,have proposed a machine learning model to accurately predict the Bulk modulus and shear modulus of noble metal HEAs,and have discussed how to improve the strength and ductility od Au Cu Ni Pd Pt alloy.It mainly includes the following research contents:(1)Using DFT calculations,we have studied phase stability,structure properties,and elastic properties of Cantor alloy under no magnetic,ferromagnetic and paramagnetic conditions,discussed the differences of predicted phase stability,structure and elastic properties of Cantor alloy at different magnetic state,and revealed the effect of Mn and Cr element on integral magnetic effects of Cantor alloy from the atomic magnetic moment and charge density distribution.The results show that the thermodynamic stability and the prediction of structural and elastic properties of Cantor alloy under paramagnetic and ferromagnetic conditions is better and more accurate than that under non-magnetic conditions,especially for paramagnetic conditions.At the same time,the difference between ferromagnetism and paramagnetism of Cantor alloy mainly came from Mn and Cr elements.(2)Using dispersion-inclusive DFT calculations and empirical criteria,we have studied the phase stability and elastic properties of quaternary equal-molar HEAs consist of Al,Au,Ag,Cu,Ni,Pd,Pt,Rh,have discussed the relationship between the elastic properties and mechanical properties,and by the charge density distribution to explains how constituent elements affect the mechanical properties of alloy.The results showed that Al can lower the stability of the solid solution of HEAs.The Ir Pt Ni series alloys have higher strength,Ni Au Pt series alloys have higher toughness,and Au Ag Cu series alloys have higher ductility.The yield strength of the alloy mainly depends on the shear modulus and lattice distortion.(3)Utilization dispersion-inclusive DFT calculations and machine learning methods,we have studied the elastic properties and mechanical properties of quinary equal-molar HEAs consist of Au,Ag,Cu,Ni,Pd,Pt,Rh,Ir,compared the difference of predicted elastic properties by the mixing rule and DFT calculations,accurately predicted strength and ductility relationship of quinary noble metal equal-molar HEAs.The results showed that the mixing rule method was unable to accurately predict the elastic properties of HEAs.The alloys containing Ni-Rh or NiIr have higher yield strength.In Au Cu Ni Pd Pt alloy,the strength and ductility could be improved by adding Ni and Au,respectively.