Brazing graphite to hastelloy N superalloy using pure-Au filler metal: Bonding mechanism and joint properties

• The graphite is successfully joined to Hastelloy N alloy using Au foil at 980 °C, 1020 °C, 1060 °C, 1100 °C for 0.5 h.
• The brazing temperatures introduced have a limited influence on the joint average shear strength.
• The maximum strength is 26 MPa obtained at 1060 °C for 0.5 h, which reaches 88% of the graphite's intrinsic strength.
• A naturally hierarchical structure in relaxing the stresses is produced in the joints, contributing to excellent bonding.

The graphite was joined to Hastelloy N alloy using Au foil at 980–1100 °C for 0.5 h. The effect of brazing temperature on microstructure and mechanical properties of the joints was investigated. The typical microstructure for the joint is given below: graphite/Zone 1/Zone 2/Zone 3/Zone 4/Hastelloy N alloy. Zone 1 is mainly composed of Au (s.s) + Ni (s.s), in which some larger blocky Mo2C are embedded. Zone 2 is constituted by Au (s.s) and Ni (s.s). Many Mo-C carbides are precipitated in Zone 3 with Mo2C in the grain interiors and tiny MoC at the grain boundaries. In Zone 4, there are still tiny MoC at the grain boundaries but no Mo2C can be observed. Zones 1 and 2 are formed in a liquid state while Zones 3 and 4 occur due to the diffusion reaction of Mo and C in a solid state. The brazing temperatures used have a limited influence on the joint shear strength. The minimum average value is 22.5 MPa, which is equal to 77% of the graphite's intrinsic strength. A naturally hierarchical structure in relieving the thermal stresses is produced in the joints, contributing to the excellent bonding.

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