Preface

High-entropy alloys (HEAs) are a new class of materials defined by crystals in which more than five elements, each with an atomic fraction between 5% and 35%, randomly occupy one crystallographic site. The concept of HEAs initially developed in single-site crystal structures such as face-centered cubic (fcc), body-centered cubic (bcc), and hexagonal-closed packing (hcp). However, the concept is now adopted in many multi-site alloys (multi-site HEAs). Due to the severe lattice distortion effect, many fcc or bcc HEAs show superior mechanical properties. The superior mechanical properties result from the cocktail effect, which means an enhancement of property beyond the simple mixture of constituent elements. The cocktail effect is also observed in a multi-site HEA; for example, outstanding thermal stability or enhancement of magnetic frustration in a high-entropy alloyed oxide. Due to the massive elemental combination of the HEA system, there are unlimited possibilities of finding new phenomena in the materials research on HEAs. The synthesis of a new single-phase or multiphase bulk sample is crucial work. In addition, fabrications of thin-film and nanocrystalline samples of well-known HEAs are important works. It is widely accepted that first-principle calculations, machine learning, and calculation of phase diagram (CALPHAD) are powerful methods for screening new compounds.

This book covers some very interesting topics concerning the mechanical, physical, and chemical properties of new HEAs, including high strength, high ductility, good thermal stability, superconductivity, exotic magnetism, and so on. It also examines potential applications of HEAs, such as coating against corrosion, biomaterials, catalysts, shape memory alloys, magnetic refrigeration materials, and more. This book provides the reader with a comprehensive overview of the frontier of materials research and the exotic properties (mechanical, physical, chemical, etc.) and exciting applications of HEAs.

The book consists of three sections. Section 1 focuses on HEA superconductors. Chapter 1 summarizes the frontier studies of multi-site HEA superconductors. The chapter focuses on HEA-type compounds with the NaCl-type, the CuAl2-type, high-Tc cuprates, and BiS2-based layered structures. In the three-dimensional structures with the NaCl-type and the CuAl2-type, the improvement of superconducting properties by introducing the HEA state is not clearly observed, whereas some interesting properties are found. However, high-Tc cuprates and BiS2-based layered HEA superconductors, characterized by two-dimensional crystal structures, exhibit improved superconducting properties due to high-entropy effects. The HEA effects depend on structural dimensionality. Chapter 2 describes the materials research on fcc and Mn5Si3-type HEA superconductors.

Section 2 is devoted to HEA composites. Chapter 3 examines an Al-B4C metal matrix composite (MMC), which is useful for shielding material for nuclear reactors. In this chapter, the authors introduce the microstructures of Al-B4C MMC samples. Furthermore, they examine the mechanical properties of samples and present the results from the viewpoint of microstructures. Chapter 4 reviews papers concerning the effect of mischmetal, La or Ce, and La+Ce additions on Al-Si

**II**

**Chapter 7 131**

Design and Development of High Entropy Alloys Using Artificial

*by Shailesh Kumar Singh and Vivek K. Singh*

Intelligence

cast alloys. The review discusses the microstructures and mechanical properties of multicomponent systems. In this chapter, the authors introduce the microstructures of many Al-Si multicomponent alloys. The mechanical properties of samples are well correlated with the results of microstructures. Chapter 5 summarizes the frontier studies of high-entropy superalloys (HESAs) and laser surface modification used to protect them. HESAs are potential alternatives to nickel superalloys and are good candidates for gas turbine applications. After presenting the advances of nickel superalloys and HESAs, the authors discuss the protection of superalloys, which is necessary for gas turbine applications. The authors focus on laser surface modifications, including laser surface melting, laser transformation hardening, laser surface alloying, and laser glazing. It is stressed that HESAs exhibit good oxidation resistance and high yield strength compared to traditional nickel superalloys.

Section 3 discusses the current state of HEA design. Chapter 6 surveys the material design of HEAs and explains the thermodynamic and electronic structure parameters. The authors review calculation and simulation methods such as density functional theory (DFT), CALPHAD, and machine learning. Finally, the authors discuss high-throughput screening methods for HEAs. Chapter 7 reports on the design of HEAs using Artificial Intelligence (AI). In materials research, it is expected that AI is quicker than other computational methods in the case of no physical model. The authors explain machine learning in the design of HEAs as well as discuss the methodology for implementing AI in HEAs.

I hope that many readers will be interested in this book and enter into the research of HEAs.

> **Jiro Kitagawa** Fukuoka Institute of Technology, Fukuoka, Japan

> > **1**

Section 1

High-Entropy Alloy

Superconductors

## Section 1
