Robust joining of FGH98 superalloy to DD5 single crystal by modulating interdiffusion behavior with Ni/high-entropy alloy hybrid interlayer

FGH98 superalloys and DD5 single crystals are highly desired to work together in high-temperature service environments. In this study, the FGH98-DD5 robust joining was enabled by diffusion bonding using a hybrid interlayer of Ni and high-entropy alloy (Ni/HEA). Elemental interdiffusion among the FGH98 superalloy, DD5 single crystal, and Ni/HEA hybrid interlayer triggered the formation of individual tiny Ni₃Al phase (γ′)-reinforced face-centered cubic (FCC) matrix composite structures, resulting in the high-strength interfacial bonding. Temperature plays a key role in regulating the elemental interdiffusion. As the temperature required for diffusion bonding increased from 1075 to 1130°C, the elemental interdiffusion intensified to promote the void closure at interfaces and the thickening of composite diffusion zones (γ′-reinforced FCC matrix composites), which favors joint strengthening. Nevertheless, the too-high diffusion temperature (1160°C) led to the formation of coarse grains and additional carbides in the FGH98 superalloy and HEA layer, deteriorating the joint performance. Ultimately, the optimal diffusion bonding condition was identified as 1130°C for 1 h with a low pressure of 2 MPa under vacuum, in which the FGH98-DD5 joint underwent a ductile fracture with a shear strength of up to 650 MPa. This study provides scientific insight for modulating interdiffusion behavior, as well as theoretical and technical support for joining FGH98 superalloys reliably with DD5 single crystals.[Figure not available: see fulltext.].