Modeling of Microstructural Evolution in an MCrAlY Overlay Coating on Different Superalloy Substrates
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INTRODUCTION
OPERATING temperatures within gas turbine engines are being raised continuously in search of higher efficiency exposing the turbine blades and vanes to increasingly harsher environments (e.g., References 1 and 2). As a result, there is greater demand for the components to be made more resistant to oxidation and corrosion. Designing components with improved performance requires a deeper understanding of how the microstructure of coated alloy systems evolves during service. Computer models can help in this regard by extending our understanding of the microstructural evolution processes without recourse to traditional but more expensive experiments at high temperatures. In addition, a well-tuned model can assist service engineers by providing valuable parameters, such as the remaining life of a blade system or their temperature limitations. The development of a model with these attributes was described in detail in an earlier paper,[3] and validated against a particular experimental system. In this article,
M.S.A. KARUNARATNE, Research Associate, and R.C. THOMSON, Professor of Materials Engineering, are with the Department of Materials, Loughborough University, Loughborough LE11 3TU, United Kingdom. Contact e-mail: [email protected] I. DI MARTINO, formerly PhD Student, with the Department of Materials, Loughborough University, is now Corporate Metallurgist/Engineer, with International Power plc, Senator House, London EC4V 4DP, United Kingdom. S.L. OGDEN, formerly PhD Student, with the Department of Materials, Loughborough University, is now Materials Engineer, with E.ON New Build and Technology, Technology Centre, Ratcliffe-on-Soar, Nottingham NG11 0EE, United Kingdom. D.L. OATES, formerly PhD Student, with the Department of Materials, Loughborough University, is now with RWE npower, Windmill Hill Business Park, Whitehill Way, Swindon, Wiltshire SN5 6PB, United Kingdom. Manuscript submitted October 11, 2010. Article published online October 18, 2011 774—VOLUME 43A, FEBRUARY 2012
the model is used in conjunction with experimental observations to demonstrate the pivotal role of the substrate in determining the microstructure evolution by considering the same MCrAlY coating composition on three different substrates. Therefore, the information contained in this article should be helpful to coating system designers for turbine blades.
II.
BACKGROUND
It is common to manufacture industrial turbine components from either Ni-, Co-, or Fe- based superalloys and protect them additionally by an MCrAlY-based coating system (M = Ni, Co and/or Ta). Such coatings owe their protection to a dense surface layer of oxide that consists primarily of Al2O3. For the long-term stability of the oxide scale and, hence, the protection of the substrate, maintaining a substantial reservoir of Al in the coating is crucial. During service, the coating loses Al to two distinct processes: First, it is consumed by the oxide scale that undergoes many cycles of spallation and reforming. Second, it diffuses into the substrate, which no
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