The effects of alloy microstructure refinement on the short-term thermal oxidation of NiCoCrAlY alloys
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I. INTRODUCTION
NiCoCrAlY alloys are used as overlay coatings on superalloy components or as bond coatings in between thermal barrier coatings and superalloy components in state-ofthe-art high-temperature coating systems.[1–4] The application of such a NiCoCrAlY coating is required to protect the components in gas turbine engines against oxidation and corrosion at elevated temperatures.[1–4] The protection offered by such a bond or overlay coating relies on the ability of this alloy to produce and maintain a stable, adherent, slowgrowing oxide layer on its surface, preferably a continuous a-Al2O3 layer.[1–4] However, generally the formation of such a slow-growing, continuous alumina layer is preceded by an initial stage of very fast oxidation. This fast oxidation regime is associated with severe Al and Cr depletion in the alloy near the oxide/metal (O/M) interface, and the simultaneous formation of a-Al2O3 and fast growing oxide phases such as Cr2O3, NiO, NiCr2O4, NiAl2O4, or u-Al2O3.[5–11] The formation of these less protective oxide phases should be suppressed, because they can contribute significantly to the total oxide layer thickness,[6,11] thereby enhancing the spallation of the oxide layer from the alloy upon thermal cycling. For typical coating compositions, the NiCoCrAlY alloys have a multiphase structure, consisting of an Al-rich b phase (NiAl) and an Al-poor g phase (Ni solid solution).[12,13] This
implies that the microstructure, composition, thickness, and phase constitution of the oxide layers that develop on these alloys are not only determined by the composition of the two principal phases (i.e., g and b), but also by their size, fraction, and distribution within the alloy.[14–18] Besides that, the adherence of the developing oxide layers to the alloy is influenced by the Y distribution in the alloy prior to oxidation.[19,20,21] To investigate the effects of the alloy microstructure on the early stages of high-temperature oxidation of NiCoCrAlY alloys, three alloys with similar average composition (Ni20Co-19Cr-24Al-0.2Y in at. pct) but with different initial microstructures were oxidized for 0.5 and 1 hours at 1373 K: (1) a coarse-grained as-cast alloy,[22] (2) a fine-grained lasersurface-melted (LSM) alloy,[22] and (3) a fine-grained electron beam–physical vapor deposition (EB-PVD) coating.[13] After oxidation, the morphology, structure, chemical composition, phase constitution, and thickness of the oxide layers were investigated using various analytical techniques. In addition to this, the performance of the three alloys was assessed by means of thermal cycling oxidation testing. The relationships between the alloy microstructure prior to oxidation and (1) the microstructure of the oxide after shortterm oxidation and (2) the performance of the alloys upon thermal cycling are discussed. II. EXPERIMENTAL A. Materials and Sample Preparation
T.J. NIJDAM, formerly Postdoctoral Student, Department of Materials Science and Engineering, Delft University of Technology, is a Postdoctoral Researcher with th
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