Microdiffusion Behavior Of Falling-down Elements In Nickel-based Brazing Joints

High temperature alloys are widely used in nuclear power,petrochemical,transportation,energy,aerospace and other engineering fields because of their excellent high temperature properties.High temperature brazing is currently the most widely used joint and repair technology for high temperature alloy,especially the repair technology of heat loss of high temperature hot end parts,for example,the high and low pressure turbine blade crack damage can only be repaired by high temperature brazing.High temperature brazing has become an important joining technology for the manufacture and repair of high temperature alloy parts.With the development of aerospace technology,the requirement of joining technology becomes higher,and high temperature brazing technology is paid more attention to.In this paper,the first-principles calculation method is used,taking BNi5 brazing alloy X as an example,the occupancy tendency of melting-down elements Si and common alloy elements（Cr,Mo,W）in the structure of Ni-based filler metalsγ-Ni,γ’-Fe Ni₃ and ternary disordered alloys was calculated by VASP first-principles,the influence of crystal structure and element types on the diffusion behavior of Si during brazing was revealed by transition state search.In addition,the formation of brittle phase of melting-down element Si and alloy elements（Ni,Cr,Mo,W）and their mechanical properties were characterized by theoretical calculation.The purpose of this paper is to explain the diffusion behavior of fusible elements and the effect of the product properties on the brazed joints from the microscopic point of view.The conclusions are as follows:（1）The stability of the crystal structure is improved when the alloying elements m（Cr,Mo,W）are doped inγ-Ni,γ’-Fe Ni₃ supercell,and the alloying elements tend to replace Fe inγ’-Fe Ni₃ supercell,the Si elements inγ-Ni andγ’-Fe Ni₃ tend to occupy the octahedral gaps where the atoms are all Ni atoms.Based on the calculation of diffusion energy barrier between Si atoms,it is found that the occupation of alloy elements causes local distortion in the octahedral gap nearγ-Ni,γ’-Fe Ni₃ cell alloy elements,and the larger the atomic radius of doped alloy elements,the more serious the lattice distortion is,the greater the diffusion energy barrier of Si atoms is,and the more difficult the diffusion of Si atoms is,which is the main factor of decreasing the diffusion of fusible Si elements inγ-Ni andγ’-Fe Ni₃.（2）According to the first-principles calculation,the Si elements in the ternary disordered alloys with a special quasi-random structure occupy more interstitial positions of the alloy elements,not only the generation energy,but also the crystal lattice distortion is the smallest,even when pure Ni is doped with Si,the Si atoms will have a serious position offset after the structure optimization,therefore,fusible Si tends to occupy more kinds of interstices in the ternary disordered alloys.It is found that the smaller the number of atoms around the diffusion path,the higher the diffusion energy barrier,which may be related to the lattice distortion and the bond energy intensity,it is inferred that the diffusion of Si in the ternary alloys is toward the location segregation diffusion of various elements.Therefore,in the actual brazing process,the brazing temperature and the brazing holding time can not be unrestrained increase,and it is very important to select appropriate brazing parameters.（3）The non-uniform diffusion products of melting-down element Si during brazing are mostly silicide,taking binary silicide as an example,the structure and thermodynamic properties of M_nSi_m（m=Ni,Cr,Mo,W）binary silicide are calculated by first-principles method.It is found that the binary silicide is stable and easy to be produced,the mechanical moduli,such as bulk modulus,shear modulus and Young’s modulus,change regularly with the increase of Si content in silicide,the results show that the thermodynamic stability of binary silicide increases with the increase of Si content,while the hardness and brittleness increase with the increase of Si content,the aggregation of melting-down element Si and the formation of silicide are the important reasons for the decrease of brazing joint quality.