PHASE TRANSFORMATION OF Al-Ti COATINGS ON Co-BASE SUPERALLOY DZ40M DURING HIGH TEMPERATURE OXIDATION

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Both the formation of simple aluminide and Ti modified aluminide coating on Co base superalloy DZ40M and the phase transformation in high temperature oxidation have been studied. The results indicate that the deposition coatings consist of an outer layer of CoAl phase and an internal layer of Cr,W,Ti rich carbide zone. The transformation of the phase for all these coatings has occurred as follows: CoAl→CoAl(majority)+ α Co→ α Co (majority) + CoAl→ α Co at a high temperature of 900～1 100 ℃. The appearance of α Co marks the beginning of degradation which is over when all of CoAl changes into α Co. The addition of Ti into reagent during deposition can lower the Al activity, reduce the Al content and thickness of coatings relevantly, and speed up the degradation of coatings.