Mechanical property comparisons between CrCoNi medium-entropy alloy and 316 stainless steels

Abstract

• The CrCoNi medium-entropy alloy shows a superior strength and ductility synergy, Charpy impact energy ( A K ), and fracture toughness at cryogenic temperatures as compared to both 316L and 316LN stainless steels. • These three alloys all rely heavily on shear transformations upon plastic straining, with deformation twinning dominating in CrCoNi while martensitic transformation involved in stainless steels. We systematically compared the mechanical properties of CrCoNi, a recently emerged prototypical medium-entropy alloy (MEA) with face-centered-cubic (FCC) structure, with hallmark FCC alloys, in particular, the well-known austenitic 316L and 316LN stainless steels, which are also concentrated single-phase FCC solid solutions and arguably next-of-kin to the MEAs. The tensile and impact properties, across the temperatures range from 373 K to 4.2 K, as well as fracture toughness at 298 K and 77 K, were documented. From room temperature to cryogenic temperature, all three alloys exhibited similarly good mechanical properties; CrCoNi increased its tensile uniform elongation and fracture toughness, which was different from the decreasing trend of the 316L and 316LN. On the other hand, the stainless steels showed higher fracture toughness than CrCoNi at all temperatures. To explain the differences in macroscopic mechanical properties of the three alloys, microstructural hardening mechanisms were surveyed. CrCoNi MEA relied on abundant mechanical twinning on the nanoscale, while martensitic transformation was dominant in 316L at low temperatures. The deformation mechanisms in the plastic zone ahead of the propagating crack in impact and fracture toughness tests were also analyzed and compared for the three alloys.