PSFC/JA-20-33

Combinatorial development of the low-density high-entropy alloy Al10Cr20Mo20Nb20Ti20Zr10 having gigapascal strength at 1000 °C

Owais Ahmed Waseema1,2, Ho JinRyu1

1 Department of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and Technology, 291 Daehakro, Yuseong-gu, Daejeon, 34141, Republic of Korea 2 Plasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge, MA, 02139, USA

March 2020

Plasma Science and Fusion Center Massachusetts Institute of Technology

Cambridge MA 02139 USA

This study is supported by NRF grant of MSIT (NRF-2015R1A2A2A01002436, NRF- 2020R1A5A6107701, NRF-2017K1A3A7A09016308), Republic of Korea, and by the Asian Office of Aerospace Research and Development (AOARD) through a grant FA2386-19-1-4009.

Submitted to Journal of Alloys and Compounds

1

Combinatorial development of the low-density high-entropy alloy

Al10Cr20Mo20Nb20Ti20Zr10 having gigapascal strength at 1000 ℃

Owais Ahmed Waseema, b, Ho Jin Ryua*

aDepartment of Nuclear and Quantum Engineering, Korea Advanced Institute of Science and

Technology, 291 Daehakro, Yuseong-gu, Daejeon 34141, Republic of Korea

bPlasma Science and Fusion Center, Massachusetts Institute of Technology, Cambridge,

Massachusetts 02139

*Corresponding Author: Tel.: +82–42–350–3812, Fax: +82–42–350–3810, E-mail address:

hojinryu@kaist.ac.kr (Ho Jin Ryu)

2

Abstract

A pseudo-ternary combinatorial approach to AlxTayVzCr20Mo20Nb20Ti20Zr10 revealed the

composition of refractory high-entropy alloys characterized by outstanding high-temperature

yield strength. Compression testing of Al10Cr20Mo20Nb20Ti20Zr10 disclosed yield strengths of

1206 MPa at 1000 ℃, one of the highest values reported for refractory high-entropy alloys. Ta-

containing AlxTayVzCr20Mo20Nb20Ti20Zr10 presented a lower high-temperature strength, while

characterization of Al10Cr20Mo20Nb20Ti20Zr10 showed C14 Al2Zr- and NbCr2-type hexagonal

Laves intermetallics, with a hardness of ~10.5 GPa (higher than that of the body centered cubic

phase, at ~9 GPa). The stronger bonds between Al and transition metals appear to give rise to

extraordinary load-bearing capabilities in Al10Cr20Mo20Nb20Ti20Zr10, at high temperatures.

Owing to this rare combination of relatively low density (6.96 g/cm3) and remarkable high-

temperature strength, Al10Cr20Mo20Nb20Ti20Zr10 has emerged as a potential material for high-

temperature structural applications.

Keywords

High-entropy alloy, refractory metals, combinatorial metallurgy, mechanical properties

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1. Introduction

Refractory high-entropy alloys (HEAs, having 5–13 elements in 5–35 at.%) [1–3] are being

explored for possible applications in the aerospace, automobile, and power industries. Several

refractory HEAs with promising heat-resistant properties, including high strength levels (e.g.,

MoNbTaW (405 MPa at 1600 ℃) and MoNbTaVW (477 MPa at 1600 ℃) [4]), and oxidation

resistance (e.g., 0.5 mg/cm2 weight gain in AlCrMoNbTi after oxidation, for one h at 900 ℃, in

air [5,6]) have been reported.

Exchanging constituents and varying element concentrations significantly affect HEA properties

[7,8]. The replacement of Ta in HfNbTaTiZr, with Mo, to develop HfMoNbTiZr, improved its

room-temperature compressive strength from 1250 MPa to 1719 MPa [9], while adding Al to

AlxNbTaTiV led to a yield strength improvement from 1092 MPa (x = 0) to 1330 MPa (x = 0.25)

[10]. Composition modifications like these have led to the development of AlMo0.5NbTa0.5TiZr,

which has shown an impressive combination of high-temperature mechanical properties, that is,

a compressive yield strength of 745 MPa, and a fracture strain of > 50% (at 1000 ℃) [11].

Although several HEAs with promising properties have been reported, the achievement of

desirable properties remains somewhat unpredictable [12]. Studying combinatorial HEA libraries

containing ranges of elements—with the aim of discovering promising compositions with

improved properties for further development and commercialization—is therefore useful. In our

previous study, we undertook the combinatorial development of the novel HEAs

AlxCryMozNbTiZr (a, b, and c: 10–30 at.%), which revealed relatively good oxidation resistance,

as Al20Cr10Mo10Nb20Ti20Zr20 and Al30Cr10Nb20Ti20Zr20 (21 mg/cm2 and 20 mg/cm2 weight gains,

after 20 h of oxidation at 1000 ℃, respectively) [13], when compared to several other refractory

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HEAs and conventional alloys. We focused on Al20Cr10Mo10Nb20Ti20Zr20, as the presence of Mo

in this HEA suggested it would exhibit good strength at high temperatures.

In this study, we locked in the base composition as Cr20Mo20Nb20Ti20Zr10, and decided to vary

the Al content, from 0 to 10 at.%, to achieve a balance between oxidation resistance and strength,

with a gradual increase/decrease of Ta and V, at the cost of 10 at.% Zr (so that oxidation

resistance could be improved [14]), considering their renown in HEA strengthening [4,15]. In

this way, we designed a pseudo-ternary combinatorial system of HEA, that is,

AlxTayVzCr20Mo20Nb20Ti20Zr10 (AlxTayVz-Q, where x + y + z = 10 at.%, and Q is the quinary

Cr20Mo20Nb20Ti20Zr10).

2. Experimental Procedures

Arc melting of 99.9% metal sources to develop AlxTayVz-Q HEA samples (Table I) was carried

out using an ACM-01 arc melting furnace (DAIA-VACUUM, Japan). Sample remelting (five

times) and further homogenization (at 1200 ℃ for 24 h, followed by air cooling) were carried out

to improve their chemical homogeneity. Microstructural examinations were carried out by X-ray

diffraction (XRD, D/MAX-2500, Rigaku, USA), scanning electron microscopy (SEM, FEI

Magellan 400, USA), and energy dispersive spectroscopy (EDS, coupled with SEM). The

contribution of each phase to the respective alloy’s mechanical properties was analyzed by

taking nano-indentation hardness measurements of each phase, using an iNano nano-indentor

(Nanomechanics, Inc., USA). Cylindrical samples, with dimensions of 6 mm × 3 mm and 8 mm

× 3 mm (length × diameter), were subjected to both room-temperature and high-temperature

(1000 ℃) compression tests, at a strain rate of 10-4 /s, using an Instron 5982 device.

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TABLE I HEA sample names, compositions, and theoretical densities

Sample

Name HEA Compositions (at.%)

Density (g/cm3)

Theoretical Measured

Al x

Ta y

Vz-

Q

(Q =

Cr 2

0M

o20N

b20T

i 20Z

r 10)

V10-Q V10Cr20Mo20Nb20Ti20Zr10 7.32 7.31

Al10-Q Al10Cr20Mo20Nb20Ti20Zr10 6.96 6.93

Ta10-Q Ta10Cr20Mo20Nb20Ti20Zr10 8.42 8.40

AlxVz-Q

(y = 0)

Al2.5V7.5-Q Al2.5V7.5Cr20Mo20Nb20Ti20Zr10 7.22 7.19

Al5.0V5.0-Q Al5.0V5.0Cr20Mo20Nb20Ti20Zr10 7.13 7.12

Al7.5V2.5-Q Al7.5V2.5Cr20Mo20Nb20Ti20Zr10 7.05 7.01

AlxTay-Q

(z = 0)

Al7.5Ta2.5-Q Al7.5Ta2.5Cr20Mo20Nb20Ti20Zr10 7.32 7.29

Al5.0Ta5.0-Q Al5.0Ta5.0Cr20Mo20Nb20Ti20Zr10 7.69 7.68

Al2.5Ta7.5-Q Al2.5Ta7.5Cr20Mo20Nb20Ti20Zr10 8.06 8.06

TayVz-Q

(x = 0)

Ta7.5V2.5-Q Ta7.5V2.5Cr20Mo20Nb20Ti20Zr10 8.15 8.05

Ta5.0V5.0-Q Ta5.0V5.0Cr20Mo20Nb20Ti20Zr10 7.88 7.75

Ta2.5V7.5-Q Ta2.5V7.5Cr20Mo20Nb20Ti20Zr10 7.60 7.53

AlxTayVz-

Q

Al2.5Ta5.0V2.5-Q Al2.5Ta5.0V2.5Cr20Mo20Nb20Ti20Zr10 7.78 7.77

Al5.0Ta2.5V2.5-Q Al5.0Ta2.5V2.5Cr20Mo20Nb20Ti20Zr10 7.42 7.35

Al2.5Ta2.5V5.0-Q Al2.5Ta2.5V5.0Cr20Mo20Nb20Ti20Zr10 7.51 7.44

3. Results and discussion

The pseudo-ternary combinatorial library of post-homogenization microstructures, along with

three representative X-ray diffraction analyses, are shown in Fig. 1. The SEM micrographs show

a granular microstructure, with secondary phases in the intergranular region. Refractory HEAs

typically show various types of hexagonal (C14) and cubic (C15) Laves intermetallics [13]. XRD

analysis of homogenized AlxTayVz-X revealed a major body-centered cubic (BCC) phase, along

with secondary phases. The representative XRD patterns for homogenized AlxTayVz-Q are

shown in Fig. 1 (b)–(d).

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Fig. 1. (a) A pseudo-ternary combinatorial library of the microstructures of homogenized

AlxTayVzCr20Mo20Nb20Ti20Zr10; XRD analysis spectra for (b) Al10-Q, (c) Ta10-Q, and (d) V10-Q

Al10-Q, Ta10-Q, and V10-Q mainly show BCC phases, with 0.303, 0.303, and 0.319 nm lattice

parameters, respectively, (Table II). Al2Zr- and NbCr2-type C14 Laves were identified in Al10-Q,

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while Cr2Zr-type C14 and Cr2Ta-type C15 Laves were observed in Ta10-Q, with V10-Q showing

(CrV)Zr-type C15 Laves. The volume% of the Laves phases in Al10-Q, Ta10-Q, and V10-Q, as

determined by image analysis, were 30 vol.%, 35 vol.%, and 22 vol.%, respectively. Laves phase

formation was observed in HEAs using Allen electronegativity differences (ΔXAllen), atomic size

differences (δr), and d-orbital energy levels (Md) exceeding 7%, 5%, and 0.915 eV, respectively

[16,17]. The occurrence of the Laves phase in HEAs is estimated by comparing ΔXAllen, δr, and

Md with these criteria, so using these methods, we calculated ΔXAllen, δr, and Md for AlxTayVz-Q,

and found them to be in the ranges of 7.38–7.74%, 6.35–6.62%, and 1.865–1.933 eV. These

values suggested the formation of Laves phases in AlxTayVz-Q.

TABLE II Analysis of solid solution and Laves phases in AlxTayVz-Q

HEA Phase

Structure,

Strukturbericht Designation,

Space Group

and JCPDS

Lattice Parameter

(nm)

Volume%

Atomic

Size Difference

(%)

Allen

Electronegativity Difference

(ΔXAllen, %)

d-orbital

energy level

(Md, eV)

Al10-Q

Solid solution BCC a = b = c = 0.303 70

6.35 7.66 1.900 Laves

intermetallics

Al2Zr, C14 hexagonal, p63/mmc (194)

[PDF#48-1384]

a = b = 0.5281,

c = 0.8743

30

NbCr2, C14, hexagonal, p63/mmc (194)

[PDF#47-1638]

a = b = 0.4976,

c = 0.8059

Ta10-Q

Solid solution BCC a = b = c = 0.323 65

6.38 7.74 1.933 Laves

intermetallics

Cr2Zr, C14, hexagonal,

p63/mmc (194)

[PDF#06-0613]

a = b = 0.5089,

c = 0.8279

35

Cr2Ta, C15, cubic,

Fd3m (227) [PDF#20-

0317]

a = b = c = 1.145

V10-Q

Solid solution BCC a = b = c = 0.319 78

6.62 7.38 1.865 Laves

intermetallics

(CrV)Zr, C15, cubic,

Fd3m (227) [01-071-

7632]

a = b = c = 0.733 22

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Fig. 2. EDS mapping ((a), (d), (g)), point EDS analyses ((b), (e), (h)), and nano-indentation

hardness results, for the solid solution and Laves intermetallic phases ((c), (f), (i)) of V10-Q,

Ta10-Q, and Al10-Q, respectively

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EDS analysis results for the Laves phases observed in V10-Q, Al10-Q, and Ta10-Q are shown in

Fig. 2 (a), (d), and (g), respectively. In addition to the main constituents of Laves phases

identified by XRD, notable concentrations of other elements were also present in the Laves

intermetallics, suggesting the possible formation of high-entropy intermetallics, which could be a

promising area for future research [18].

The literature shows higher high-temperature mechanical properties for Laves phase

intermetallics than for BCC solid solutions [19]. The Laves phases maintain their strength at

elevated temperatures and enhance high-temperature strength in Laves-containing HEAs [20,21].

In order to confirm the role of the continuous network of Laves phase intermetallics, with regard

to the increased strength of Al10-Q, we carried out nano-indentation hardness tests on the

dendritic (BCC) and interdendritic (Laves intermetallics) regions of V10-Q, Al10-Q, and Ta10-Q.

These tests were repeated five times at different points in each region, and the average results are

shown in Fig. 2 (c), (f), and (i). High scattering in the nano-indentation hardness data was

observed near the surface, and the hardness decreased with increasing indentation depth. The

nano-indentation hardness data were processed using the Nix-Gao model, to extract the hardness

of the sample. The Nix-Gao model, expressed as shown in Eq. (i) showed a gradual decrease in

the measured hardness (H) up to the bulk hardness (H0), with increased indentation:

𝐻 = 𝐻0(1 +𝑑∗

𝑑)0.5 ----------------- (i)

The hardness at a critical depth (d*) is regarded as the representative or bulk-equivalent hardness

of the samples, with values listed in Table III.

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Table III. Estimated Vickers hardness of Al10-Q, V10-Q and Ta10-Q

HEA Phase

Bulk-equivalent hardness

(H, as GPa)

V10-Q

BCC 8.0

Laves 11.9

Al10-Q

BCC 9.0

Laves 10.5

Ta10-Q

BCC 8.4

Laves 11.0

Although nano-indentation data can have errors—because the stress field generated during

indentation is not limited to a single phase—it can be used for comparative analysis. The

comparison of the hardness values of the BCC and Laves phases clearly showed significantly

higher hardnesses for the Laves phases than those for BCC phases, which demonstrated the

significant contribution of the Laves phases to the mechanical behavior of AlxTayVz-Q.

This behavior, under a compressive load, is shown in Fig. 3. The interchange between 10 at.% V,

Al, and Ta did not impart any significant alteration of the room-temperature compressive yield

strength (Fig. 3 (a)), as V10-Q, Al10-Q, and Ta10-Q showed similar strengths—that is, 1572, 1692,

and 1626 MPa, respectively. It should be pointed out that the newly developed RHEAs show

brittleness at room temperature with the maximum compressive strain around 5%. However, as

there are some promising examples of ductile RHEAs showing tensile ductility at room

temperatures such as HfNbTiZr and HfNbTaTiZr [22][23][24], a further study for the

ductilization of the strong RHEA compositions should be necessary for their industrial

applications in the future.

The ductilization of the promising composition can be achieved by microstructural engineering.

There have been remarkable achievements for the ductilization of RHEAs to overcome the

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conflict between high-temperature strength and room-temperature ductility. One such example

has been reported by Soni et al. [25]. The change in strength due to phase transformation is

inevitable, therefore the HEAs-based composites are also being exploited as reported in Mileiko

et al. [26]. Waseem et al. were able to achieve significant enhancement in the toughness of

W0.5TaTiVCr without sacrificing strength [27], which suggests that the extrinsic toughening can

potentially play an important role in the development of novel RHEAs with a good combination

of high-temperature strength and room-temperature ductility.

Fig. 3. Compressive stress–strain curves obtained at (a) room temperature, and (b) 1000 ℃

High-temperature strength is one of the most important characteristics of refractory HEAs;

therefore, compression tests for AlxTayVz-Q were also carried out at 1000 ℃, to assess the role of

Al, Ta, and V in imparting high strength in AlxTayVz-Q.

The results showed that Al10-Q exhibited a strength of ~ 1200 MPa, at 1000 ℃ (Fig. 3 (b)), while,

in contrast, V10-Q and Ta10-Q achieved values of ~ 830 MPa and ~ 500 MPa, respectively, under

the same conditions. Published HEA literature includes reports of improved high-temperature

strength due to the addition of Ta; for instance, HfMoNbTaTiZr and HfMoNbTiZr exhibited 814

MPa and 600/721 MPa strengths, respectively, at 1000 ℃ [22,23,24], although our study has

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revealed rather different Ta behavior—that is, reduction in high temperature strength due to Ta

addition.

Fig. 4. Comparison of the high-temperature (1000 ℃) yield strengths and densities in refractory

HEAs

In order to compare the high-temperature yield strength and density of various HEAs, we

extracted data from the literature, as shown in Fig. 4 [4,6,15–31]. We divided these HEAs into

several groups, based on the degree of similarity between their compositions and ours. For

example, the group/region marked as AlCrNbTiV (Zr) represents the high-temperature

compressive strengths and densities of the HEAs AlCrNbTiV, AlCr0.5NbTiV, AlCrNbTiV,

AlCr1.5NbTiV, Al0.5CrNbTi2V0.5, Al0.25CrNbTiVZr, Al0.5CrNbTiVZr, and AlCrNbTiVZr, in

which Al, Cr, Nb, Ti, and V were present in every alloy, whereas Zr may or may not have been

present. The actual HEA compositions are also shown in the legend. The strength–density

regions of the various HEAs (as shown in Fig. 4) show the extent to which the strength and

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density of certain HEAs can be varied by adding, removing, and/or changing the concentration(s)

of the elements shown in parentheses. Reported refractory HEAs with density levels < 7 g/cm3

showed relatively low high-temperature yield strength, which would hinder their high-

temperature structural applications. In contrast, some stronger HEAs showed high density levels,

and therefore would not suit the application to the automobile or aerospace industries, where

light alloys are needed.

The strength–density region of the AlxTayVz-Q HEA system can be seen to have covered a wide

range of high-strength values, while maintaining relatively low density, indicating that AlxTayVz-

Q provided the opportunity to design numerous new HEAs characterized by desirable

combinations of high-temperature strength and lower density levels. These data also highlighted

the outstanding high-temperature compressive strength of Al10-Q HEAs (higher than all other

refractory HEAs known thus far).

The mismatch between the atomic sizes of Al and the other Al10-Q constituents results in lattice

distortion—and so, consequently, the mechanical strength of Al10-Q increases. However, the

atomic size difference was relatively higher in the case of V10-Q (Table II), so that the mismatch

induced in Al10-Q by the atomic size of Al cannot have been the only reason for the promising

strength of Al10-Q.

In order to reveal the mechanism behind the strengthening brought about by the addition of Al in

refractory HEAs that incorporate transition metals and Al, Qui et al. performed density

functional theory (DFT) calculations [44]; these calculations revealed the hybridization of p and

s states from Al atoms, with d states from transition metals, which resulted in the formation of

strong, directionally angular bonds between the Al atoms and their neighbors in transition metals.

Such strong bonds were not detected between the constituents of Al-free refractory HEAs [44],

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supporting the fact that the addition of V and Ta in Cr20Mo20Nb20Ti20Zr10—to form V10-Q and

Ta10-Q, respectively—did not result in a remarkably strong HEA. In addition to the hybridization

of the p and s states from Al atoms, the presence of Mo in AlxTayVz-Q also facilitated high

temperature strength, while Mo-free RHEAs, such as AlNbTiV(Zr) and/or AlCrNbTiV(Zr)

(Figure 4) showed remarkably low strength, at 1000 ℃.

We have carried out combinatorial synthesis and analysis of a refractory HEA (AlxTayVz-Q), and

explored a novel HEA, Al10Cr20Mo20Nb20Ti20Zr10, which exhibited gigapascal strength and low

density, at 1000 ℃, and showed that it possessed characteristics considered very desirable for the

application to the aerospace, automobile, and power industries. To understand this potential

better, additional research into high-temperature oxidation resistance is needed

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4. Conclusions

In summary, combinatorial synthesis and analysis of AlxTayVzCr20Mo20Nb20Ti20Zr10 (x, y, and

z: 0–10 at.%) high-entropy alloys were carried out. Arc-melted samples were homogenized at

1200 ℃ for 24 h and air cooled. Considering the potential load-bearing applications of HEAs,

quasi-static compression tests were conducted, at room temperature and 1000 ℃, and analysis of

the compressive stress–strain curves revealed outstanding high-temperature strength, for

Al10Cr20Mo20Nb20Ti20Zr10 (Al10-Q) (1206 MPa), along with low density (6.96 g/cm3). Thorough

characterization of the Al10-Q microstructure, using XRD and EDS analyses, revealed Al2Zr- and

NbCr2-type hexagonal Laves. The BCC phase revealed a bulk equivalent hardness of 9 GPa,

while for Laves intermetallics, the result was 10.5 GPa. These promising mechanical

characteristics suggest that this low-density, ultra-strong HEA (Al10-Q) has very good potential

for future, high-temperature applications.

Declaration of Interest

None.

Author Contributions

Both authors contributed to the manuscript preparation. Owais Ahmed Waseem performed the

experiments and analyzed results under the direct supervision of Ho Jin Ryu. Both authors

reviewed the manuscript.

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Acknowledgments

This study is supported by NRF grant of MSIT (NRF-2015R1A2A2A01002436, NRF-

2020R1A5A6107701, NRF-2017K1A3A7A09016308), Republic of Korea, and by the Asian

Office of Aerospace Research and Development (AOARD) through a grant FA2386-19-1-4009.

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