Open access peer-reviewed chapter

Materials Research on High-Entropy Alloy Superconductors

By Jiro Kitagawa, Naoki Ishizu and Shusuke Hamamoto

Submitted: November 27th 2020Reviewed: July 29th 2021Published: August 17th 2021

DOI: 10.5772/intechopen.99693

Downloaded: 44

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].

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.

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 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 rare-earth 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, 12, 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 Tc 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 Tc values in the valley of the curve of the Matthias rule [16]. HEA superconductors with simple crystal structures have been found in bcc [17, 18, 19, 20, 21, 22] and hcp [23, 24, 25, 26]. The Tc 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 Tc would be substantially reduced in the VEC dependence of Tc 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 Tc 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-Mn-type, A15, NaCl-type, σ-phase and CuAl2-type HEA superconductors are reported [29, 30, 31, 32, 33, 34, 35, 36, 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, 39, 40, 41, 42, 43, 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 P63/mcm, and the M atoms occupy the 4d(for M1 atom) and 6 g(for M2 atom) Wyckoff positions and the X atom another 6 gone. 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.

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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.

ElementSupply companyPurity (%)Crystal structureAtomic radius (Å)VEC
ZrSoekawa Chemicals, Tokyo, Japan99A3 (hcp)1.60254
NbNilaco, Tokyo, Japan99.9A2 (bcc)1.4295
VKojundo Chemical Laboratory, Sakado, Japan99.9A2 (bcc)1.3165
RuSoekawa Chemicals, Tokyo, Japan99.9A3 (hcp)1.33848
IrFuruya Metal, Tokyo, Japan99.99A1 (fcc)1.35739
RhSoekawa Chemicals, Tokyo, Japan99.9A1 (fcc)1.3459
PdTanaka Kinzoku Kogyo, Tokyo, Japan99.9A1 (fcc)1.375410
CuSoekawa Chemicals, Tokyo, Japan99.99A1 (fcc)1.27811
ScFuruya Metal, Tokyo, Japan99.9A3 (hcp)1.6413
TiNilaco, Tokyo, Japan99.9A3 (hcp)1.46154
GaKojundo Chemical Laboratory, Sakado, Japan99.99All1.3923
SiSoekawa Chemicals, Tokyo, Japan99.999A41.1534
GeSoekawa Chemicals, Tokyo, Japan99.999A41.244
PtTanaka Kinzoku Kogyo, Tokyo, Japan99.9A1 (fcc)1.38710

Table 1.

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

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 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 χac (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.

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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 δand VEC were calculated as follows:

No.SampleComposition of Phase I, II or IIIδVEC
1Cu20Nb15Pd25Rh30V103.528.65
Phase ICu8.3(8)Nb21.2(8)Pd21.6(4)Rh42.9(5)V6.0(5)3.148.29
Phase IICu14.4(5)Nb12.6(5)Pd28(1)Rh26.4.(8)V18.6(5)3.268.32
Phase IIICu65(5)Nb2(1)Pd28(2)Rh2(1)V3(1)3.5610.38
2Cu21Ir21Nb15Pd22Rh213.459.04
Phase ICu1.4(6)Ir36.4(8)Nb24.3(4)Pd10.1(9)Rh27.8(3)2.528.16
Phase IICu8.5(7)Ir6(1)Nb12.5(6)Pd46(3)Rh27(2)2.669.13
Phase IIICu40(5)Nb4(1)Pd52(3)Rh4(1)3.7410.16
3Cu21Nb15Pd22Rh21Zr218.007.99
Phase ICu19.3(3)Pd37.2(2)Rh19.2(4)Zr24.3(5)8.308.54
Phase IICu7(1)Nb41(1)Pd8(1)Rh27(1)Zr17(1)6.606.73
Phase IIICu57(1)Pd13(1)Rh9(1)Zr21(1)9.339.22
4Cu20Nb15Pd24Rh25V10Zr65.628.34
Phase ICu14.5(3)Nb15.7(5)Pd31(1)Rh23.1(5)V7(1)Zr8.7(5)5.988.26
Phase IICu15.0(5)Nb24(1)Pd18(1)Rh27.2(5)V15.8(8)3.727.89
Phase IIICu86(1)Pd14(1)2.6210.86
5Cu40Nb20Pd30V104.448.9
Phase ICu21.1(2)Nb27.6(2)Pd39.7(7)V11 5(7)4.058.25
Phase IICu89.2(5)Pd10.8(5)2.3510.89
6Ir10Nb17Pd33Rh28Ru122.218.54
Phase IIr15.5(2)Nb17.7(8)Pd20.3(7)Rh29.7(2)Ru16.8(5)2.308.33
Phase IIIr4.0(6)Nb16.0(5)Pd51(1)Rh22(1)Ru7.0(7)2.028.8

Table 2.

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

δ=100×i=1nci1rir¯2E1

and

VEC=i=1nciVECi,E2

where ci, ri, and VECiare the atomic fraction, the atomic radius, and the VEC of element i, respectively, and r¯is the composition-weighted average atomic radius. The parameter δmeans 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 δvalues, 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 (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.

Figure 2.

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

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 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 dendritic-like morphology. The compositions of all phases determined by EDX are listed in Table 2.

Figure 3.

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

χac (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 δand VEC, which is summarized in Table 2. We also calculate these parameters for the phases detected by EDX. The values of δwere 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 δis reduced in Cu20Nb15Pd25Rh30V10 and Cu21Ir21Nb15Pd22Rh21, each sample shows three phases, all with rather low δvalues. 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 δand no Cu atom was expected to show a single fcc phase; however, two phases were detected in the sample. The detected phases possess reduced δvalues and a similar VEC. Thus, it may not be easy to synthesize a single-phase Nb-containing fcc HEA.

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

No.SampleComposition of main phaseComposition of minor phaseVEC
1(NbScTiVZr)(GaGeSi)(Nb13.0(1)SC15.5(1)Ti11.2(1)V4.6(2)Zr19.0(1))(Ga4.4(2)Ge19.3(1)Si13.1(1))Ga7(1)Ge6(1)Nb19(1)Sc8(1) Si7(1)Ti17(1)V28(1)Zr8(1)4
2(Nb1.25Sc1.25Ti1.25Zr1.25) (Ge1.8Si1.2)(Nb17(1)Sc14(1)Ti16(1)Zr17(1)) (Ge22(1)Si14(1))4
3(Nb1.25Sc1.25Ti1.25Zr1.25) (Ge1.55Ru0.47Si0.98)(Nb18.0(2)Sc13.5(4)Ti12.3(5)Zr20.5(6)) (Ge21.4(2)Si14.3(4))Nb28(4)Ru31(4)Sc6(2)T31(3)Zr4(1)4.234
4(Nb1.4ScTiZr1.6) (Ge1.6Pt0.3Si1.1)(Nb20.8(3)Sc8.4(5)Ti10.9(3)Zr24(1)) (Ge19.9(5)Pt1.0(4) Si15.0(5))Ge5.5(7)Sc37(2)Si1.7(2)Ti15(1)Pt40.7(7)4.275
5(Nb1.4ScTiZr1.6) (Ge1.6Ir0.3Si1.1)(Nb20.4(2)Sc8.9(5)Ti10.9(1)Zr24.5(4)) (Ge20.3(3)Si15.0(2))Ge5.3(5)Ir27.7(9)Nb9.3(5) Sc20.5(9)Si4.0(6)Ti16.4(5) Zr16.7(8)4.238

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.

Figure 6.

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 ofχac of Mn5Si3-type HEA samples.

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)). χac (T) measurements of these Mn5Si3-type HEA do not show a superconducting signal down to 3 K.

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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 Tc 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 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.

  • 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 composition.

  • 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 eutectic phase in HEA superconductors might be interesting.

  • 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 route to obtain single-phase HEA superconductors.

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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, Tcat 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 Tcby introducing the high-entropy state is not observed [28]. It would be interesting to pursue the cocktail effect of Tc in multisite HEAs.

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Acknowledgments

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

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Conflict of interest

The authors declare no conflict of interest.

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

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Jiro Kitagawa, Naoki Ishizu and Shusuke Hamamoto (August 17th 2021). Materials Research on High-Entropy Alloy Superconductors, Advances in High-Entropy Alloys - Materials Research, Exotic Properties and Applications, Jiro Kitagawa, IntechOpen, DOI: 10.5772/intechopen.99693. Available from:

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