Materials Research on High-Entropy Alloy Superconductors

*Jiro Kitagawa, Naoki Ishizu and Shusuke Hamamoto*

## **Abstract**

The first purpose of this chapter is materials research on face-centered-cubic (fcc) high-entropy alloy (HEA) superconductors, which have not yet been reported. We have investigated several Nb-containing multicomponent alloys. Although we succeeded in obtaining Nb-containing samples with the dominant fcc phases, no superconducting signals appeared in these samples down to 3 K. The microstructure analyses revealed that all samples were multi-phase, but the existence of several new Nb-containing HEA phases was confirmed in them. The second purpose is the report of materials research on the Mn5Si3-type HEA superconductors. This hexagonal structure offers various intermetallic compounds, which often undergo a superconducting state. The Mn5Si3-type HEA is classified into the multisite HEA, which possesses the high degree of freedom in the materials design and is good platform for studying exotic HEA superconductors. We have successfully found a single-phase Mn5Si3-type HEA, which, however, does not show a superconducting property down to 3 K. The attempt of controlling the valence electron count was not successful.

**Keywords:** high-entropy alloys, superconductivity, face-centered-cubic, niobium, Mn5Si3-type

## **1. Introduction**

High-entropy alloys (HEAs) are a new class of materials and have attracted a great deal of attention [1, 2]. The concept of HEA was originally proposed for a face-centered-cubic (fcc), body-centered-cubic (bcc), or hexagonal-closed packing (hcp) structure. The most prominent feature of a HEA is that more than five elements, each having an atomic percentage between 5% and 35%, randomly occupy one crystallographic site (see also **Figure 1(a)**). This produces a large mixing entropy, and HEAs exhibit the combination of high yield strength and ductility [3], high strength at elevated temperatures [4], strong resistance to corrosion and oxidation [5], and so on. The high-entropy concept is extensively adapted in various materials such as oxides, chalcogenides, and halides [6, 7].

One of the novelties of HEAs is a cocktail effect, which indicates an enhancement of physical properties beyond the simple mixture of those of components. For example, several bcc HEAs show superior mechanical properties compared to conventional hard materials. Another example is found in magnetic spinel oxide (Mg0.2Co0.2Ni0.2Cu0.2Zn0.2)Al2O4. The high-entropy type spinel oxide interestingly

**Figure 1.**

*Crystal structure of compounds with (a) fcc and (b) Mn5Si3-type structures. In (a), a multicolored ball means a random occupation by constituent elements.*

shows enhanced magnetic frustration [8]. The cocktail effect is also reported in the structural stability of high-entropy-type materials. A γ-type disilicate structure is stable from room temperature to 1900°C in (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7. The outstanding thermal stability is ascribed to the high-entropy state at the rareearth site [9]. The other novelty of HEAs is the tuning of physical properties via the change of microstructure. The manufacturing process of HEAs considerably affects their microstructures, which are often deeply related to their physical properties. Fe15Co15Ni20Mn20Cu30 shows a spinodal decomposition after the heat treatment [10]. The spinodally decomposed sample exhibits enhanced Curie temperature and magnetization compared to the homogenized single-phase sample. The tuning of magnetic properties is also reported in dual-phase HEAs [11–13].

One of the new research topics in HEA is the superconductivity found in 2014 [14]. Transition metal-based superconductors, forming simple crystalline structures, follow the so-called Matthias rule. When the superconducting critical temperature *T*c is plotted as a function of valence electron count per atom (VEC), this rule shows broad peak structures at the specified VEC [15]. On the other hand, transition metal amorphous superconductors do not follow this rule and frequently show relatively high *T*c values in the valley of the curve of the Matthias rule [16]. HEA superconductors with simple crystal structures have been found in bcc [17–22] and hcp [23–26]. The *T*c vs. VEC plots of these HEAs seem to fall between a crystalline curve and an amorphous one [27, 28]. Thus, HEA superconductors will shed light on the study of the relationship between crystalline and amorphous compounds.

In the typical HEAs with fcc, bcc, or hcp structure, the superconductivity seems to appear in bcc or hcp HEAs. According to the classification by VEC, single-phase fcc HEA is stabilized for VEC larger than 8.0 [1, 2], where *T*c would be substantially reduced in the VEC dependence of *T*c observed in the Matthias rule. So it may be unrealistic to search for an fcc HEA superconductor. However, this is valuable to challenge because an fcc HEA superconductor would contribute to the deep understanding of HEA and/or the relationship between crystalline and amorphous compounds. In this chapter, we introduce our attempt at the search for an fcc HEA superconductor. Our strategy is to employ a rather high *T*c element because the HEA superconductors reported to date contain superconducting elements [28]. We focused on the Nb-containing HEAs.

The concept of HEA is now used in superconducting materials with the crystal structures possessing multiple Wyckoff positions. For example, CsCl-type, α-Mntype, A15, NaCl-type, σ-phase and CuAl2-type HEA superconductors are reported

**23**

**Table 1.**

*[47], and VEC are also listed.*

*Materials Research on High-Entropy Alloy Superconductors*

atom forming a one-dimensional atomic chain along the *c*-axis.

[29–37]. High degree-of-freedom in such a multisite HEA design would promote the investigations of multisite HEA superconductors. The second purpose of this chapter is the materials research on the hexagonal Mn5Si3-type HEAs, possessing multiple Wyckoff positions. Recently, several superconductors with the Mn5Si3 type—or its ordered derivative Ti5Ga4-type—structure have been found and attract much attention [38–44]. Besides, many intermetallic compounds are crystallizing into these crystal structures [45, 46]. **Figure 1(b)** shows the crystal structure of the Mn5Si3-type compound represented by M5X3. The space group is *P*63/*mcm*, and the M atoms occupy the 4*d* (for M1 atom) and 6 *g* (for M2 atom) Wyckoff positions and the X atom another 6 *g* one. The M2 atoms form a face-sharing octahedral chain along the *c*-axis. The X atoms also form another octahedron, which encloses the M1

In this chapter, we report the synthesis and characterization of the fcc and the Mn5Si3-type HEA samples. The measurement of AC magnetic susceptibility checked the superconducting state. We also present the phase analyses of both kinds of samples. Finally, the future direction of materials research on superconducting

All samples were synthesized by a home-made arc furnace in an Ar atmosphere. The constituent elements as listed in **Table 1** were arc-melted on a water-cooled Cu hearth. The samples were turned over and melted several times. The Mn5Si3-type

A powder X-ray diffractometer (XRD-7000 L, Shimadzu, Kyoto, Japan) with Cu-Kα radiation was employed to detect the X-ray diffraction (XRD) patterns of

Zr Soekawa Chemicals, Tokyo, Japan 99 A3 (hcp) 1.6025 4 Nb Nilaco, Tokyo, Japan 99.9 A2 (bcc) 1.429 5

Ru Soekawa Chemicals, Tokyo, Japan 99.9 A3 (hcp) 1.3384 8 Ir Furuya Metal, Tokyo, Japan 99.99 A1 (fcc) 1.3573 9 Rh Soekawa Chemicals, Tokyo, Japan 99.9 A1 (fcc) 1.345 9 Pd Tanaka Kinzoku Kogyo, Tokyo, Japan 99.9 A1 (fcc) 1.3754 10 Cu Soekawa Chemicals, Tokyo, Japan 99.99 A1 (fcc) 1.278 11 Sc Furuya Metal, Tokyo, Japan 99.9 A3 (hcp) 1.641 3 Ti Nilaco, Tokyo, Japan 99.9 A3 (hcp) 1.4615 4

Si Soekawa Chemicals, Tokyo, Japan 99.999 A4 1.153 4 Ge Soekawa Chemicals, Tokyo, Japan 99.999 A4 1.24 4 Pt Tanaka Kinzoku Kogyo, Tokyo, Japan 99.9 A1 (fcc) 1.387 10

*Materials used in this study. The supply company, purity, crystal structure at room temperature, atomic radius* 

**(%)**

**Crystal structure**

99.9 A2 (bcc) 1.316 5

99.99 All 1.392 3

**Atomic radius (Å)** **VEC**

HEAs were annealed at 800°C for four days in evacuated quartz tubes.

**Element Supply company Purity** 

Japan

V Kojundo Chemical Laboratory, Sakado,

Ga Kojundo Chemical Laboratory, Sakado,

Japan

*DOI: http://dx.doi.org/10.5772/intechopen.99693*

HEAs is mentioned.

**2. Materials and methods**

*Materials Research on High-Entropy Alloy Superconductors DOI: http://dx.doi.org/10.5772/intechopen.99693*

*Advances in High-Entropy Alloys - Materials Research, Exotic Properties and Applications*

shows enhanced magnetic frustration [8]. The cocktail effect is also reported in the structural stability of high-entropy-type materials. A γ-type disilicate structure is stable from room temperature to 1900°C in (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2Si2O7. The outstanding thermal stability is ascribed to the high-entropy state at the rareearth site [9]. The other novelty of HEAs is the tuning of physical properties via the change of microstructure. The manufacturing process of HEAs considerably affects their microstructures, which are often deeply related to their physical properties. Fe15Co15Ni20Mn20Cu30 shows a spinodal decomposition after the heat treatment [10]. The spinodally decomposed sample exhibits enhanced Curie temperature and magnetization compared to the homogenized single-phase sample. The tuning of

*Crystal structure of compounds with (a) fcc and (b) Mn5Si3-type structures. In (a), a multicolored ball means* 

magnetic properties is also reported in dual-phase HEAs [11–13].

One of the new research topics in HEA is the superconductivity found in 2014 [14]. Transition metal-based superconductors, forming simple crystalline structures, follow the so-called Matthias rule. When the superconducting critical temperature *T*c is plotted as a function of valence electron count per atom (VEC), this rule shows broad peak structures at the specified VEC [15]. On the other hand, transition metal amorphous superconductors do not follow this rule and frequently show relatively high *T*c values in the valley of the curve of the Matthias rule [16]. HEA superconductors with simple crystal structures have been found in bcc [17–22] and hcp [23–26]. The *T*c vs. VEC plots of these HEAs seem to fall between a crystalline curve and an amorphous one [27, 28]. Thus, HEA superconductors will shed light on the study of the relationship between crystalline and amorphous

In the typical HEAs with fcc, bcc, or hcp structure, the superconductivity seems to appear in bcc or hcp HEAs. According to the classification by VEC, single-phase fcc HEA is stabilized for VEC larger than 8.0 [1, 2], where *T*c would be substantially reduced in the VEC dependence of *T*c observed in the Matthias rule. So it may be unrealistic to search for an fcc HEA superconductor. However, this is valuable to challenge because an fcc HEA superconductor would contribute to the deep understanding of HEA and/or the relationship between crystalline and amorphous compounds. In this chapter, we introduce our attempt at the search for an fcc HEA superconductor. Our strategy is to employ a rather high *T*c element because the HEA superconductors reported to date contain superconducting elements [28]. We

The concept of HEA is now used in superconducting materials with the crystal structures possessing multiple Wyckoff positions. For example, CsCl-type, α-Mntype, A15, NaCl-type, σ-phase and CuAl2-type HEA superconductors are reported

**22**

compounds.

**Figure 1.**

*a random occupation by constituent elements.*

focused on the Nb-containing HEAs.

[29–37]. High degree-of-freedom in such a multisite HEA design would promote the investigations of multisite HEA superconductors. The second purpose of this chapter is the materials research on the hexagonal Mn5Si3-type HEAs, possessing multiple Wyckoff positions. Recently, several superconductors with the Mn5Si3 type—or its ordered derivative Ti5Ga4-type—structure have been found and attract much attention [38–44]. Besides, many intermetallic compounds are crystallizing into these crystal structures [45, 46]. **Figure 1(b)** shows the crystal structure of the Mn5Si3-type compound represented by M5X3. The space group is *P*63/*mcm*, and the M atoms occupy the 4*d* (for M1 atom) and 6 *g* (for M2 atom) Wyckoff positions and the X atom another 6 *g* one. The M2 atoms form a face-sharing octahedral chain along the *c*-axis. The X atoms also form another octahedron, which encloses the M1 atom forming a one-dimensional atomic chain along the *c*-axis.

In this chapter, we report the synthesis and characterization of the fcc and the Mn5Si3-type HEA samples. The measurement of AC magnetic susceptibility checked the superconducting state. We also present the phase analyses of both kinds of samples. Finally, the future direction of materials research on superconducting HEAs is mentioned.

### **2. Materials and methods**

All samples were synthesized by a home-made arc furnace in an Ar atmosphere. The constituent elements as listed in **Table 1** were arc-melted on a water-cooled Cu hearth. The samples were turned over and melted several times. The Mn5Si3-type HEAs were annealed at 800°C for four days in evacuated quartz tubes.

A powder X-ray diffractometer (XRD-7000 L, Shimadzu, Kyoto, Japan) with Cu-Kα radiation was employed to detect the X-ray diffraction (XRD) patterns of


#### **Table 1.**

*Materials used in this study. The supply company, purity, crystal structure at room temperature, atomic radius [47], and VEC are also listed.*

prepared samples. The microstructure of each sample was examined by a field emission scanning electron microscope (FE-SEM, JSM-7100F; JEOL, Akishima, Japan). The atomic compositions of the samples were checked by an energy dispersive X-ray (EDX) spectrometer equipped to the FE-SEM.

To confirm the diamagnetic signal due to the superconducting state, the temperature dependence of the AC magnetic susceptibility *χ*ac (T) was measured by a home-made system in a GM refrigerator (UW404, Ulvac cryogenics, Kyoto, Japan) between 3 and 300 K. The amplitude and the frequency of the AC field were 5 Oe and 800 Hz, respectively.

## **3. Results and discussion**

### **3.1 Nb-containing fcc HEAs**

The starting compositions of prepared Nb-containing samples were determined, considering the conventional design rule [1, 2]: a *δ*-value less than 5% and a VEC larger than 8.0. To realize the requirements, fcc elements were predominantly used (see also **Tables 1** and **2**). The parameters of *δ* and VEC were calculated as follows:


**25**

**Figure 2.**

*Materials Research on High-Entropy Alloy Superconductors*

δ

2

(1)

*r*

*r*

,

=∑ (2)

*i i*

<sup>1</sup> 100 1 *<sup>n</sup> <sup>i</sup> i i*

=× − <sup>∑</sup>

1

where *c*i, *r*i, and VECi are the atomic fraction, the atomic radius, and the VEC of element *i*, respectively, and *r* is the composition-weighted average atomic radius. The parameter *δ* means the degree of the atomic size difference among the constituent elements. The calculated parameters for the prepared samples are listed in **Table 2**, in which the samples are named as their starting compositions. Cu20Nb15Pd25Rh30V10, Cu21Ir21Nb15Pd22Rh21 and Ir10Nb17Pd33Rh28Ru12 fulfill the design requirements. To investigate the effect of larger δ values, two samples,

*i VEC c VEC* =

Shown in **Figure 2** is the XRD patterns of prepared samples. In the upper five samples, all containing Nb, Pd, and Cu atoms, Cu20Nb15Pd25Rh30V10 and Cu21Ir21Nb15Pd22Rh21 possess dominant fcc phases. On the other hand, the XRD patterns of Zr-containing samples (Cu21Nb15Pd22Rh21Zr21 and Cu20Nb15Pd24Rh25V10Zr6) cannot be characterized by fcc phases. These results suggest that Zr is unfavorable for the formation of an fcc structure. In order to further investigate the formation condition of the single fcc phase, the quaternary alloy Cu40Nb20Pd30V10 was synthesized. As shown in **Figure 2**, this sample exhibits two fcc phases with quite different lattice parameters. The XRD pattern of the sample with no Cu atom

*XRD patterns of Nb-containing samples. The origin of each pattern is shifted by an integer value.*

*n*

=

*c*

*DOI: http://dx.doi.org/10.5772/intechopen.99693*

including a Zr atom, were prepared.

and

#### **Table 2.**

δ *and VEC of Nb-containing samples and phases detected by EDX measurements.*

*Materials Research on High-Entropy Alloy Superconductors DOI: http://dx.doi.org/10.5772/intechopen.99693*

$$\mathcal{S} = \mathbf{100} \times \sqrt{\sum\_{i=1}^{n} c\_i \left(\mathbf{1} - \frac{r\_i}{\overline{r}}\right)^2} \tag{1}$$

and

*Advances in High-Entropy Alloys - Materials Research, Exotic Properties and Applications*

prepared samples. The microstructure of each sample was examined by a field emission scanning electron microscope (FE-SEM, JSM-7100F; JEOL, Akishima, Japan). The atomic compositions of the samples were checked by an energy disper-

To confirm the diamagnetic signal due to the superconducting state, the temperature dependence of the AC magnetic susceptibility *χ*ac (T) was measured by a home-made system in a GM refrigerator (UW404, Ulvac cryogenics, Kyoto, Japan) between 3 and 300 K. The amplitude and the frequency of the AC field were 5 Oe

The starting compositions of prepared Nb-containing samples were determined, considering the conventional design rule [1, 2]: a *δ*-value less than 5% and a VEC larger than 8.0. To realize the requirements, fcc elements were predominantly used (see also **Tables 1** and **2**). The parameters of *δ* and VEC were calculated as follows:

**No. Sample Composition of Phase I, II or III** *δ* **VEC** 1 Cu20Nb15Pd25Rh30V10 3.52 8.65

2 Cu21Ir21Nb15Pd22Rh21 3.45 9.04

3 Cu21Nb15Pd22Rh21Zr21 8.00 7.99

4 Cu20Nb15Pd24Rh25V10Zr6 5.62 8.34

5 Cu40Nb20Pd30V10 4.44 8.9

6 Ir10Nb17Pd33Rh28Ru12 2.21 8.54

δ *and VEC of Nb-containing samples and phases detected by EDX measurements.*

Phase I Cu8.3(8)Nb21.2(8)Pd21.6(4)Rh42.9(5)V6.0(5) 3.14 8.29 Phase II Cu14.4(5)Nb12.6(5)Pd28(1)Rh26.4.(8)V18.6(5) 3.26 8.32 Phase III Cu65(5)Nb2(1)Pd28(2)Rh2(1)V3(1) 3.56 10.38

Phase I Cu1.4(6)Ir36.4(8)Nb24.3(4)Pd10.1(9)Rh27.8(3) 2.52 8.16 Phase II Cu8.5(7)Ir6(1)Nb12.5(6)Pd46(3)Rh27(2) 2.66 9.13 Phase III Cu40(5)Nb4(1)Pd52(3)Rh4(1) 3.74 10.16

Phase I Cu19.3(3)Pd37.2(2)Rh19.2(4)Zr24.3(5) 8.30 8.54 Phase II Cu7(1)Nb41(1)Pd8(1)Rh27(1)Zr17(1) 6.60 6.73 Phase III Cu57(1)Pd13(1)Rh9(1)Zr21(1) 9.33 9.22

Phase I Cu14.5(3)Nb15.7(5)Pd31(1)Rh23.1(5)V7(1)Zr8.7(5) 5.98 8.26 Phase II Cu15.0(5)Nb24(1)Pd18(1)Rh27.2(5)V15.8(8) 3.72 7.89 Phase III Cu86(1)Pd14(1) 2.62 10.86

Phase I Cu21.1(2)Nb27.6(2)Pd39.7(7)V11 5(7) 4.05 8.25 Phase II Cu89.2(5)Pd10.8(5) 2.35 10.89

Phase I Ir15.5(2)Nb17.7(8)Pd20.3(7)Rh29.7(2)Ru16.8(5) 2.30 8.33 Phase II Ir4.0(6)Nb16.0(5)Pd51(1)Rh22(1)Ru7.0(7) 2.02 8.8

sive X-ray (EDX) spectrometer equipped to the FE-SEM.

and 800 Hz, respectively.

**3. Results and discussion**

**3.1 Nb-containing fcc HEAs**

**24**

**Table 2.**

$$\text{VECC} = \sum\_{i=1}^{n} c\_i \text{VEC}\_i,\tag{2}$$

where *c*i, *r*i, and VECi are the atomic fraction, the atomic radius, and the VEC of element *i*, respectively, and *r* is the composition-weighted average atomic radius. The parameter *δ* means the degree of the atomic size difference among the constituent elements. The calculated parameters for the prepared samples are listed in **Table 2**, in which the samples are named as their starting compositions. Cu20Nb15Pd25Rh30V10, Cu21Ir21Nb15Pd22Rh21 and Ir10Nb17Pd33Rh28Ru12 fulfill the design requirements. To investigate the effect of larger δ values, two samples, including a Zr atom, were prepared.

Shown in **Figure 2** is the XRD patterns of prepared samples. In the upper five samples, all containing Nb, Pd, and Cu atoms, Cu20Nb15Pd25Rh30V10 and Cu21Ir21Nb15Pd22Rh21 possess dominant fcc phases. On the other hand, the XRD patterns of Zr-containing samples (Cu21Nb15Pd22Rh21Zr21 and Cu20Nb15Pd24Rh25V10Zr6) cannot be characterized by fcc phases. These results suggest that Zr is unfavorable for the formation of an fcc structure. In order to further investigate the formation condition of the single fcc phase, the quaternary alloy Cu40Nb20Pd30V10 was synthesized. As shown in **Figure 2**, this sample exhibits two fcc phases with quite different lattice parameters. The XRD pattern of the sample with no Cu atom

**Figure 2.** *XRD patterns of Nb-containing samples. The origin of each pattern is shifted by an integer value.*

#### *Advances in High-Entropy Alloys - Materials Research, Exotic Properties and Applications*

#### **Figure 3.**

*Back-scattered electron (15 keV) images of (a) Cu20Nb15Pd25Rh30V10, (b) Cu21Ir21Nb15Pd22Rh21, (c) Cu21Nb15Pd22Rh21Zr21, (d) Cu20Nb15Pd24Rh25V10Zr6, (e) Cu40Nb20Pd30V10, and (f) Ir10Nb17Pd33Rh28Ru12, respectively.*

(see the bottom of **Figure 2**) can be explained by an fcc phase. The lattice parameters of all fcc phases were obtained by the least-square method [48, 49] and are shown in **Figure 2**.

**Figures 3(a)**–**(f)** display the SEM images of samples, all indicating multi-phases. In Cu20Nb15Pd25Rh30V10 (**Figure 3(a)**) and Cu21Ir21Nb15Pd22Rh21 (**Figure 3(b)**), three contrast phases I, II and III were detected. In each case, the brightest area (phase I) showed a dendritic morphology, which is surrounded by phase II with the median contrast. The darkest area (phase III) would be the precipitate that formed in the final solidification process. A part of Cu21Nb15Pd22Rh21Zr21 (**Figure 3(c)**) or Cu20Nb15Pd24Rh25V10Zr6 (**Figure 3(d)**) showed a eutectic-like structure formed by phase I and phase II (see, for example, the green elliptic closed-curve). As shown in **Figure 3(e)**, Cu40Nb20Pd30V10 possesses two phases, both of which would be fcc

**27**

**Table 3.**

*Materials Research on High-Entropy Alloy Superconductors*

easy to synthesize a single-phase Nb-containing fcc HEA.

**No. Sample Composition of main** 

1 (NbScTiVZr)(GaGeSi) (Nb13.0(1)SC15.5(1)Ti11.2(1

2 (Nb1.25Sc1.25Ti1.25Zr1.25) (Ge1.8Si1.2) (Nb17(1)Sc14(1)Ti16(1

4 (Nb1.4ScTiZr1.6) (Ge1.6Pt0.3Si1.1) (Nb20.8(3)Sc8.4(5)Ti10.9(3)

5 (Nb1.4ScTiZr1.6) (Ge1.6Ir0.3Si1.1) (Nb20.4(2)Sc8.9(5)

3 (Nb1.25Sc1.25Ti1.25Zr1.25)

*starting composition is also shown.*

(Ge1.55Ru0.47Si0.98)

phases taking into account the XRD results. Ir10Nb17Pd33Rh28Ru12 displays two contrast areas (see phases I and II in **Figure 3(f)**). The shape of the main phase has a dendriticlike morphology. The compositions of all phases determined by EDX are listed in

*χ*ac (*T*) measurements of all prepared samples suggested no superconductivity down to 3 K, which means that a Nb-containing fcc-HEA might be an inadequate strategy for searching fcc HEA superconductors. The appearance of superconductivity in a Nb-containing fcc compound might be a rare event because almost all Nb-based superconductors form bcc-related structures. NbN or NbC superconductor is a rare example, crystallizing into a NaCl-type structure related to the fcc structure [50, 51]. Although our results would be negative for the research of Nb-containing fcc HEA superconductors, it is to be noted that phase II in sample no. 1, phase I in sample no. 6 and possibly phases I and II in sample no. 4 are new

Here, we discuss the fcc phase stability, viewed from the parameters of *δ* and VEC, which is summarized in **Table 2**. We also calculate these parameters for the phases detected by EDX. The values of *δ* were very large in Cu21Nb15Pd22Rh21Zr21 and Cu20Nb15Pd24Rh25V10Zr6 due to the larger atomic radius of Zr, which would lead to no fcc phase in each sample. While *δ* is reduced in Cu20Nb15Pd25Rh30V10 and Cu21Ir21Nb15Pd22Rh21, each sample shows three phases, all with rather low *δ* values. In each case, going from phase I to phase III, the VEC value increases, which accompanies the decrease (increase) of the Nb (Cu) atomic fraction. This suggests that the combination of Nb and Cu is not recommended even with a reduced *δ*, because an Nb-rich phase and a Cu-rich phase are stabilized for a smaller VEC and a larger VEC, respectively. Probably due to this reason, quaternary Cu40Nb20Pd30V10 does not show a single fcc phase. We note here that the addition of Cu leads to the breakdown of single-phase fcc CoCrFeNi into two fcc phases. This is ascribed to the positive enthalpy of mixing between the Cu and several elements [52]. Ir10Nb17Pd33Rh28Ru12 with substantially suppressed *δ* and no Cu atom was expected to show a single fcc phase; however, two phases were detected in the sample. The detected phases possess reduced *δ* values and a similar VEC. Thus, it may not be

**phase**

)V4.6(2)Zr19.0(1))(Ga4.4(2) Ge19.3(1)Si13.1(1))

)Zr17(1)) (Ge22(1)Si14(1))

(Nb18.0(2)Sc13.5(4 )Ti12.3(5)Zr20.5(6)) (Ge21.4(2)Si14.3(4))

Zr24(1)) (Ge19.9(5)Pt1.0(4) Si15.0(5))

> Ti10.9(1)Zr24.5(4)) (Ge20.3(3)Si15.0(2))

*Chemical compositions of phases detected by EDX measurements for Mn5Si3-type HEA samples. VEC of each* 

**Composition of minor phase**

Ga7(1)Ge6(1)Nb19(1 )Sc8(1) Si7(1)Ti17(1)V28 (1)Zr8(1)

Nb28(4)Ru31(4)Sc6(2)T3 1(3)Zr4(1)

Ge5.5(7)Sc37(2)Si1.7(2)Ti 15(1)Pt40.7(7)

Ge5.3(5)Ir27.7(9)Nb9.3(5) Sc20.5(9)Si4.0(6)Ti16.4(5) Zr16.7(8)

— 4

**VEC**

4

4.234

4.275

4.238

*DOI: http://dx.doi.org/10.5772/intechopen.99693*

**Table 2**.

members of HEA.

*Advances in High-Entropy Alloys - Materials Research, Exotic Properties and Applications*

(see the bottom of **Figure 2**) can be explained by an fcc phase. The lattice parameters of all fcc phases were obtained by the least-square method [48, 49] and are

*Back-scattered electron (15 keV) images of (a) Cu20Nb15Pd25Rh30V10, (b) Cu21Ir21Nb15Pd22Rh21, (c) Cu21Nb15Pd22Rh21Zr21, (d) Cu20Nb15Pd24Rh25V10Zr6, (e) Cu40Nb20Pd30V10, and (f) Ir10Nb17Pd33Rh28Ru12,* 

In Cu20Nb15Pd25Rh30V10 (**Figure 3(a)**) and Cu21Ir21Nb15Pd22Rh21 (**Figure 3(b)**), three contrast phases I, II and III were detected. In each case, the brightest area (phase I) showed a dendritic morphology, which is surrounded by phase II with the median contrast. The darkest area (phase III) would be the precipitate that formed in the final solidification process. A part of Cu21Nb15Pd22Rh21Zr21 (**Figure 3(c)**) or Cu20Nb15Pd24Rh25V10Zr6 (**Figure 3(d)**) showed a eutectic-like structure formed by phase I and phase II (see, for example, the green elliptic closed-curve). As shown in **Figure 3(e)**, Cu40Nb20Pd30V10 possesses two phases, both of which would be fcc

**Figures 3(a)**–**(f)** display the SEM images of samples, all indicating multi-phases.

**26**

shown in **Figure 2**.

**Figure 3.**

*respectively.*

phases taking into account the XRD results. Ir10Nb17Pd33Rh28Ru12 displays two contrast areas (see phases I and II in **Figure 3(f)**). The shape of the main phase has a dendriticlike morphology. The compositions of all phases determined by EDX are listed in **Table 2**.

*χ*ac (*T*) measurements of all prepared samples suggested no superconductivity down to 3 K, which means that a Nb-containing fcc-HEA might be an inadequate strategy for searching fcc HEA superconductors. The appearance of superconductivity in a Nb-containing fcc compound might be a rare event because almost all Nb-based superconductors form bcc-related structures. NbN or NbC superconductor is a rare example, crystallizing into a NaCl-type structure related to the fcc structure [50, 51]. Although our results would be negative for the research of Nb-containing fcc HEA superconductors, it is to be noted that phase II in sample no. 1, phase I in sample no. 6 and possibly phases I and II in sample no. 4 are new members of HEA.

Here, we discuss the fcc phase stability, viewed from the parameters of *δ* and VEC, which is summarized in **Table 2**. We also calculate these parameters for the phases detected by EDX. The values of *δ* were very large in Cu21Nb15Pd22Rh21Zr21 and Cu20Nb15Pd24Rh25V10Zr6 due to the larger atomic radius of Zr, which would lead to no fcc phase in each sample. While *δ* is reduced in Cu20Nb15Pd25Rh30V10 and Cu21Ir21Nb15Pd22Rh21, each sample shows three phases, all with rather low *δ* values. In each case, going from phase I to phase III, the VEC value increases, which accompanies the decrease (increase) of the Nb (Cu) atomic fraction. This suggests that the combination of Nb and Cu is not recommended even with a reduced *δ*, because an Nb-rich phase and a Cu-rich phase are stabilized for a smaller VEC and a larger VEC, respectively. Probably due to this reason, quaternary Cu40Nb20Pd30V10 does not show a single fcc phase. We note here that the addition of Cu leads to the breakdown of single-phase fcc CoCrFeNi into two fcc phases. This is ascribed to the positive enthalpy of mixing between the Cu and several elements [52]. Ir10Nb17Pd33Rh28Ru12 with substantially suppressed *δ* and no Cu atom was expected to show a single fcc phase; however, two phases were detected in the sample. The detected phases possess reduced *δ* values and a similar VEC. Thus, it may not be easy to synthesize a single-phase Nb-containing fcc HEA.


#### **Table 3.**

*Chemical compositions of phases detected by EDX measurements for Mn5Si3-type HEA samples. VEC of each starting composition is also shown.*

#### **Figure 4.**

*XRD patterns of Mn5Si3-type HEAs. The simulation pattern is also shown. Each pattern is shifted by an integer value for clarity.*

#### **Figure 5.**

*Back-scattered electron (15 keV) images of (a) (NbScTiVZr)(GaGeSi), and (b) (Nb1.25Sc1.25Ti1.25Zr1.25) (Ge1.8Si1.2), respectively. The elemental mappings are also shown.*

**29**

**Figure 7.**

**Figure 6.**

*Materials Research on High-Entropy Alloy Superconductors*

*Back-scattered electron (15 keV) images of (a) (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.55Ru0.47Si0.98), (b) (Nb1.4ScTiZr1.6)*

*(Ge1.6Pt0.3Si1.1), and (c) (Nb1.4ScTiZr1.6)(Ge1.6Ir0.3Si1.1), respectively.*

*Temperature dependences of χac of Mn5Si3-type HEA samples.*

*DOI: http://dx.doi.org/10.5772/intechopen.99693*

*Materials Research on High-Entropy Alloy Superconductors DOI: http://dx.doi.org/10.5772/intechopen.99693*

#### **Figure 6.**

*Advances in High-Entropy Alloys - Materials Research, Exotic Properties and Applications*

*XRD patterns of Mn5Si3-type HEAs. The simulation pattern is also shown. Each pattern is shifted by an* 

*Back-scattered electron (15 keV) images of (a) (NbScTiVZr)(GaGeSi), and (b) (Nb1.25Sc1.25Ti1.25Zr1.25)*

*(Ge1.8Si1.2), respectively. The elemental mappings are also shown.*

**28**

**Figure 5.**

**Figure 4.**

*integer value for clarity.*

*Back-scattered electron (15 keV) images of (a) (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.55Ru0.47Si0.98), (b) (Nb1.4ScTiZr1.6) (Ge1.6Pt0.3Si1.1), and (c) (Nb1.4ScTiZr1.6)(Ge1.6Ir0.3Si1.1), respectively.*

**Figure 7.** *Temperature dependences of χac of Mn5Si3-type HEA samples.*

## **3.2 Mn5Si3-type HEAs**

We have prepared five Mn5Si3-type HEAs as listed in **Table 3**, and the XRD patterns are given in **Figure 4**. All XRD patterns are well indexed by the hexagonal Mn5Si3-type structure, and the determined lattice parameters are displayed in **Figure 4**. The SEM images of all samples are presented in **Figures 5** and **6**, and *χ*ac (*T*) of each sample is shown in **Figure 7**.

We have started from (NbScTiVZr)(GaGeSi), which shows a diamagnetic signal (see **Figure 7**). However, as shown in **Figure 5(a)**, the elemental mapping has revealed the inhomogeneous distribution of constituent elements, which is obviously signaled by the V atom. The atomic compositions determined by EDX are (Nb13.0(1)Sc15.5(1)Ti11.2(1)V4.6(2) Zr19.0(1))(Ga4.4(2)Ge19.3(1)Si13.1(1)) for the V-poor phase and Ga7(1)Ge6(1)Nb19(1)Sc8(1)Si7(1)Ti17(1)V28(1) Zr8(1) for the V-rich phase, respectively. The separately synthesized latter phase crystallizes into a bcc structure. This compound also shows the diamagnetic signal at approximately 5 K, which is identical to that of (NbScTiVZr)(GaGeSi). Therefore, (NbScTiVZr)(GaGeSi) would be an intrinsically normal state down to 3 K. The result of the chemical composition of the Mn5Si3-type phase in (NbScTiVZr)(GaGeSi) suggests the difficulty of incorporation of V and Ga atoms in a Mn5Si3-type HEA. Taking into account this experimental result, we have synthesized (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2). As shown in **Figure 4**, the sample is almost single phase, which is also supported by homogeneous elemental mapping (see also **Figure 5(b)**). The determined atomic composition is (Nb17(1)Sc14(1)Ti16(1)Zr17(1))(Ge22(1)Si14(1)), which agrees well with the starting composition. While the single-phase Mn5Si3-type HEA is successfully obtained, the diamagnetic signal cannot be confirmed down to 3 K, as shown in **Figure 7**.

One of the conceivable reasons for no superconductivity in the samples mentioned above is that the VEC value is slightly less than the optimal value (see also **Table 3**). As pointed out in the review [28], multisite HEA superconductors follow the respective Matthias rule, which means the important role of the density of states at the Fermi level. The VEC values of Mn5Si3-type superconductors Zr5Sb3 and Zr5Ge2.5Ru0.5 are 4.375 and 4.25, respectively [40, 42], while the VEC value of (NbScTiVZr)(GaGeSi) or (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2) is 4. Thus, aiming at increasing the VEC, we substituted Ru, Pt, or Ir atoms at the Si site of Mn5Si3 type HEA. The prepared samples were (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.55Ru0.47Si0.98), (Nb1.4ScTiZr1.6)(Ge1.6Pt0.3Si1.1), and (Nb1.4ScTiZr1.6)(Ge1.6Ir0.3Si1.1) with the respective VEC value of 4.234, 4.275, and 4.238. In each sample, the main phase of XRD pattern is well characterized by the Mn5Si3-type structure (see **Figure 4**). However, the atomic composition, deviating from the starting one, as shown in **Table 3**, indicates that Ru, Pt, or Ir atoms cannot replace the atoms at the Si site. The SEM images of these samples show the precipitation of impurity phases at the grain boundaries of hexagonal-shaped main phases (see **Figure 6(a)**–**(c)**). *χ*ac (*T*) measurements of these Mn5Si3-type HEA do not show a superconducting signal down to 3 K.

#### **4. Summary**

We have carried out materials research on the fcc and the Mn5Si3-type HEA superconductors. In the study of fcc HEA superconductors, we employed the Nb element, taking into account that the inclusion of rather high *T*c elements is advantageous. Although some Nb-containing samples showed dominant fcc phases, single-phase ones could not be obtained. While we have found several new Nb-containing HEA phases in the multi-phase samples, no superconductivity

**31**

*Materials Research on High-Entropy Alloy Superconductors*

appeared in each HEA phase down to 3 K. Considering that the Nb element forms a bcc structure at room temperature, and there are only several examples of fcc-related Nb-based superconductors, the discovery of Nb-containing fcc HEA superconductors would be a hard task. For the Mn5Si3-type HEA samples, we have successfully confirmed the single phase in (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2) with VEC = 4. However, this HEA compound does not show the diamagnetic signal down to 3 K. Some Mn5Si3-type superconductors possess the VEC value of 4.25 or 4.375. Thus Ru, Ir, or Pt element was substituted at the Si site to increase VEC, resulting in the unsuccessful attempt. We need a strategy to adjust the VEC value for the

The conclusions regarding the manufacturing process or microstructure in HEA

• If one wants to obtain a single-phase sample with bcc, hcp, or fcc type structure by the arc-melting method, it would be a rather hard task due to an appearance of secondary phase and/or of phase with a slightly different

eutectic phase in HEA superconductors might be interesting.

route to obtain single-phase HEA superconductors.

the evaluation of the CALPHAD method in the future.

cocktail effect of *T*c in multisite HEAs.

• Eutectic HEAs receive much attention due to the rich functions arising from the microstructures [53]. In some cases, eutectic superconductors show enhanced superconducting critical temperatures. Therefore, the study of the

• Mechanical alloying has been widely used to produce HEAs [54]. The mechanical alloying process is different from the arc-melting one. So this is another

The formation of single-phase fcc HEA is realized at VEC larger than 8.0. According to the Matthias rule of transition metal alloys, *T*c at such a large VEC value is substantially reduced. Therefore, the measurement of physical properties at much lower than 3 K is desired. Because the synthesis of quinary alloy with the single-phase might be a hard task, research on ternary or quarternary fcc multicomponent superconductor would be necessary. In the research area of HEAs, the CALPHAD (calculation of phase diagram) method is rapidly used for the prediction of HEAs or the study of the phase relation between HEAs and other alloys. If the thermodynamic data of various compounds in the present Nb-containing multicomponent systems are sufficiently collected, the CALPHAD method will elucidate the stability of a HEA in this system. Thus, our results will greatly assist in

Nb5Ir3O, crystallizing into the Ti5Ga4-type structure, which is the ordered derivative of Mn5Si3-type structure, is well known as a two-band superconductor [41]. By substituting Pt into Ir, the crossover to single-band superconductivity is observed [44], which is a rare phenomenon. This result promotes us to investigate multisite HEA superconductors for further search of the crossover phenomenon, and the high-entropy state may be a new route of controlling the superconducting band. Another interesting aspect of HEA is the cocktail effect. In bcc HEA superconductors, we have shown that the peculiar enhancement of *T*c by introducing the high-entropy state is not observed [28]. It would be interesting to pursue the

*DOI: http://dx.doi.org/10.5772/intechopen.99693*

Mn5Si3-type HEAs.

composition.

**5. Future directions**

superconductors are bulleted below.

*Advances in High-Entropy Alloys - Materials Research, Exotic Properties and Applications*

We have prepared five Mn5Si3-type HEAs as listed in **Table 3**, and the XRD patterns are given in **Figure 4**. All XRD patterns are well indexed by the hexagonal Mn5Si3-type structure, and the determined lattice parameters are displayed in **Figure 4**. The SEM images of all samples are presented in **Figures 5** and **6**, and *χ*ac

(see **Figure 7**). However, as shown in **Figure 5(a)**, the elemental mapping has revealed the inhomogeneous distribution of constituent elements, which is obviously signaled by the V atom. The atomic compositions determined by EDX are (Nb13.0(1)Sc15.5(1)Ti11.2(1)V4.6(2) Zr19.0(1))(Ga4.4(2)Ge19.3(1)Si13.1(1)) for the V-poor phase and Ga7(1)Ge6(1)Nb19(1)Sc8(1)Si7(1)Ti17(1)V28(1) Zr8(1) for the V-rich phase, respectively. The separately synthesized latter phase crystallizes into a bcc structure. This compound also shows the diamagnetic signal at approximately 5 K, which is identical to that of (NbScTiVZr)(GaGeSi). Therefore, (NbScTiVZr)(GaGeSi) would be an intrinsically normal state down to 3 K. The result of the chemical composition of the Mn5Si3-type phase in (NbScTiVZr)(GaGeSi) suggests the difficulty of incorporation of V and Ga atoms in a Mn5Si3-type HEA. Taking into account this experimental result, we have synthesized (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2). As shown in **Figure 4**, the sample is almost single phase, which is also supported by homogeneous elemental mapping (see also **Figure 5(b)**). The determined atomic composition is (Nb17(1)Sc14(1)Ti16(1)Zr17(1))(Ge22(1)Si14(1)), which agrees well with the starting composition. While the single-phase Mn5Si3-type HEA is successfully obtained, the

diamagnetic signal cannot be confirmed down to 3 K, as shown in **Figure 7**.

these Mn5Si3-type HEA do not show a superconducting signal down to 3 K.

We have carried out materials research on the fcc and the Mn5Si3-type HEA superconductors. In the study of fcc HEA superconductors, we employed the Nb element, taking into account that the inclusion of rather high *T*c elements is advantageous. Although some Nb-containing samples showed dominant fcc phases, single-phase ones could not be obtained. While we have found several new Nb-containing HEA phases in the multi-phase samples, no superconductivity

One of the conceivable reasons for no superconductivity in the samples mentioned above is that the VEC value is slightly less than the optimal value (see also **Table 3**). As pointed out in the review [28], multisite HEA superconductors follow the respective Matthias rule, which means the important role of the density of states at the Fermi level. The VEC values of Mn5Si3-type superconductors Zr5Sb3 and Zr5Ge2.5Ru0.5 are 4.375 and 4.25, respectively [40, 42], while the VEC value of (NbScTiVZr)(GaGeSi) or (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2) is 4. Thus, aiming at increasing the VEC, we substituted Ru, Pt, or Ir atoms at the Si site of Mn5Si3 type HEA. The prepared samples were (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.55Ru0.47Si0.98), (Nb1.4ScTiZr1.6)(Ge1.6Pt0.3Si1.1), and (Nb1.4ScTiZr1.6)(Ge1.6Ir0.3Si1.1) with the respective VEC value of 4.234, 4.275, and 4.238. In each sample, the main phase of XRD pattern is well characterized by the Mn5Si3-type structure (see **Figure 4**). However, the atomic composition, deviating from the starting one, as shown in **Table 3**, indicates that Ru, Pt, or Ir atoms cannot replace the atoms at the Si site. The SEM images of these samples show the precipitation of impurity phases at the grain boundaries of hexagonal-shaped main phases (see **Figure 6(a)**–**(c)**). *χ*ac (*T*) measurements of

We have started from (NbScTiVZr)(GaGeSi), which shows a diamagnetic signal

**3.2 Mn5Si3-type HEAs**

(*T*) of each sample is shown in **Figure 7**.

**30**

**4. Summary**

appeared in each HEA phase down to 3 K. Considering that the Nb element forms a bcc structure at room temperature, and there are only several examples of fcc-related Nb-based superconductors, the discovery of Nb-containing fcc HEA superconductors would be a hard task. For the Mn5Si3-type HEA samples, we have successfully confirmed the single phase in (Nb1.25Sc1.25Ti1.25Zr1.25)(Ge1.8Si1.2) with VEC = 4. However, this HEA compound does not show the diamagnetic signal down to 3 K. Some Mn5Si3-type superconductors possess the VEC value of 4.25 or 4.375. Thus Ru, Ir, or Pt element was substituted at the Si site to increase VEC, resulting in the unsuccessful attempt. We need a strategy to adjust the VEC value for the Mn5Si3-type HEAs.

The conclusions regarding the manufacturing process or microstructure in HEA superconductors are bulleted below.


## **5. Future directions**

The formation of single-phase fcc HEA is realized at VEC larger than 8.0. According to the Matthias rule of transition metal alloys, *T*c at such a large VEC value is substantially reduced. Therefore, the measurement of physical properties at much lower than 3 K is desired. Because the synthesis of quinary alloy with the single-phase might be a hard task, research on ternary or quarternary fcc multicomponent superconductor would be necessary. In the research area of HEAs, the CALPHAD (calculation of phase diagram) method is rapidly used for the prediction of HEAs or the study of the phase relation between HEAs and other alloys. If the thermodynamic data of various compounds in the present Nb-containing multicomponent systems are sufficiently collected, the CALPHAD method will elucidate the stability of a HEA in this system. Thus, our results will greatly assist in the evaluation of the CALPHAD method in the future.

Nb5Ir3O, crystallizing into the Ti5Ga4-type structure, which is the ordered derivative of Mn5Si3-type structure, is well known as a two-band superconductor [41]. By substituting Pt into Ir, the crossover to single-band superconductivity is observed [44], which is a rare phenomenon. This result promotes us to investigate multisite HEA superconductors for further search of the crossover phenomenon, and the high-entropy state may be a new route of controlling the superconducting band. Another interesting aspect of HEA is the cocktail effect. In bcc HEA superconductors, we have shown that the peculiar enhancement of *T*c by introducing the high-entropy state is not observed [28]. It would be interesting to pursue the cocktail effect of *T*c in multisite HEAs.

## **Acknowledgements**

J.K. is grateful for the support provided by Comprehensive Research Organization of Fukuoka Institute of Technology.

## **Conflict of interest**

The authors declare no conflict of interest.

## **Author details**

Jiro Kitagawa\*, Naoki Ishizu and Shusuke Hamamoto Department of Electrical Engineering, Faculty of Engineering, Fukuoka Institute of Technology, Fukuoka, Japan

\*Address all correspondence to: j-kitagawa@fit.ac.jp

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

**33**

*Materials Research on High-Entropy Alloy Superconductors*

oxide. Scr. Mater. 2020;186:366-369. DOI: 10.1016/j.scriptamat.2020.04.027

[9] Sun L, Luo Y, Ren X, Gao Z, Du T, Wu Z, Wang J. A multicomponent γ-type (Gd1/6Tb1/6Dy1/6Tm1/6Yb1/6Lu1/6)2 Si2O7 disilicate with outstanding thermal stability. Mater. Res. Lett. 2020;8:424- 430. DOI: 10.1080/21663831.2020.

[10] Rao Z, Dutta B, Körmann F, Lu W, Zhou X, Liu C, Kwiatkowski da Silva A, Wiedwald U, Spasova M, Farle M, Ponge D, Gault B, Neugebauer J, Raabe D, Li Z. Beyond solid solution high-entropy alloys: tailoring magnetic properties via spinodal decomposition. Adv. Funct. Mater. 2021;31:2007668. DOI: 10.1002/adfm.202007668

[11] Quintana-Nedelcos A, Leong Z, Morley N A. Study of dual-phase functionalisation of NiCoFeCr-Alx multicomponent alloys for the enhancement of magnetic properties and magneto-caloric effect. Mater. Today: Energy 2021;20:100621. DOI: 10.1016/j.mtener.2020.100621

[12] Jung C, Kang K, Marshal A, Gokuldoss Pradeep K, Seol J–B, Lee H

composition and elemental partitioning

AlFeCoCrMn high entropy alloys. Acta Mater. 2019;171:31-39. DOI: 10.1016/j.

[14] Koželj P, Vrtnik S, Jelen A, Jazbec S, Jagličić Z, Maiti S, Feuerbacher M, Steurer W, Dolinšek J. Discovery of a superconducting high-entropy alloy. Phys. Rev. Lett. 2014;113:107001. DOI: 10.1103/PhysRevLett.113.107001

M, Choi P–P. Effects of phase

on soft magnetic properties of

[13] BaBa K, Ishizu N, Nishizaki T, Kitagawa J. Magnetic and transport properties of new dual-phase highentropy alloy FeRhIrPdPt. Materials 2021;14:2877. DOI: 10.3390/ma14112877

actamat.2019.04.007

1783007

*DOI: http://dx.doi.org/10.5772/intechopen.99693*

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[2] Murty B S, Yeh J W, Ranganathan S, Bhattacharjee P P. High-Entropy Alloys. 2nd ed. Amsterdam: Elsevier; 2019.

[3] Yeh J W, Chen S K, Lin S J, Gan J Y, Chin T S, Shun T T, Tsau C H, Chang S Y. Nanostructured high-entropy alloys with multiple principal elements: Novel alloy design concepts and outcomes. Adv. Eng. Mater. 2004;

[4] Otto F, Yang Y, Bei H, George E P. Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys. Acta Mater. 2013;61:2628-2638. DOI: 10.1016/j.

[5] Rost C M, Sachet E, Borman T, Moballegh A, Dickey E C, Hou D, Jones J L, Curtarolo S, Maria J P. Entropystabilized oxides. Nat. Commun. 2015;6:8485. DOI: 10.1038/ncomms9485

[6] Musicό B L, Gilbert D, Ward T Z, Page K, George E, Yan J, Mandrus D, Keppens V. The emergent field of high entropy oxides: Design, prospects, challenges, and opportunities for tailoring material properties. APL Materials 2020;8:040912. DOI:

[7] Ying T, Yu T, Shiah Y-S, Li C, Li J, Qi Y, Hosono H. High-entropy van der Waals materials formed from mixed metal dichalcogenides, halides, and phosphorus trisulfides. J. Am. Chem. Soc. 2021;143:7042-7049. DOI: 10.1021/

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*Advances in High-Entropy Alloys - Materials Research, Exotic Properties and Applications*

J.K. is grateful for the support provided by Comprehensive Research

Organization of Fukuoka Institute of Technology.

The authors declare no conflict of interest.

**32**

**Author details**

**Acknowledgements**

**Conflict of interest**

Technology, Fukuoka, Japan

Jiro Kitagawa\*, Naoki Ishizu and Shusuke Hamamoto

\*Address all correspondence to: j-kitagawa@fit.ac.jp

provided the original work is properly cited.

Department of Electrical Engineering, Faculty of Engineering, Fukuoka Institute of

© 2021 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/ by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, [1] Gao M C, Yeh J W, Liaw P K, Zhang Y. High-entropy alloys: fundamentals and applications Swizerland: Springer; 2015.

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[4] Otto F, Yang Y, Bei H, George E P. Relative effects of enthalpy and entropy on the phase stability of equiatomic high-entropy alloys. Acta Mater. 2013;61:2628-2638. DOI: 10.1016/j. actamat.2013.01.042

[5] Rost C M, Sachet E, Borman T, Moballegh A, Dickey E C, Hou D, Jones J L, Curtarolo S, Maria J P. Entropystabilized oxides. Nat. Commun. 2015;6:8485. DOI: 10.1038/ncomms9485

[6] Musicό B L, Gilbert D, Ward T Z, Page K, George E, Yan J, Mandrus D, Keppens V. The emergent field of high entropy oxides: Design, prospects, challenges, and opportunities for tailoring material properties. APL Materials 2020;8:040912. DOI: 10.1063/5.0003149

[7] Ying T, Yu T, Shiah Y-S, Li C, Li J, Qi Y, Hosono H. High-entropy van der Waals materials formed from mixed metal dichalcogenides, halides, and phosphorus trisulfides. J. Am. Chem. Soc. 2021;143:7042-7049. DOI: 10.1021/ jacs.1c01580

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[10] Rao Z, Dutta B, Körmann F, Lu W, Zhou X, Liu C, Kwiatkowski da Silva A, Wiedwald U, Spasova M, Farle M, Ponge D, Gault B, Neugebauer J, Raabe D, Li Z. Beyond solid solution high-entropy alloys: tailoring magnetic properties via spinodal decomposition. Adv. Funct. Mater. 2021;31:2007668. DOI: 10.1002/adfm.202007668

[11] Quintana-Nedelcos A, Leong Z, Morley N A. Study of dual-phase functionalisation of NiCoFeCr-Alx multicomponent alloys for the enhancement of magnetic properties and magneto-caloric effect. Mater. Today: Energy 2021;20:100621. DOI: 10.1016/j.mtener.2020.100621

[12] Jung C, Kang K, Marshal A, Gokuldoss Pradeep K, Seol J–B, Lee H M, Choi P–P. Effects of phase composition and elemental partitioning on soft magnetic properties of AlFeCoCrMn high entropy alloys. Acta Mater. 2019;171:31-39. DOI: 10.1016/j. actamat.2019.04.007

[13] BaBa K, Ishizu N, Nishizaki T, Kitagawa J. Magnetic and transport properties of new dual-phase highentropy alloy FeRhIrPdPt. Materials 2021;14:2877. DOI: 10.3390/ma14112877

[14] Koželj P, Vrtnik S, Jelen A, Jazbec S, Jagličić Z, Maiti S, Feuerbacher M, Steurer W, Dolinšek J. Discovery of a superconducting high-entropy alloy. Phys. Rev. Lett. 2014;113:107001. DOI: 10.1103/PhysRevLett.113.107001

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[21] Zhang X, Winter N, Witteveen C, Moehl T, Xiao Y, Krogh F, Schilling A, von Rohr F O. Preparation and

characterization of high-entropy alloy (TaNb)1−x(ZrHfTi)x superconducting films. Phys. Rev. Res. 2020;2:013375. DOI: 10.1103/PhysRevResearch.2.013375

[22] Harayama Y, Kitagawa J.

s10948-021-05966-z

Superconductivity in Al-Nb-Ti-V-Zr multicomponent alloy. J. Supercond. Nov. Magn. 2021 DOI:10.1007/

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[32] Mizuguchi Y. Superconductivity in high-entropy-alloy telluride AgInSnPbBiTe5. J. Phys. Soc. Jpn. 2019;88:124708. DOI: 10.7566/ JPSJ.88.124708

[33] Kasem M R, Hoshi K, Jha R, Katsuno M, Yamashita A, Goto Y, Matsuda T D, Aoki Y, Mizuguchi Y. Superconducting properties of highentropy-alloy tellurides M-Te (M: Ag, In, Cd, Sn, Sb, Pb, Bi) with a NaCl-type structure. Appl. Phys. Express 2020;13:033001. DOI: 10.35848/1882- 0786/ab7482

[34] Yamashita A, Jha R, Goto Y, Matsuda T D, Aoki Y, Mizuguchi Y. An Efficient way of increasing the total entropy of mixing in high-entropy-alloy compounds: a case of NaCl-type (Ag,In,Pb,Bi)Te1-xSex (x = 0.0, 0.25, 0.5) superconductors. Dalton Trans. 2020;49:9118-9122. DOI: 10.1039/ D0DT01880E

[35] Liu B, Wu J, Cui Y, Zhu Q, Xiao G, Wu S, Cao G-h, Ren Z. Superconductivity and paramagnetism in Cr-containing tetragonal highentropy alloys. J. Alloys Compd. 2021;869:159293. DOI: 10.1016/j. jallcom.2021.159293

[36] Mizuguchi Y, Kasem M R, Matsuda T D. Superconductivity in CuAl2-type Co0.2Ni0.1Cu0.1Rh0.3Ir0.3Zr2 with a high-entropy-alloy transition metal site. Mater. Res. Lett. 2020:9;141-147. DOI: 10.1080/21663831.2020.1860147

[37] Kasem M R, Yamashita A, Goto Y, Matsuda T D, Mizuguchi Y. Synthesis of high-entropy-alloy-type superconductors (Fe,Co,Ni,Rh,Ir)Zr2 with tunable transition temperature. J. Mater. Sci. 2021;56:9499-9505. DOI: 10.1007/s10853-021-05921-2

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**37**

Section 2

High-Entropy Alloy

Composites

## Section 2
