In-situ synthesized (Nb,W,Al)Si2 composite coatings on Nb521 high-temperature alloy via hot-dipping: microstructural characteristics, formation mechanism and growth kinetics

Nb521 alloy is promising for aerospace ultra-high-temperature components but limited by severe oxidation above 600 °C. Despite the effectiveness of silicide coatings for protection, the growth behavior of hot-dipped (Nb, W, Al)Si 2 composite coatings on Nb521 remains unclear. In this work, dense (Nb, W, Al)Si 2 composite coatings were in-situ prepared on Nb521 substrates via hot-dipping in Al-10Si melt. Effects of deposition temperature (700-775 °C) and time (3-12 min) on phase composition, microstructure, and surface roughness were characterized, with growth kinetics and mechanism discussed via thermodynamic calculations. The results show that the coating phase evolves from Al-rich (Al, Si) 3 Nb intermediate to (Nb, W, Al)Si 2 solid solution with increasing temperature and prolonged time, and substrate-derived W dissolves uniformly in the silicide lattice. The surface roughness increases monotonically with grain nucleation and coarsening, while the Sdr peaks at 9 min due to Ostwald ripening. All coatings are dense with good metallurgical bonding, and a double-layered structure (Al-enriched outer layer + low-Al inner layer) forms at deposition time over 9 min caused by interfacial Si depletion. The coating growth follows a diffusion-controlled parabolic law with a K p of 1.514 × 10 −13 m 2 /s at 750 °C, which is significantly higher than that of conventional pack cementation. The growth process includes four stages: contact incubation, intermediate phase formation, silicide transformation and steady double-layer growth, with (Al, Si) 3 Nb acting as a metastable transition phase. This clarifies multi-scale evolution of hot-dipped multicomponent silicide coatings, guiding controllable preparation of high-temperature protective coatings on Nb-based alloys.