Optimization of holding time for stable spinel-chromia oxide formation in TLP-bonded Hastelloy X: A diffusion-controlled approach

This study investigates the influence of holding time on the microstructural evolution and oxidation resistance of transient liquid phase (TLP)-bonded Hastelloy X (HX) joints utilizing a 100 μm BNi-2 interlayer. Bonding was conducted at 1070 °C for 80, 320, and 640 min, followed by isothermal oxidation at 900 °C for 150 h. Microstructural characterization revealed that extended holding times facilitated complete isothermal solidification and the dissolution of brittle eutectic intermetallics such as Ni 3 B and Ni 3 Si. Among the tested conditions, the joint bonded for 320 min exhibited the most favorable characteristics, including a refined γ-Ni matrix and minimal boride formation in the diffusion-affected zone (DAZ). In contrast, shorter holding times (<320 min) retained residual eutectics that degraded oxidation resistance, while longer times (>320 min) caused chromium depletion in the substrate and microstructural coarsening. Oxidation kinetics followed a parabolic behavior, with the 320 min sample displaying the lowest parabolic rate constant, indicative of a stable and protective oxide scale. Surface and cross-sectional analyses confirmed the development of a continuous Cr 2 O 3 /NiCr 2 O 4 bilayer in the optimized condition. These findings underscore the critical role of optimized holding time in enabling thermodynamically stable oxide formation and enhancing the high-temperature reliability of Ni-based superalloy joints in aggressive environments.