Kinetics and Microstructural Investigation of High-Temperature Oxidation of IN-738LC Super Alloy
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JMEPEG DOI: 10.1007/s11665-016-2487-4
Kinetics and Microstructural Investigation of High-Temperature Oxidation of IN-738LC Super Alloy S. Hamidi, M.R. Rahimipour, M.J. Eshraghi, S.M.M. Hadavi, and H. Esfahani (Submitted June 13, 2016; in revised form November 11, 2016) The present study was carried out to investigate the kinetics and the surface chemistry of the oxide layers formed on the IN-738LC super alloy during high-temperature oxidation at 950 °C in air from 1 to 260 h. Oxidation kinetics were studied by mass gain measurement. The oxide layers were characterized by field emission scanning electron microscope, elemental distribution map, energy-dispersive spectroscopy as well as x-ray diffractometry (XRD). The oxidation kinetics followed the parabolic law. The XRD analysis revealed that the oxide scale contained mainly NiO, Ni (Cr, Al)2O4, Al2O3, TiO2 and Cr2O3. The oxide structure, from the top surface down to the substrate, was clarified by elemental map distribution studies as Ni-Ti oxides, Cr-Ti oxides, Cr2O3 oxide band, Ni-Co-Cr-W oxide and finally a blocky Al2O3 region. The oxidation scales were composed of three distinct layers of the outer and mid layers enriched by TiO2 and Cr2O3, NiCr2O4 oxide, respectively, and the innermost layer was composed of Al2O3 and matrix alloy. The depleted gamma prime layer was formed under the oxidation scales due to the impoverishment of Al and Ti which were induced by the formation of Al2O3 and TiO2. Keywords
depleted gamma prime, morphology, oxidation kinetics, super alloy
1. Introduction Super alloys, especially Nickel-based ones, are widely used in many applications because of excellent properties at the elevated temperatures. However, these materials are vulnerable to surface attacks after exposing to stringent working conditions resulting in erosion, hot corrosion as well as hightemperature oxidation. Because of the high cost of exchanging the pieces, the worthwhile repair process is carried out on the oxidized parts. Therefore, investigation of the oxide scales formed on the super alloys is an important direction of maintenance of pieces prior to the repairing process (Ref 1-3). Oxidation not only deteriorates the surface integrity, but also depletes the surface from Al, Cr and Ni elements, causing the non-uniform distribution of gamma prime precipitates at the surface close to the oxide scale. This in turn results in more severe weakness in surface strength. The main oxides of NiO, Ni2O3, Al2O3, Cr2O3, TiO2, NiAl2O4 and NiCr2O4 are reported to be formed during oxidation of various super alloys at high temperatures (Ref 4-7). The gamma prime precipitate phase of (Ni3(Ti, Al)) is a major corresponding phase strengthening of the Ni-based super alloys substrate by blocking dislocation movements at elevated temperatures (Ref 8, 9). The gamma prime phase beneath the oxide scale is decomposed during the oxidation process and leave behind a depleted area of alloy S. Hamidi, M.R. Rahimipour, and S.M.M. Hadavi, Ceramic Department, Materials and Energy Research Center, Albo
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