CoCrFeNi High-Entropy Alloy (HEA) at cryogenic temperatures*

HEAs are solid solution alloys containing five or more principal elements in equal or near equal atomic percent. The uniaxial compression of nanopillars fabricated from the single crystals of the Face-Centered Cubic and Body-Centered Cubic phases present in Al_0.7CoCrFeNi HEA were studied at room temperature, 143 K and 40 K. Higher yield stresses were observed at lower temperatures with the smallest pillar size for both phases. The “smaller is stronger” effect was observed at the temperatures investigated. An isothermal size dependence was observed at the temperatures investigated which is calculated as a poswer law exponent: - 0.28 for BCC which is low compared to commonly studied BCC system and - 0.66 which is typical for FCC systems due to the ease of glide dislocation motion. The deformation mechanisms of both phases differed at low temperatures. Deformation in the FCC phase can be explained by collective dislocation glide and nucleation-governed plasticity while that of the BCC phase is due to dislocation cross-slip.This is further investigated by TEM analysis to show the twinning elements. We discuss these results in the framework of nanoscale plasticity and the intrinsic lattice resistance through the interplay of the microstructural and dimensional size effects.