In situ synthesis of dispersed boride-induced high-performance brazed DD5 superalloy via the formation of a novel sandwich-structured interlayer

Abstract

The aggregation of boron into continuous brittle phases during the brazing of single-crystal superalloys remains a critical challenge for fabricating aero-engine turbine components. Here, the in situ synthesis of dispersed borides was achieved using a novel sandwich-structured interlayer (SS-interlayer) of NiCrSiB/NiCrWMoAlRe/NiCrSiB. The liquid NiCrSiB interlayer eliminated the initial interface gaps via its excellent wettability, thereby facilitating the interdiffusion of B, Al and Ta atoms and inducing the precipitation of numerous γ′ nanoparticles. The SS-interlayer, used for brazing at 1100 °C for 20 min, increased the shear strength by 74.4% to 753.2 MPa compared with the traditional NiCrSiB interlayer brazed joint. The ultimate tensile strength of the SS-joint was 791.5 MPa at room temperature and decreased to 601.3 MPa at 850 °C. The polycrystalline NiCrWMoAlRe layer functioned as an effective boron getter, with its abundant grain boundaries providing favorable sites for boride nucleation. This led to the initial precipitation of borides at the grain boundaries, followed by their subsequent formation within the γ channels, resulting in a dispersed boride zone (DBZ) embedded in an equiaxed polycrystalline γ + γ′ matrix. The dispersed borides formed semi-coherent interfaces with the matrix, as characterized by specific orientation relationships, such as [00]M₃B₂//[310]FCC and (00) M₃B₂//(00) FCC, which promoted uniform strain partitioning and inhibited the formation of a continuous brittle eutectic structure. The residual zone (RZ) formed in the SS-joint, characterized by a single-crystal/polycrystalline (S/P) interface and coarse Ni₂₃B₆- and M₃B₂-type borides, acts as the dominant brittle failure zone of the joint. The phases within the RZ exhibited semi-coherent orientation relationships, [00] M₃B₂//[110] Ni₂₃B₆, [00] M₃B₂//[110] γ, and [110] Ni₂₃B₆//[110] γ, corresponding to lattice mismatches of 15.3%, 7.3%, and 5.2%, respectively. The fracture mode transformed from brittle cleavage to a ductile-brittle mixed mode with abundant ductile dimples. This strategy provides a viable pathway for fabricating high-performance brazed single-crystal joints.