| Summary: | The main disadvantage of fcc (face-centred cubic lattice) high-entropy alloys is the low stress level at the yield point (<i>σ</i><sub>0.1</sub>) at a test temperature above room temperature. This restricts their practical application at high test temperatures from 773 K to 973 K. In this study, we found that a high stress level was reached at the yield point <i>σ</i><sub>0.1</sub> ≈ G/100–G/160 (G is the shear modulus) of the [001]- and <inline-formula><math display="inline"><semantics><mrow><mo stretchy="false">[</mo><mover accent="true"><mn>1</mn><mo>¯</mo></mover><mn>44</mn><mo stretchy="false">]</mo></mrow></semantics></math></inline-formula>-oriented crystals of the Co<sub>23.36</sub>Cr<sub>23.29</sub>Fe<sub>23.80</sub>Ni<sub>21.88</sub>Al<sub>7.67</sub> (Al<sub>0.3</sub>CoCrFeNi) high-entropy alloy (HEA) within a wide temperature range of 77–973 K under tension, due to the occurrence, of nanotwins, multipoles, dislocations under plastic deformation at 77 K and the subsequent precipitation of ordered L1<sub>2</sub> and B2 particles. It was shown that grain boundaries are not formed and the samples remain in a single-crystal state after low-temperature deformation and subsequent ageing at 893 K for 50 h. Achieving a high-strength state in the Al<sub>0.3</sub>CoCrFeNi HEA single crystals induces the orientation dependence of the critical resolved shear stresses (<i>τ</i><sub>cr</sub>) at T ≥ 200 K (<i>τ</i><sub>cr</sub><inline-formula><math display="inline"><semantics><mrow><mo stretchy="false">[</mo><mover accent="true"><mn>1</mn><mo>¯</mo></mover><mn>44</mn><mo stretchy="false">]</mo></mrow></semantics></math></inline-formula> > <i>τ</i><sub>cr</sub>[001]), which is absent in the initial single-phase crystals, weakens the temperature dependence of <i>σ</i><sub>0.1</sub> above 573 K, and reduces plasticity to 5–13% in the <inline-formula><math display="inline"><semantics><mrow><mo stretchy="false">[</mo><mover accent="true"><mn>1</mn><mo>¯</mo></mover><mn>44</mn><mo stretchy="false">]</mo></mrow></semantics></math></inline-formula> orientation and 15–20% in the [001] orientation.
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