Study On Microstructure And Failure Mechanism Of Vacuum Brazing Joint Of Ni-base Single Crystal Superalloy

Based on the application requirements for high-performance Ni-based single crystal superalloy brazed joints in the manufacture of aero-engine hot-end components,a novel Ni-based brazing filler metal was designed and prepared in this study,which was adopted for the brazing and the post-bond heat treatment of Ni-based single crystal superalloys.Using scanning electron microscopy（SEM）,X-ray diffraction analysis（XRD）,electron microprobe analysis（EMPA）and high-temperature metallography（HTM）,the microstructure evolution mechanism during the formation and heat treatment of brazed joints was deeply explored.Furthermore,microhardness testing and creep testing were employed to evaluate the mechanical properties of as-bonded and post-bond heat-treated joints as well as to investigate their failure mechanisms.The joints were consisted of an isothermal solidification zone（ISZ）formed during brazing and heat preservation and an athermally solidified zone（ASZ）formed during furnace cooling.The phase composition in the ASZ was quite complex.In the order of formation during cooling,these phases were:skeleton-like M₃B₂ phase with their nearby Al-richγ-Ni matrix,blocky M₃B₂phases,γ+γ′eutectic,M₂₃B₆ phase,and Al-poor single-phaseγ-Ni solid solution.The slow cooling resulted in a low initial nucleation rate ofγ′phases in the matrix near the skeleton-like M₃B₂ in the ASZ,whose growth was controlled by diffusion tip effect to present flower-like clumps of different sizes.Owing to the continuous increase of supersaturation in the matrix,secondary precipitation behavior ofγ′phases with high nucleation rate occurred within the matrix channels near the flower-likeγ′phases.These secondaryγ′phases were controlled by the interfacial energy in order to form a subspheroidal shape.Due to the element concentration gradient of the matrix in the ISZ,flower-likeγ′phases of gradient sizes and numbers were precipitated.Besides,their low formation temperature and high nucleation density inhibited the continuous growth of theγ′flower-like clumps while preventing the precipitation behavior of secondaryγ′phases via the rapid depletion of matrix supersaturation.The post-bond heat-treated joints was consisted of an ASZ with a greatly reduced width,an ISZ with a significantly increased width,and a diffusion-affected zone（DAZ）transformed from base metal in the near brazing seam.Acicular M₃B₂ phases of different sizes and numbers were precipitated at the edge ISZ and the DAZ,respectively.With retention of some blocky M₃B₂ phases only,the low-melting-point microstructures in the ASZ were completely eliminated.High-temperature metallographic experiments confirmed the melting behavior of the low-melting-point microstructures in the ASZ during heat treatment,while the significantly expanded width of the ISZ following heat treatment validated the secondary growth behavior of the ISZ.Theγ′phase precipitation in the ISZ was controlled by the interfacial energy,and theγ′phases grew was controlled by Ostwald ripening.Theγ′phases in the central ISZ began to precipitate later,which showed non-uniform nucleation positions as well.Accordingly,they were smaller in size and more scattered in distribution than those in the marginal ISZ.Due to the rapid cooling following heat preservation,theγ′phases in this zone did not undergo flower-like transformation.The extensive precipitation of borides in the diffusion-affected zone contributed to theγ′phase coarsening in this zone,while the rapid cooling following heat preservation resulted in the precipitation of substantial finely dispersed secondaryγ′phases from the Al-richγmatrix channels.After heat treatment of joints,the microhardness of various areas generally decreased,and the areas with lowest microhardness were all located at the center matrix of the joints.These areas were also the locations where the as-bond and post-bond heat-treated joints respectively fractured during the creep testing.The microhardness variations at different areas were attributable to the strengthening of different solid solutions caused by compositional disparity and the second phase strengthening effect.The creep fracture mechanisms of both as-bonded and post-bond heat-treated joints were ductile/quasi-cleavage fractures dominated by ductile fractures.The final creep life of the joints was improved by 29%owing to the elimination of low-melting-point phases and increase of ISZ area caused by the post-bond heat treatment.