Microstructural evolution of the nickel platinum-aluminide bond coat on electron-beam physical-vapor deposition thermal-
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. INTRODUCTION
IT is well known that modification of Ni-aluminide bond coats by Pt additions significantly improves their resistance to high-temperature oxidation and hot corrosion.[1–4] Such property enhancements have led to widespread application of Ptaluminide bond coats on superalloy components of advanced gas turbine engines. Some studies[5–8] have indicated several coupled effects of the Pt addition in these Ni-aluminide bond coats. First, Pt increases the coating stability by preventing inward diffusion of Al and outward diffusion of Ni and refractory elements like Mo, V, Ta, and W.[5] Second, Pt increases the diffusivity of Al in the outer region of the bond coat, resulting in the formation of a purer thermally grown oxide (TGO).[6] In addition, Pt improves oxide adherence.[7,8] Last, Pt prevents the -NiAl alloy from transforming into a -Ni3Al–based alloy, thereby improving the oxidation resistance of the bond coat.[6] The aluminide bond coats deposited on single-crystal superalloy turbine blades involve several matrix and precipitate phases, primarily -NiAl and -Ni3Al matrix phases and , , and precipitates (the superalloy substrate itself is, of course, a / Ni-based alloy). Precipitation of the and phases usually depletes the substrate of the refractory strengthening elements Ta, W, Mo, Cr, and Co and can, therefore, be detrimental to mechanical properties of the superalloy. However, these precipitates do have a beneficial effect on coating stability by restricting inward diffusion of Al and outward L.C. ZHANG, formerly Research Associate with the Department of Materials Science and Engineering, Case Western Reserve University, is Postdoctoral Researcher, University of Connecticut, Storrs, CT 06269-3136. A.H. HEUER, University Professor and Kyocua Professor of Ceramics, is with the Department of Materials Science and Engineering, Case Western Reserve University, Cleveland, OH 44106-7204. Contact e-mail: heuer@ case.edu Manuscript submitted December 17, 2003. METALLURGICAL AND MATERIALS TRANSACTIONS A
diffusion of Ni.[9] It is worth noting that the phase preferentially dissolves W and Ta and the phase preferentially dissolves Cr and Co. Furthermore, -Cr precipitates are commonly encountered in the simple aluminide bond coat,[10] whereas -W precipitates have been found in a Pt-aluminide bond coat,[11] where they formed by outward diffusion of W from the superalloy substrate. Carbide precipitates are present in some bond coats when sufficient carbon is present.[12] In order to investigate the mechanisms of degradation of these bond coats, laboratory studies have been carried out involving isothermal exposure,[10,13–15] thermal fatigue,[15] and thermomechanical fatigue[16] under various conditions. All these studies have shown that two processes dominate the microstructural evolution: interdiffusion between the bond coat and substrate and oxidation of the bond coat, leading to the so-called TGO layer beneath the ZrO2-based thermal-barrier coating (TBC) topcoat. Microstructural charact
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