High temperature oxidation behavior of Ta-Mo-Cr-Ti-Al refractory high entropy alloys with different Ta concentrations
Refractory High Entropy Alloys (RHEAs) are regarded to be promising candidates for high temperature structural applications primarily due to their high melting points and outstanding high temperature strength. Though oxidation mechanisms are not always properly understood, some RHEAs possess excellent oxidation resistance despite the high content of refractory metals. In this contribution, the oxidation behavior of several alloys xTa-Mo-Cr-Ti-Al with different Ta concentrations (x = 0, 5, 10, 15, 20 at. %) was investigated to understand the effect of Ta. The oxidation kinetics at 1200°C was measured continuously using thermogravimetric systems. The corrosion products were characterized using standard techniques, such as X-ray diffraction and scanning electron microscopy. The oxidation kinetics of alloys with 15 and 20 at. % Ta obeys the parabolic rate law, while alloys with lower Ta concentrations shows complex kinetics as the formation of gaseous Mo-oxides becomes more significant with decreasing Ta content. The amount of evaporated Mo was determined quantitatively for each alloy. It was found that the evaporation of Mo oxides can only be neglected for the equiatomic alloy with 20 at. % Ta. The structure of the oxide layers formed on the alloys studied and, therefore, the oxidation mechanism changes drastically depending on the Ta content. While a dense CrTaO4 scale in addition to the nearly continuous Al2O3 and Cr2O3 layers protects the alloy with 20 at. % Ta, porous and quickly growing layers consisting of mixed oxides form on the alloys with the Ta content lower than 10 at. % allowing evaporation of Mo oxides. The experimental results clearly show that the oxidation mechanism alters if the Ta concentration exceeds 15 at. %. It was concluded that at least 15 at. % Ta is required to enable the formation of a dense and protective CrTaO4 layer and thus to protect RHEAs from severe oxidation. In our future studies, the critical concentration of Cr to allow the formation of CrTaO4 layers on RHEAs within the system Ta-Mo-Cr-Ti-Al will be determined.