Segregation Behaviors Of Alloying Elements And Their Effects On The Thermodynamic Stability And Fracture Strength Of ?-Ni/?'-Ni₃Al Phase Interface In Nickel-based Single-crystal Superalloys

Nickel-based single-crystal(SC)superalloys are commonly used in the construction of turbine blades for most sophisticated aircraft engines and industrial gas turbines because of their numerous advantages,like excellent high-temperature strength,low density,strong oxidation resistance,high melting point,and good chemical stability.The thermodynamic instability and lower mechanical strength of?-Ni/?'-Ni3Al phase interfaces in Ni-based SC superalloys threaten the service safety of the superalloys severely.For this reason,the phenomenon that alloying elements can segregate to surfaces or interfaces spontaneously was utilized in this work and the segregation behaviors of alloying elements at the?-Ni/?'-Ni₃Al phase interface of Ni-based SC superalloys and their effects on the interfacial properties were studied from the composition-structure-properties viewpoint.At present,computational methods based on first-principles calculations provide powerful insights into the local structures,energetics,mechanical and electronic properties of various solid-solid interfaces,and the first-principles calculation has been used successfully in several metals to predict the fundamental characteristics of segregated grain boundaries.Therefore,first-principles calculations based on density functional theory(DFT)were performed in this dissertation to investigate the segregation behaviors of alloying elements at the?-Ni/?'-Ni3Al phase interface and to gain insight into the micro-mechanism of the effect of interfacial segregation on the thermodynamic stability and fracture strength of the?-Ni/?'-Ni₃Al interface.In summary,the main findings of this dissertation are as follows:(1)In view of the urgent need to improve the thermodynamic stability and fracture strength of the?-Ni/?'-Ni₃Al phase boundary,we systematically studied 12 alloying elements X(X=Re,Ta,Ti,Mo,Ru,Cr,Co,Hf,Nb,W,Y and Zr)in the?-Ni/?'-Ni3Al phase interface single segregation trend and its influence on the interface formation energy and Griffith fracture work,and in-depth discussion of the potential microscopic mechanism.The study found that,all alloying elements X can segregate to the corner-point site of the?-Ni(001)layer except the element Y.The Re-,Ti-,Mo-,W-,Cr-,Nb-,Ta-,Hf-and Zr-segregated interfaces are more stable than the unalloyed interface due to the presence of pseudogap and lower values of densities of states at the Fermi level.The segregation of Ru,Co,Re,Mo,W,Cr,Nb and Ta strengthens the interfacial fracture strength,which can be mainly attributed to the enhanced bonding strengths of X-Ni bonds formed in these segregated interfaces.The interfacial segregation of Cr,Re,Mo,W,Nb and Ta can not only improve the thermodynamic stability but also enhance the fracture strength of the phase interface.The segregation of Ta and Re can improve the thermodynamic stability and fracture strength of the?-Ni/?'-Ni3Al interface to the maximum extent.This provides a useful reference for phase boundary optimization and composition design of nickel-based single crystal superalloys.(2)Using the first-principles calculation method based on density functional theory,on the basis of the above work,we further studied the co-segregation behavior of Zr-Re and Zr-W in the?-Ni/?'-Ni3Al phase interface and their relative phases.It is found W and Re can segregate at the?-L₁-3.52-cp site of the?-Ni(001)layer in the Zr-segregated interface,which is the most stable.The stability of Zr-W/Re co-segregated interface is improved and becomes more stable than clean and Zr-segregated interfaces,due to the existence of pseudogaps at the Fermi level.Compared with the?-Ni/?'-Ni3Al phase interface of Zr single segregation,Zr-W/Re co-segregation will not change the final fracture position of the Zr-segregating phase interface,but will slightly increase the fracture strength of the Zr-segregating phase interface.This work points out the direction for the composition optimization and performance improvement of nickel-based single crystal superalloys.(3)The co-segregation behavior of W-Re and W-Cr atoms in the?-Ni/?'-Ni₃Al interface and their effects on the interfacial formation energy and Griffith fracture work are investigated.Calculations found that both Re and Cr atoms tend to segregate to the?-L₁-3.52-cp site of the W-segregated interface;compared with the W single segregation phase interface and the co-segregation phase interface,the W-Re/Cr co-segregation can be significant Improve the thermodynamic stability of the phase interface.Compared with the W single segregation phase interface,W/Re-Cr co-segregation does not change the final fracture position of the phase interface,but it slightly weakens the fracture strength of the phase interface.In addition,the basic electronic density of states and charge density analysis,we analyzed the influence mechanism of W/Re-Cr co-segregation on phase interface stability and fracture strength.