Unravelling the metal borides evolution in the transient liquid phase bonding of Ni-based alloys via high-throughput transmission electron microscopy and first-principles thermo-kinetic calculations

Abstract

The understanding of temperature and time-dependent metal borides precipitation/dissolution is crucial for the design of the transient liquid phase (TLP) bonding process of Ni-based alloys. It however remains elusive despite substantial research efforts for many years mainly owing to the uncertainty on the precipitated metal borides and the complex thermo-kinetics in the process. In this paper, we have unambiguously constructed the micro/nanoscale map of the precipitated metal borides in the TLP bonded Ni-based Inconel 718 superalloy via a high-throughput transmission electron microscopy (TEM) analysis. Five types of metal borides were found to precipitate in the diffusion affected zone (DAZ) when the isothermal solidification is completed. They are the M5B3 with stacking faults, Ti-rich M3B4, and Nb-rich M3B4 at the grain boundaries as well as single-crystalline M5B3 and M3B2 inside the grains. Notably, the crystal structure of the faulted M5B3 was rationalized by a hybrid modelling approach integrating first-principles calculation and TEM experiments. The sublattice model, with the optimized thermodynamic model parameters, was used to reproduce the metal borides precipitation map in the TLP process using DICTRA software. Coupling with multiscale simulation and experimental data, the present work built a modified thermo-kinetic model, which enables the design of the TLP bonding process of Ni-based alloys that often involves complicated time-temperature schedules and the precipitation/dissolution of a variety of different phases. The strategy can be applied to the design of the brazing process of other alloys or hybrid materials such as ceramics and metal.