Microstructure and oxidation behavior of the aluminized coating on K447A nickel-based superalloy prepared by AlF₃-activated pack cementation

The function of AlF₃ activator, the aluminized coating formation mechanism on K447A nickel-based superalloy during the pack cementation process, and their oxidation behaviors have been systematically investigated. Comprehensive characterization by XRD, SEM, TEM and EDS analyses with HSC thermochemical calculation reveals four key findings: AlF₃ demonstrates the optimal aluminizing capability and generates active Al atoms via the disproportionation reaction of AlF gas. The aluminized coating exhibits a multilayer structure, with the outer layer composed of high Al content Ni₂Al₃ or NiAl₃ and the inner layer consisting of NiAl. Precipitated phases such as α-(Cr, W), σ phase, and carbides are distributed in the aluminides, confirming an Al-dominated inward diffusion mechanism characteristic of a “high-activity” process. The coating thickness exhibits linear relationships to the reciprocal of temperature and holding time. Ni₂Al₃ is formed before NiAl, transforms into NiAl, and then is regenerated. After oxidizing at 1150 °C for 100 h, a protective ridged α-Al₂O₃ scale forms on the aluminide coating, effectively shielding the substrate.