Influence of bonding temperature on interfacial microstructure and high-temperature oxidation behavior of transient liquid phase (TLP) bonded Hastelloy X joints

The high-temperature performance of Hastelloy X (HX) components critically depends on joint integrity and oxidation resistance. In this study, transient liquid phase (TLP) bonding was applied at 1070, 1110, and 1160 °C for 40 min using a 38 µm BNi-2 interlayer. Oxidation tests were conducted at 900 °C for up to 150 h. Microstructural evolution was characterized using optical microscopy (OM), field emission scanning electron microscopy equipped with energy-dispersive X-ray spectroscopy (FESEM/EDS), and X-ray diffraction (XRD). A fully austenitic γ-matrix with Cr-rich M23C6 and Mo-rich M6C carbides was observed in the base metal. All joints showed complete isothermal solidification without the formation of an athermal solidification zone (ASZ). Increasing bonding temperature thickened the isothermal solidification zone (ISZ: 37 → 45 µm) and reduced the diffusion-affected zone (DAZ: 70 → 41 µm), consistent with enhanced diffusion of B and Si. Cr- and Mo-rich borides exhibited multiple morphologies, though their density markedly decreased at 1160 °C. Oxidation kinetics followed a parabolic trend; the 1160 °C sample exhibited the lowest mass gain (≈60 % lower than 1070 °C), with a uniform Cr2O3/SiO2 scale and minimal spallation. XRD confirmed dominant NiO, Cr2O3, and NiCr2O4 phases, with intensified spinel peaks at 1160 °C. Optimizing bonding temperature thus significantly enhances oxidation resistance and joint durability.