Reactive air aluminizing of a nickel-based superalloy (IN738LC): Coating formation mechanism

Abstract

Reactive air aluminizing (RAA) based on a slurry route is a low-cost method with high efficiency to form aluminide coatings on surfaces where final coating microstructures are temperature and time-dependent. This study aims to incorporate the RAA method into standard heat-treatment cycles of IN738LC. Painted samples are solutionized, solutionized then aged, and only aged. Different samples are prepared at different stages of aluminide formation. FE-SEM surface morphology, BSE-SEM cross-sectional observations, surface XRD characterization, and comparing EDS elemental composition with Al-Ni-Cr phase diagrams are utilized to discuss the formation mechanism thoroughly. A classical high activity type of aluminide coatings with a triple-layered structure of β-NiAl and Cr-rich precipitates at the top coat, β-NiAl in the middle coat, and an IDZ is formed by the RAA method. A three-stage formation mechanism, including the reactive stage (interaction between molten Al and solid substrate), Al-inward diffusion at low and Ni-outward, and Al-inward diffusions at high temperatures, is proposed in detail. After heat-treatment cycles, a ~3 μm aluminum oxide protective scale is detected on the sample surfaces. γ-Al2O3 and α-Al2O3 are formed during only aging at 845 °C and solutionizing at 1120 °C on sample surfaces, respectively.