DECOMPOSITION OF THE B2-TYPE MATRIX OF ALUMINIDE DIFFUSION COATINGS ON SINGLE-CRYSTAL NICKEL-BASE SUPERALLOY SUBSTRATES

The microstructural stability of aluminide diffusion coatings, prepared by means of a two-stage pack-aluminization treatment on single-crystal nickel-base superalloy substrates, is considered in this article. Edge-on specimens of coated superalloy are studied using transmission electron microscopy (TEM). The effects of coating thickness and post-coating heat treatment (duration, temperature, and atmosphere) on coating microstructure are examined. The article discusses the partial transformation of the matrix of the coating, from a B2-type phase (nominally NiAl) to a L1(2) phase (nominally Ni3(Al,Ti)), during exposure at temperatures of 850-degrees-C and 950-degrees-C in air and in vacuum for up to 138 hours. Three possible processes that can account for decomposition of the coating matrix are investigated, namely, interdiffusion between the coating and the substrate, oxidation of the coating surface, and aging of the coating. Of these processes, aging of the coating is shown to be the predominant factor in the coating transformation under the conditions considered.