Co-base superalloys are commonly used for vanes and parts of the combustion chamber in gas turbine engines. The Co-base superalloys are primarily solid solution strengthened and have good resistance to hot corrosion, creep and thermal fatigue. In particular, Co-base alloy X-40 was used to fabricate the first stage NGV airfoils of T56 series engines; inspections after service have revealed that X-40 airfoils suffered from severe thermal fatigue damages. In this study, a new braze repair scheme is proposed; in which Ni-base alloys are used to repair the X-40 substrate in both narrow and wide gap configurations. Metallographic examination, X-ray mapping and EDS semi-quantitative compositional analyses were carried out to study the microstructures in the braze joint in the asbrazed condition and after thermal exposure at 950°C. The results obtained so far suggest the formation of Cr-rich borides, eutectic phases and various carbides in the joint. No TCP phases were found in the brazed joint and base metals adjacent to the joint. The high carbon content in the alloy X-40 may have played an important role in preventing the formation of TCP phases during brazing and subsequent thermal exposure.

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