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<p>High entropy alloys (HEAs) are promising candidates for high-temperature applications. One potential application is in concentrated solar power (CSP) systems, where native oxides of cost-effective FeMnNiAlCr HEAs can serve as high-efficiency solar thermal absorbers. However, the large and repetitive temperature change of the CSP system in day-night cycles greatly challenges the thermal stability of this native oxide solar absorber layer. In this paper, the cyclic oxidation behavior of FeMnNiAlCr high entropy alloys (HEA) with and without cold working and subsequent annealing is investigated to evaluate the optical performance and thermal stability of their native oxides as a solar absorber for next-generation CSP systems. As the cold working reduction increases, we find that the stability of the native surface oxides improves. The native oxides on cold rolled and annealed HEA samples (rolling reduction of 30% followed by annealing at 1000&#xa0;°C for 1&#xa0;h) sustained &gt; 20 simulated day-night thermal cycles without degradation, each cycle comprising 12&#xa0;h at 750&#xa0;°C in air and 12&#xa0;h natural cooling to 25&#xa0;°C. This is in sharp contrast to significant oxide spallation at &lt; 5 cycles for the as-cast HEAs, as well as notable spallation after 20 thermal cycles for HEAs with cold rolling reduction of 18% of the same composition. The enhanced thermal stability is explained by increased short-cut diffusion pathways induced by a higher density of dislocations and subsgrains, as well as the formation of a new A12 phase after cold working and annealing. The enhanced short-cut diffusion overcomes the sluggish diffusion effects in HEAs and helps maintain the protective alumina layer through a steady Al diffusion flux to the oxide/HEA interface. Furthermore, the enhanced Al out-diffusion also enables the formation of Al<sub>2</sub>O<sub>3</sub> pegs into the Mn<sub>2</sub>O<sub>3</sub> solar absorber layer, improving its adhesion to the HEA substrate upon thermal cycling. Therefore, phase and microstructure engineering through cold working and annealing effectively prevents excessive oxidation and oxide spallation of FeMnNiAlCr HEAs, an important step towards long-term cyclic thermal stability for CSP applications.</p>

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Cold Working and Annealing Enhanced Cyclic Thermal Stability of Native Oxide Solar Thermal Absorbers on a FeMnNiAlCr High Entropy Alloy

  • 期刊论文

  • 发表日期：

    2025年10月9日

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  • Xiaoxue Gao,
  • Edwin Jiang,
  • Andrew Pike,
  • Shang Liu,
  • Geoffroy T. F. Hautier,
  • Ian Baker,
  • Jifeng Liu

摘要

High entropy alloys (HEAs) are promising candidates for high-temperature applications. One potential application is in concentrated solar power (CSP) systems, where native oxides of cost-effective FeMnNiAlCr HEAs can serve as high-efficiency solar thermal absorbers. However, the large and repetitive temperature change of the CSP system in day-night cycles greatly challenges the thermal stability of this native oxide solar absorber layer. In this paper, the cyclic oxidation behavior of FeMnNiAlCr high entropy alloys (HEA) with and without cold working and subsequent annealing is investigated to evaluate the optical performance and thermal stability of their native oxides as a solar absorber for next-generation CSP systems. As the cold working reduction increases, we find that the stability of the native surface oxides improves. The native oxides on cold rolled and annealed HEA samples (rolling reduction of 30% followed by annealing at 1000 °C for 1 h) sustained > 20 simulated day-night thermal cycles without degradation, each cycle comprising 12 h at 750 °C in air and 12 h natural cooling to 25 °C. This is in sharp contrast to significant oxide spallation at < 5 cycles for the as-cast HEAs, as well as notable spallation after 20 thermal cycles for HEAs with cold rolling reduction of 18% of the same composition. The enhanced thermal stability is explained by increased short-cut diffusion pathways induced by a higher density of dislocations and subsgrains, as well as the formation of a new A12 phase after cold working and annealing. The enhanced short-cut diffusion overcomes the sluggish diffusion effects in HEAs and helps maintain the protective alumina layer through a steady Al diffusion flux to the oxide/HEA interface. Furthermore, the enhanced Al out-diffusion also enables the formation of Al2O3 pegs into the Mn2O3 solar absorber layer, improving its adhesion to the HEA substrate upon thermal cycling. Therefore, phase and microstructure engineering through cold working and annealing effectively prevents excessive oxidation and oxide spallation of FeMnNiAlCr HEAs, an important step towards long-term cyclic thermal stability for CSP applications.