First-principles Study On Structural Stability,Mechanical And Thermodynamic Properties Of For ?' Phase In Co-Al-W And Co-Nb Based Superalloys With Transition Elements

Ni-based superalloys are an important high-temperature material in the aerospace industry.However,due to the melting point limitations,their service temperature is difficult to meet the temperature-bearing capacity requirements of next-generation engine turbine blade materials.The discovery of novel ?'/? two-phase coherent-strengthened Co-based superalloys provides new ideas for the further development of superalloys,and attracts wide attentions of the researchers all over the world.This ?' phase,which is similar to the Ni3Al phase in Ni-based superalloys,is the key to determine the comprehensive mechanical properties of the novel Cobased superalloys.However,most of the ?'-phases in current research reports are metastable,and it is difficult to study and characterize them experimentally.Therefore,it is necessary to study the ?'-phase from the perspective of theoretical calculation,to guide the design and development of novel Co-based superalloys.In this paper,the first-principles calculation method based on density functional theory(DFT)was used to systematically study the structural stability and mechanical properties of the ?'-phase in Co-Al-W and Co-Nb-based superalloys by alloying various transition elements.The specific works are summarized as follows:1)The site preference of 21 kinds of alloying elements X(X=Sc,Ti,V,Cr,Mn,Fe,Ni,Y,Zr,Nb,Mo,Tc,Ru.Rh,Pd,Hf,Ta,Re,Os,Ir and Pt)in ?'-Co3(Al,W)phase were investigated,and the effect of transition element on the structural stability,elastic and thermodynamic properties of the ?'-Co3(Al,W)phase were analyzed.The calculation results show that the elements of Sc,Ti,V,Cr,Mn,Y,Zr,Nb,Mo,Tc,Hf,Ta,Re and Os prefer to occupy the Al site,while Fe,Ni,Ru,Rh,Pd,Ir and Pt prefer to occupy the Co site.All Xsubstituted compounds are metastable at 0 K.By comparing to the X-free Co3(Al,W)phase,the addition of X element,especially Ti,Ta,Hf,Nb,V and Zr,can improve the structural stability of Co3(Al,W).According to the charge density difference(CCD)results,symmetric electron configuration is beneficial to increase the shear modulus of the ?'-phase.By analyzing the difference in Gibbs free energy between ?'-phase and its competitive phase,it can be found that the addition of Hf,Ti,Ta,Zr,Nb and Mo elements can greatly improve the stability of the?'-phase at high temperatures.2)The site preference of 21 transition elements(Sc,Ti,V,C.r,Mn,Fe,Ni,Y,Zr,Mo,Tc,Ru,Rh,Pd,Hf,Ta,W,Re,Os,Ir and Pt)in the ?'-Co3Nb phase was investigated,and the effect of these transition elements on the structural stability,elastic and thermodynamic properties of?'-Co3Nb phase were analyzed.The calculation results show that the elements of Sc,Ti,V,Cr,Y,Zr,Mo,Hf,Ta,W and Re prefer to occupy the Nb site,while Mn,Fe,Ni,Tc,Ru,Rh,Pd,Os,Ir and Pt prefer to occupy the Co site.The addition of transition elements except of Cr,Mn,Fe,Y,Tc,and Os can improve the structural stability of the ?'-Co3Nb phase,and the stabilization effect is sorted as follows:Ti>Ta>Hf>Pt>Ir>Zr>Rh>V>Ni>W>Sc>Mo>Pd>Re>Ru.The symmetric electron distribution is beneficial to increase the shear modulus of ?'Co3Nb.By analyzing the difference in Gibbs free energy between ?' phase and its competitive phase,it can be found that Ti,Ta,Hf,Pt,Ir,Zr and V can greatly improve the high-temperature phase stability of the ?'-Co3Nb,and have little effect on the temperature sensitivity of the thermodynamic properties of ?' phase.