Synergistic effects of Si and Zr on the microstructure of aluminide coatings on IN792 superalloy by out-of-pack aluminizing: A thermodynamic and experimental study

ABSTRACT This study investigates the synergistic effects of Si and Zr on the microstructure of aluminide coatings on the IN792 Ni-based superalloy. A halide-activated, out-of-pack aluminization process was employed utilizing powder mixtures of Al, Si, ZrO 2 , NH 4 Cl, and Al 2 O 3 . Thermodynamic analysis confirms the spontaneous formation of gaseous precursors (AlCl, SiCl 2 , ZrCl 4 ) and their subsequent deposition, as indicated by negative Gibbs free energy changes. Experimentally, low Al-activity powder mixtures produced double-layer, Ni-rich β-NiAl coatings (32.6–35.3 µm) via outward Ni diffusion. Conversely, high Al-activity powder mixtures produced thicker (55.2–70.2 µm) Al-rich β-NiAl coatings via an inward Al-diffusion mechanism. Crucially, a powder mixture with high nominal Al (15 wt.%), Si (22.5 wt.%), and ZrO 2 (9 wt.%) content resulted in an Al-rich β-NiAl coating (51.4 μm) governed by outward Ni diffusion. This suggests that high modifier concentrations can suppress local Al activity, effectively overriding the influence of nominal Al content. Energy-dispersive spectroscopy (EDS) indicated the successful co-deposition, revealing a uniform distribution of Zr throughout the coating and the formation of discrete Ti-Si-rich phases. X-ray diffraction (XRD) identified β-NiAl as the dominant phase, with minor Mo 3 Cr 3 Si 2 phases in high-activity coatings. The novelty of this work is the demonstration via coupled thermodynamic modeling and experiment that high local concentrations of Si and Zr can act as a thermodynamic sink for halide activator species, suppressing local Al activity and thereby altering the inward/outward diffusion regime in out‑of‑pack aluminizing.