Conditions for Formation of Coherent Aluminide Coatings on γ-Tiai By Pack Cementation Process

Abstract

Thermochemical analyses were undertaken for a series of pack powder mixtures for depositing aluminide coatings on γ-TiAl by the pack cementation process. Particular attention was focused on the effects of halide activators on the deposition characteristics of the pack powder mixtures. The activators investigated included the halide salts NH₄F, NH₄Cl, AlF₃ and AlCl₃. Based on the results of the analyses, a series of coating deposition experiments was carried out to identify optimum pack powder mixtures for depositing adherent and coherent aluminide coatings on γ-TiAl. It was observed that aluminide coatings consisting of an outer TiAl₃ layer and an inner TiAl₂ layer with a coherent structure free from microcracking could be deposited on γ-TiAl via inward diffusion of aluminium at 1000 ^° C using pack powder mixtures containing 4 wt-%Al or less and 2 wt-%AlCl₃ with Al₂O₃ as the inert filler. A discussion is presented on the conditions that need to be carefully controlled to ensure the deposition of aluminide coatings on γ-TiAl with a coherent structure free from microcracking by the pack cementation process. The oxidation resistance and the thermal stability of the aluminide coatings deposited were evaluated in air at temperatures up to 850 ° C. The results are discussed in relation to the oxide scale formation process and the high temperature stability of the diffusion triplet of TiAl₃/TiAl₂/TiAl.