First-principles Studies Of The Structural Stability And Mechanical Properties Of The γ" Phase In Inconel 718 Superalloy

Ni-base superalloys are extensively used in the hot-end components of aircraft engines due to their excellent chemical and physical properties.With the continuous improvement of the thrust-weight ratio of modern aircraft engines,the service conditions for hot-end components are becoming harsher and harsher,which makes the performance improvement of existing Ni-base superalloys and design new superalloys more urgent.In recent researches,it was found that the γ"-Ni3Nb ordered phase(D022 structure)in Inconel 718 superalloy,a precipitation-strengthening structure,has a great potential in exploiting the novel superalloys.The structural stability and mechanical properties of the γ" phase directly affect the comprehensive mechanical properties of the alloy.However,it is much more difficult to do experimental studies on the γ"-Ni3Nb phase and thus it has not been sufficiently studied.By using the first-principles method,the structural stability,thermodynamic properties,mechanical properties and alloying effects of the γ"-Ni3Nb phase can be systematically studied.It will provide theoretical support for optimizing the high-temperature stability and mechanical properties of Inconel 718 and other Ni-base superalloys,and accelerating the design of new superalloys based on the γ"-Ni3Nb phase.In this paper,the structural stability,thermodynamic properties,mechanical properties and alloying effects of the γ"-Ni3Nb phase were systematically studied from first-principles calculations based on the density functional theory.Selecting the beneficial elements for improving the structural stability and mechanical properties of the γ"-Ni3Nb phase was then carried out.In addition,the microscopic mechanism of temperature affecting the mechanical properties of the γ"-Ni3Nb phase was discussed.The main contents are as follows:(1)The site preference of twenty-one transition metal elements in the γ" Ni3Nb phase,such as Sc,Ti,V,Cr,Fe,Y,Mo,W,and Os,and alloying effects on the structural stability of the γ"-Ni3Nb phase were studied by first-principles.It shows that Sc,Ti,Y,Zr,Hf,and Ta strongly tend to occupy the Nb site,and Rh,Ir,and Pt have a strong preference for the Ni site,while V,Mo,W,Cr,Mn,Fe,Co,Tc,Ru,Pd,Re,and Os can substitute both the Nb and Ni sites.The structural stability of the γ"-Ni3Nb phase is improved when alloyed with Sc,Ti,W,Y,Zr,Mo,Hf,Ta,W,Pt,Rh,or Ir.(2)The stress-strain method was applied to investigate the effects of alloying on the mechanical properties(elastic constant,modulus,toughness/brittleness,hardness,elastic anisotropy,etc.)for the γ"-Ni3Nb phase at 0 K.The charge density difference was also used to reveal the relationship between the mechanical properties and the electronic structure.It is found that the Ni3Nb-X phases exhibit mechanical stability,intrinsic ductility,and strong elastic anisotropy.The largest increase of elastic properties of the γ"-Ni3Nb phase is ascribed to element W,followed by Os,Ta,and Mo,while the maximum decrease of those is attributed to element Y,followed by Cr,and Sc.The hardness of the γ"-Ni3Nb phase decreases when alloyed with Pd,Hf,Zr,Y,Mn,or Sc.(3)Combining the quasi-harmonic Debye model with the stress-strain method,the thermodynamic and high-temperature mechanical properties of the γ"-Ni3Nb and NbNb-X(X=W,Mo,Ti,Fe,and Cr)phases were explored.The microscopic mechanism of temperature weakening mechanical properties is further explained from the electronic structure.The results indicate that Fe and Cr make the γ"-Ni3Nb phase easier to expand throughout the temperature range(0-1000 K).High temperature causes the mechanical properties reducing and the elastic anisotropy weakening of the y"Ni3Nb and Ni3Nb-X phases.Element W always contributes to the improvement of the elastic properties of the γ"-Ni3Nb phase.The γ"-Ni3Nb phase still maintains intrinsic ductility at elevated temperatures.At high temperatures,the delocalization of electrons between Nb-Ni and X-Ni atoms results in the decrease of the mechanical properties of the γ"-Ni3Nb and Ni3Nb-X phases.(4)The stress-strain relationships,ideal tensile strengths,ideal shear strengths were quantified for the γ"-Ni3Nb phase along the[001],[100],[221]and(112)[111]direction.In addition,the effect of W,Mo,Ti,Fe,and Cr on the ideal strengths were analyzed.The ideal tensile strength along the[001]direction is the largest while that along the[221]is the smallest.The ideal shear strength along the(112)[111]direction is significantly smaller than those tensile strengths.The alloying generally induces the decrease of the ideal strengths for the γ"-Ni3Nb phase,except for that Fe is beneficial to increase its ideal shear strength.