Yasuo Matsunaga

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

Growth behavior of coatings formed by AlF3 activated vapor phase aluminizing on a Ni-base superalloy substrate was investigated at 1223-1353 K for up to 4 h using FeAl, FeAl2, Fe2Al5, and FeAl3 pellets as aluminum donors, with an aim at understanding the kinetics under different aluminum activities. The coatings consist of an outer δ-Ni2Al3 layer, a middle β-NiAl layer, and an inner diffusion layer of γ′-Ni3Al. The amount of aluminum deposition w shows parabolic time dependence at 1353 K, and the parabolic rate constant increases with an increase of the aluminum content of the pellet. The linear relationship is approved between ln (w2) and reciprocal of absolute temperature, 1/T. This implies that aluminum deposition is a thermally activated process. When the Fe2Al5 or FeAl3 pellet is used, the activation energy is similar to those of the aluminum diffusion in the binary Ni-Al alloys. Therefore, the rate of aluminum deposition might be dominantly controlled by the aluminum diffusion in the coating. When the FeAl pellet is used, the activation energy is much smaller. It is supposed that the aluminum supply from the gas phase to the coating surface is smaller than the aluminum diffusion in the coating. In this case, the processes other than the aluminum diffusion in the coating might contribute to the rate-controlling step. The rate falling occurs in the last stage owing to the phase transformations of FeAl3 → Fe2Al5 → FeAl2 → FeAl caused by the depletion of aluminum in the pellet. When the pellet surface is completely covered by FeAl, the rate-controlling step might change.