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In this study, oxidation behavior in as‐cast polycrystalline HfNbTaTi3, HfNbTaTiZr, HfMoTaTiZr, and NbMoTaTiZr refractory high‐entropy alloys (RHEAs), and the amorphous form of NbMoTaTiZr RHEA, is investigated. Herein, the surfaces of samples undergoing static oxidation experiments at 700, 800, and 900 °C using scanning electron microscopy, energy‐dispersive X‐Ray spectroscopy, X‐Ray photoelectron spectroscopy, and X‐Ray diffraction are explored. In the corresponding findings, the presence of three different oxidation behaviors in the studied RHEAs is shown: formation of a non‐protective scale, powderization and pesting of the sample, and a distinct amorphous oxidation behavior compared to its polycrystalline counterpart. Notably, Nb appears to induce a detrimental effect on the oxidation properties of samples due to the high ratio of volume change between the oxide and the metal. In the current findings, it is also evidenced that Mo can cause catastrophic failure in RHEAs that lack a protective oxide layer. Overall, the results constitute a step forward in enhancing the understanding of oxidation behavior in RHEAs and developing novel oxidation‐resistant RHEAs. [ABSTRACT FROM AUTHOR]

Arc-melting (AM) as a primary method for casting high entropy alloys (HEAs) ensures rapid alloy screening with minimal material input, high cost-effectiveness, and high cooling rates. However, the limitations of AM on a laboratory scale, particularly its constrained sample size and the necessity for remelting steps to ensure homogeneity, hampers thorough mechanical and functional testing of bulk materials. Therefore, this study features a comparative analysis between AM and vacuum induction-melting (VIM) techniques for High Entropy Shape Memory Alloys (HE-SMAs) production, focusing on the senary alloy Ti16.6Zr16.6Hf16.6Co10Ni20Cu20, known for its potential functional applications and high sensitivity to material inhomogeneity. The alloy's composition, including high-melting point elements like Hf, Ti and Zr, makes it a well-suited candidate for assessing the capabilities of VIM in producing homogeneous bulk materials. The employment of binary pre-alloys in both AM and VIM processes reduced the necessity for remelting steps and ensured better initial quality for subsequent heat treatments. A homogenization treatment at 900 °C for 100 h of an AM-produced senary alloy showed only slight improvements compared to the same alloy produced via VIM, largely due to the slow diffusion of the larger Hf and Zr atoms from the dendrites into the solid solution. This suggests that VIM can achieve comparable levels of homogenization in substantially less time than required for AM-treated samples. The findings finally indicate that by using VIM, when combined with binary pre-alloys, one achieves more homogeneous alloys with reduced heat-treatment time, making it a viable method for HE-SMA production. [ABSTRACT FROM AUTHOR]

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