Evolution of aluminide coating microstructure on nickel-base cast superalloy CM-247 in a single-step high-activity aluminizing process

Abstract

This study deals with the aluminizing of a directionally cast Ni-base superalloy, namely CM-247, by a single-step process using a high-activity pack. It is observed that significant incorporation of Al into the substrate surface during aluminizing continues over a period of about 1 hour and is not restricted merely to the first few minutes, as reported in the literature. Based on the microstructural details of the coatings formed at various stages of aluminizing, it is concluded that the coating growth in the above process takes place primarily by inward Al diffusion initially, followed by an intermediate stage when the growth involves both inward Al and outward Ni diffusion. In the final stages, the outward diffusion of Ni dominates the coating formation process. The above mechanism of coating formation is different from the one that prevails in the conventional two-step high-activity coating process in that the reaction front for the formation of NiAl remains spatially stationary despite the outward diffusion of nickel during the intermediate stage. It is also shown in the present study that the content of the Al source in the pack affects the coating structure significantly. It is further demonstrated that the microstructure of the aluminide coatings depends not only on the amount of Al incorporated in the sample during aluminizing but also on the time over which the uptake of this Al takes place.

Microstructure and High Temperature Oxidation Performance of Silicide Coating on Nb-Based Alloy C-103

“…Such large difference in the diffusivity between Nb and Si in NbSi 2 has led to the formation of the NbSi 2 layer almost exclusively by the inward diffusion of Si in the present case. Similar inward growth in case of aluminide (B2-NiAl) coatings on Ni-base superalloys by pack aluminizing method has been well documented [33,34]. In case of aluminide coating, the comparatively much larger diffusivity of Al as compared to that of Ni in hyperstoichimetric (Al-rich) B2-NiAl phase causes the inward coating growth [33,35].…”

“…This layer has formed towards the end of siliconizing treatment. As typical of a pack process, the availability the coating forming atoms (Si in the present case) decreases drastically after initial few hours [34]. For example, the Si pick-up by the sample during siliconizing increased from zero to 5.8 mg cm -2 during the first 2 h while during the remaining 4 h of the siliconizing process, the Si pick-up increased by only 4.5 mg cm -2 out of the total pick-up value of 10.3 mg cm -2 .…”

Aluminide Coatings on Titanium-based Alloy IMI-834 for High Temperature Oxidation Protection

“…Further, unlike the precipitate-rich IDZ typically found in aluminide coatings on superalloys (see Fig. 3) 16,17 , both plain aluminide ( Fig. 2(a)) and PtAl [ Fig.…”

“…In the present case of plain aluminide coating, no such rapidly oxidising second phase was present and the coating had a singlephase Al 3 Ti structure. The additional PtAl 2 phase present in the PtAl coating is also known to possess extremely good high temperature oxidation resistance based on the reported studies on Pt-aluminide coated Ni-base superalloys 14,15,17 . Therefore, unlike in the case 28 of bulk Al 3 Ti, a normal behaviour of oxidation rate rising with increasing temperature was observed for both the above coatings on IMI-834 alloy (Table 2).…”

Cyclic Oxidation Behavior of Iridium-Modified Aluminide Coatings for Nickel-Base Single Crystal Superalloy TMS-75

“…1,2) Aluminide coatings have been regarded as the most basic and the most important among various kinds of high temperature coatings. To improve performance of the aluminide coatings, Pt-modified aluminide coatings have been applied for turbine blades and vanes, and they have been found to exhibit excellent oxidation resistance and hot corrosion resistance.…”