JMEPEG (2001) 10:258–262 ASM International Microstructural Characterization of a Pt-Aluminide Coated IN738LC I. Calliari, M. Dabala `, and A. Zambon (Submitted 7 December 1999; in revised form 4 January 2000) In this paper, the microstructural and chemical characterization of the IN738LC superalloy, coated with a Pt-Cr modified aluminide coating, is presented. The effects of aging in air at 850 C on superalloy and coating microstructures were investigated. The growth of  ' precipitates in IN738LC follows the Wagner- Lifshits model. The positive effect of Pt in preventing refractory element diffusion into the outer coating is not influenced by the aging. A moderate precipitation of TCP phases has been noted at the coating- superalloy interface. In this study, the microstructural and chemical characteriza- Keywords aging, aluminide coating, microstructure, nickel tion of the IN738LC superalloy, coated with a commercially alloy IN738LC, platinum-modified coatings produced (RT22 type) Pt-Cr [8] modified aluminide coating (Pt deposited by electroplating and codeposition of Cr-Al by the 1. Introduction pack cementation process), is presented. The effects of aging on superalloy and coating microstructures were investigated Given the extremely aggressive conditions in the combustion after heat treatment at 850 °C for 500 to 2000 h. environment of industrial gas turbines, some fuel and atmos- pheric contaminants, such as sulfur and chlorine, cause acceler- 2. Materials and Methods ated oxidation and hot corrosion attack of Ni-base superalloys turbine components, operating at temperatures as high as 900 °C. To increase the useful service life for the components, hot The composition of IN738LC superalloy is reported in Table corrosion resistant coatings have been studied and produced. 1. The samples were aged at 850 °C in air for 500, 1000, 1500, Aluminide coatings, [1,2] obtained via the pack cementation proc- and 2000 h. All the specimens were characterized using a ess, are commonly employed to protect  '-strengthened Ni- scanning electron microscope (SEM, Cambridge Stereoscan based blades. The pack normally consists of the coating element 440) equipped with an energy dispersive spectrometer (EDS, source, an activator, usually a halide salt, and an inert filler PV9800) and a CDU detector, (Philips Electronic Instruments material, often alumina, to prevent the source of sintering at Corp., Mahwah, NJ). For the phase quantitative analysis, an high temperature. Solid-state diffusion, when the element source electron beam current of 500 pA, a voltage of 25 kV, and a reacts with the substrate alloy, and gas phase diffusion of the standardless procedure with ZAF (Z (atomic number) Absorp- halides of this element are the main phenomena involved in tion Fluorescence) correction were employed. The accuracy of coating formation. Typically, the outer coating layer is formed the method was checked on the bulk of the same superalloy. of -NiAl and a solid solution of elements originating from For the line profiles across the coating, the IMIX system the substrate. By adding platinum, the protective nature of (Princeton Gamma Tech., NJ) was employed. The microstruc- aluminide coating is significantly improved. In this case, the ture was investigated after chemical (Kalling’s etchant) and outer coating contains intermetallic phases such as PtAl 2 , Pt 2 Al 3 , electrolytic etchings (H 3 PO 4 20 vol.% aqueous solution for 10 or PtAl. [3,4] Platinum increases the coating stability toward s at 3 V). The EDS analysis and line profiles were performed inward diffusion of aluminum or outward diffusion of nickel, without etching. and prevents refractory transition elements such as Mo, V, and W from diffusing into the outer coating layers [5,6] and eliminates 3. Results and Discussion the Cr-rich precipitates in the outer coating layers. [7] 3.1 IN738LC I. Calliari, M. Dabala `, and A. Zambon, Department of Mechanical The microstructure of a new and an aged (2000 h) sample and Management Innovation (DIMEG), University of Padua, 35131 Padova, Italy. Contact e-mail: irene.calliari@unipd.it. of IN738 are reported in Fig. 1 and 2, and the  ' (cuboidal Table 1 Chemical composition (wt.%) of IN738LC Cr Co Ti Al W Ta Mo Nb Zr B C Ni 15.7–16.3 8.0–9.0 3.2–3.7 3.2–3.7 2.4–2.8 1.5–2.0 1.5–2.0 0.6–0.11 0.06–0.10 0.008–0.014 0.09–0.13 Bal 258—Volume 10(3) June 2001 Journal of Materials Engineering and Performance