Segregation Behaviors Of Alloying Elements At Grain Boundaries Of Nickel-based Superalloys And Their Effects On Grain Boundary Properties

The thermodynamic instability as well as the lower fracture strength compared with the single crystal limit the feasibility of strengthening the properties of nickel-based superalloy by introducing grain boundary.In order to reduce or even eliminate the adverse effects of grain boundaries in nickel-based superalloy,this paper uses the grain boundary segregation engineering as a promising approach for accurately manipulating the chemical composition,structure and properties of grain boundaries.The first principles calculation method was used to systematically study the segregation behavior of 3d(Ti-Co),4d(Zr-RH)and 5d(Hf-Ir)transition metal elements in NiΣ3[110](111)and Ni Σ5[001](210)grain boundary as well as the effect of grain boundary segregation of transition metal elements on the thermodynamic stability and fracture strength of the grain boundary.Finally,the mechanism of stabilization and strengthening caused by grain boundary segregation of alloying elements is clarified by the microscopic view.This work could provide theoretical basis and guidance for manipulating the grain boundary and composition of Ni-based superalloys based on the concept of grain boundary segregation engineering.The main results are as follows:(1)The segregation energies of the alloying elements in the same period in NiΣ3[110](111)and Ni Σ5[001](210)grain boundary show a "concave-up" trend of first decrease and then increase and a "convex-down" trend of first increase and then decrease with the increase of the atomic number.Tc and Re element which with half full extranuclear electron configuration are located at the extreme points of concave or convex,and have the lowest or highest segregation energies.Different grain orientations in Ni Σ3[110](111)and Ni Σ5[001](210)grain boundary lead to different disorder degrees of nickel atoms in the grain boundaries,which makes the same alloying elements present different segregation behaviors in different grain boundaries.For the Ni Σ5[001](210)grain boundary with higher atomic disorder,the larger atomic radius difference and electronegativity difference between alloying elements and Ni promote the alloying elements to segregate into the grain boundary.(2)In the Ni Σ3[110](111)and Σ5[001](210)grain boundary,the grain boundary energy(γGB)of segregated grain boundary and the segregation energy(Eseg)of corresponding elements show a linear relationship.The expressions are γGB=0.74Eseg+0.17 and γGB=1.35Eseg+0.59,which indicate that the stronger the grain boundary segregation tendency of alloying elements,the higher the thermodynamic stability of segregated grain boundary.The segregation of Re and Zr elements can improve the thermodynamic stability of Ni Σ3[110](111)and Ni Σ5[001](210)grain boundary to the greatest extent,respectively.The segregation reduced thermodynamic stability provement can be attributed to the increase of the low-energy bonding states below Fermi level and the decrease of the high-energy anti bonding states.(3)The element Mn and W has the most significant effect on improving the fracture strength of Ni Σ3[110](111)and Σ5[001](210)grain boundary,respectively.The strengthening effect of alloying segregation on the fracture strength of Ni Σ3[110](111)and Σ5[001](210)grain boundary can be attributed to the accumulation of charge density in the bonding direction between bonding atoms near the grain boundary,while the alloying elements segregation reduced weakening effect on the fracture strength of Ni Σ3[110](111)grain boundary can be due to the depletion of charge density near the grain boundary.