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 metall
urgical structure has been studied with a special emphasis on the natu
re of the phases and distribution of the various elements throughout t
he coating section. A two-layer structure similar to that formed on si
mple aluminide coatings is observed whatever the type of aluminizing t
reatment (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
PdNi aluminide. Palladium concentration profiles, which significantly
differ when using low or high activity cements, and the results of a s
tudy of the ternary Ni-Pd-Al phase diagram, provide qualitative indica
tions concerning the coatings' growth processes. Palladium, if present
at a sufficient level, enhances Al diffusion in the beta phase. In th
e case of low activity processes, a consequence of this increase of Al
diffusion flux is the location of an NiAl reaction zone inside the co
ating.