  High entropy alloys (HEAs) have attracted substantial interest in recent
years. Thus far, most investigations have focused on their applications as
structural materials rather than functional materials. In this paper, we show
that FeMnNiAlCr HEAs can potentially be applied as both a structural and
functional material for high-efficiency concentrated solar thermal power (CSP)
systems working at >700 degrees C. The HEA itself would be used in
high-temperature tubing to carry working fluids, while its surface oxide would
act as a high-efficiency solar thermal absorber. These HEAs have demonstrated
yield strengths 2-3x greater than that of stainless steel at 700 degrees C and
a creep lifetime >800 h at 700 degrees C under a typical CSP tubing mechanical
load of 35 MPa. Their Mn-rich surface oxides maintain a high optical-to-thermal
conversion efficiency of ~87% under 1000x solar concentration for 20 simulated
day-night thermal cycles between 750 degrees C and environmental temperature.
These HEAs have also sustained immersion in unpurified bromide molten salts for
14 days at 750{\deg}C with <2% weight loss, in contrast to 70% weight loss from
a 316 stainless steel reference. The simultaneous achievement of promising
mechanical, optical, and thermochemical properties in this FeMnNiAlCr system
opens the door to new applications of HEAs in solar energy harvesting.
