Designing high entropy alloy-ceramic eutectic composites of MoNbRe0.5TaW(TiC)x with high compressive strength

Journal Pre-proof Designing high entropy alloy-ceramic eutectic composites of MoNbRe0.5TaW(TiC)x with high compressive strength Qinqin Wei, Guoqiang Luo, Jian Zhang, Shijing Jiang, Pingan Chen, Qiang Shen, Lianmeng Zhang PII:

S0925-8388(19)34092-7

DOI:

https://doi.org/10.1016/j.jallcom.2019.152846

Reference:

JALCOM 152846

To appear in:

Journal of Alloys and Compounds

Received Date: 22 August 2019 Revised Date:

16 October 2019

Accepted Date: 28 October 2019

Please cite this article as: Q. Wei, G. Luo, J. Zhang, S. Jiang, P. Chen, Q. Shen, L. Zhang, Designing high entropy alloy-ceramic eutectic composites of MoNbRe0.5TaW(TiC)x with high compressive strength, Journal of Alloys and Compounds (2019), doi: https://doi.org/10.1016/j.jallcom.2019.152846. This is a PDF file of an article that has undergone enhancements after acceptance, such as the addition of a cover page and metadata, and formatting for readability, but it is not yet the definitive version of record. This version will undergo additional copyediting, typesetting and review before it is published in its final form, but we are providing this version to give early visibility of the article. Please note that, during the production process, errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain. © 2019 Published by Elsevier B.V.

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Designing high entropy alloy-ceramic eutectic composites of

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MoNbRe0.5TaW(TiC)x with high compressive strength Qinqin Wei a, Guoqiang Luo a, Jian Zhang a, Shijing Jiang a, Pingan Chen b, Qiang Shen a, ∗,

3 4

Lianmeng Zhang a

5

(a State Key Lab of Advanced Technology for Materials Synthesis and Processing, Wuhan University of

6

Technology, Wuhan 430070, China; b

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The State Key Lab of Refractories and Metallurgy, Wuhan University of Science and Technology, Wuhan

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430070, China)

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Abstract: Combining the advantages of composites and eutectic high entropy alloys, novel

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MoNbRe0.5TaW(TiC)x high entropy alloy-ceramic eutectic composites with high strength were

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successfully designed and investigated based on the computer assisted thermodynamic calculation.

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The MoNbRe0.5TaW(TiC)x composites are composed of body-centered cubic (BCC) solid solution

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and multi-component (MC) carbide with face-centered cubic (FCC) structure. With increasing TiC

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addition, microstructure exhibits an evolution from hypo-eutectic to eutectic and then to

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hyper-eutectic, which is explained by the solidification process analysis in calculated equilibrium

17

phase

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MoNbRe0.5TaW(TiC)1.0 fully eutectic composite has the highest compressive strength of 1943 ± 13

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MPa with high yield strength of 1496 ± 17 MPa. The strengthening mechanism is contributed by the

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synthetic effect of second-phase and boundary strengthening.

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Keywords: Composite materials; Computer simulations; Phase diagrams; Microstructure;

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Mechanical properties

diagram.

The

MoNbRe0.5TaW(TiC)x

composites

possess

high

strength.

The

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1． ．Introduction

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High entropy alloys (HEAs) become one of the research hotspots [1-2]. Due to their high

∗

Corresponding author. E-mail address: [email protected] (Q. Shen)

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strength and excellent softening resistance properties at elevated temperature, HEAs are regarded as

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attractive high-temperature structural materials [3-5]. Many HEAs with promising properties have

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been investigated [6-10], such as Co1.5CrFeNi1.5Ti HEA with high wear-resistant [6], MoNbTaW and

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MoNbTaWV HEAs with high-strength at elevated temperatures [8-9]. However, in the traditional

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single phase HEAs made by casting, the poor castability performance and component segregation are

31

commonly existed, which reduce their mechanical properties and affect engineering applications

32

[11-12]. Meanwhile, single-phased HEAs are known to have limited properties in general.

33

Metal-ceramic composites combine the excellent performance of metals and ceramics and have

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superior properties. A few studies on HEA-ceramic composites have been carried out to enhance the

35

mechanical properties [13-18]. For example, Wei et al. [13] synthesized a MoNbRe0.5W(TaC)0.5

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composite with addition of TaC, increasing the yield strength, compressive strength and ductility to

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1202 MPa, 2067 MPa and 10.25%, respectively. Zhou et al. [14] improved the compressive strength

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of ultrafine grained WC carbides to 4395 MPa by adding 10wt% AlCrFeCoNi. In addition, eutectic

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high entropy alloys (EHEAs) [12, 19-22], by adding appropriate elements to HEA to form in-situ

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eutectic structure with a solid-solution and a hard phase (such as intermetallic), have been developed

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to improve the properties of HEAs. The design of in-situ EHEAs makes the direct casting of good

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quality industrial scale HEA ingots possible [12, 19]. Meanwhile, the eutectic structures have many

43

other advantages such as low energy phase interface, fine controllable microstructures and good

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high-temperature creep resistance [23]. Combining the advantages of HEA-ceramic composites and

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EHEAs, we designed and fabricated novel HEA-ceramic eutectic composites with better mechanical

46

properties.

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In the recent past, mixing enthalpy, existing binary alloy systems and CALPHAD (CALculation

48

of PHAse Diagrams) methods were successfully used to predict EHEAs and locate the eutectic point

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[24-26], indicating that CALPHAD method is feasible in the design of eutectic materials. MoNbTaW

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HEA has excellent mechanical properties at elevated temperature. Re with high melting point can be

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fully solid soluble in the MoNbTaW system HEAs [13]. For TiC, eutectic microstructures of (Ti, M)

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solid solution and (Ti, M)C carbide are formed according to the Ti-C-M (M = Mo, Nb, Ta or W)

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ternary alloy phase diagrams [27-30]. All these indicate that HEA-ceramic eutectic microstructure

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would be formed in the composite with Mo, Nb, Re, Ta, W and TiC. Herein, in this work, the

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CALPHAD approach is utilized to design novel MoNbRe0.5TaW(TiC)x refractory HEA-ceramic

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eutectic composites. The composites with different TiC addition are prepared to verify the method.

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Meanwhile, the effect of TiC addition on the microstructure and mechanical properties of the as-cast

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composites is studied and discussed.

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2． ．Experimental procedures

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MoNbRe0.5TaW(TiC)x composite ingots were prepared by arc melting in high-purity argon

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atmosphere. The raw materials of Mo, Nb, Re, Ta, W and TiC with high-purity of 99.9 wt% were

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used. The ingots were re-melted for five times to ensure the chemical homogeneity and then

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drop-cast in a water-cooled copper mold. The dimensions of composite ingots were about 25 mm

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(diameter) × 8 mm (length).

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The crystal structure was identified by an X-ray diffraction (XRD, Bruker D8 Advance) with Cu

66

Kα radiation at a scanning rate of 4 °/min. The microstructure was carried out by scanning electron

67

microscope (SEM, Quanta FEG250) under back-scatter electron (BSE) mode and transmission

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electron microscope (TEM, Talos F200S). The chemical compositions were analyzed by electron

69

probe microanalyzer (EPMA, JOEL JXA-8230) and TEM with energy dispersive X-ray spectroscopy

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(EDS). The compressive property were tested on ∅ 2 mm × 4 mm samples by a testing machine

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(Instron 5966) with a strain rate of 2 × 10-3 s-1 at room temperature. Four samples were tested for

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each composite on the same conditions.

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3． ．Results

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3.1 CALPHAD modeling

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The design of new eutectic composite system was carried out using the CALPHAD approach,

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utilizing Factsage software [31] and SGTE database. Considering MoNbRe0.5TaW has no

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compositional fluctuation, (MoNbRe0.5TaW)-TiC may compose a pseudo binary composite. Fig. 1(a)

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shows the predicted pseudo binary equilibrium phase diagram of (MoNbRe0.5TaW)-TiC composites.

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A eutectic composition point of TiC-0.6 is predicted and the fully eutectic composite consists of BCC

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and FCC phases. When the TiC addition is below 0.6, the composite has a hypo-eutectic composition

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with BCC proeutectic phase, while FCC proeutectic phase would occur in the hyper-eutectic

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composite with TiC addition higher than 0.6. These two phases are stable in the high temperature

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range. With temperature decreases, a hexagonal close-packed (HCP) phase appears.

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Considering the fast cooling rate of melting, the non-equilibrium solidification simulation was

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carried out using the Equilib Module in Scheil-Gulliver cooling and the simulated result of phase

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constituent for the MoNbRe0.5TaW(TiC)0.6 eutectic composite is shown in Fig. 1(b). Only BCC and

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FCC phases are formed during the entire non-equilibrium solidification with no HCP phase.

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Meanwhile, element segregation tendency in each phase is also predicted, as seen from Fig. 2. The

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BCC phase contains six metal elements and a very small amount of C, while the FCC phase has lots

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of C, Ti, Ta, Nb and some Mo, W. By comparing the element composition in the two phases,

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elements Re, Mo and W are enriched in the BCC phase, while element C is rich in the FCC phase.

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Meanwhile, the compositions of Ti, Ta and Nb in the FCC phase are slightly higher than those in the

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BCC phase. Herein, the BCC phase is predicted to contain more W, Mo and Re, and the FCC phase

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contains more C, Ti, Ta and Nb.

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3.2 Microstructure and phase identification

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Based on the thermodynamic equilibrium calculation (as given in Fig. 1a), the

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MoNbRe0.5TaW(TiC)x composites (x = 0.2, 0.5, 0.8 and 1.5 in molar ratio, namely as I0.2, I0.5, I0.8

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and I1.5, respectively) were chosen to be prepared by arc melting and the actual microstructures

99

were investigated, whose TiC additions are near the hypo-eutectic, eutectic and hyper-eutectic

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composition point, respectively. Fig. 3 presents the actual microstructures of MoNbRe0.5TaW(TiC)x

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composites obtained by SEM-BSE. The composites are composed of typical proeutectic phase and

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eutectic microstructure. The I0.2, I0.5 and I0.8 are hypo-eutectic composites with white proeutectic

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phase. With the increase of TiC addition in the hypo-eutectic composites, the volume fraction of

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eutectic structures increases. With further increase of TiC addition, the I1.5 composite becomes a

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hyper-eutectic composite with black proeutectic phase. Thus, it can be inferred that the real eutectic

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point would be between x = 0.8 and 1.5. Then, the MoNbRe0.5TaW(TiC)1.0 composite (namely as

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I1.0) was synthesized and the microstructure is shown in Fig. 3(e). It has fully eutectic structure,

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indicating that the eutectic composition point of the MoNbRe0.5TaW(TiC)x composites stays at the

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point of TiC-1.0.

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Fig. 3(f) presents the XRD patterns of the MoNbRe0.5TaW(TiC)x composites. As it was

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identified by XRD analysis, the composites are composed of BCC and FCC phases. Meanwhile, the

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diffraction peaks of FCC phase match well with carbide TiC (with JCPDS card No. 65-0966) and are

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identified to be MC-type carbide. With increasing TiC addition, the peak intensity of the MC phase

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increases, illustrating that the content of MC phase increases accordingly. In accordance with the

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SEM results, the proeutectic phase in the hypo-eutectic composites is BCC phase and the one in the

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hyper-eutectic composite is MC phase. The chemical compositions of the BCC and MC phases

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obtained by EPMA are shown in Table 1. The BCC phase contains all six metal elements, while the

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MC phase contains element C and five metal elements except Re. Herein, the BCC phase can be

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identified as a solid solution of (Mo, Nb, Re, Ta, W, Ti), and the MC phase is a multi-component

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carbide solid solution of (Nb, Ta, Ti, Mo, W)C.

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Table 1 Chemical compositions of phases in the MoNbRe0.5TaW(TiC)x composites (at%) Composite

Phase

Mo

Nb

Re

Ta

W

Ti

C

I0.2

BCC

22.0±0.8

21.7±1.1

10.6±0.5

21.2±0.8

21.9±0.8

2.6±0.4

0

MC

7.0±3.3

41.9±1.3

0

23.2±1.5

4.8±1.4

15.7±1.9

7.4±1.4

BCC

21.5±0.2

18.2±0.4

11.2±0.3

19.6±0.2

23.3±0.8

6.3±0.5

0

MC

5.3±2.0

30.7±1.6

0

29.1±1.7

6.3±2.3

21.3±1.7

7.3±0.5

BCC

20.7±0.9

15.5±0.3

11.4±0.5

18.3±1.7

26.5±0.5

7.6±0.3

0

MC

6.4±1.3

27.7±1.6

0

25.4±2.8

5.2±0.5

27.1±1.8

8.2±1.7

BCC

20.5±1.2

14.1±0.9

12.1±0.8

16.1±0.3

28.5±0.7

8.6±0.4

0

MC

3.8±0.4

24.9±0.2

0

30.1±0.3

5.8±0.4

28.3±0.4

7.2±0.5

BCC

22.9±0.8

12.9±0.2

13.0±0.6

12.6±0.2

26.3±0.9

12.3±0.6

0

MC

4.0±0.8

22.2±1.3

0

25.5±2.5

6.0±1.2

34.5±3.0

7.8±0.8

I0.5

I0.8

I1.0

I1.5

123 124

TEM was carried out to further analyze the eutectic structure and phase composition. Fig. 4(a)

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presents a dark-field TEM image and corresponding element distribution maps of the I1.0 fully

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eutectic composite. The composite consists of BCC and MC phases. The BCC phase is rich in Mo,

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Re, W, whereas MC phase contains more Ti, Nb, Ta and C. This result is in good agreement with the

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ones obtained from the previous XRD and EPMA. High resolution TEM (HRTEM) image and EDS

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compositional profile of the inter-phase are shown in Fig. 4(b)-(c). An interesting nanostructure

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consisting of two fully coherent nanoscale regions is observed at the phase interface. The

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nanostructure is a diffuse coherent interface with all elements continuously changing from BCC to

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MC without abrupt interface (diffuse interface width ~ 8nm). Fig. 4(d)-(e) show the quantitative

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analyses of the two phases. The following orientation relation holds along the interface, i.e. (110)BCC

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// (200)MC, [001]BCC // [001]MC. Furthermore, the interplane distance d110 = 0225 nm for BCC phase

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and d020 = 0.218 nm for MC phase are determined, and the angle of orientation misfit between the

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close packed planes of the two phases is about 2.38°. Herein, the misfit value between BCC and MC

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phases can be calculated by δ =

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lattice misfit dislocations can be observed and the phase interface exhibits good bonding.

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3.3 Mechanical properties

0.225-0.218/cos(2.38) 0.218/cos(2.38)

≈ 0.0335 [32]. Due to the low lattice mismatch, few

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Fig. 5(a) presents the compressive engineering stress-strain curves of MoNbRe0.5TaW(TiC)x

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composites and the strengths are listed in Fig. 5(b). With TiC addition increased, the yield strength is

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improved accordingly and goes up to 1543 ± 19 MPa in the I1.5 composite. The compressive

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strength increases as the TiC addition increases from 0.2 to 1.0, and then decreases with further

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increase of TiC addition. The I1.0 fully eutectic composite reaches the maximum compressive

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strength of 1943 ± 13 MPa and has high yield strength of 1496 ± 17 MPa. Furthermore, we estimate

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the strength of these composites and compare them with refractory MoNbTaW(V) HEAs [8] and

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MoNbRe0.5W(TaC)x composites [13]. According to the comparison shown in Fig. 5(c), the

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MoNbRe0.5TaW(TiC)x eutectic composites obviously have higher yield strength.

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4． ．Discussion

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4.1 Phase formation and microstructure evolution

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By comparing the CALPHAD computed and experimental results, it is concluded that the

152

thermodynamics calculation can well predict phase composition and microstructure evolution of

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MoNbRe0.5TaW(TiC)x composites within a wide range of compositions. Nevertheless, the calculated

154

eutectic composition point (TiC-0.6) is lower than experimental result (TiC-1.0), which would be due

155

to the thermodynamic database. The SGTE database used in this study has lots of binary alloy

156

systems and few higher-order systems [31]. However, the base phase in MoNbRe0.5TaW(TiC)x

157

composites was MoNbRe0.5TaW-rich phase, which has no special database, resulting in the change of

158

thermodynamic parameters of the phases and the incorrect prediction of eutectic point. Meanwhile,

159

only BCC and MC phases form without phase transformation during the real solidification process,

160

which is consistent with the prediction of phase formation by non-equilibrium solidification. The

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high thermal stability of the phases may be due to the high entropy effect [33] along with the rapid

162

cooling of as-cast structure, thus inhibiting the phase transformation. Here, we clarify the difference

163

between thermodynamic predictions and experimental observation, and verify the reliability of

164

thermodynamic calculations, which provide guidance for the design and development of the

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HEA-ceramic eutectic composites.

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The real microstructure evolution of MoNbRe0.5TaW(TiC)x composites can be clearly explained

167

by solidification process analysis shown in Fig. 1(a). For the I0.2 to I0.8 hypo-eutectic composites,

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the proeutectic BCC phase first nucleates and grows during the solidification process. When the

169

liquidus temperature drops to the eutectic temperature, eutectic reaction starts. BCC and MC phases

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nucleate and grow cooperatively and arrange alternately to form eutectic structure. Thus, the final

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microstructures consist of proeutectic BCC phase and eutectic structure of BCC and MC phases. In

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the I1.0 fully eutectic composite, BCC and MC phases form simultaneously from the composite

173

melts. The microstructure is a binary eutectic crystallization, which requires synergistic nucleation

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and growth of BCC and MC phases. Meanwhile, eutectic can minimize the interface energy through

175

maximizing the area of the low-energy facets [34], which causes the above phase orientation

176

relationship. As for the I1.5 hyper-eutectic composite, the formation process of microstructure is

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similar to that of the I0.2 to I0.8 hypo-eutectic composites, but with MC phase as proeutectic phase.

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The final microstructure is proeutectic MC phase and eutectic structure of MC and BCC phases. The

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crystallization process can be well illustrated by the schematic diagram shown in Fig. 6.

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4.2 Relationship between microstructures on mechanical properties

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Fig. 7 shows the relationship between the volume fraction of hard MC phase and the strength of

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MoNbRe0.5TaW(TiC)x composites. The results display that the addition of TiC has obvious beneficial

183

effect on improving the strength. With the increase of TiC addition, the volume fraction of the MC

184

phase increases, leading to the increase of the yield and compressive strength. Thus, the increase of

185

yield and compressive strength is mainly ascribed to the formation of MC phase and second-phase

186

strengthening is the main strengthening mechanism. Meanwhile, the I1.0 composite has the highest

187

compressive strength, which would be attributed to the fully eutectic microstructure and proper

188

content of MC phase. Low volume fraction of MC phase in the hypo-eutectic composites weakens

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the second-phase strengthening effect. Large amount of MC phases in the hyper-eutectic composite

190

make the compressive strength decrease as the proeutectic MC phase has numerous defects of cracks

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and holes (as shown in Fig. 3d), which will cause micro-cracks to originate in the coarse MC phase

192

and then propagate. In addition, the I1.0 composite with fine and fully eutectic microstructure has

193

many boundaries to prevent dislocation from moving and improve the strength. Thus, the strength of

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the I1.0 fully eutectic composite is enhanced. The strengthening mechanism is the combination of the

195

second-phase and boundary strengthening.

196

5． ．Conclusions

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Based on the thermodynamic calculation, novel MoNbRe0.5TaW(TiC)x HEA-ceramic eutectic

198

composites were successfully designed and synthesized by arc melting. The effect of TiC addition on

199

the microstructure and mechanical properties is also studied.

200

(1) The MoNbRe0.5TaW(TiC)x composites consist of BCC solid solution and MC

201

multi-component carbide with FCC structure. With increasing TiC addition, the microstructure

202

evolves from hypo-eutectic structure to eutectic structure and then to hyper-eutectic structure. The

203

microstructure evolution can be well clarified from the analysis of solidification process based on the

204

thermodynamics equilibrium phase diagram.

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(2) The phase compositions obtained by Factsage calculations (expecially using Equilib Module

206

in Scheil-Gulliver cooling) and the experimentally results are in a decent agreement. The calculated

207

eutectic composition point is lower than experimental result due to the limitation of thermodynamic

208

database.

209

(3) The yield strength of composites improves accordingly with the increase of TiC addition.

210

The MoNbRe0.5TaW(TiC)1.0 fully eutectic composite reaches the maximum compressive strength of

211

1943 ± 13 MPa and the second highest yield strength of 1496 ± 17 MPa. The strengthening

212

mechanism is attributed to second-phase and boundary strengthening.

213

214

Acknowledgements

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This work was supported by the National Natural Science Foundation of China [Grant No.

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51521001, 51572208, 51932006], the 111 Project [Grant No. B13035], and the Joint Fund [Grant No.

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6141A02022255]. The S/TEM work was performed at the Nanostructure Research Center (NRC) at

218

Wuhan University of Technology.

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1-19.

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Figure Captions

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Fig. 1. (a) The pseudo binary equilibrium phase diagram of MoNbRe0.5TaW(TiC)x composites; (b) mole fraction of

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phases of MoNbRe0.5TaW(TiC)0.6 composite during non-equilibrium solidification.

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Fig. 2. Non-equilibrium solidification of MoNbRe0.5TaW(TiC)0.6 fully eutectic composite using Equilib Module in

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Scheil-Gulliver cooling: (a) BCC composition, (b) FCC composition.

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Fig. 3. SEM images and XRD patterns of the MoNbRe0.5TaW(TiC)x composites: (a) I0.2, (b) I0.5, (c) I0.8, (d) I1.5,

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(e) I1.0.

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Fig. 4. (a) Dark-field TEM image of the MoNbRe0.5TaW(TiC)1.0 composite and corresponding element distribution

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maps, (b) HRTEM image of the inter-phase, (c) EDS compositional profile showing the element distributions from

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BCC to MC, (d) the Fast Fourier Transformation (FFT) image of BCC and MC phases obtained from (b); (e) the

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selected diffraction spots (designated with arrows in (d)) for FFT filtered images from the region of (b).

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Fig. 5. (a) Compressive stress-strain curves and (b) strength of the MoNbRe0.5TaW(TiC)x composites, (c) Map of

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strength combinations of refractory HEAs and composites.

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Fig. 6. The schematic diagram of crystallization process of MoNbRe0.5TaW(TiC)x composites.

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Fig. 7. Effect of the volume fraction of MC phase on the strength of MoNbRe0.5TaW(TiC)x composites.

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Highlights HEA-ceramic eutectic composites of MoNbRe0.5TaW(TiC)x are designed and prepared. The calculated results agree reasonably well with the experimental results. Eutectic structure consists of BCC phase and MC carbide with good phase interface. Eutectic composite exhibits excellent strength. Strengthening mechanism is combination of second-phase and boundary strengthening.

Declaration of interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.