Abstract
The main disadvantage of fcc (face-centred cubic lattice) high-entropy alloys is the low stress level at the yield point (σ0.1) 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 σ0.1 ≈ G/100–G/160 (G is the shear modulus) of the [001]- and [1¯44]-oriented crystals of the Co23.36Cr23.29Fe23.80Ni21.88Al7.67 (Al0.3CoCrFeNi) 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 L12 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 Al0.3CoCrFeNi HEA single crystals induces the orientation dependence of the critical resolved shear stresses (τcr) at T ≥ 200 K (τcr[1¯44] > τcr[001]), which is absent in the initial single-phase crystals, weakens the temperature dependence of σ0.1 above 573 K, and reduces plasticity to 5–13% in the [1¯44] orientation and 15–20% in the [001] orientation.
Highlights
High-entropy alloys (HEAs) are a new class of alloys that have a combination of unique properties, namely, high strain hardening, good plasticity and good ductile fracture strength [1,2,3,4,5,6]
Single-phase fcc HEAs are only comparable to high-strength materials (σ0.1 ~G/140–G/220; here G is the HEA shear modulus, equal to 85, 81 and 67 GPa, respectively, at 77, 296 and 773 K [7]), which include materials with a yield point σ0.1 ~G/100, and they are low-strength (σ0.1 > G/360) at T ≥ 296 K, due to the strong temperature dependence of the yield point σ0.1 (T) [2,3,4,5,6]
Transmission electron microscopy (TEM) investigations have shown that a planar dislocation structure, with dislocation pile ups, developed in the quenched Al0.3 CoCrFeNi HEA single crystals after undergoing strain at 77 K [5]
Summary
High-entropy alloys (HEAs) are a new class of alloys that have a combination of unique properties, namely, high strain hardening, good plasticity and good ductile fracture strength [1,2,3,4,5,6]. The fcc Al0.3 CoCrFeNi alloy is one representative of the HEAs, which is characterized by medium stacking fault energy γ0 = 0.051 J/m2 and demonstrates a unique combination of properties, such as high strength, work hardening, good plasticity in tension at room and cryogenic temperatures, corrosion resistance, high oxidation resistance and excellent fatigue resistance [5,8,9,10,11,12,13,14]. The high level of strength properties at the yield point, achieved in the Al0.3 CoCrFeNi HEA due to Al alloying in the single-phase state, led to the development of twinning at T < 296 K. In poly- and single crystals of the Al0.3 CoCrFeNi HEA, twinning when interacting with slip provides a strong strain
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