Effect of Cyclic Oxidation Exposure on Tensile Properties of a Pt-Aluminide Bond-Coated Ni-Base Superalloy
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INTRODUCTION
DIRECTIONALLY solidified (DS) Ni-base superalloy aerofoil components are used in the hot sections of advanced gas turbine engines. Although these alloys possess good elevated temperature strength, they are moderate to poor in terms of their resistance against high-temperature oxidation, especially under cyclic heating and cooling conditions that typically prevail in gas turbine engines. Inadequate oxidation resistance of these alloys can cause large-scale material loss because of oxide formation and, therefore, lead to reduction in the load bearing cross section of the superalloy component.[1,2] Further, as observed elsewhere,[1] the oxidation-related surface damage to the superalloy can result in significant reductions in the tensile properties of the alloy, especially its ductility. It was shown that the DS superalloy CM-247LC can lose its tensile ductility from about 10 to 3 to 4 pct after the cyclic oxidation exposure of 100 hours at 1373 K (1100 C) in air.[1] In order to protect these superalloys against the deleterious effects of high-temperature oxidation, superalloy components are typically provided with a oxidation-resistant bond coat.[2–4] At present, Pt-modified aluminide (PtAl) bond coats are widely being used in thermal barrier coating systems applied on turbine components such as blades and nozzle guide vanes.[2–4] The protective nature of PtAl bond coats is derived from their ability to form a continuous regenerative layer of a-Al2O3 on the surface.[5–8] The presence of Pt in the coating enhances the Md. ZAFIR ALAM, N. HAZARI, and DIPAK K. DAS, Scientists, are with the Defence Metallurgical Research Laboratory, Kanchanbagh, Hyderabad-500058, India. Contact e-mail: zafir@ dmrl.drdo.in VIJAY K. VARMA, Scientist, is with the Centre for Military Airworthiness & Certification, Bangalore-560093, India. Manuscript submitted January 2, 2011. Article published online September 17, 2011 4064—VOLUME 42A, DECEMBER 2011
adherence of the alumina scale and, thereby, significantly improves the protective capability of the coating.[5,9–13] The microstructural characteristics and oxidation behavior of PtAl bond coats were widely reported.[7,8,14–17] PtAl bond coats are constituted of brittle b-(Ni,Pt)Al intermetallic phase and, typically, have a brittle-toductile transition temperature (BDTT) above 923 K (650 C).[18–22] These coatings are known to degrade the mechanical properties of the substrate superalloy.[4,23–25] Veys and Mervel reported that the room temperature (RT) ductility of single-crystal alloy CMSX-2 decreased drastically by the application of a diffusion aluminide bond coat. However, the strength was found to remain unaffected up to about 1323 K (1050 C).[24] During use at high temperatures, the bond coats undergo considerable microstructural changes, driven primarily by the loss of Al from the coating (toward scale formation) and also by extensive interdiffusion of elements between the coating and the substrate. The b-NiAl structure of the coating gradually transforms to c¢-Ni3Al and c-Ni.[26–29]
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