PACK CEMENTATION ALUMINIDE COATINGS ON SUPERALLOYS - CODEPOSITION OF CR AND REACTIVE ELEMENTS

A single-step chloride-activated pack cementation process has been developed f or growing a Cr/RE(Y, Zr)-modified or a RE(Y, Zr, Hf)-doped aluminide diffusion coating on commercial Ni-base superalloys. The coatings consisted of an outward-grown hypostoichiometric beta-NiAl layer with a substantial enrichment in Cr and a low reactive element content. The Cr was both dissolved in the beta-NiAl matrix and precipitated as alpha-Cr second-phase particles. Aluminide coatings grown outwards in a pack permitting powder contact with the substrate were embedded with Al2O3 and reactive element (RE) oxide pack particles. Physical separation of the substrates from the powder mixture eliminated powder entrapment but reduced the growth rate and the amount of Cr enrichment in the coating. Isolation of the substrate from the pack introduced partial rate control by gaseous diffusion.