Effects of Short-Range Ordering on Mechanical Properties of Single Crystals of the Equiatomic CrCoNi Medium-Entropy Alloy

The plastic deformation behavior of single crystals of the FCC equatomic CrCoNi medium-entropy alloy has been investigated in a temperature range of 10-1273 K, paying special attention to possible variations in mechanical properties with heat-treatment that may cause short-range ordering of the constituent elements. Deformation occurs via slip of the {111}<110> system exclusively in the whole temperature range regardless of different heat treatments. The CRSS values increase with decreasing temperature, especially below room temperature, so that the concept of ‘stress equivalence’ is obeyed. This is a clear indication that the strength of these alloys should be described by a mechanism based on solid-solution hardening. Dislocations are smoothly curved in the slip plane without any preferred line orientation, indicating no significant anisotropy in the mobility of edge and screw segments. Planar ½<110>{111} dislocations dissociate widely into Shockley partials for all alloys investigated, indicating the low stacking fault energy of 13 ± 3 mJ/m²for CrMnFeCoNi and CrCoNi, respectively. 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 10 K for single crystals of CrCoNi and its related quaternary (CrFeCoNi) and quinary (CrMnFeCoNi) alloys. 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. 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.