Alloy design for strong and ductile Mn-alloyed Cr-Co-Ni medium-entropy alloys for room temperature applications

Abstract

The equiatomic Cr-Co-Ni medium-entropy alloy (MEA) is known to exhibit the highest strength and ductility among high- and medium-entropy alloys in the Cr-Mn-Fe-Co-Ni and its subset systems and many attempts have been made to further improve its strength and ductility. A new strategy for alloy design is proposed for Cr-Co-Ni-based MEA by incorporating Mn as the alloying element with the utilization of Mean-Square Atomic Displacement (MSAD) and Stacking Fault Energy (SFE) as predictors for strength and ductility. Our investigation reveals that the addition of 5 at.% Mn results in a modest reduction in MSAD and hence in 0 K yield stress, but the room temperature strength reaches the level achieved by the strongest Cr-rich Cr-Co-Ni MEA (45Cr-20Co-35Ni (in at.%) in our previous study), as the dulling of the temperature dependence of yield strength occurs below room temperature by the addition of Mn. Furthermore, at 77 K, a critical SFE value (−35 mJ/m²) is identified below which the tensile ductility drops significantly due to the change in the additional deformation mode from deformation twinning to ε-martensite transformation, while at room temperature the tensile ductility increases monotonously with decreasing SFE. The designed alloy with a composition of 40Cr-5Mn-27.5Co-27.5Ni thus achieves an optimal balance of strength and ductility at room temperature, improving the tensile ductility of the 45Cr-20Co-35Ni alloy by as much as 24 % without scarifying the yield strength.