Effect of palladium-based undercoat on the formation, structure and properties of diffusion aluminide coatings

Abstract
Coatings with Pd-Ni predeposit alloys have been processed using various aluminizing treatments : low and high activity cements, pack and vapor phase coatings. Their metallurgical structure has been investigated with a special emphasis on the repartition of the various metallic elements throughout the coating section. During a first step, aluminizing of the palladium rich underlayer takes place, followed by simultaneous diffusion of nickel and aluminum through this first aluminized layer. Palladium concentration profiles - which significantly differ when using L.A. or H.A. cements - provide qualitative indications concerning the overall "activity" of aluminum in the aluminizing process. Palladium, if present at a sufficient level, enhances Al diffusion in the beta phase. In the case of L.A. processes, a consequence of this increase of Al diffusion flux is the location of NiAl reaction zone inside the coating.