Abstract
The effects of prolonged exposure to air at 1073 K on Al20Cr25Nb20Ti20Zr15 and Al20Cr25Nb19Ti20Zr15Y1 (at.%) high-entropy alloys (HEAs) were investigated in this work. Combined scanning electron microscopy and energy-dispersive x-ray spectroscopy (SEM–EDS) analysis revealed that scales containing all major elements in the systems are formed during the oxidation process. Thermogravimetric analysis carried out under the above-mentioned corrosive conditions indicates an initial parabolic kinetics course in the case of the sample with 1 at.% Y addition. However, after a certain period of time, the correlation between mass gain per unit surface area and time becomes linear. In the other case, oxidation proceeds according to the parabolic rate law for the entire process duration. The addition of 1 at.% Y decreases the parabolic rate constants by 1 order of magnitude, thereby improving the chemical stability of the studied Al-Cr-Nb-Ti-Zr system. This is confirmed by visual evaluation, as well as SEM–EDS cross-sectional analysis. Additionally, x-ray diffraction studies indicate that a multiphase oxide scale is formed on the metallic core of both samples. This means that selective oxidation does not occur and all constituent elements took part in the reaction. Taking all of the above into account, it can be concluded that more research is required to fully understand and improve the corrosion resistance of Ti-Al-Cr-Nb-based HEAs.
Highlights
High-entropy alloys (HEAs) were first officially introduced to the scientific world in 2004 by Yeh and his team (Ref [1,2,3,4])
From all the obtained initial results, it can be concluded that much work is still needed before the studied HEA materials capable of operating at high temperatures in corrosive atmospheres are fully understood or ready for practical use
The fact that all the constituent elements of the Ti-Al-Cr-Nb-based HEAs react with oxygen during oxidation at 1073 K indicates that the chemical stability of the samples at elevated temperatures requires improvement
Summary
High-entropy alloys (HEAs) were first officially introduced to the scientific world in 2004 by Yeh and his team (Ref [1,2,3,4]). Configurational entropy depends on the principle element concentrations in accordance with the following equation: DSconf 1⁄4 ÀRRXi ln Xi ðEq 2Þ where R is the universal gas constant and Xi—atom fraction of principle element i. This approximation for liquid and solid alloys near the melting temperature suggests that maximum DSconf can be obtained by mixing the elements together in equimolar ratios. Non-equimolar compositions can be classified as HEAs provided they meet the following criteria: DSconf ‡ 1.5R (Ref 5)
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