Single-Crystal Mechanical Properties of FCC High-Entropy Alloys

The plastic deformation behavior of single crystals of the FCC equatomic CrMnFeCoNi high-entropy alloy and its derivative quaternary (CrFeCoNi) and ternary (CrCoNi) medium-entropy alloys has been investigated in a temperature range of 10-1273 K. Deformation occurs via slip of the {111}<110> system exclusively in the whole temperature range for all alloys investigated. The CRSS values increase with decreasing temperature, especially below room temperature, so that the concept of ‘stress equivalence’ is obeyed for all alloys investigated. This is a clear indication that the strength of these alloys should be described by a mechanism based on solid-solution hardening. The CRSS values extrapolated to 0 K for polycrystals of equiatomic quinary, quaternary and ternary alloys are reported to be well scaled with the mean-square atomic displacement from the regular FCC lattice points (calculated based on density-functional theory). This seems also the case for the CRSS values at 0 K for single crystals of the present three alloys, although some modifications are definitely needed. Deformation twinning occurs on the conjugate system in the form of the Lüders type deformation in the later stage of deformation at low temperatures in all of the three alloys. The correlation between the twinning stress and the stacking-fault energy in these alloys will be discussed. The occurrence of short-range ordering was confirmed in CrCoNi through the measuring of electrical resistivity, while conventional x-ray and electron diffraction failed to detect short-range ordering because of the very small difference in atomic scattering factor for the constituent elements in a wide range of scattering angle. The extent of short-range oredering in CrCoNi was proved to be the largest after annealing at 673 K. However, no significant variation of the CRSS values, twinning stress and the stacking-fault energy was noted upon the formation of short-range order by heat-treatment.