**Abstract**

The lightweight and flexible materials can improve people's quality of daily life; in addition, the materials can be widely used in aerospace, automotive, consumer electronics, etc. Recently, high-entropy alloys had become hot issues in materials science with many excellent properties; therefore, we can combine the design ideas of highentropy alloys with lightweight materials and flexible materials, taking into account the advantages of two types of materials, and promoting the development and progress of new materials. In the chapter, we will elaborate on the relationship between the microstructure and properties of lightweight high-entropy alloys and the design ideas of high-entropy alloys with flexible materials that were investigated in recent years. Furthermore, as the microstructure and mechanical properties of the alloys exhibit the nonlinear behaviors with entropy on high-entropy alloys, we would like to define the lightweight high-entropy alloy as the density is lower than 6 g/cm3 , the mix-entropy of these alloys is higher than 1R (here, R is gas constant), and the number of components is four or more. Finally, it is expected to broaden the research field of high-entropy alloys and provide some new directions for the development of new materials.

**Keywords:** lightweight, high-entropy alloys, solid solution, alloy design, flexible materials

### **1. Introduction**

Materials have always been a necessary progressive factor in human development; the progress of human society is often accompanied by advances in materials. From the Stone Age to the Bronze Age and then to the Iron Age, the emergence of each new materials has brought major changes in people's productivity. Nowadays, a series of materials have been used in various fields. The traditional structural materials such as steel, aluminum alloys, titanium alloys, magnesium alloys, etc. were still the most widely used materials. However, these materials cannot be applied in some specific areas. In addition, new materials have been developed such as composite material, nanostructure materials, carbon materials, bulk metallic glasses, high-entropy alloys, etc., as high-entropy alloys have been developed since 2004 by Yeh et al. [1] and Cantor et al. [2]. Due to the extremely complex composition of these alloys, the alloys also exhibit excellent properties that are difficult to achieve with many conventional alloys, such as high strength, high hardness, high fracture toughness, corrosion resistance, high temperature oxidation resistance, good low temperature performance, etc. In recent years, these high-entropy alloys such as AlCoCrFeNiCu, CoCrFeMnNi, CoCrFeNi (Ti, Al), NbMoTaW, CoCrNi (AlSi), etc. have been developed and studied [3, 4]. As these alloys also have large proportion of transition metal elements, they also show high density. However, lightweight materials in the aerospace, automotive (especially electric vehicles), consumer electronics, and other fields have become an important development direction. However, designing novel lightweight materials with the concept of high-entropy alloys has become a hot issue, which will promote the development and applications [5, 6].

In view of the excellent performance of high-entropy alloys, we firmly believe that the lightweight high-entropy alloys have superior performance than traditional lightweight materials such as aluminum alloys, titanium alloys, magnesium alloys, etc. The general definition of lightweight materials generally uses the density of titanium alloy as the limit. The existing elements with lower density than titanium (4.51 g/cm3 ) are mainly lithium (0.53 g/cm3 ), beryllium (1.85 g/cm3 ), boron (2.46 g/cm3 ), sodium (0.97 g/cm3 ), carbon (2.26 g/cm3 ), magnesium (1.74 g/cm3 ), aluminum (2.70 g/cm3 ), silicon (2.33 g/cm3 ), potassium (0.86 g/cm3 ), calcium (1.55 g/cm3 ), yttrium (2.99 g/cm3 ), rubidium (1.53 g/cm3 ), strontium (2.64 g/cm3 ), strontium (4.47 g/cm3 ), barium (3.51 g/cm3 ), etc., and most of these elements are main group elements, which tend to have a higher chemical activity, with larger atomic radius, also with large difference in melting point and boiling point (lower melt point such as rubidium 39.3°C and higher melt point as titanium 1668°C). Also as we design the lightweight high-entropy alloys, these elements are not exactly used for the new alloy systems. Therefore, the development of lightweight high-entropy alloys often shows more difficulty than that of traditional high-entropy alloys.

In addition, compared with rigid materials, flexible materials are also widely used, which include foils, fibers, films, ribbons, etc., and usually they are made of organic matter. The inorganic materials such as silica, bulk metallic glasses, and metal materials, etc. tend to exhibit the characteristics of rigid materials. However, after being made into fibers or films, such materials can often undergo bending deformation due to the size effect and can also exhibit the characteristics of flexible materials. Nowadays, there is an increasing demand for flexible electronic materials in the field of electronics, especially in the field of wearable electronics. High-entropy alloys have demonstrated excellent overall performance as a new class of alloy materials in the field of rigid materials. Combined with the design concept of high-entropy alloy, can high-entropy open up a new research field in terms of flexible materials?

Nowadays, some scholars have also carried out a lot of research works; therefore, we will give a brief review on the relevant research works (mainly based on the research works of our own research group) and put forward our own opinions on the design and preparation of lightweight high-entropy alloys and high-entropy flexible materials.
