Abstract
Refractory high-entropy alloys (RHEAs) are widely studied because of their promising potential for ultrahigh-temperature applications. One key challenge towards the development of RHEAs as high-temperature structural materials is to concurrently achieve optimum oxidation resistance and mechanical properties. Here in this work, the effect of alloying on the oxidation behavior of ductile RHEAs was studied. Specifically, a ductile RHEA, Al0.5Cr0.25Nb0.5Ta0.5Ti1.5, was alloyed with Al and Zr aiming to improve its oxidation resistance. The two modified RHEAs, Al0.75Cr0.25Nb0.5Ta0.5Ti1.5 and Al0.5Cr0.25Nb0.5Ta0.5Ti1.5Zr0.01, indeed show enhanced oxidation resistance at 800 °C and 1,100 °C, compared with Al0.5Cr0.25Nb0.5Ta0.5Ti1.5. In addition, all three RHEAs studied here show an excellent oxidation resistance at 800 °C compared with other RHEAs, although there is still a large space to further improve their performance at 1,100 °C. Internal oxidation and even nitridation are still present after oxidation exposure, indicating further efforts are required to form protective oxide scales on the surface of ductile RHEAs. Nevertheless, the work is expected to shed some light on future directions of improving the oxidation of ductile RHEAs, via the alloying route.
Highlights
Refractory high-entropy alloys (RHEAs) [1e3], consisting of refractory metals with high melting points, constitute one particular group of high-entropy alloys (HEAs) [4e6]
The oxidation results for three RHEAs at 800 C and 1,100 C are given in Fig. 2(a), showing the mass change as a function of time (h)
All three RHEAs show parabolic oxide growth during their complete exposure time at 800 C, suggesting that the oxygen ingress is effectively limited, with the oxidation rate controlled by diffusion [13]
Summary
Refractory high-entropy alloys (RHEAs) [1e3], consisting of refractory metals with high melting points, constitute one particular group of high-entropy alloys (HEAs) [4e6]. RHEAs have gained increasing attention because of their inherent high strength and softening resistance at elevated temperatures, surpassing those of commercialized superalloys such as Haynes 230 and Inconel 718 [1,3,8]. Despite these advantages, RHEAs suffer from room-temperature brittleness and/or insufficient oxidation resistance, and RHEAs that possess a balanced ductility and decent d These authors contributed to this work. To quote a few examples here, NbCrMo0.5Ta0.5TiZr after 100 h exposure at 1,000 C forms complex oxides with an exceptionally high mass gain of 120 mg/cm2 [13]; AlCrMoTiW containing about 80 at.% of refractory elements shows a parabolic oxidation kinetics during 40 h of exposure to air at 1,000 C with the mass gain of ~ 7.8 mg/cm, significantly higher than that of Ni-based superalloys [14,15]; AlCrNbMoTi changes its oxidation kinetics from nearly parabolic to linear behavior at 1,000 C with the mass gain of ~ 28 mg/cm after
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