Effects of elemental combination, stacking fault energy and temperature on the tensile deformation behavior of single crystals of quinary, quaternary and ternary equiatomic high- and medium-entropy alloys derived from the Cr-Mn-Fe-Co-Ni system

Abstract

The effects of elemental combination, stacking fault energy (SFE) and temperature on the deformation behavior of single crystals of equiatomic high- and medium-entropy alloys (HEA and MEAs) with the face-centered cubic structure derived from the Cr-Mn-Fe-Co-Ni system have been investigated in tension at room temperature and 77 K. The SFE of these alloys varies from 83 mJ/m² to 14 mJ/m² and the efficiency in decreasing the SFE increases in the order of Cr>Co>>Mn>Fe. For all the HEA and MEAs investigated, the critical resolved shear stress for slip increases remarkably from room temperature to 77 K but does not exhibit any significant compression-tension asymmetry at both temperatures. Deformation in Stage I occurs in the form of Lüders band, the extent of which is temperature-independent but increases with the extent of solid-solution strengthening (SSS). The extent of yield drop increases also with the extent of SSS and with decreasing temperature. The work hardening rate of Stage II does not vary significantly from alloy to alloy but is slightly higher at 77 K than at room temperature. Deformation twinning occurs only in the Cr-Co-Ni MEA at room temperature, but at 77 K, it occurs in six HEA and MEAs with SFE≤32 mJ/m². Consequently, while at room temperature only the Cr-Co-Ni MEA exhibits remarkably superior tensile elongation, the tensile elongation at 77 K tends to increase with decreasing SFE in particular for those (with SFE≤32 mJ/m²) twin. The effects of twinning mechanisms (nucleation- and propagation-controlled twinning) on the twinning stress-SFE relationship are discussed.