Alloy Design of Cr-Co-Ni-Based Medium-Entropy Alloys for High Strength and High Ductility

High entropy alloys (HEAs) in the Cr-Mn-Fe-Co-Ni system and medium entropy alloys (MEAs) in its subsystems have attracted considerable interest because of their excellent combination of strength and ductility. Previous studies have revealed that these excellent mechanical properties rely neither on the number of constituent elements nor on the equiatomic composition. For example, the Cr-Co-Ni and Cr-Mn-Co-Ni MEAs are proved to possess superior mechanical properties to the Cr-Mn-Fe-Co-Ni HEA. The strength of these face-centered cubic (FCC) HEAs and MEAs is believed to relate to how significantly the FCC lattice is distorted, while the ductility is related to the low stacking fault energy that promotes deformation twinning. That means combinations of strength and ductility can be tailored by tuning the lattice distortion and stacking fault energy. We have proposed that mean-square atomic displacement (MSAD) is a nice scaling factor to describe the lattice distortion, and thereby the strength. In the present study, we investigate the MSAD-yield strength as well as the SFE-ductility relationships for series of Cr-Co-Ni and Cr-Mn-Co-Ni MEA polycrystals produced by cold-rolling and subsequent annealing, in order to deduce a composition range where excellent combinations of strength and ductility are obtained. The 0 K yield strengths evaluated from compression tests made in a temperature range of 10-300 K are found well correlated with the calculated MSAD values and the ductility obtained by tensile tests at 77 K is generally higher when the stacking fault energy, measured from the dislocation dissociation widths, is lower. We discuss how quickly we can determine a composition range where excellent combinations of strength and ductility are gained.