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Thermal barrier coatings (TBCs) are widely used for hot section components of gas turbine engines to protect the underlying metals from the high operating temperatures; they serve to both increase the engine efficiency and improve the durability of components. TBC systems typically consist of a Ni-based superalloy substrate, an alumina forming-alloy bond coat and an yttria-stabilized zirconia topcoat. When TBCs are exposed to high temperatures in oxidizing environments, a thermally grown oxide (TGO) develops on the bond coat surface underneath the top coat. Fracture of TBCs progresses in the vicinity of the TGO, which attains a critical thickness during thermal cycling operations (Evans et al., 2001). Thus, suppressing oxidation of the bond coat is anticipated to enhance the durability of TBCs. Alumina-forming alloys tend to form metastable Al2O3 polymorphs such as gammaand theta-phases in oxidizing environments at 1100-1450 K (Brumm et al., 1992, Tolpygo et al., 2000). The thermodynamically stable alpha-Al2O3 typically forms at longer oxidation times and/or higher temperatures. Metastable oxide scales consisting of the gammaand thetaphases contain lattice defects so that they hardly act as a protective layer to further oxidation of the alloys compared with a alpha-Al2O3 scale (Brumm et al., 1992). It is also well known that the transformation of the alpha-phase from metastable polymorphs involves large changes in volume (~13%) and morphology. Metastable oxides tend to form on the bond coat during coating with the topcoat in an oxygen-containing atmosphere at high temperatures, so that they are present at the interface between the bond coat and topcoat. The transformation of these oxides to the stable alphaphase at higher operating temperatures promotes topcoat spalling. Thus, in order to enhance the durability of TBCs, the as-processed TBC microstructures should contain a thin layer of alpha-Al2O3 at the interface with no the metastable oxides. The standard gritblasting procedure that is used in practical applications to prepare the bond coat for topcoat deposition promotes the formation of alpha-Al2O3. However, it also results in severe contamination of the coating surface and accelerates oxidation of the bond coat (Tolpygo et al., 2001). Some studies have revealed that a pre-oxidation step that forms a alpha-Al2O3 TGO on the bond coat without grit blasting reduces further oxidation of the bond coat and improves the durability of the TBC (Tolpygo et al., 2005, Nijdam et al., 2006, Matsumoto et al., 2006, 2008). Although it is generally preferable to perform pre-oxidation above 1450 K to produce a thin alpha-Al2O3 oxide and avoid producing metastable oxides, high temperature

Control of Polymorphism and Mass-Transfer in Al2O3 Scale Formed by Oxidation of Alumina-Forming Alloys