Fabrication of ceramics/high-entropy alloys gradient composites by combustion synthesis in ultra-high gravity field 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 China b Key Laboratory of Cryogenics, Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, Beijing 100190, PR China c Nuclear Engineering and Radiological Sciences, University of Michigan, Ann Arbor, MI 48108, USA article info Article history: Received 9 May 2018 Received in revised form 23 July 2018 Accepted 11 August 2018 Available online 12 August 2018 Keywords: Gradient composites Ceramic composites Combustion synthesis Ultra-high gravity High-entropy alloy Functional abstract The gradient ceramic/metal composite of TiC-TiB 2 /Al 0.3 CoCrFeNi was prepared by combustion synthesis under ultra-high gravity field. Right after the formation of the mixture by combustion, the ceramics and metals are separated in an ultra-gravity field due to the difference in mass density, resulted in a gradient distribution of ceramics (TiC-TiB 2 ) along the gravitational field in the high-entropy alloy (Al 0.3 CoCrFeNi) matrix. The hardness of the material shows a significant gradient change. Ó 2018 Published by Elsevier B.V. 1. Introduction Recently, high-entropy alloys (HEAs), which contain at least four principal metals, have been explored and attracted more and more attentions [1,2]. Compared with conventional alloy materials, HEAs possess excellent structural stability and compre- hensive properties because of the high entropy effect and sluggish diffusion effect [3,4]. Thus, HEAs become a focus in many fields of scientific 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 the preparation technology, ceramic/HEAs gradient composites, which combine the advantages of ceramics and high-entropy alloys, have been rarely reported, although they are desirable for protective structural materials including armor and aerospace shell materials. In this LETTER, a novel method has been adopted to prepared TiC-TiB 2 / Al 0.3 CoCeFeNi gradient composites by ultra-high gravity combustion synthesis, an efficiently approach to produce ceramics, cermet, grade alloys, HEAs, etc. [8–11]. Metal melt and Al 2 O 3 melt are formed through aluminothermic reactions, as well as TiC-TiB 2 , which are rapidly separated under the influence of ultra-high grav- ity field, forming the TiC-TiB 2 /Al 0.3 CoCrFeNi gradient composites after solidification. The microstructures, mechanical properties and synthesis mechanism of this material are investigated. 2. Experimental procedures Commercial powders of Co 3 O 4 , Fe 2 O 3 , Cr 2 O 3 , NiO, Ti (99.5%pu- rity), B 4 C(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 metal cylindrical mold for press-forming and then put into a graphite crucible. A gravitation field of 1500 g (g = 9.8 m/s 2 ) was applied by a high-gravity casting apparatus for approximately 3 min. The final composite system was TiC-TiB 2 /67 vol%Al 0.3 CoCrFeNi (nomi- nal composition). The reactions are follows. 2Al þ 3NiO ¼ Al 2 O 3 þ 3Ni; 2Al þ Fe 2 O 3 ¼ Al 2 O 3 þ 2Fe; 2Al þ Cr 2 O 3 ¼ Al 2 O 3 þ 2Cr; 8Al þ 3Co 3 O 4 ¼ 4Al 2 O 3 þ 9Co; 3Ti þ B 4 C ¼ TiC þ 2TiB 2 ð1Þ X-ray diffraction (XRD, Rigaku) was used to identify the crystal structure, the scanning rate of 4°/min with a step of 0.02°, and the 2 h scanning range was 10°–90°. JSM-6510A (Japan) scanning elec- tron microscopy equipped with EDS was used for microstructure characterization. The hardness of the material was measured using a micro hardness tester. A load of 0.2 kg applied and the loading time was 10 s. https://doi.org/10.1016/j.matlet.2018.08.059 0167-577X/Ó 2018 Published by Elsevier B.V. ⇑ Corresponding authors. E-mail addresses: lijiangtao@mail.ipc.ac.cn (J. Li), qpeng@umich.edu (Q. Peng). Materials Letters 233 (2018) 4–7 Contents lists available at ScienceDirect Materials Letters journal homepage: www.elsevier.com/locate/mlblue