Evolution of aluminide coating microstructure on nickel-base cast superalloy CM-247 in a single-step high-activity alumi
- PDF / 3,316,727 Bytes
- 16 Pages / 612 x 792 pts (letter) Page_size
- 40 Downloads / 302 Views
I.
INTRODUCTION
The pack aluminizing technique is being used worldwide for the formation of protective aluminide coatings on nickel-base superalloy components. The formation of these aluminide coatings is carried out using either a low-activity or a high-activity pack. Conventionally, a low-activity coating is developed in one step, i.e., aluminizing is carried out at a temperature above 1000 7C for the required duration (typically 3 to 4 hours) to produce a NiAl coating layer. No subsequent treatment is essential in this case to obtain the NiAl phase that acts as a reservoir of aluminum during high-temperature oxidation so that a protective layer of alumina is maintained on the surface of the component. In high-activity aluminizing, however, incorporation of aluminum (aluminizing) is conventionally carried out at a much lower temperature (700 7C to 850 7C) which is then followed by a diffusion treatment above 1000 7C for achieving the NiAl coating structure. The typical times for aluminizing and diffusion treatment are 2 hours and 4 hours, respectively, for the preceding two-step process. Therefore, while a conventional low-activity coating process is also referred to as a single-step aluminizing process, the high-activity process is referred to as a two-step process. The above two aluminizing processes are also known as high-temperature low-activity (HTLA) and low-temperature high-activity (LTHA) processes, respectively, based on the temperature of aluminizing and the activity of the pack used. In both the above aluminizing processes, the D.K. DAS and S.V. JOSHI, Scientists, are with the Defence Metallurgical Research Laboratory, Hyderabad -500 058, India. VAKIL SINGH, Professor, Department of Metallurgical Engineering, Benaras Hindu University, Varanasi, 221005 India. Manuscript submitted May 28, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
resultant coating contains NiAl as the bulk phase. The term activity mentioned earlier in defining an aluminizing pack mixture is related to the concentration of aluminium in the Al-containing powder (Al source) used in the pack. If the amount of Al is greater than 60 at. pct in the aluminum source, the corresponding pack is usually called a highactivity pack.[1,2,3] Otherwise, the pack is referred to as a low-activity one.[1,2,3] Although high-activity coatings are normally developed in a two-step process, as mentioned earlier, there are also numerous reports of these coatings having been developed in a single step by carrying out aluminizing directly above 1000 7C.[3–10] Such a method of forming the high-activity coatings appears attractive, because it eliminates one processing step and leads to the formation of the final NiAl coating structure in a single treatment, as in the case of a low-activity coating. The mechanism of plain aluminide coating formation on Ni-base superalloys has been extensively reported for both HTLA and LTHA processes.[11–14] In these studies, the inert carbides originally present in the substrate have been treated as natural markers, and the
Data Loading...
