REVIEW ARTICLES CURRENT SCIENCE, VOL. 118, NO. 10, 25 MAY 2020 1520 *For correspondence. (e-mail: npgurao@iitk.ac.in; kbiswas@iitk.ac.in) High-entropy materials: critical review and way forward Nilesh P. Gurao* and Krishanu Biswas* Department of Materials Science and Engineering, Indian Institute of Technology Kanpur, Kanpur 208 016, India High-entropy materials and related complex concen- trated materials are considered to be the new ‘avatar’ in physical metallurgy and materials engineering, inspiring novel ideas and approaches to design mate- rials by expanding the compositional space in multi- component and multiprincipal systems. This article is intended to provide critical analyses of the field, summarizing the principles underlying the birth and growth of these novel materials, progress over the last 15 years, and expansion of the field from metallic alloys to ceramics. Finally, the challenges ahead and possible way forward are discussed. Keywords: Ceramics, compositional space, high-entropy materials, metallic alloys. ALLOYING is the greatest gift for the development of human civilization on our planet. We have been using alloying (by the addition of alloying elements to base metals) even since the serendipitous discovery of Cu–Sn bronzes to modify properties for betterment of perfor- mance of any material 1 . For example, steels contain iron as the primary element into which carbon, silicon and chromium are added in minute amounts to obtain high strength, corrosion resistance, ductility; copper is added to aluminium to obtain strong and tough duralumin for aircraft; copper–beryllium alloys are made for non- sparking applications in explosive environment, etc. However, the traditional alloys we use are primarily based on one or two elements into which alloying addi- tions are made. In the last decade of the 20th century, it was acknowledged that this primary element-based approach severely restricts the choice of elements and their addition, and hence the field of high-entropy mate- rials (HEMs) started in 2004 in order to ‘expand the compositional space’ of the multicomponent system 2,3 . This field, including high-entropy alloys (HEAs) and high-entropy ceramics (HECs) 4,5 has gathered momentum as well as scientific and technological curiosity, due to the desire to address fundamental issues achieving extraordinary physical, structural and functional proper- ties for engineering applications from the explored com- positional space in different materials 6,7 . The approach is based on mixing multiprincipal elements in relatively high concentration, which is distinctly different from the traditional alloying design approach. Hence, it has opened up new possibilities by providing access to hither- to unexplored compositional and thereby virgin property space. The fundamentals of our understanding of physical metallurgy, thermodynamic, kinetics and properties have been thrown up with challenges that they have prevailed, but our understanding of these aspects has improved with the advent of HEAs 8 . It is no surprise that the new field of HEAs has gradually expanded to include complex con- centrated alloys (CCAs) 9,10 , which comprise of astronom- ically large number of combinations. The initial focus from metallic alloys has gradually shifted to ceramics, and other complex concentrated materials (CCMs), lead- ing to evolution of the field. The biggest contribution of HEAs has been towards ‘exploring the unexplored compositional space’ by thermodynamic modelling, high throughput experiments and combinational alloy devel- opment. All the tools and techniques developed for the study of HEAs are expected to yield novel results, and lead serendipitous and tedious alloy development from an art form to a thorough scientific domain. It is, therefore important to look backward in order to chart our way forward in the field of HEAs. There have been many seminal reviews on the topic discussing various aspects related but not limited to fundamental scientific issues, processing, properties and applications of these materials in engineering applications. The focus of this article is to portray our journey in this highly exciting field, which we hope shall encourage young researchers in India and elsewhere to take a plunge into research on HEMs. Basic concepts The concept of HEAs and multiprincipal component alloys was proposed independently in 2004 by Yeh and co-workers 11,12 and Cantor et al. 13 . However, the concept of exploring central regions of multicomponent phase diagrams has been attempted since the 1960s, particularly with respect to bulk metallic glasses. The major driving force for research was to explore the unexplored regions of the multicomponent phase diagrams, and entropy was not mentioned by Cantor. However, Yeh’s approach was to maximize configurational entropy to avoid intermetal- lic phases which are generally found in the central region