Growth Behavior of Coatings Formed by Vapor Phase Aluminizing Using Fe-Al Pellets of Varying Composition

Abstract

Growth behavior of coatings formed by AlF3 activated vapor phase aluminizing on an Ni-base superalloy substrate was investigated at 1353 K for up to 4 h using FeAl, FeAl2 ,F e 2Al5, and FeAl3 pellets as aluminum donors, with an aim of understanding the kinetics under the different aluminum activities. The coatings consist of an outer � -Ni2Al3 layer, a middle � -NiAl layer, and an inner diffusion layer of � 0 -Ni3Al. The thickness of the � -Ni2Al3 layer remarkably increases with an increase of the coating time and of the aluminum content of the donor pellet. Contrarily, the aluminum concentration at the coating surface increases only a little, which might be due to the strong dependence of the aluminum activity on composition of solid Ni-Al alloys. The amount of aluminum deposition shows parabolic time dependence in all the cases, and the deposition rate becomes higher for the donor pellet of higher aluminum content. Except for the case of the FeAl pellet, the parabolic rate constants are similar to the reported value calculated from the diffusion rate of aluminum in solid Ni-Al alloys. This suggests that the rate of aluminum deposition is dominantly controlled by solid diffusion in the coating. However, the rate falling occurs in the last stage due to the phase transformations of FeAl3 ! Fe2Al5 ! FeAl2 ! FeAl caused by the depletion of aluminum in the pellet. It is supposed that the gaseous diffusion of aluminum or the other process might contribute to the rate-controlling when a sufficiently thick FeAl layer covers the pellet surface. [doi:10.2320/matertrans.47.2341]