Exceptional damage-tolerance of a medium-entropy alloy CrCoNi at cryogenic temperatures

TL;DR

This study demonstrates that the CrCoNi medium-entropy alloy exhibits exceptional damage-tolerance and improved mechanical properties at cryogenic temperatures, surpassing many high-entropy and multi-phase alloys.

Contribution

It reveals that a simple three-element medium-entropy alloy can achieve superior strength, ductility, and toughness, especially at cryogenic temperatures, due to continuous strain hardening and dislocation activity.

Findings

Strength exceeds 1.3 GPa at cryogenic temperatures

Failure strains reach up to 90% at low temperatures

Fracture toughness (KJIc) improves to 275 MPa.m1/2 at cryogenic temperatures

Abstract

High-entropy alloys are an intriguing new class of metallic materials that derive their properties from being multi-element systems that can crystallize as a single phase, despite containing high concentrations of five or more elements with different crystal structures. Here we examine an equiatomic medium-entropy alloy containing only three elements, CrCoNi, as a single-phase face-centered cubic (fcc) solid solution, which displays strength-toughness properties that exceed those of all high-entropy alloys and most multi-phase alloys. At room temperature the alloy shows tensile strengths of almost 1 GPa, failure strains of ~70%, and KJIc fracture-toughness values above 200 MPa.m1/2; at cryogenic temperatures strength, ductility and toughness of the CrCoNi alloy improve to strength levels above 1.3 GPa, failure strains up to 90% and KJIc values of 275 MPa.m1/2. Such properties appear to result from continuous steady strain hardening, which acts to suppress plastic instability, resulting from pronounced dislocation activity and deformation-induced nano-twinning.