The microstructure and oxidation resistance of the aluminide coatings deposited by the CVD method on pure nickel and hafnium-doped nickel superalloys

Aluminide coating were deposited on pure nickel and hafnium-doped nickel superalloys Mar M247, Mar M200 and CMSX 4 by means of the CVD method. All coatings consisted of two layers: an outer, comprising the β-NiAl phase and the interdiffusion one. The interdiffusion layer on pure nickel consisted of the γ′-Ni3Al phase and of the NiAl phase on superalloys. MC and M23C6 carbides besides the NiAl phase were found in the interdiffusion zones on Mar M247 and Mar M200, whereas topologically close-packed phases, such as the TCP σ-phase and the R phase were found in the interdiffusion zone on CMSX 4. Coatings on substrates containing more hafnium (Mar M247 and Mar M200) were more resistant to degradation during the cyclic oxidation. The amount of 1.5–1.8 wt.% hafnium in the substrate let to the HfO2 ‘pegs’ formation in the oxide scale during oxidation of aluminized Mar M247 and Mar M200 superalloys. The improvement of lifetime of coated CMSX 4 superalloy was obtained by platinum modification. Platinum decreased diffusion of alloying elements such as Ti and Ta from the substrate to the coating and oxide scale, stabilized the NiAl phase and delayed the NiAl → Ni3Al phase transformation.