Development of oxidation resistant refractory high entropy alloys within the system Ta-Mo-Cr-Ti-Al

Abstract

Refractory High Entropy Alloys (RHEA) are considered novel promising high temperature materials for structural applications at ultra-high temperatures primarily due to their attractive mechanical properties. By contrast, the oxidation behavior of RHEA has raised concern owing to pest oxidation, significant weight changes, scale spallation or even complete oxidation at elevated temperatures. In this contribution, results on high temperature oxidation behavior of RHEA within the alloy system Ta-Mo-Cr-Ti-Al will be presented. The isothermal oxidation kinetics of alloys was continuously recorded in thermogravimetric devices. The oxidation experiments were performed in laboratory air in wide temperature range from 500°C to 1500°C. The crystal structures of corrosion products were analyzed using X-ray diffraction. The morphology of oxides scales and internal precipitates was characterized applying scanning electron microscopy in conjunction with energy dispersive X-ray spectroscopy. The high-resolution images of the oxide layers were generated in transmission electron microscope. In order to identify the oxidation states, electron energy-loss spectroscopy was utulized in combination with scanning transmission electron microscopy. The experimental results show that the equiatomic alloy Ta-Mo-Cr-Ti-Al exhibits very high oxidation resistant despite the high amount of refractory metals. It was found that the superior oxidation behavior of this alloy is attributed to formation of the rutile-type oxide (Cr,Ta,Ti)O2. The studies on oxidation mechanism of alloys in the system Ta-Mo-Cr-Ti-Al revealed that the growth of this oxide is governed by the inward diffusion of oxygen via oxygen vacancies. A pronounced zone of internal corrosion was, however, observed underneath the oxide scale indicating the notable inward diffusion of oxygen and nitrogen. Due to additions of higher valency metals, e.g. W, the thickness of the zone of internal corrosion was successfully reduced. This experimental finding confirmed the hypothesis that the doping of higher valency cations of n-type oxides leads to decrease in the oxygen vacancy concentration. This study showed that while many RHEA indeed suffer from poor oxidation resistance, some RHEA, e.g. the equiatomic Ta-Mo-Cr-Ti-Al alloy, exhibit very good protectiveness which is attributed to the formation of rarely encountered Ta-Cr-Ti-based complex oxides which possess high thermodynamic stability and low growth rates.