Microstructural aspects of aluminized coatings on nickel-base alloys

Slattery, G.F.

doi:10.1179/030716983803291325

Cited by 34 publications

(11 citation statements)

References 7 publications

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“…[11][12][13][14] In these studies, the inert carbides originally present in the substrate have been treated as natural markers, and their position in the coating has been used for identifying the direction of motion of the various elements such as Al (from the pack) and Ni (from the substrate) during aluminization. [12,13,14] With the help of such natural markers, it has been shown that the coating formation in a low-activity aluminizing process results primarily by the outward diffusion of nickel from the substrate (as illustrated in Figure 1) and its reaction with aluminum available from the pack.…”

Section: Introductionmentioning

confidence: 99%

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

Das

Joshi

Singh

1998

Metall Mater Trans A

133

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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.

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Section: Introductionmentioning

confidence: 99%

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

Das

Joshi

Singh

1998

Metall Mater Trans A

133

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“…The variation in elemental concentrations across the coating is very similar to that in the nickel alloy 718. 2 Metallographic evaluation revealed that the uniform coating thickness of about 50 mm was achieved over the entire length of the specimens. The variation of coating thickness along the length of the strip is shown in Fig.…”

Section: Resultsmentioning

confidence: 95%

“…The variation in elemental concentrations across the coating is very similar to that in the nickel alloy 718. 2…”

Section: Resultsmentioning

confidence: 99%

“…Aluminising is a surface modification process in which an intermetallic layer of NiAl is formed in the temperature range between 1123 and 1373 K with very high hardness and excellent resistance to oxidising environments. 2 The coating acts as a barrier between the surface of the material to be protected and the hostile environments. 3 In the context of liquid sodium-water steam generators, these coatings provide surface properties such as low friction and resistance to wear and galling.…”

Section: Introductionmentioning

confidence: 99%

“…Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam 603102, India2 Formerly with Indira Gandhi Centre for Atomic Research (IGCAR), Kalpakkam 603102, India…”

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Development of process for aluminising nickel alloy 718 strips for tube bundle support structures in liquid sodium–water steam generators

Srinivasan

Shankar

Bhaduri

et al. 2011

Materials Science and Technology

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The tubes in liquid sodium–water steam generators of the Indian prototype fast breeder reactor (PFBR) will be supported by corrugated nickel alloy 718 strips. Aluminisation of nickel alloy 718 strips has been chosen for this application because of the excellent performance of aluminide coatings in reducing impact fretting wear of the tubes due to flow induced vibrations and compatibility of the coating with liquid sodium at the operating temperature of the steam generators. Aluminisation of nickel alloy 718 strips for steam generator tube bundle support structures has been developed using a procedure involving thermal spraying of aluminium followed by diffusion heat treatment in vacuum atmosphere. One of the advantages of the technique is that it will coat only the desired surfaces of the strips, whereas in conventional pack cementation process, significant precautions have to be ensured. Furthermore, this process has enabled aluminisation to be carried out at a much lower cost than the conventional process of pack aluminising. The problems encountered during the initial trials and technology development, such as coating thickness and distortion, are discussed. A process flow chart for this procedure to take the job on an industrial scale is also reported. This process (under patenting) has been adopted for the aluminisation of corrugated strips for the support structures of one steam generator module and the steam generator for a test facility during the technology development phase, as also of all the steam generators being fabricated for the PFBR.

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