Atomic-Scale Investigation of the Borides Precipitated in a Transient Liquid Phase-Bonded Ni-Based Superalloy

Boron is widely used as a melting point depressant (MPD) element in filler materials for utilization in transient liquid phase (TLP) bonding. Various kinds of borides will precipitate in the diffusion-affected zone (DAZ) and influence the integral mechanical properties of bonded materials. In this study, we systematically investigate the boride precipitates formed in DAZ in a TLP-bonded Ni-based superalloy using electron diffraction and atomic-scale high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). The M3B2- and M5B3-type precipitates are observed, both of which obey definitive crystallographic orientation relationships (ORs) with the matrix. The atomic structures of some interfaces associated with these borides are defined. The dendrite morphological feature comprising M3B2 and M5B3 is examined in detail. The dendrite trunks are always composed of two phases including M3B2 and M5B3 borides. The core of the trunk is always M3B2 phase enclosed by M5B3 boride. The dendrite branches consist of M5B3 phase. The M3B2 in the dendrite trunk keeps a definitive OR with the neighboring M5B3. The interfacial features among M3B2, M5B3, and matrix, and the unique intergrowth of M3B2 and M5B3 are further rationalized by phase transformation crystallography.