High Temperature Oxidation Behavior of CrMnFeCoNi High-Entropy Alloy

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Thin films of the CrMnFeCoNi high entropy alloy were deposited by magnetron sputtering from a sintered equimolar target. The substrate temperature and bias were varied during deposition, and the structure, morphology and elemental distribution were studied in detail. All films formed phase mixtures of multiple crystal structures. This contrasts with studies on the bulk alloy, where it typically forms a single phase with a simple cubic closed packed (ccp) structure, with other phases precipitating only after long annealing times. For higher substrate temperatures, we observed a mixture of phases with ccp and bcc (body centered cubic) structures, and the intermetallic phases σ-phase and L10, the first three being the predicted equilibrium phases at the deposition temperature. For room temperature depositions, we found evidence of very limited diffusion of metal atoms during the deposition. These films formed a mixture of a ccp and the intermetallic χ-phase. Two mechanisms can be distinguished that govern the phase formation at lower and higher temperatures. From the present results and comparisons with the literature, we also discuss why the small grain size, the low process temperature, and the fast surface diffusion during synthesis causes magnetron sputtering to yield different results compared to bulk synthesis from the melt. These principles explain why it is easier to form the equilibrium phases by sputtering, and why a single ccp phase should not be expected as a rule for this deposition method. Following the thermodynamic principles of high entropy alloys, this may also be the case in other high entropy alloy systems.

High-temperature oxidation behavior of any materials depends on the stability of the protective oxide formed on the surface of the materials. The chromium oxide, Cr2O3, formed on Fe-Cr alloys is unstable above 850 °C, whereas the alumina, Al2O3, scale formed on Fe-Al alloys is stable up to 1350 °C. Consequently, Fe-Cr alloys are not suitable for high-temperature applications, especially above 850 °C. In contrast, Fe-Al alloys containing a sufficient amount of Al exhibit an excellent oxidation resistance at high temperatures due to the formation of a continuous and robust layer of Al2O3. However, the high amount of Al required for the formation of a robust layer of Al2O3 on Fe-Al alloys makes the alloys brittle and restricts their structural application. The required critical content of Al for the formation of a protective layer of Al2O3 on Fe-Al alloys can be reduced by the addition of the “third element” like Cr, which improves the ductility of the alloy. In addition, the required critical Al content can further be reduced by decreasing the alloy grain size to the nano-regime. Based on the available literature, it can be hypothesized that the nanocrystalline (NC) structure can enhance the protective oxide formation, and further reduce the required critical content of Al for the formation of a continuous layer of Al2O3 on Fe-Cr-Al alloys. Although the oxidation behavior of Fe-Cr-Al alloys has been studied by several researchers, the studies are limited to the microcrystalline structure of the alloys. The effect of NC structure on the oxidation behavior of Fe-Cr-Al alloys has yet not been reported. Therefore, the present work primarily focuses on the study the oxidation resistance of NC Fe-Cr-Al alloys vis-a-vis the oxidation behavior of their microcrystalline (MC) counterparts.The NC Fe-Cr-Al alloys powders (of compositions Fe-20Cr-5Al, Fe-20Cr-3Al Fe-10Cr-5Al and Fe-10Cr-3Al) were synthesized using high-energy ball milling followed by a rapid consolidation using spark plasma sintering at 900 °C with the application of a pressure of 90 MPa. Out of these four alloys, Fe-20Cr-5Al and Fe-20Cr-3Al alloys were selected to study the effect of the NC structure on their high-temperature oxidation behavior. The oxidation behavior of the NC Fe-20Cr-(3,5)Al alloys at temperatures range (500-900 °C) in 60 h of oxidation was compared with that of their MC counterparts. The oxide scales formed on the NC and MC alloys were analyzed for morphology, chemical composition, and thickness of oxide using different characterization techniques. The post-oxidation characterization shows a remarkable effect of NC structure on the oxidation behavior of the Fe-20Cr-(3,5)Al alloys. The NC Fe-20Cr-(3,5)Al alloys exhibit superior oxidation resistance at high temperatures than that of their MC counterparts due to the formation of a continuous protective layer on the NC alloy. Contrary to the oxidation behavior of common steels at high temperatures, the Fe-20Cr-(3,5)Al alloys show better oxidation resistance at high temperatures (800 and 900 °C) than that at relatively low oxidation temperatures (500 and 700 °C) due to the formation of a considerably more protective oxide layer at high temperatures (800 and 900 °C). Further, the NC structure also influences the “third element effect” of Cr. Consequently, the protective oxide formed on NC Fe-20Cr-5Al alloy with the assistance of the “third element effect” of Cr at 500 and 700 °C, whereas the oxide formed on the NC Fe-20Cr-5Al alloy without the assistance of the “third element effect” of Cr at 800 and 900 °C. On the other hand, the oxide formed on MC Fe-20Cr-(3,5)Al alloys without the assistance of the “third element effect” of Cr at all oxidation temperatures (500-900 °C). Based on the post-oxidation characterization and available literature, the mechanisms for the formation of oxide scale on the Fe-Cr-Al alloys are proposed. The thesis provides the evidence to validate the hypothesis that NC structure can extensively enhance the formation of protective oxide on the Fe-Cr-Al alloys at high temperatures. In addition, the thesis also presents that the NC structure influences the role of the “third element effect” of Cr for the formation of protective oxide on the Fe-Cr-Al alloys at high temperatures. Thus, the present research work has provided a comprehensive overview of the oxidation behavior of NC and MC Fe-Cr-Al alloys at high temperatures.

High temperature oxidation and corrosion behaviour of Ni/Ni–Co–Al composite coatings