Palladium‐Modified Aluminide Coatings: Mechanisms of Formation

Abstract

A systematic investigation of the influence of Pd‐Ni predeposit alloys on the microstructure and composition of aluminum diffusion coatings has been conducted on Ni base superalloys (mainly IN738). Their metallurgical structure has been studied with a special emphasis on the nature of the phases and distribution of the various elements throughout the coating section. A two‐layer structure similar to that formed on simple aluminide coatings is observed whatever the type of aluminizing treatment (low and high aluminum activity, pack of vapor‐phase coating). The superficial layer is, however, very different from that observed in simple aluminide coatings, since it is constituted with a ternary aluminide. Palladium concentration profiles, which significantly differ when using low or high activity cements, and the results of a study of the ternary Ni‐Pd‐Al phase diagram, provide qualitative indications concerning the coatings' growth processes. Palladium, if present at a sufficient level, enhances Al diffusion in the beta phase. In the case of low activity processes, a consequence of this increase of Al diffusion flux is the location of an reaction zone inside the coating.