The aluminization of platinum and platinum-coated IN-738
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THEenvironment which exists in the hot-gas portion of industrial gas turbines is extremely agressive. 1 T e m p e r a t u r e s in the range of 900~ are sufficient to cause accelerated oxidation/hot corrosion attack of Ni-base superatloy turbine components by the fuel and atmospheric constitutents, such as sulfates, chlorides, vanadates and sodium and lead ions. To increase the useful service life for superaltoy components, there is a continuing need for improved h o t - c o r r o s i o n r e sistant coatings. Diffusion aluminide and CoCrA1Y overlay coatings have proven useful in turbine envirom e n t s J '2 Another coating system for nickel-base superatloys has been described by Lebalert and Memhardt. This system is reported to give a 230 pct improvement in h o t - c o r r o s i o n r e s i s t a n c e over simple aluminide c o a t ings. The platinum-aluminide coating s y s t e m is applied by first electrolytically depositing a platinum layer less than 10/~ m thick, then pack aluminizing the substrate to form a platinum-aluminide-type coating. Aluminizing p r o c e s s e s have been reviewed extensively e l s e w h e r e . 7-~ The degradation p r o c e s s for aluminide coatings involves loss of aluminum in two directions; (1) outward loss by oxide formation and spallation, and (2) inward loss by interdiffusion with the substrate. The Pt-A1 coating was developed during a search for a "diffusion b a r r i e r " to prevent or r e t a r d the migration of aluminum from the coating into the substrate. However, the compositional profile found by Lehnert and Meinhardt 3 indicated that, after aluminization, the platinum remained concentrated in the outermost region of the duplex coating and was not a diffusion barrier. The goal of this study was to gain an understanding of the effects of an initial platinum layer on the f o r m a tion of aluminide coatings on IN-738.* The aluminiza9
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*59,6a/o N1,8.2 Co, 17.5Cr, 7~2AI,4.0 Ti, 1,0 Mo,0.8 W,05 Nb, 0.6 Ta, 0.03 Zr, 0.6 C. tion of platinum was studied to provide supplemental data. The pack aluminization of IN-738 has been treated M, R. JACKSONand J. R. RAIRDENare MetaJlurglsts with General Electric Company,Corporate Research and Development, Schenectady, NY 12345, Manuscript submitted October 20, 1975. METALLURGICALTRANSACTIONSA
in detail previously. *~ Aluminum was observed to deposit v e r y rapidly initially, with deposition slowing to a much lower rate soon after the aluminizing pack reaches the aluminizing temperature. F u r t h e r exposure is p r i m a r i l y a solid-state diffusion anneal. The phases formed during the entire pack p r o c e s s a r e substantiatty those predicted front ternary phase diag r a m s . The manner in which the sequence of events is altered by the presence of a Pt layer on IN-738 can be understood in a similar fashion, and is the subject of this study. EXPERIMENTAL TECHNIQUES The IN-738 s u b s t r a t e s were pins cast to 0.47 cm diameter and centerless ground to 0.43 c m diameter. The platinum substrates were cut from 0.1 cm thi
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