Abstract: Hastelloy X superalloy was bonded with Ni-Cr-Fe based brazing filler metal by activated diffusion brazing(ADB). Microstructure and phase composition of brazed joints were analyzed by scanning electron microscopy,energy dispersive spectrometer and X-ray diffraction,and effects of brazing temperatures and holding times on microstructure evolution of brazed joints were investigated. The results showed that the brazed joint mainly includes solidification zone,diffusion zone and base material. The solidification zone was mainly composed of γ solid solution phase and complex boride M₃ B₂(M=Cr,Mo),while the diffusion zone was made up of γ solid solution phase and three different forms of Cr and Mo borides. With the increase of brazing temperature and the extension of holding time,the number of boride precipitated in the grain interior of the diffusion zone gradually decreased,while the number of precipitating phase increased at the grain boundary. The maximum hardness value of the joint bonded at 1 170℃ for 30 min appeared at the position of precipitation phase in the solidification zone,which was about406 HV1.