The Regulation Strategy Of The High-temperature Stability And Mechanical Properties For γ/γ’ High Entropy Alloys

Novel structural materials with combined high strength and good ductility at extreme operating circumstances are of importance for the development of aerospace industry.Comparing to traditional superalloys,the newly developed γ/γ’ high-entropy alloys(HEAs)exhibit excellent synthesis of strength and ductility,therefore,attract much attention in academic world.Especially for the recently reported(FeCoNi)86Ti7Al7 HEAs,they yield superior strengths of 1.5 GPa and ductility as high as 50%at ambient temperature,offering a paradigm for the development of next generation of new high-temperature structural materials.Similar to the nickel-based and cobalt-based superalloys,the outstanding mechanical properties of γ/γ’ HEAs strongly rely on the thermal stability of the γ’ precipitate phase,especially at high temperatures.However,the present experimental researches on the high-temperature stability of γ’ phase are still in the initial stage of "trial and error method" due to the limited development time.By using the first-principle calculations,the theoretical exploration of strategy on improving the high-temperature stability and intrinsic mechanical properties of γ’ phase is beneficial value for the discovery of new hightemperature structural materials.In this dissertation,the valence electron concentration(VEC)and lattice distortion constant δ are firstly employed to check the phase constituent rules of 40 γ/γ’ HEAs.It’s found that the VEC is not suitable for the regulation of "FCC+L12" structures,but the further consideration of δ is effective for the control of their phase constituent.Based on the exact muffin-tin orbitals(EMTO)method in combination with the coherent potential approximation(CPA),the site preference,structural stability and effects of the order-disorder transformation behavior of the(FeCoNi)86Ti7Al7 HEAs are systematically investigated.The results show that the atomic structural model of the L1 2 structure can be expressed as(Fe,Co,Ni)3(Ti,Al,Fe).The FCC-L12 enthalpic difference of γ’ precipitation phase is much larger than that of the the y matrix phase,therefore,prefers to form a partially ordered L1 2 structure.However,the high entropic difference as well,which is adverse for the ordered structures formation,leads to the transformation to the completely disordered FCC structure at high temperatures.The competition between entropy and enthalpy strictly determines the structural selection,and this competitiveness is closely related to their specific composition.To understand the process of composition and structure,the competition of entropy and enthalpy is taken here as the bridge.The effects of Fe,Co,Ni,Al/Ti elements on the Curie temperature,mechanical stability and order-disorder transition temperature(Tod)of the γ’ phase in(FeCoNi)86Ti7Al7 HEAs are systematically evaluated.The results show that Fe is the unfavorable element for the γ’ phase stability.The increase of Fe element will enable the alloys to transform from paramagnetism to ferromagnetism,and leads to the mechanical instability in high content.The analysis of the entropic and enthalpic contribution indicates that the configuration entropy is not sensitive to the change of the composition,while the enthalpy and vibrational entropy are the critical roles for the order-disordered transformation of the γ’ phase.Therein,the enthalpy is the inherent character of the alloys and the vibration entropy has a positive relationship with δ.Based on this,the ideal condition for attaining the stable γ’ precipitation phase at high temperatures is "high enthalpic difference but low lattice distortion".By considering the standard of "low δ" high melting point,and low density" with the reference from successful experiments,the transition elements of Nb,Mo,Ta,W,V,Cr,Mn and Cu are screened out to be the possibly beneficial elements for the high temperature stability of γ’ phase.Based on the density functional theory,the elements effects on the Tod and mechanical properties are systematically explored,and the mechanism of the both strength and plasticity is explained from generalized stacking faults.The calculated results show that Nb and Ta are the most ideal alloying elements,which can simultaneously improve the high-temperature stability,strength and ductility of the γ’ phase.