There have been very limited studies on plastic deformation mechanisms in single-crystalline high-entropy alloys (HEAs) with body-centered cubic (BCC) phases. We performed in situ uniaxial compression on single-crystalline BCC AlCrFeCo-Ni micropillars/nanopillars with three orientations (including [100], [110], and [111]) and diameters of 270−1583 nm, inside a scanning electron microscope. The experimental results showed the significant size effects on yield/flow stress and the remarkable strain hardening in these HEA micropillars/nanopillars. Especially, HEA micropillars/nanopillars with ⟨100⟩ orientation exhibited higher strain hardening exponents than BCC pure metals and Al 0.7 CrCoFeNi counterparts. A combination of transmission electron microscopy observations and large-scale atomistic simulations revealed that dislocation slip, reaction, tangling and accumulation, and solid solution effects are responsible for the observed size effects on yield/flow stress and remarkable strain hardening, but these dislocation mechanisms are dependent on nanopillar orientation. Our present study sheds light on the underlying deformation mechanisms in BCC HEA single crystals.

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High-entropy alloys, a new class of metallic materials, exhibit excellent mechanical properties at high temperatures. In spite of the worldwide interest, the underlying mechanisms for temperature dependence of mechanical properties of these alloys remain poorly understood. Here, we systemically investigate the mechanical behaviors and properties of Al 1.2 CrFeCoNi (comprising a body-centered cubic phase) and Al 0.3 CrFeCoNi (comprising a face-centered cubic phase) single-crystal micropillars with three orientations ([100], [110], and [111]) at temperatures varying from 300 to 675 K by using in situ compression of micropillars inside a scanning electron microscope. The results show that the yield stresses of Al 1.2 CrFeCoNi micropillars are insensitive to temperature changes, and their flow stresses and work hardening rates increase slightly with increasing temperature from 300 to 550 K, which differs from the typical temperature dependence of yield/flow stresses in metals and alloys. In contrast, Al 0.3 CrFeCoNi micropillars exhibit typical thermal softening. Furthermore, it is found that the Al 1.2 CrFeCoNi micropillars exhibit a transition from homogenous deformation to localized deformation at a critical temperature, while the Al 0.3 CrFeCoNi micropillars always maintain a well-distributed and fine slip deformation. Detailed transmission electron microscopy analyses reveal that dynamic recrystallization (involving dislocation tangles, and formation of dislocation cell structures and sub-grains) plays a key role in the observed temperature insensitivity of the yield stress and increasing flow stress (and work hardening rate) with increasing temperature in the Al 1.2 CrFeCoNi micropillars, and that thermally activated dislocation slip leads to thermal softening of the Al 0.3 CrFeCoNi micropillars. The differences in deformation modes and temperature dependence of the mechanical properties between Al 1.2 CrFeCoNi and Al 0.3 CrFeCoNi essentially originate from the differences in dislocation activities and slip systems since the two alloys adopt different phases. Our findings provide key insights in the temperature dependence of mechanical properties and deformation behaviors of high-entropy alloys with body-centered cubic and face-centered cubic phases.