Metallurgical aspects of high entropy alloys

Abstract

Alloying traditionally enhances material properties by adding small amounts of secondary elements to a primary base. Over the past decade and a half, a revolutionary strategy has emerged: combining multiple principal elements in high concentrations to create high-entropy alloys (HEAs). This approach unlocks a vast, largely unexplored compositional space, leading to the discovery of alloys with exceptional properties. This work provides a comprehensive review of recent advancements in HEA research, focusing on metallurgical aspects, highlighting key findings, and identifying future research directions. We reviewed the design philosophy of conventional alloys and its transition to HEAs, emphasizing metallurgical differences and similarities. Key topics include the fundamentals of high-entropy alloys design for variety of HEAs, such as equiatomic and non-equiatomic, eutectic, metastable, refractory, lightweight, spinodal HEAs, and high-entropy superalloys. The review explores microstructural features of HEAs, including as-cast wrought and additively manufactured morphologies, texture development, precipitation, and dispersion-based microstructures. The design of HEAs involves understanding and manipulating these microstructural characteristics to achieve desirable properties. Metallurgical properties of HEAs are summarised, including tensile and compressive properties, fatigue properties, creep, superplastic, corrosion behavior and weldability. Typical deformation mechanisms such as slip, twinning, twinning-induced plasticity, transformation-induced plasticity, and precipitation-assisted deformation, which are found to be mainly active in HEAs, are also discussed. Despite the progress made, the potential of HEAs is far from fully realized. Future directions conceive the concept of high-entropy conventional alloys (HECAs), which merge the high-entropy effect in HEAs with the conventional alloy design approach, thereby introducing high entropy phases in the conventional alloy matrix or vice-versa. Thus, the novel concept of HECA provides a future pathway to materials design and foster enhanced metallurgical properties utilizing the HEA concept for practical applications. To explore the extensive compositional and microstructural possibilities of HEAs and HECAs, high-throughput experimental techniques and computational methods are essential. This review aims to serve as a valuable resource for new researchers and provides insights to guide future investigations in the field of high-entropy alloys and high-entropy conventional alloys, with a strong emphasis on their metallurgical aspects.