Microstructural evolution and micro-mechanical properties of non-isothermal solidified zone in TLP bonded Ni-based superalloy joints

Transient liquid phase (TLP) bonding is a promising process for the joining and repairing of nickel-base superalloys. One of the most important parameters in TLP bonding is the bonding time required for sufficient isothermal solidification which prevents the formation of undesirable precipitated phases. In the present work, the effect of bonding time on the microstructure, type, and evolution of precipitates in the non-isothermal solidified zone (NSZ) and their effect on micro-mechanical properties were systematically investigated using multi-scale tests in TLP bonded Mar-M247 superalloy joints with Ni-15.2Cr-3.74B interlayer at 1230 ℃. For a bonding time of 5 min, dual-phase M23(C, B)6 - γ/γ’ (where M is a mixture of Hf, Ta, Cr, and Ni) with eutectic configuration was formed in NSZ. With the increase in bonding time, the evolution of NSZ microstructure can be summed up as eutectic M23(C, B)6 - γ/γ’, semi-striping dual-phase M23(C, B)6 - γ/γ’, discontinuously striping M23(C, B)6 - γ/γ’, followed by the disintegration of NSZ. As the NSZ counterpart, the isothermal solidified zone (ISZ) is mainly composed of γ/γ’. Accompanied by the dissolution of M23(C, B)6 in the centerline, the proportion of the ISZ increases greatly until the joints are completely occupied by ISZ. Finally, a bamboo-like structure with domain size of ∼100 μm was formed in the joint centerline, along with γ’ reorganized themselves all into cubic shapes and distributed homogeneously. Mechanical property tests demonstrated that in comparison to samples with longer bonding time, the NSZ of the shortest bonding time (5 min) has the highest strength and a subsequent decrease in strength was observed with prolonging the bonding time and post-bond heat treatment. Furthermore, possible solidification/transformation path, segregation behavior, and formation mechanism of NSZ/ISZ evolution were discussed.