Accelerated emergence of CoNi-based medium-entropy alloys with emphasis on their mechanical properties

The concept of alloying has evolved over the centuries and in the past decade and a half, the emergence of the high entropy alloying concept has completely changed our perception of alloy design. This alloying strategy has been found to exhibit exciting properties such as high strength, excellent corrosion resistance, high cryogenic fracture toughness, thermal stability, and irradiation resistance. While the fcc-structured equiatomic CrMnFe-CoNi has been very popular over the years, the discovery of the superior properties by a ternary CoNiCr alloy, kick-started a new era for medium-entropy alloy-focused research in the last 5-10 years due to the realization that medium is better. Here we review the recent progress made in the development of medium entropy alloys from a binary CoNi building block (CoNi-M, where M is Fe, Cr, or V), which are prototype systems of medium -entropy alloys. We discuss the relationship between their microstructure and properties (mainly mechanical ones), and how the stacking fault energy, and/or short-range order (SRO) determines the corresponding deformation mechanism. The influence of minor-alloying on their crystal structure and variations in deformation modes are critically discussed. Lastly, some insights and challenges are outlined.

Automated summary

The concept of alloying has evolved over the centuries, and the emergence of high entropy alloying has revolutionized alloy design. Medium entropy alloys, such as the ternary CoNiCr alloy, have shown superior properties and have opened up new avenues of research. This review explores the development of medium entropy alloys using a binary CoNi building block and discusses the relationship between microstructure, properties, and deformation mechanisms.