Optimizing strength-ductility synergy in dissimilar superalloy joint via low-temperature spark plasma diffusion bonding and post-bonding heat treatment

Dissimilar joining of GH5188 and GH3536 superalloys faces the long-standing problem of interfacial brittleness and limited ductility. This challenge mainly originates from oxide-film retention, insufficient diffusion, and carbide accumulation at the bonding interface. To resolve these issues, we developed a low-temperature spark plasma diffusion bonding (SPDB) route combined with a post-bond heat treatment, where pulsed-current-induced local heating, oxide-film disruption and short-range mass transport provide clear processing advantages over conventional diffusion bonding. Key experiments demonstrate that a defect-free joint can be produced at 850 °C within only 10 min, forming a straight bonded line containing MnCr₂O₄ spinel, M₂₃C₆ carbides and deformed solid solutions, with a tensile strength of 524 MPa but limited elongation (15.2 %). Subsequent heat treatment at 1100 °C for 1 h triggers interfacial recrystallisation, cross-interface grain growth, and partial dissolution/redistribution of interfacial M₂₃C₆ carbides, transforming the sharp bond line into a recrystallized and compositionally graded diffusion zone. As a result, the joint achieves a strength of 721 MPa and an elongation of 33.8 % at room temperature. At 700 °C, the post-treated joint maintains a strength of 428 MPa and an elongation of 18.2 %, which are 1.75 and 3.37 times higher than those of the as-bonded joint, accompanied by a fracture-mode transition from interfacial cleavage to ductile failure. Overall, this study demonstrates a SPDB + heat treatment strategy capable of overcoming the metallurgical incompatibility of Co-/Ni-based superalloys and achieving a stable strength-ductility synergy at both ambient and elevated temperatures.