Theoretical Study On The Structural Stability And Thermodynamic Properties Of γ’ Phase In New Co-V-based Superalloys

As an important pillar of Superalloy field,Ni-based superalloy is widely used in aerospace industry because of its good mechanical properties and anti-oxidation corrosion ability in high temperature environment,which is due to the existence of coherent stableγ+γ’phase in it.By adding W element to the Co-Al binary alloy,the researchers have successfully introduced theγ+γ’two-phase coherent strengthening mechanism into the Co-based superalloy.The ordered precipitation phaseγ’-Co₃（Al,W）in the new Co-Al-W ternary system is similar to theγ’-Ni₃Al in Ni-based superalloy.Its stability and mechanical properties are the key factors affecting the high temperature mechanical properties of Co-based superalloy.This discovery provides an idea for the development of new multicomponent Co-based superalloys.Stableγ’strengthening phase can be obtained by stabilizing and improving the metastableγ’in Co-based superalloys.Therefore,it is of great significance to study the effect of alloying elements on the metastableγ’phase in Co-based superalloys.In order to carry out theoretical research on Co-based superalloy and provide guidance for alloy composition design,based on density functional theory,the effects of transition metal elements on the structural stability,elastic mechanics and thermodynamic properties ofγ’precipitates in Co-V-Ti and Co-V-Ir system of new Co-V-based superalloys were systematically studied based on density functional theory.The main research contents are as follows:（1）The occupation tendency of 20 transition metal elements TM（TM:Sc,Cr,Mn,Fe,Ni,Y,Zr,Nb,Mo,Tc,Ru,Rh,Pd,Hf,Ta,Re,Os,Ir and Pt）inγ’-Co₃（V,Ti）phase was studied.The occupation tendency of transition elements inγ’phase and its effects on structural stability,mechanical properties and thermodynamic properties ofγ’phase were analyzed.The results show that Mn,Fe,Ni,Ru,Rh,Pd,Os,Ir,Pt tend to occupy Co site in L1₂structure,while Sc,Cr,Y,Zr,Nb,Mo,Tc,Hf,Ta,W,Re are more inclined to Ti site.In the 0 K ground state,theγ’-Co₃（V,Ti）phase is stable,and theγ’phase doped with all transition elements is also stable.The doping of Hf,Ta,Os and Pt can significantly work on the stability ofγ’in the ground state.Through the charge density difference analysis,it can be seen that a symmetrical charge distribution between atoms and a large charge density can improve the shear modulus ofγ’phase.By analyzing the Gibbs free energy difference between the competing phase D0₁₉,γ’-Co₃（V,Ti）is still metastable at higher temperature.Os,Y,Sc,Pt,Pd,Rh,Ir,Mn,Hf and Ni can improve the relative thermodynamic stability ofγ’phase at high temperature,especially Sc,Y,Os.（2）The occupation tendency of 20 transition elements（Sc,Ti,Cr,Mn,Fe,Ni,y,Zr,Mo,Tc,Ru,Rh,Pd,Hf,Ta,W,re,Os,and Pt）in theγ’-Co₃（V,Ir）phase were investigated.The effects of transition elements doping on the structural stability,mechanical properties and thermodynamic properties ofγ’phase were analyzed.The results show that Cr,Ti,Sc,Mn,Fe,Ni,Nb,Mo,Tc,Ru,Rh,Pd,Ta,W,Os,Pt prefer Co site inγ’-Co₃（V,Ir）phase at 0 K ground state.Sc,Y,Zr,Hf elements have a strong V-site tendency and Re element has a weak tendency to occupy Ir atom site.Except for Pd,other transition elements can improve the thermodynamic stability ofγ’-Co₃（V,Ir）phase at 0 K,and the order of stabilization effect is as follows:Os>Hf>Sc>Zr>Ta>Mn>Cr>Nb>W>Ti>Mo>Tc>Rh>Pt>Fe>Ni>Re>Ru>Y.In addition to the higher charge density,the symmetrical charge distribution between atoms can also improve the shear modulus ofγ’-Co₃（V,Ir）.By analyzing the Gibbs free energy difference between competing phases,it can be seen that the relative stability ofγ’-Co₃（V,Ir）phase can be improved by doping other elements except Pd,Pt,Ni,Re,Rh and Os.