Materials Letters 233 (2018) 4–7

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Materials Letters

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Fabrication of ceramics/high-entropy alloys gradient composites bycombustion synthesis in ultra-high gravity field

https://doi.org/10.1016/j.matlet.2018.08.0590167-577X/� 2018 Published by Elsevier B.V.

⇑ Corresponding authors.E-mail addresses: lijiangtao@mail.ipc.ac.cn (J. Li), qpeng@umich.edu (Q. Peng).

Wenrui Wang a, Huifa Xie a, Lu Xie a, Xiao Yang b, Jiangtao Li b,⇑, Qing Peng c,⇑a School of Mechanical Engineering, University of Science and Technology Beijing, Beijing 100083, PR ChinabKey Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR ChinacNuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48108, USA

a r t i c l e i n f o

Article history:Received 9 May 2018Received in revised form 23 July 2018Accepted 11 August 2018Available online 12 August 2018

Keywords:Gradient compositesCeramic compositesCombustion synthesisUltra-high gravityHigh-entropy alloyFunctional

a b s t r a c t

The gradient ceramic/metal composite of TiC-TiB2/Al0.3CoCrFeNi was prepared by combustion synthesisunder ultra-high gravity field. Right after the formation of the mixture by combustion, the ceramics andmetals are separated in an ultra-gravity field due to the difference in mass density, resulted in a gradientdistribution of ceramics (TiC-TiB2) along the gravitational field in the high-entropy alloy (Al0.3CoCrFeNi)matrix. The hardness of the material shows a significant gradient change.

� 2018 Published by Elsevier B.V.

1. Introduction

Recently, high-entropy alloys (HEAs), which contain at leastfour principal metals, have been explored and attracted moreand more attentions [1,2]. Compared with conventional alloymaterials, HEAs possess excellent structural stability and compre-hensive properties because of the high entropy effect and sluggishdiffusion effect [3,4]. Thus, HEAs become a focus in many fields ofscientific research, such as heat-resistant HEAs, cryogenic HEAs,HEA wire, HEA film, high entropy ceramics, superconducting HEAs,eutectic HEAs, etc. [5–7]. However, due to the limitations of thepreparation technology, ceramic/HEAs gradient composites, whichcombine the advantages of ceramics and high-entropy alloys, havebeen rarely reported, although they are desirable for protectivestructural materials including armor and aerospace shell materials.

In this LETTER, a novel method has been adopted to preparedTiC-TiB2/ Al0.3CoCeFeNi gradient composites by ultra-high gravitycombustion synthesis, an efficiently approach to produce ceramics,cermet, grade alloys, HEAs, etc. [8–11]. Metal melt and Al2O3 meltare formed through aluminothermic reactions, as well as TiC-TiB2,which are rapidly separated under the influence of ultra-high grav-ity field, forming the TiC-TiB2/Al0.3CoCrFeNi gradient composites

after solidification. The microstructures, mechanical propertiesand synthesis mechanism of this material are investigated.

2. Experimental procedures

Commercial powders of Co3O4, Fe2O3, Cr2O3, NiO, Ti (99.5%pu-rity), B4C(97% purity), and Al (99.9% purity) are fully mixed accord-ing to the formula in Eq. (1). Take 0.2 kg powder into a metalcylindrical mold for press-forming and then put into a graphitecrucible. A gravitation field of 1500 g (g = 9.8 m/s2) was appliedby a high-gravity casting apparatus for approximately 3 min. Thefinal composite system was TiC-TiB2/67 vol%Al0.3CoCrFeNi (nomi-nal composition). The reactions are follows.

2Al þ 3NiO ¼ Al2O3 þ 3Ni; 2Al þ Fe2O3 ¼ Al2O3 þ 2Fe;2Al þ Cr2O3 ¼ Al2O3 þ 2Cr; 8Al þ 3Co3O4 ¼ 4Al2O3 þ 9Co;

3Ti þ B4C ¼ TiC þ 2TiB2

ð1ÞX-ray diffraction (XRD, Rigaku) was used to identify the crystal

structure, the scanning rate of 4�/min with a step of 0.02�, and the2 h scanning range was 10�–90�. JSM-6510A (Japan) scanning elec-tron microscopy equipped with EDS was used for microstructurecharacterization. The hardness of the material was measured usinga micro hardness tester. A load of 0.2 kg applied and the loadingtime was 10 s.

W. Wang et al. /Materials Letters 233 (2018) 4–7 5

3. Results and discussion

3.1. X-ray diffraction

The material installation diagram is illustrated in Fig. 1(a), thewhole part is the put into the equipment for combustion synthesisunder high gravity and adjust the counterweight to achieve equilib-rium (Fig. 1b). An ultra-high gravity field G = 1500 g is applied byhigh-speed rotation. Once the raw material is ignited by the tung-sten wire, the chemical reactions take place immediately and thecombustion wave is transmitted along the direction of the ultra-high gravity field. The reaction process is shown in Fig. 1(c), the finalsample was naturally layered, as shown in Fig. 1(d). The upper layeris porous, presumably Al2O3, and the lower layer is a dense alloyingot, which may be a cermet composite.

Phase compositions are examined by XRD on four spots(Fig. 1e). The main phase of the top layer is Al2O3 (region Ⅰ). TheII, III, and IV regions were mainly composed of TiC, TiB2 and highentropy alloy with FCC structure. The diffraction peak of the fine(Cr,Fe)B borides phase was detected, indicating that the region isa composite structure composed of a HEA and ceramics. Alongthe direction of the ultra-high gravity field, the correspondingdiffraction peak relative intensity of the ceramics material (TiC-TiB2) is weakening gradually.

3.2. Microstructure

Microstructures and materials along the direction of the ultra-high gravity field are characterized, as showed in Fig. 2(a–f). Thenumber of precipitates on the substrate gradually decreases. The

Fig. 1. (a) The materials installation diagram. (b) The schematic drawing of the equipmAl2O3-gradient alloy. (e) XRD pattern.

elemental composition of every region (Fig. 2b) was determinedseveral times using EDS, as summarized in Table 1.

The EDS results show that region 1 is the TiB2 and region 2 is theTiC, the TiB2 is significantly larger than TiC. Region 3 containsnearly equal elements of Al, Co, Ni and Fe, which is a typical HEAphase with FCC structure. Region 4 has significant Cr enrichmentand a certain amount of B and Fe, with the composition close to(Cr, Fe) B borides. The slightly high ratio of B left in this regionmight due to the unintended supplement of C from graphite cru-cibles during the reaction. The concentrations of the ceramics grad-ually decrease and shows a significant gradient distribution alongthe direction of the ultra-high gravity field.

Next, in order tomeasure thehardness and ceramics volumecon-tent distributions, the specimenwas divided into 11 layers along thedirection of the ultra-high gravity field. The volume content of eachlayer and the hardness of the composite aremeasured five times forgood statistics at each of these 11 layers. The ceramic volume con-tent and hardness of composites both have a continuous downwardtrend along the direction of the ultra-high gravity field, as shown inFig. 2(g). The volume content of the ceramics has dropped from41 vol% to 4.2 vol%, and the hardness of the composites has droppedfrom1200HV to 552HV. The two curves are consistent to each otherwith some correlations. The highest hardness region has the highvolumetric ceramics. This is reasonable since the ceramic is muchstiffer than metal in this case. Therefore, the volume content of theceramics is amajor factor for themechanical properties of this com-posite. At the end of the curve, the volume content and hardness ofthe materials change smoothly and even slightly increased. Wespeculate that the graphite crucible supplied the C element duringthe reactions, resulting in additional amount of TiC.

ent for combustion synthesis under high gravity. (c) Reaction process diagram. (d)

2µm

1

2 3

4

1(b)

20µm

(c) (d)

20µm

(e)

20µm

(f)

20µm

(a)

20µm

(g)

Fig. 2. (a)–(f) are the microstructures along the direction of the ultra-high gravity field. (b) is microscopic structure enlargement of (a), the numbers of 1,2,3,4 representdifferent phase regions, respectively. (g) changes in ceramic volume content and hardness of materials along the ultra-high gravity field.

Table 1Elemental composition for regions in Fig. 2(b), (at.%).

Region. Phase Al Co Cr Fe Ni B C Ti Error

1 TiB2 – – – – – 48 13 35 1.5%2 TiC – – – – – 8 40 48 0.8%3 FCC 16 12 2 10 21 22 15 1 0.35%4 (Cr,Fe)B borides – 3 43 8 – 31 5 1 0.15%

6 W. Wang et al. /Materials Letters 233 (2018) 4–7

3.3. Synthesis mechanism

The principle of the combustion synthesis under ultra-highgravity field to preparation gradient material is shown in Fig. 3.The combustion process releases a large amount of heat, the calcu-lated adiabatic temperature for the combustion reaction system is2938 K, the direction of the propagation of the combustion wave isconsistent with the direction of applied gravitation field. Therefore,the gravity field promotes the flow of heat and matter inside thereaction system, thereby accelerating the propagation of the com-

Co3O4 Cr2O3 NiO Fe2O3Al

(a) (b)

Fig. 3. Reaction Mechanism. (a) Before the start of th

bustion wave. Powdered raw materials react with each other athigh temperatures to produce TiC, TiB2, Al2O3 and metal structures,respectively. The subsequent separation of these phases in ultra-high gravity field obeys the Stock’s law [12].

Due to the difference in density between ceramics (4.9 g/cm3

for TiC and 4.5 g/cm3 for TiB2) and melts(>7g/cm3), directional flowoccurs in the ultra-high gravity field, high-density metals flow inthe direction of the gravitational field, and low-density ceramicsflow in the opposite direction, resulting in volume-mass segrega-tion of metals and ceramics. Due to the large density difference

TiC TiB2B4C Ti Al2O3

(c)Al2O3

G

Melt

HEA

e reaction. (b) Reaction process. (c) After cooling.

W. Wang et al. /Materials Letters 233 (2018) 4–7 7

between Al2O3 melt and metal, liquid phase separation occurs,resulting in a complete segregation of Al2O3 on one side (Fig. 1d).The difference in density between TiC/TiB2 particulates and HEAis relatively smaller and compounding can be achieved. Ceramicsmaterials have a much higher melting point (3430 K for TiC,3500 K for TiB2) than HEAs and much less thermal expansion coef-ficients. Therefore, ceramics have less volumetric change, and thefinal solidification of the metal structure shrinks.

4. Conclusions

A novel functionally gradient material TiC-TiB2/Al0.3CoCrFeNiwhich was successfully prepared in-situ by combustion synthesisunder ultra-high gravity field. The ceramic volume content andhardness of composites both monotonically decrease, along thedirection of the ultra-high gravity field. The ceramic volume con-tent drops from 41 vol% to 4.2 vol%, and the hardness of the corre-sponding composites has dropped from 1200 HV to 552 HV.Ceramics-metal gradient composites have both high hardnessand toughness of the ceramics, and the gradient change of mechan-ical properties is tunable. With these unique structural andmechanical properties, this gradient ceramic/HEA composite haspromising applications in the field of protective structural materi-als, including armored protective materials and aerospace shells.

Acknowledgements

The authors are grateful for the financial support provided bythe Fundamental Research Funds for the Central Universities(FRF-GF-17-B21 and FRF-TP-16-044A1) and National NaturalScience Foundation of China (51702332 and 21703007) and Key

Laboratory of Cryogenics, TIPC, CAS (CRYOQN201705 andCRYOQN201507).

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