**2. Lightweight high-entropy alloy systems**

#### **2.1 Al-Mg-Li lightweight high-entropy alloy systems**

Nowadays, the most commonly used lightweight metal materials are aluminum alloys, titanium alloys, magnesium alloys, etc. As the lithium alloys is the lightest structural metal material, which magnesium and aluminum are the common lightweight metal materials, our group firstly design the two lightweight high-entropy alloys systems (AlLiMgZnCu and AlLiMgZnSn) by Yang et al. [7].

With the design concept of traditional high-entropy alloys, we hope to form a multicomponent solid solution by alloy design. In recent years, these factors such as ΔS*mix*, ΔH*mix*, δ, Ω, Δχ, VEC, Tm, etc., have made significant effects on the formation of solid solution as the design of high-entropy alloys. Therefore, in order to

**127**

*Light-Weight and Flexible High-Entropy Alloys DOI: http://dx.doi.org/10.5772/intechopen.88332*

with compressive plasticity better than 15%.

shown in **Table 1**.

ρ (g/cm3

**Table 1.**

3.08 g/cm3

enhance the formation of the solid solution in lightweight high-entropy alloys, we firstly considered these factors and made some relevant calculations, which are

*Atomic radius (r), standard atomic weight (A), crystal structure, electronegativity (χ), value electron concentration (VEC), density (ρ), and melting temperature (Tm) for constituent elements in present alloys [7].*

) 2.70 0.54 1.74 7.13 8.93 7.37 Tm (K) 933.5 453.7 922 692.7 1358 505.1

**Alloy design elements Al Li Mg Zn Cu Sn** r (10<sup>−</sup>10 m) 1.43 1.56 1.60 1.39 1.28 1.55 A (g/mol) 26.98 6.94 24.31 65.39 63.55 118.7 Crystal structure FCC BCC HCP HCP FCC Tetragonal χ 1.61 0.98 1.31 1.65 1.90 1.96 VEC 2 1 2 12 11 4

We had designed six lightweight high-entropy alloys that were AlLiMgZnSn, AlLi0.5MgZn0.5Sn0.2, AlLi0.5MgZn0.5Cu0.2, AlLi0.5MgCu0.5Sn0.2, Al80Li5Mg5Zn5Sn5, and Al80Li5Mg5Zn5Cu5, and the densities of these alloys are 4.23, 3.22, 3.73, 3.69, 3.05, and

The XRD pattern analysis of these alloys shows that the single-phase solid solution did not appear as the main phase under the condition of high mixing entropy; however, a large number of intermetallic compounds are produced during the smelting process. Only when the addition of aluminum reach to 80 at.%, the alloys show in a single face center cube(FCC) solid solution as the aluminum alloys. The SEM photos of these alloys which are shown in **Figure 1**, we can see a lot of intermetallic become the main phase of these alloys with high entropy, these show that the entropy did not victory when competition with the enthalpy, the solid solution did not form, also a lot of crack were found in the compounds, which cause the plasticity of these alloys are poor, also we when the aluminum become the main element of these alloy the α-Al (FCC) solid solution become the main phase in dendrite, some compounds which were rich in Cu or Sn in inter dendrite. The compression test of the alloys is shown in **Figure 2**, and the Al80Li5Mg5Zn5Sn5 and Al80Li5Mg5Zn5Cu5 alloys show good strength with higher than 800 MPa and yield strength higher than 400 MPa,

Also, the rare-earth elements lanthanum and cerium were added in these alloys to improve the solid solution formation ability of the alloys. In addition, Bridgeman directional solidification technology is also used in these alloys. However, these do not work in these systems. In order to further understand the formation law of solid solution of these lightweight high-entropy alloys, the δ, Ω, Δχ, and VEC of these low-density high-entropy alloys are shown in **Figure 3**. Comparing the formation regions of solid solutions and intermetallic compounds with the conventional highentropy alloys, we find that for the lightweight high-entropy alloys, which tend to have higher mixing enthalpy and electronegativity with smaller Ω and VEC, the δ of the alloy is in an intermediate region, often close to the critical region where the solid solution phase forms. In addition, the δ-Δχ can be a better way to predict the phase formation ability of these alloys. When Δχ < 0.175, the solid solution will become the main phase of these alloys. Mainly, we found that the Al-Mg-Li system low-density high-entropy alloys had high chemical activity, which made it easier to form intermetallic compounds with other elements. Finally, we found that with the study of composition design, microstructure performance, and phase

, respectively; the density of these alloys is lower than that of titanium.

*Light-Weight and Flexible High-Entropy Alloys DOI: http://dx.doi.org/10.5772/intechopen.88332*


#### **Table 1.**

*Engineering Steels and High Entropy-Alloys*

titanium (4.51 g/cm3

boron (2.46 g/cm3

calcium (1.55 g/cm3

high-entropy alloys.

flexible materials.

(1.74 g/cm3

(2.64 g/cm3

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

) are mainly lithium (0.53 g/cm3

), carbon (2.26 g/cm3

), rubidium (1.53 g/cm3

), silicon (2.33 g/cm3

), barium (3.51 g/cm3

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

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,

Nowadays, the most commonly used lightweight metal materials are aluminum alloys, titanium alloys, magnesium alloys, etc. As the lithium alloys is the lightest structural metal material, which magnesium and aluminum are the common lightweight metal materials, our group firstly design the two lightweight high-entropy

With the design concept of traditional high-entropy alloys, we hope to form a multicomponent solid solution by alloy design. In recent years, these factors such as ΔS*mix*, ΔH*mix*, δ, Ω, Δχ, VEC, Tm, etc., have made significant effects on the formation of solid solution as the design of high-entropy alloys. Therefore, in order to

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

), sodium (0.97 g/cm3

), yttrium (2.99 g/cm3

), aluminum (2.70 g/cm3

), strontium (4.47 g/cm3

**2. Lightweight high-entropy alloy systems**

**2.1 Al-Mg-Li lightweight high-entropy alloy systems**

alloys systems (AlLiMgZnCu and AlLiMgZnSn) by Yang et al. [7].

), beryllium (1.85 g/cm3

), magnesium

), potassium (0.86 g/cm3

), etc., and most of these

), strontium

),

),

**126**

*Atomic radius (r), standard atomic weight (A), crystal structure, electronegativity (χ), value electron concentration (VEC), density (ρ), and melting temperature (Tm) for constituent elements in present alloys [7].*

enhance the formation of the solid solution in lightweight high-entropy alloys, we firstly considered these factors and made some relevant calculations, which are shown in **Table 1**.

We had designed six lightweight high-entropy alloys that were AlLiMgZnSn, AlLi0.5MgZn0.5Sn0.2, AlLi0.5MgZn0.5Cu0.2, AlLi0.5MgCu0.5Sn0.2, Al80Li5Mg5Zn5Sn5, and Al80Li5Mg5Zn5Cu5, and the densities of these alloys are 4.23, 3.22, 3.73, 3.69, 3.05, and 3.08 g/cm3 , respectively; the density of these alloys is lower than that of titanium. The XRD pattern analysis of these alloys shows that the single-phase solid solution did not appear as the main phase under the condition of high mixing entropy; however, a large number of intermetallic compounds are produced during the smelting process. Only when the addition of aluminum reach to 80 at.%, the alloys show in a single face center cube(FCC) solid solution as the aluminum alloys. The SEM photos of these alloys which are shown in **Figure 1**, we can see a lot of intermetallic become the main phase of these alloys with high entropy, these show that the entropy did not victory when competition with the enthalpy, the solid solution did not form, also a lot of crack were found in the compounds, which cause the plasticity of these alloys are poor, also we when the aluminum become the main element of these alloy the α-Al (FCC) solid solution become the main phase in dendrite, some compounds which were rich in Cu or Sn in inter dendrite. The compression test of the alloys is shown in **Figure 2**, and the Al80Li5Mg5Zn5Sn5 and Al80Li5Mg5Zn5Cu5 alloys show good strength with higher than 800 MPa and yield strength higher than 400 MPa, with compressive plasticity better than 15%.

Also, the rare-earth elements lanthanum and cerium were added in these alloys to improve the solid solution formation ability of the alloys. In addition, Bridgeman directional solidification technology is also used in these alloys. However, these do not work in these systems. In order to further understand the formation law of solid solution of these lightweight high-entropy alloys, the δ, Ω, Δχ, and VEC of these low-density high-entropy alloys are shown in **Figure 3**. Comparing the formation regions of solid solutions and intermetallic compounds with the conventional highentropy alloys, we find that for the lightweight high-entropy alloys, which tend to have higher mixing enthalpy and electronegativity with smaller Ω and VEC, the δ of the alloy is in an intermediate region, often close to the critical region where the solid solution phase forms. In addition, the δ-Δχ can be a better way to predict the phase formation ability of these alloys. When Δχ < 0.175, the solid solution will become the main phase of these alloys. Mainly, we found that the Al-Mg-Li system low-density high-entropy alloys had high chemical activity, which made it easier to form intermetallic compounds with other elements. Finally, we found that with the study of composition design, microstructure performance, and phase

#### **Figure 1.**

*SEM secondary electron images of low-density multicomponent alloys. (a) AlLiMgZnSn; (b) AlLi0.5MgZn0.5Sn0.2; (c) AlLi0.5MgZn0.5Cu0.2; (d) AlLi0.5MgCu0.5Sn0.2; (e) Al80Li5Mg5Zn5Sn5; and (f) Al80Li5Mg5Zn5Cu5 alloys [7].*

#### **Figure 2.**

*Compressive engineering stress-strain curves of AlLiMgZnSn, AlLi0.5MgZn0.5Sn0.2, Al80Li5Mg5Zn5Sn5, and Al80Li5Mg5Zn5Cu5 alloys at room temperature. The initial strain rate was 5 × 10<sup>−</sup><sup>4</sup> s<sup>−</sup><sup>1</sup> [7].*

formation of multicomponent alloys, for low-density high-entropy alloys, high mixing entropy is not the key factor in the formation of solid solution structures of these alloys. Compared with the traditional high-entropy alloys (mostly composed of transition metal elements), the solid solution phase formation conditions of the

**129**

the intermetallic compounds.

*(Al0.5Mg0.5)100-xLi*x*,* x *= 5, 10, 15, 25, and 33.33) [7].*

**Figure 3.**

*Light-Weight and Flexible High-Entropy Alloys DOI: http://dx.doi.org/10.5772/intechopen.88332*

Al-Mg-Li-based lightweight high-entropy alloys are more severe. The solid solution formation of these alloys can be predicted by electronegativity (Δχ); when the Δχ < 0.175, it is easier to form the solid solution; and as Δχ ≥ 0.175, it tends to form

*Phase constituent prediction maps: (a) δ-Hmix, (b) δ-Ω, (c) δ-Δχ, and (d) δ-VEC plots for multicomponent alloys in this work overlaid on cross-hatched regions developed in previous HEA investigations. (For* 

Li et al. also studied the microstructure and properties of the Al-Mg-Li highentropy alloy system by using super-gravity technology [8]. Under different

conditions with the super-gravity experiments, which found that supergravity does not separate the heavy elements of the alloy from the light elements; however, the microstructure of the alloy changed, which caused different properties. The alloy structure is still composed of α-Al solid solution structure and intermetallic compounds, and with supergravity, the microstructure changes to the eutectic microstructure. As supergravity is one entopic force, there are a variety of entropic forces in the process of alloy during solidification. Since gravity increases with distance, there are pressure and viscosity gradients in the molten metal. Meanwhile, due to the high mixing entropy of the alloy and the combination of various factors, the microstructure of intermetallic compounds and solid solution will change during solidification. In addition, these effects also made the grain refinement of the alloy along the direction of gravity to a certain extent, resulting in an enhancement of the strength of the alloy. Nevertheless, the alloy still does not form a single-phase

*Light-Weight and Flexible High-Entropy Alloys DOI: http://dx.doi.org/10.5772/intechopen.88332*

*Engineering Steels and High Entropy-Alloys*

**128**

**Figure 2.**

**Figure 1.**

*(f) Al80Li5Mg5Zn5Cu5 alloys [7].*

formation of multicomponent alloys, for low-density high-entropy alloys, high mixing entropy is not the key factor in the formation of solid solution structures of these alloys. Compared with the traditional high-entropy alloys (mostly composed of transition metal elements), the solid solution phase formation conditions of the

*Compressive engineering stress-strain curves of AlLiMgZnSn, AlLi0.5MgZn0.5Sn0.2, Al80Li5Mg5Zn5Sn5, and* 

 *s<sup>−</sup><sup>1</sup> [7].*

*Al80Li5Mg5Zn5Cu5 alloys at room temperature. The initial strain rate was 5 × 10<sup>−</sup><sup>4</sup>*

*SEM secondary electron images of low-density multicomponent alloys. (a) AlLiMgZnSn;* 

*(b) AlLi0.5MgZn0.5Sn0.2; (c) AlLi0.5MgZn0.5Cu0.2; (d) AlLi0.5MgCu0.5Sn0.2; (e) Al80Li5Mg5Zn5Sn5; and* 

#### **Figure 3.**

*Phase constituent prediction maps: (a) δ-Hmix, (b) δ-Ω, (c) δ-Δχ, and (d) δ-VEC plots for multicomponent alloys in this work overlaid on cross-hatched regions developed in previous HEA investigations. (For (Al0.5Mg0.5)100-xLi*x*,* x *= 5, 10, 15, 25, and 33.33) [7].*

Al-Mg-Li-based lightweight high-entropy alloys are more severe. The solid solution formation of these alloys can be predicted by electronegativity (Δχ); when the Δχ < 0.175, it is easier to form the solid solution; and as Δχ ≥ 0.175, it tends to form the intermetallic compounds.

Li et al. also studied the microstructure and properties of the Al-Mg-Li highentropy alloy system by using super-gravity technology [8]. Under different conditions with the super-gravity experiments, which found that supergravity does not separate the heavy elements of the alloy from the light elements; however, the microstructure of the alloy changed, which caused different properties. The alloy structure is still composed of α-Al solid solution structure and intermetallic compounds, and with supergravity, the microstructure changes to the eutectic microstructure. As supergravity is one entopic force, there are a variety of entropic forces in the process of alloy during solidification. Since gravity increases with distance, there are pressure and viscosity gradients in the molten metal. Meanwhile, due to the high mixing entropy of the alloy and the combination of various factors, the microstructure of intermetallic compounds and solid solution will change during solidification. In addition, these effects also made the grain refinement of the alloy along the direction of gravity to a certain extent, resulting in an enhancement of the strength of the alloy. Nevertheless, the alloy still does not form a single-phase

#### **Figure 4.**

*Microstructure of the content of different element and hardness of the AlZn0.4Li0.2Mg0.2Cu0.2 alloy with different supergravity experiments: (a) Sample 1; (b) Sample 2, and (c) Sample 3 [8].*

solid solution structure; therefore, the optimal structure of the alloy is the eutectic structure with the intermetallic compound and the solid solution with grain refine. **Figure 4** shows the microstructure, the composition of different elements by X-ray photoelectron spectroscopy, and the hardness of the alloy with the distance of gravity.

#### **2.2 The Al-Mg-Zn-Cu-Si lightweight high-entropy alloy system**

Based on the Al-Mg-Li study, our research group Shao et al. [9] used the Si exchange of Li, in order to reduce the cost of the alloy and expected to achieve lightweight, low-cost, high-entropy alloys; therefore, we studied the Al-Mg-Si system lightweight high-entropy alloys. Based on Δχ, we designed the AlMgZnCuSi alloys, and these alloy samples were prepared by vacuum induction melting. We have found that the alloy forms a eutectic structure of solid solution and intermetallic compound when the content of Al is less than 80 at.%; however, these alloys show high strength with low ductility, and as the Al condition is higher than 80 at.%, they become α-Al face center cube solid solution. These alloys also have high strength with good compressive ductility. Which found that the Al85Mg10.5Zn2.025Cu2.025Si0.45 alloy shows good toughness when the strength is higher than 800 MPa with ductility more than 20%. Currently, Δχ also predicts the phase formation of the alloy. We also found some serrated flow phenomena in the compressive strain curve of

**131**

10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup>

**Figure 5.**

are shown in **Figure 5**.

*The compressive stress-strain curves at room temperature [9].*

*Light-Weight and Flexible High-Entropy Alloys DOI: http://dx.doi.org/10.5772/intechopen.88332*

the alloy and will do some further research on the mechanism of serration behavior with the alloy. This research shows that this inexpensive alloy system is the research direction of another high-strength lightweight high-entropy alloy. The compressive stress-strain curves of a series of alloys at room temperature with a strain rate of

In addition, some other researchers have also studied similar alloy systems based

on this type of lightweight high-entropy alloy system. Baek et al. [10] used the ultrasonic melt treatment to prepare lightweight Al70Mg10Si10Cu5Zn5 alloy, this alloy also forms a large number of other precipitated phases in the aluminum matrix, and the effect of solution treatment of this alloy on the microstructure and properties of the alloy was investigated, which found that the alloy has an excellent performance at both room temperature and 350°C; however, the microstructure of the alloy is finely refined by the precipitation phase size by ultrasonic melt treatment technology, and mainly due to the introduction of trace amounts of Ti, the grain size is refined. In addition, mechanical properties of the alloy at room temperature have been improved with the solution treatment at 440°C, but the mechanical properties with high temperature (350°C) deteriorate. Through solution treatment, the Zn atoms redissolve into the second phases, which not only leads to the formation of fine super-saturated clusters in the matrix, but also spheroidizes the primary Si and Mg2Si phases, thereby improving the room temperature mechanical properties of the alloy. They also studied the effects of Al-6Mg-9Si-10Cu-10Zn-3Ni alloy aging treatment on properties and microstructure of alloys at different aging temperatures, and at 120°C, they found that the GP zone in the alloy with aging time was replaced by a Zn-rich metastable elliptical cluster to form a stable Zn precipitate containing a part of Cu atoms [11]. Besides, the aging precipitation behavior under different temperatures had also been studied [12]; as the aging temperature is below 70°C, a series of fine clusters and precipitates were formed, which greatly improves the strength of the composite. On the other hand, due to the coarsening of the precipitate, and the softening by the reduced volume fraction and the periodization of the second phase, a small strengthening effect was observed above 170°C. Sanchez et al.[13] have done some research on Al65Cu5Mg5Si15Zn5X5 and Al70Cu5Mg5Si10Zn5X5 systems and reported the effect of Fe, Ni, Cr, Mn, and Zr elements on the phase

*Engineering Steels and High Entropy-Alloys*

solid solution structure; therefore, the optimal structure of the alloy is the eutectic structure with the intermetallic compound and the solid solution with grain refine. **Figure 4** shows the microstructure, the composition of different elements by X-ray photoelectron spectroscopy, and the hardness of the alloy with the distance of

*Microstructure of the content of different element and hardness of the AlZn0.4Li0.2Mg0.2Cu0.2 alloy with* 

Based on the Al-Mg-Li study, our research group Shao et al. [9] used the Si exchange of Li, in order to reduce the cost of the alloy and expected to achieve lightweight, low-cost, high-entropy alloys; therefore, we studied the Al-Mg-Si system lightweight high-entropy alloys. Based on Δχ, we designed the AlMgZnCuSi alloys, and these alloy samples were prepared by vacuum induction melting. We have found that the alloy forms a eutectic structure of solid solution and intermetallic compound when the content of Al is less than 80 at.%; however, these alloys show high strength with low ductility, and as the Al condition is higher than 80 at.%, they become α-Al face center cube solid solution. These alloys also have high strength with good compressive ductility. Which found that the Al85Mg10.5Zn2.025Cu2.025Si0.45 alloy shows good toughness when the strength is higher than 800 MPa with ductility more than 20%. Currently, Δχ also predicts the phase formation of the alloy. We also found some serrated flow phenomena in the compressive strain curve of

**2.2 The Al-Mg-Zn-Cu-Si lightweight high-entropy alloy system**

*different supergravity experiments: (a) Sample 1; (b) Sample 2, and (c) Sample 3 [8].*

**130**

gravity.

**Figure 4.**

**Figure 5.** *The compressive stress-strain curves at room temperature [9].*

the alloy and will do some further research on the mechanism of serration behavior with the alloy. This research shows that this inexpensive alloy system is the research direction of another high-strength lightweight high-entropy alloy. The compressive stress-strain curves of a series of alloys at room temperature with a strain rate of 10<sup>−</sup><sup>3</sup> s<sup>−</sup><sup>1</sup> are shown in **Figure 5**.

In addition, some other researchers have also studied similar alloy systems based on this type of lightweight high-entropy alloy system. Baek et al. [10] used the ultrasonic melt treatment to prepare lightweight Al70Mg10Si10Cu5Zn5 alloy, this alloy also forms a large number of other precipitated phases in the aluminum matrix, and the effect of solution treatment of this alloy on the microstructure and properties of the alloy was investigated, which found that the alloy has an excellent performance at both room temperature and 350°C; however, the microstructure of the alloy is finely refined by the precipitation phase size by ultrasonic melt treatment technology, and mainly due to the introduction of trace amounts of Ti, the grain size is refined. In addition, mechanical properties of the alloy at room temperature have been improved with the solution treatment at 440°C, but the mechanical properties with high temperature (350°C) deteriorate. Through solution treatment, the Zn atoms redissolve into the second phases, which not only leads to the formation of fine super-saturated clusters in the matrix, but also spheroidizes the primary Si and Mg2Si phases, thereby improving the room temperature mechanical properties of the alloy. They also studied the effects of Al-6Mg-9Si-10Cu-10Zn-3Ni alloy aging treatment on properties and microstructure of alloys at different aging temperatures, and at 120°C, they found that the GP zone in the alloy with aging time was replaced by a Zn-rich metastable elliptical cluster to form a stable Zn precipitate containing a part of Cu atoms [11]. Besides, the aging precipitation behavior under different temperatures had also been studied [12]; as the aging temperature is below 70°C, a series of fine clusters and precipitates were formed, which greatly improves the strength of the composite. On the other hand, due to the coarsening of the precipitate, and the softening by the reduced volume fraction and the periodization of the second phase, a small strengthening effect was observed above 170°C. Sanchez et al.[13] have done some research on Al65Cu5Mg5Si15Zn5X5 and Al70Cu5Mg5Si10Zn5X5 systems and reported the effect of Fe, Ni, Cr, Mn, and Zr elements on the phase

formation, microstructure, and properties of these alloys. These researches all showed that this kind of alloy system has a good prospect in foundry industry.

## **2.3 High temperature application of lightweight high-entropy alloy**

The light-weight metal elements as beryllium, scandium, titanium, yttrium etc., and the light-weight non-metallic elements such as carbon, boron, silicon etc., in addition to aluminum have a higher boiling point; therefore, these elements were also used for the design of high temperature application of lightweight high-entropy alloy. Some researchers have done a series of research work on these alloys.

Tseng et al. [14] studied the Al20Be20Fe10Si15Ti35 lightweight high-entropy alloy with a vacuum-arc-melting, and this alloy showed a single hexagonal close-packed (HCP) structure solid solution phase, with high hardness ~8.9 GPa, high strength ~2.976 GPa, with a density of ~3.91 g/cm3 ; in addition, this alloy showed an excellent oxidation resistance at both 700 and 900°C, which is much better than the normal Ti-6Al-4 V alloy. Another way to prepare lightweight, high-temperature, high-entropy alloys is to reduce alloy density by adding lightweight elements Ti and Al to conventional alloys. These alloys usually have a higher density, however, lighter than the conventional superalloys, usually less than 6 g/cm3 . These lightweight high entropy alloys, such as NbTiVTaAl*x* [15], CrNbTiVZr [16], AlNbTiV [17], Al1.5CrFeMnTi [18, 19], AlTiVCr [20, 21] etc., which tend to have a singlephase solid solution structure with lower density, good plasticity, and high temperature properties. In addition, these are a powerful alternative to the next generation of superalloys, with great potential to replace existing superalloys. The application of such alloys will bring a big leap in materials for the aviation industry.

#### **2.4 Other lightweight high-entropy alloy systems**

Finally, we will briefly explain the existing research on other lightweight highentropy alloys. Youssef et al. [22] made an investigation on Al20Li20Mg10Sc20Ti30 lightweight high-entropy alloy with mechanical alloying. Since such alloys were prepared by mechanical alloying, the alloy structure exhibited an ultrafine grain structure with only 12 nm, and the alloy exhibited an ultra-high hardness of 5.9 GPa, and its density was only 2.67 g/cm3 ; which shows a single face center cube (FCC) solid solution structure, when the power was milled without N, O, the alloy has a face centered cube (FCC) transformation into a hexagonal close-packed (HCP) structure with 500°C annealing treatment, however with N, O this transformation did not happen. Li et al. [23, 24] made an investigation on Mg*x*(MnAlZnCu)100-*x* lightweight high-entropy alloys, the microstructure of Mg20(MnAlZnCu) alloy was consistent with HCP solid solution and Al-Mn icosahedral quasicrystal phase, and the compressive strength of these Mg*x*(MnAlZnCu)100-*x* alloys were high; however, the plasticity of alloys was poor. In addition, the microstructure and properties of the Mg20(MnAlZnCu) alloy under different solidification conditions were also studied, and they found that with the faster cooling rate, the Al-Mn icosahedral quasicrystal phase was refined, which enhanced the strength of this alloy; however, with the brittleness of the HCP alloy, even the fast cooling rate can improve the plastic deformation ability of the alloy. The plasticity of this alloy is still poor, and with this work, they found that the high entropy can enhance the formation ability of icosahedral quasicrystal [24]. Du et al. [25] investigated the MgCaAlLiCu alloy, which shows a mainly single solid solution phase with tetragonal symmetry lattice structure, and the density of this alloy is ~2.2 g/cm3 , with high compressive strength ~910 MPa. Jia et al. investigated the AlLiMgCaSi high-entropy alloys and they found that the density of these alloys were 1.46 to 1.70 g/cm3 and the strength was higher than 450 MPa, especially, as the Al15Li38Mg45Ca0.5Si1.5 and Al15Li39Mg45Ca0

**133**

than that of ceramics.

**Figure 6.**

the toughness of these alloys.

**3. High-entropy flexible materials**

*Light-Weight and Flexible High-Entropy Alloys DOI: http://dx.doi.org/10.5772/intechopen.88332*

.5Si0.5 alloy exhibited good plasticity ~45 and ~60%, which is much higher than most of the lightweight high-entropy alloys [26]. Sanchez et al. investigated on the as-cast high-entropy aluminums, and they found these alloys showed high hardness than other lightweight alloys [27, 28]. **Figure6** shows the area of lightweight high-entropy alloys in the Ashby diagram of strength vs. density for structural materials, which we found that the strength of lightweight high-entropy alloys was much higher and the density much lower than some ceramics such as the SiC, Al3N, etc.; however, the ductility is better

*The area of lightweight high-entropy alloys in the Ashby diagram of strength vs. density for structural* 

*materials [29], a) the strength of these alloys was equivalent as the hardness/3.*

There are still many problems in the existing lightweight high-entropy alloys to be solved. First, the formation conditions of conventional high-entropy alloy solid solution need to be corrected. In addition, lightweight high-entropy alloys tend to exhibit high strength and poor room temperature plasticity, and we need to improve

High-entropy alloys tend to have a solid solution structure, which means that these alloys had good plastic deformation capacity. The face center cubic (FCC) high-entropy alloys such as CoCrFeMnNi, Al0.3CoCrFeNi, CoCrFeNi, etc. [30–36], show the excellent tensile plasticity whish can exceed 50% at room temperature. Therefore, these alloys can be deformed by plastic deformation such as rolling,

*Light-Weight and Flexible High-Entropy Alloys DOI: http://dx.doi.org/10.5772/intechopen.88332*

#### **Figure 6.**

*Engineering Steels and High Entropy-Alloys*

~2.976 GPa, with a density of ~3.91 g/cm3

formation, microstructure, and properties of these alloys. These researches all showed that this kind of alloy system has a good prospect in foundry industry.

The light-weight metal elements as beryllium, scandium, titanium, yttrium etc., and the light-weight non-metallic elements such as carbon, boron, silicon etc., in addition to aluminum have a higher boiling point; therefore, these elements were also used for the design of high temperature application of lightweight high-entropy

Tseng et al. [14] studied the Al20Be20Fe10Si15Ti35 lightweight high-entropy alloy with a vacuum-arc-melting, and this alloy showed a single hexagonal close-packed (HCP) structure solid solution phase, with high hardness ~8.9 GPa, high strength

; in addition, this alloy showed an excel-

. These light-

, with high com-

and the strength

**2.3 High temperature application of lightweight high-entropy alloy**

alloy. Some researchers have done a series of research work on these alloys.

lent oxidation resistance at both 700 and 900°C, which is much better than the normal Ti-6Al-4 V alloy. Another way to prepare lightweight, high-temperature, high-entropy alloys is to reduce alloy density by adding lightweight elements Ti and Al to conventional alloys. These alloys usually have a higher density, however,

weight high entropy alloys, such as NbTiVTaAl*x* [15], CrNbTiVZr [16], AlNbTiV [17], Al1.5CrFeMnTi [18, 19], AlTiVCr [20, 21] etc., which tend to have a singlephase solid solution structure with lower density, good plasticity, and high temperature properties. In addition, these are a powerful alternative to the next generation of superalloys, with great potential to replace existing superalloys. The application

Finally, we will briefly explain the existing research on other lightweight highentropy alloys. Youssef et al. [22] made an investigation on Al20Li20Mg10Sc20Ti30 lightweight high-entropy alloy with mechanical alloying. Since such alloys were prepared by mechanical alloying, the alloy structure exhibited an ultrafine grain structure with only 12 nm, and the alloy exhibited an ultra-high hardness of 5.9 GPa, and its density

structure, when the power was milled without N, O, the alloy has a face centered cube (FCC) transformation into a hexagonal close-packed (HCP) structure with 500°C annealing treatment, however with N, O this transformation did not happen. Li et al. [23, 24] made an investigation on Mg*x*(MnAlZnCu)100-*x* lightweight high-entropy alloys, the microstructure of Mg20(MnAlZnCu) alloy was consistent with HCP solid solution and Al-Mn icosahedral quasicrystal phase, and the compressive strength of these Mg*x*(MnAlZnCu)100-*x* alloys were high; however, the plasticity of alloys was poor. In addition, the microstructure and properties of the Mg20(MnAlZnCu) alloy under different solidification conditions were also studied, and they found that with the faster cooling rate, the Al-Mn icosahedral quasicrystal phase was refined, which enhanced the strength of this alloy; however, with the brittleness of the HCP alloy, even the fast cooling rate can improve the plastic deformation ability of the alloy. The plasticity of this alloy is still poor, and with this work, they found that the high entropy can enhance the formation ability of icosahedral quasicrystal [24]. Du et al. [25] investigated the MgCaAlLiCu alloy, which shows a mainly single solid solution phase with tetragonal

pressive strength ~910 MPa. Jia et al. investigated the AlLiMgCaSi high-entropy alloys

was higher than 450 MPa, especially, as the Al15Li38Mg45Ca0.5Si1.5 and Al15Li39Mg45Ca0

symmetry lattice structure, and the density of this alloy is ~2.2 g/cm3

and they found that the density of these alloys were 1.46 to 1.70 g/cm3

; which shows a single face center cube (FCC) solid solution

lighter than the conventional superalloys, usually less than 6 g/cm3

of such alloys will bring a big leap in materials for the aviation industry.

**2.4 Other lightweight high-entropy alloy systems**

**132**

was only 2.67 g/cm3

*The area of lightweight high-entropy alloys in the Ashby diagram of strength vs. density for structural materials [29], a) the strength of these alloys was equivalent as the hardness/3.*

.5Si0.5 alloy exhibited good plasticity ~45 and ~60%, which is much higher than most of the lightweight high-entropy alloys [26]. Sanchez et al. investigated on the as-cast high-entropy aluminums, and they found these alloys showed high hardness than other lightweight alloys [27, 28]. **Figure6** shows the area of lightweight high-entropy alloys in the Ashby diagram of strength vs. density for structural materials, which we found that the strength of lightweight high-entropy alloys was much higher and the density much lower than some ceramics such as the SiC, Al3N, etc.; however, the ductility is better than that of ceramics.

There are still many problems in the existing lightweight high-entropy alloys to be solved. First, the formation conditions of conventional high-entropy alloy solid solution need to be corrected. In addition, lightweight high-entropy alloys tend to exhibit high strength and poor room temperature plasticity, and we need to improve the toughness of these alloys.

### **3. High-entropy flexible materials**

High-entropy alloys tend to have a solid solution structure, which means that these alloys had good plastic deformation capacity. The face center cubic (FCC) high-entropy alloys such as CoCrFeMnNi, Al0.3CoCrFeNi, CoCrFeNi, etc. [30–36], show the excellent tensile plasticity whish can exceed 50% at room temperature. Therefore, these alloys can be deformed by plastic deformation such as rolling,

**Figure 7.** *High-entropy alloy ribbons by vacuum suspension quenching.*

extrusion deformation, drawing deformation, etc., which can be made into foils, ribbon filaments, etc.; such materials tend to have the characteristics of flexible materials. In addition, further methods to obtain an alloy of a flexible material is the use of melt spinning method, or coating.

Ma et al. made the single-crystal structure the Al0.3CoCrFeNi alloy with the Bridgman solidification which found that the elongation of this alloy ~80%, the alloy shows an excellent plastic deformation capacity [32]. Li et al. found that Al0.3CoCrFeNi alloy shows the elongation more than ~60% with forging [33]. Based on these studies the Li et al. formed the fibers with this alloy [34]. In addition, the high-entropy alloy ribbons and fibers can also be prepared by vacuum suspension quenching system, which Zhao et al. [35] use this technology prepared the CoFeNi(AlBSi)*x* ribbons. High entropy alloy films can be prepared by chemical vapor deposition and physical vapor deposition, the thickness of these films tend to be 0.5 μm to 2 μm, which become two-dimensional materials [37], also such films after separation from the substrate will be a flexible materials. Xing et al. [38] corrected the phase formation of the film materials with the concept of cooling rate. **Figure 7** shows the CoFeNi(AlBSi)*x* high-entropy alloy ribbons by vacuum suspension quenching.
