Study on the mechanisms of formation of aluminized diffusion coatings on a Ni-base superalloy using different pack aluminization procedures

The mechanisms of formation and microstructure of pack aluminized diffusion coatings depend on a wide range of parameters. These impact coatings differently depending on the processing mode selected (out-of-pack or in-pack) and are still the subject of debate. The need to systematize data on the influence of processing mode on the mechanisms of the formation of aluminized coatings was the motivation for this study, which reviews the influence of processing mode based on an analysis of experimental data for aluminized coatings processed in-pack and out-of-pack on a Ni alloy with Al at unit activity at 800 °C [low temperature with high Al activity (LTHA)] and 1100 °C [high temperature with high Al activity (HTHA)]. The results show that processing mode determined the amount of Al deposited at the surface and that the formation of a larger halide-depleted zone during out-of-pack diffusion accounted for the reduced deposition of Al. Processing temperature and Al concentration at the surface of the alloy determined the mobility of the elements in the coating and, consequently, the coating thickness and dominant diffusion mechanisms. Aluminization with the specimen immersed in the pack resulted in a preferential inward diffusion of Al for both processing temperatures. In contrast, coatings processed by out-of-pack diffusion showed a dependence on processing temperature. When a HTHA process was used, the coatings were thinner and formed in a two-stage diffusion process, while a LTHA process failed to form aluminide diffusion coatings.