High-Entropy Alloys – A New Era of Exploitation

Jien-Wei Yeh 1, a , Yu-Liang Chen 1, b , Su-Jien Lin 1, c and Swe-Kai Chen 2, d 1Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu 300,

Taiwan

2 Materials Science Center, National Tsing Hua University, Hsinchu 300, Taiwan

a jwyeh@https://www.360docs.net/doc/6e18280304.html,.tw, b

d907519@https://www.360docs.net/doc/6e18280304.html,.tw, c sjlin@https://www.360docs.net/doc/6e18280304.html,.tw, d skchen@https://www.360docs.net/doc/6e18280304.html,.tw

Keywords: High-entropy alloys, high entropy effect, nanostructure, amorphous phase.

Abstract. A high-entropy alloy (HEA) has been defined by us to have at least five principal elements, each of which has an atomic concentration between 5% and 35%. In the exploration on this new alloy field, we find that HEAs are quite simple to analyze and control, and they might be processed as traditional alloys. There exist many opportunities to create novel alloys, better than traditional ones in a wide range of applications. In this paper, we review the basic microstructural features of HEAs and discuss the mechanisms of formation. Instead of multiple intermetallic phases, the HEAs tend to form simple solid solution phases mainly of cubic crystal structure, especially at elevated temperatures. This tendency is explained by the high entropy effect based on the simple relation: ?G mix = ?H mix – T?S mix , and the second law of thermodynamics. Moreover, nanostructures and amorphous phases are easily formed in HEAs. This tendency is explained by kinetics theory as due to slow atomic diffusion.

Introduction

Human civilization progresses as materials progress. The Metal Age comprises the Bronze Age and the Iron Age and spans about five thousands years till now. Even in this new Silicon Age which arrived just several decades ago, we still rely largely upon metals, especially upon high-performance metals. It is interesting to note that most high-performance metals were developed in the last 150 years. This is a very short time as compared with the whole Metal Age.

Up to the 1970s, almost all traditional alloys had been developed and provided a wide spectrum of properties and performance. There have been about 30 commonly used traditional alloy systems, including steel, aluminum, copper, etc., as depicted, for example, in the ASM Metals Handbooks. However, they were unsatisfactory in many aspects of application. Therefore, many efforts have been exerted to develop new metals in the last four decades. Three routes are chosen for this: to create new compositions, to invent new processes, and to use new combinations of compositions and processes. Thus, intermetallics, metal-matrix composites, metallic glasses, thermomechanical processing, rapid solidification, mechanical alloying, spray deposition, equal-channel angular pressing, reciprocating extrusion, superplastic forming, nanotechnology, etc. were proposed and investigated. Even so, there still exist many bottlenecks to overcome since the requirements for high performance become more and more strict in most applications.

When examining the design concept for traditional alloys, it is found that almost all alloys are based on one principal metallic element and seldom have more than three principal metallic elements [1, 2]. For example, steel is based on iron; superalloys are based on Ni, Co, or Fe; intermetallics are based on Ni-Al compounds, Ti-Al compounds, Fe-Al compounds, etc.; metal-matrix composites are based on Ni, Ti, or Al; and metallic glasses have nine different bases: Pd-, Mg-, La-, Zr-, Ti-, Cu-, Fe-, Co-, and Ni-base [3]. Thus, under the traditional concept, metallurgists make and process the alloys, study their microstructure and properties, and provide them for suitable applications. Obviously, the degrees of freedom in alloy development are confined by this alloy concept. In the end of the last century, the development of new metals

To overcome this constraint, we proposed a new approach for alloy design, “high-entropy alloys”, and started to explore this new alloy field in Tsing Hua University of Taiwan since 1995

[2]. High-entropy alloys are defined to have at least 5 principal metallic elements, each of which has an atomic percentage between 5 % and 35 %. For example, we could have a 6-element equimolar alloy as AlCoCrCuFeNi and a nonequimolar alloy as AlCo 0.5CrCuFe 1.5Ni 1.2. Furthermore, we might modify the alloys with minor elements such as AlCo 0.5CrCuFe 1.5Ni 1.2B 0.1C 0.15. With this brand new concept of alloy design, a huge number of new alloys may be designed and studied, and many new phenomena, new properties, and new applications may be discovered.

From our study, we conclude that high-entropy alloys could be made, processed and analyzed like conventional alloys. Moreover, they exhibit several interesting features as previously reported

[1,2,4-11]:

? Tend to form simple solid solution phases, such as FCC and BCC phases, with nanostructures or even amorphous structures.

? Range from 100 to 1100 in hardness (VHN).

? Have microstructures with good thermal stability.

? Deform by a nano-twinning deformation mechanism.

? Have excellent resistance to anneal softening.

? Can have high-temperature precipitation hardening between 500 and 1000°C.

? Can have a positive temperature coefficient of strength and hence maintain a high strength level at elevated temperatures.

? Can possess excellent corrosion resistance, wear resistance and oxidation resistance.

Due to these special properties, they have many potential applications [2,4-11]: tools, molds, dies, mechanical parts and furnace parts requiring the properties of high strength, thermal stability, and wear and oxidation resistances; anticorrosive high-strength materials in chemical plants, IC foundries, and even marine applications for piping and pump components requiring excellent corrosion resistance; functional coatings such as hard-facing of golf heads and rolls, hard and anti-sticking coating for molds and tools, diffusion barrier for Cu interconnects in ultra large-scale integrated circuits, and soft magnetic films for ultra-high-frequency communications.

In this paper, basic microstructural features of HEAs are reviewed, and their tendency to form simple nanostructures and amorphous structures is discussed

Tendency to Form Simple Microstructures

Based on physical metallurgy and experience, people easily come to the conclusion that HEAs should contain many intermetallic compounds and become too brittle to be of use. However, the experimental evidences have revealed that a high entropy enhances the formation of more ductile solid-solution phases and thus simplifies the microstructure [2]. Figure 1 shows XRD analyses of as-cast AlxCoCrCuFeNi alloys with different Al contents (x values). The major phases are FCC and BCC. As x exceeds 0.8, the increasing aluminum content increases the volume fraction of BCC and ordered BCC (B2) phases, and spinodal decomposition of the BCC or B2 phase occurs around 600°C [2, 8].

For another example, Fig. 2 shows the x-ray diffraction patterns of binary to 7-element equimolar alloys demonstrating the simplicity of the microstructure. These six alloys were prepared by arc melting and adding each time one element in the sequence of Cu-Ni-Al-Co-Cr-Fe-Si. It is noticed that their constituent phases do not become numerous as the number of elements increases. 6-element equimolar alloy, AlCoCrCuFeNi, is chosen to illustrate the phase simplicity. Table 1 lists the 44 possible binary intermediate phases found in the corresponding binary phase diagrams, most of them being intermetallic compounds [12]. Obviously, there are more, multicomponent phases formed by the elements in this alloy [12]. However, all such phases are actually inhibited and the

BCC and FCC phases are the main phases found by XRD. Interestingly, the number of occurring phases is significantly lower than the maximum number of phases predicted by the Gibbs phase rule.

2030405060708090100110

01000200030004000500060007000o o o o o ???x = 2.0

x = 2.3

x = 3.0

x = 0

x = 0.3

x = 0.5

x = 0.8

x = 1.0

x = 1.3

x = 1.5

x = 1.8

x = 2.5

?(220)

Ordered

(100)(111)(311)

(220)(211)(200)(200)(110)

(111)x = 2.8

I n t e n s i

t y (c p s )2θ (degrees)

Figure 1. XRD analyses of as-cast AlxCoCrCuFeNi alloys with different x values: , XRD peaks of FCC phase; O, XRD peaks of BCC phase (order peak indicated) [2].

Figure 2. XRD patterns of 2- to 7-element equimolar high-entropy alloys.

To emphasize the simple solid-solution phases formed, several compositions (>5 principal elements) and processes including conventional casting, sputtering and splat-quenching were used to produce the single-phase microstructure, as revealed by the XRD patterns (a), (b), (c), (d), and (f) shown in Fig. 3. This indicates that multi-principal elements could form solid solutions with a simple FCC or BCC crystal structure [6]. That is, all different atoms are regarded as solutes and expected to randomly distribute (or, at most, exhibit short-range ordering) in the crystal lattices 20 40 60

80 100

Cu-Ni-Al-Co-Cr-Fe-Si

Cu-Ni-Al-Co-Cr-Fe

Cu-Ni-Al-Co-Cr Cu-Ni-Al-Co Cu-Ni-Al

Cu-Ni

n t e

n s

i t

(

.u .)

2θ (degrees)

under the occupancy probability of a statistical average without a “matrix or host” element being that the conventional crystal structure can be extended for alloys with multi-principal elements, as shown in Fig. 4.

Table 1 Possible binary intermediate phases in the AlCoCrCuFeNi equimolar alloy [12]. Co Ni Cr Al Fe

Cu ε’ ___ ___ θ, η1, η2, ζ1, ζ2 , ε1, ε2, δ, γ,β, Al 3Cu 2 ___

Co ___ σ、α、

ε CoAl, Co 2Al 5, CoAl 3, Co 4Al 13, Co 2Al 9 ___

Ni Ni 2Cr, σ Ni 3Al, Ni 5Al 3, NiAl, Ni 2Al 3, NiAl 3 Ni 3Fe, NiFe,

NiFe 3

Cr Cr 2Al, Cr 5Al 8, Cr 4Al 9

, CrAl 4, Cr 2Al 11,

CrAl 7 σ

Al Al 3Fe, Al 5Fe 2,

Al 2Fe, AlFe,

AlFe 3, Al 2Fe 9,

Al 6Fe

2030405060708090

100

25002502502502502500

I n t e n s i t y (c p s )

2θ (degrees)

Figure 3. XRD patterns of (a) as-cast CoCrCuFeNi bulk alloy, (b) as-cast Al 0.5CoCrCuFeNi bulk alloy, (c) as-cast AlCoCrCu 0.5Ni bulk alloy, (d) as-sputtered AlCrCoCu 0.5Ni film, (e) as-cast AlCoCrCuFeNiTiV alloy, and (f) as-splat-quenched AlCoCrCuFeNiTiV foil (α:BCC,β: FCC) [6].

Thermodynamics

The principal factor which makes the constituent phases of most high-entropy alloys become simple is the entropy of mixing [2]. The explanation is based on the Second Law of Thermodynamics for a system in an isothermal and isobaric process: equilibrium is attained when the Gibbs free energy, G, reaches its minimum value. Considering the case in which an alloy forms

Figure 4. Extended crystal structure, (a) BCC, (b) FCC [6].

from its constituent elements under isobaric condition, the free energy change of mixing from the elemental state to the alloyed state can be expressed as ΔG mix = ΔH mix － T ΔS mix . Thus, the equilibrium state is the state of the lowest free energy. Following Boltzmann’s relation between entropy and system complexity, ΔS mix per mole for the formation of a disordered n-element equimolar solid solution, can be derived to be Rln(n) where R is the gas constant. Figure 5 shows the increase in the entropy of mixing with the number of elements for equimolar alloys. It illustrates that the entropy of mixing for solution phases increases from the small values for conventional alloys to large values of high-entropy alloys even for nonequimolar compositions. Since the solid solution state with multi-principal elements has a large ΔS mix , whereas the intermetallic compound state has a small ΔS mix , the solid solution state tends to be more stable than the compound state, especially at elevated temperatures, because of its effect in lowering the free energy of mixing. On the other hand, the enthalpy of mixing, ΔH mix , of multielement random solid

Figure 5. Change in the entropy of mixing as a function of the number of elements in equimolar

solutions is not substantially lower compared to that of the ordered intermetallic compounds, since the former have abundant pairs of unlike atoms, as does the latter. This makes the entropy of mixing very competitive in lowering the free energies of mixing of random solid solutions, except for the cases that stronger (covalent, ionic) bonds prevail among some constituent elements (e.g. Al reacting with O), which would induce strong compound phases such as oxides, borides, carbides, nitrides, and silicides dispersed in the alloy.

Based on the effect of the entropy of mixing, we propose to divide the alloy world into three fields, as shown in Fig 6. Low-entropy alloys are traditional alloys. High-entropy alloys are the alloys with at least five major elements. And medium-entropy alloys are the alloys with 2-4 major elements. The high entropy effect of promoting the occurrence of the disordered solution phase is essentially found in the high-entropy alloy field and should be less prevalent in the medium-entropy alloys. The stabilization of the simple solid solution phase is very significant in terms of the microstructure and properties which can be obtained in these materials.

Figure 6. The alloy world divided by the entropy of mixing.

Tendency to Form Nanostructures and Amorphous Phases

In addition to the high entropy effect enhancing the formation of simple solution phases, multi-principal elements also cause the tendency to form nanophases. Figure 7 shows the microstructures of as-cast, equimolar AlCoCrCuFeNi alloy [8]. By using TEM bright field and dark field images, numerous nanoprecipitates are seen within the matrix. In fact, it is a common observation to find nanophases in the matrix of high-entropy cast alloys. This is hardly seen in the as-cast microstructure of conventional alloys.

Figure 7. TEM microstructures of as-cast AlCoCrCuFeNi alloy, (a) bright field, (b) dark field [8].

(a) (b)

The effect of kinetics is also explained. There are two ways for diffusional transformations to occur in the matrix: (1) spinodal decomposition and (2) precipitation. In the transformation process, the formation of a new phase requires cooperative diffusion of many different kinds of atoms to accomplish the partitioning of composition. In high-entropy alloys, the movement of many kinds of substitutional solute atoms for such a partition is expected to be so sluggish that phase separation proceeds slowly resulting in nanophases [8]. This is related to the “confusion principle” involved in multielement alloys [13].

Figure 8 shows the phase formation sequence during cooling of an alloy of the Al x CoCrCuFeNi system with different aluminum contents [8]. For low aluminum content, the liquid phase solidifies forming dendritic and interdendritic solid solution phases, followed by (nano)precipitation. For high aluminum content, the liquid phase solidifies and forms again dendrititic and interdendritic solid solution phases, followed by spinodal decomposition which yields submicron modulated structures and, subsequently, nanoprecipitates.

Figure 8. Depiction of phase formation sequence during cooling of Al x CoCrCuFeNi alloys with different aluminum contents [8].

As an example in film coating, a series of alloy targets were produced from binary alloy Cu 0.5Ni to 7-element alloys Cu 0.5NiAlCoCrFeTi by increasing one element in sequence. And then, they were sputtered to deposit their corresponding films on Si substrate to see the microstructural evolution from traditional alloys with 2-4 elements to HEAs with 5-7 elements. Figure 9 shows the XRD analysis revealing that as the number of element increases, the peaks not only become lower in intensity but also become broader. This means the film structure becomes finer with smaller nanograins and eventually amorphous with increased number of elements. This also reflects the enhancement of glass formability with increased number of elements, since the atomic size difference among the seven elements is medium and changes slightly from 3-element to 7-element

alloys.

20406080100

5001000

15002000250030003500

4000450050005500amorphous

BCC FCC

ordered BCC amorphous FCC Cu 0.5NiAlCoCrFeTi

Cu 0.5NiAlCoCrFe

0.5NiAlCoCr Cu 0.5NiAlCo

Cu 0.5NiAl

Cu 0.5Ni

n t e n s i t y (a r b . u n i t )2￡K (degrees)

Conclusions

High-entropy alloys are countless in number, in contrast to only about 30 commonly used traditional alloys. They might be processed and analyzed like conventional metals. High entropy enhances the formation of solution phases and multi-principal elements cause sluggish diffusion, and the resulting microstructures frequently consist of nanophases and even amorphous phases. Because of their special microstructure and properties, many opportunities in creating novel alloys, better than traditional ones in a wide range of applications, are waiting for exploitation. Acknowledgements

The authors gratefully acknowledge the financial support for this research from the National Science Council of Taiwan under grants NSC-93-2120-M-007-006 and the ministry of Economic Affairs of Taiwan under grant 93-EC-17-A-08-S1-0003.

References

[1] S. Ranganathan: Current Science Vol. 8 (2003), p. 1404.

[2] J. W. Yeh, S. K. Chen, S. J. Lin, J. Y. Gan, T. S. Chin, T. T. Shun, C. H. Tsai and S. Y Chang: Adv. Eng. Mater. Vol. 6 (2004), p. 299.

[3] A. Inoue: Acta Mater. Vol. 48 (2000), p. 279.

[4] P. K. Huang, J. W. Yeh, T. T. Shun and S. K. Chen: Adv. Eng. Mater. Vol. 6 (2004), p. 74.

[5] C. Y. Hsu, J. W. Yeh, S. K. Chen and T. T. Shun: Metall. Mater. Trans. A Vol. 35A (2004), p. 1465.

[6] J.-W. Yeh, S.-K. Chen, J.-Y. Gan, S.-J. Lin, T.-S. Chin, T.-T. Shun, C.-H. Tsau and S.-Y. Chang: Metall. Mater. Trans. A Vol. 35A (2004), p. 2533.

Figure 9. XRD patterns of sputtered films for the alloy series from Cu 0.5Ni to Cu 0.5NiAlCoCrFeTi.

[7] T. K. Chen, T. T. Shun, J. W. Yeh and M. S. Wong: Surf. Coatings Technol,. Vol. 188-189 (2004), p. 193.

[8] C.-J. Tong, S.-K. Chen, J.-W. Yeh, T.-T. Shun, C.-H. Tsau, S.-J. Lin and S.-Y. Chang: Metall. Mater. Trans. A Vol. 36A (2005), p. 881.

[9] C.-Ji Tong, M.-R. Chen, S.-K. Chen, J.-W. Yeh, T.-T. Shun, S.-J. Lin and S.-Y. Chang: Metall. Mater. Trans. A 36A (2005), p. 1263.

[10] Y. Y. Chen, T, Duval, U. D. Hung, J. W. Yeh and H. C. Shih: Corrosion Sci. Vol. 47 (2005), p. 2257.

[11] Y.Y. Chen, U.T. Hong, H.C. Shih, J.W. Yeh and T. Duval: Corrosion Sci. Vol. 47 (2005), p. 2679.

[12] H. Baker: ASM Handbook, Alloy Phase Diagrams, Vol. 3 (ASM International, USA 1992).

[13] A. Lindsay Greer: Nature Vol. 366 (1993), p. 303.

Advanced Structural Materials III

doi:10.4028/https://www.360docs.net/doc/6e18280304.html,/MSF.560

High-Entropy Alloys – A New Era of Exploitation

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