1. Introduction
High entropy alloys [
1] consist of at least five elements, according to the atomic fraction, is greater than 5% and not more than 35% of composition [
2]. Due to the high-entropy effect, lattice distortion effect, hysteretic diffusion effect and cocktail effect among alloying elements, high-entropy alloys show excellent characteristics different from other alloys in mechanical properties [
3], high temperature properties [
4] and magnetic properties [
5]. The most prominent feature is that the sintered porous metal materials [
6] contain a large number of semi-connected and connected voids and it has been widely used in oil storage components and filtration and separation of the self-lubricating system in the industrial production process [
7]. The metal porous materials have already obtained the characteristics of high strength, weld ability, high temperature resistance, corrosion resistance and easy processing at present [
8].
At present, powder metallurgy, vacuum melting and mechanical alloying are the most commonly used methods to prepare high-entropy alloys. Kumar et al. [
9] prepared the phase evolution and properties of CoCrCuFeNiSi
x (
x = 0, 0.3, 0.6 and 0.9 atomic ratios) high entropy alloys prepared by a powder metallurgy route is investigated. The X-ray diffraction analysis reveals the presence of mixed phases of the face-centered and body-centered cubic phase after 20 h of milling. The addition of Si (0.3, 0.6 and 0.9) favors the formation of the body-centered cubic structure during mechanical alloying. Liu et al. [
10] prepared an AlCrFeNiCu high-entropy alloy by the means of vacuum arc furnace melting, and analyzed its wear-resistance in different environments. Al
xCoCrCuFeNi (
x = 0.45, 1, 2.5, 5) high-entropy alloy powder materials were prepared by Sriharitha et al. [
11] and they used XRD to analyze the phase evolution of high-entropy alloy with the increase of Al content. Huang et al. [
12] prepared AlSiTiCrFeCoNiMo
0.5 and AlSiTiCrFeNiMo
0.5 high-entropy alloys by thermal spraying technology and their micro structures, properties, porosity, wear resistance and oxidation resistance were studied. Sharma et al. [
13] investigated the atomic origin of structural phase transition in AlCrFeNiCo series high-entropy alloy by using classical molecular dynamics simulation. It is not uncommon to add metal elements such as Mn to AlCrFeNi to prepare a high-entropy alloy [
14]. However, the studies on adding Si elements to prepare CrFeNiAlSi series high-entropy alloy are rarely reported. The lattice distortion effect and comprehensive properties of high-entropy alloys can be enhanced by substituting non-metallic elements such as Si for metallic elements such as Cu, Co, Ti and Mn. The high-entropy alloy material used in this paper mainly selects Cr, Fe, Ni and Al elements that are similar to the composition of natural chromite to prepare AlCrFeNiSi
x series high-entropy alloy by simulating natural chromite powder.
At present, the study of laser ignited self-propagating sintering of AlCrFeNiSi high-entropy alloys is the first time at home and abroad. Compared with the traditional method of preparing high-entropy alloys, the laser-ignited self-propagating sintering technology adopted in this paper is simple in equipment and process. Besides the energy needed for ignition reaction, it does not need external heat, saves energy, reduces manufacturing cost, shortens the reaction time of self-propagating high-temperature synthesis, and has high production efficiency. After reaction, it can obtain high activity metastable production, which is difficult to obtain by conventional technology. These characteristics are beneficial to the preparation of high-entropy alloys.
The purpose of this study is to explore the effect of Si content on micro structure, phase structure and mechanical properties of AlCrFeNiSix porous high-entropy alloy materials, and to carry out preliminary theoretical reserve and process design for the preparation of high-entropy alloy by short process processing of natural chromium iron powder. The porous high-entropy alloy is a kind of functional structure integrated material, which has the properties of high-entropy alloy, high hardness, high wear resistance and low density and high porosity. In addition, it is expected to be applied to the oil storage components, shell, skeleton and other structures in the self-lubricating system of aerospace spacecraft development.
3. Improved Preparation Methods
After the raw material powder is weighed according to the proportion, the QM-3SP2 planetary ball mill is used. The steel balls for ball milling are composed of steel balls with diameters of 15, 10 and 6 mm, respectively. The gradation ratio is 3:5:10. The ball milling speed is 200 r/min, the ball milling speed is 3 h, and the ball milling ratio is 1:4, so as to ensure the uniformity of powder mixing. In the process of ball milling, 48 min ball milling and 12 min intermittent ball milling are used to ensure that the powder is fully mixed and uniform. Using the self-made die as shown in
Figure 1, the powder with uniform mixing is pressed into a cylindrical compact of Ø15 mm × 10 mm by 100 kN pressure on WE-30 universal experimental machine (Mitutoyo, Tokyo, Japan). To ignite the self-propagating reaction, the compacted billet is irradiated with HL-1500 helium-free transverse flow CO
2 laser for 10 s under the condition of 1000 W power and 10 mm spot diameter. Using XRD—6100 X-ray diffractometer phase analysis, the X-ray source is a Cu target, scanning angle is 20 to 90 degrees, and the scan rate is 10 degrees/min. The microstructure features of the samples are observed using the OLYMPUS laser confocal metallographic microscope (Olympus Corporation, Shibuya, Japan). Under the condition of JSM-7500F field emission scanning electron microscope (JEOL, Tokyo, Japan.) and acceleration voltage of 15 kV, the microstructure and and pore distribution characteristics of porous high entropy alloy are observed and an EDS test is carried out.
The Archimedes drainage method is used to measure the volume method, and the density of the alloy is calculated after mass weighing on the FA1004N electronic balance (accuracy: 0.0001 g, Shanghai Precision Instruments, Shanghai, China). The porosity of materials and the calculation method of porosity are further calculated. The calculation method of porosity is shown in Equation (1):
In the formula, η is the porosity (%) of sintered high-entropy alloys, ρ is the actual density of the tested samples (g·cm−3), and ρth is the theoretical density of sintered high-entropy alloys (g·cm−3).
The Vickers hardness of high-entropy alloy is tested by an HV-1000Z automatic turret micro-Vickers hardness tester (Suzhou Eisen Instrument and Equipment, Suzhou, China). The load is 0.5 kg and the holding load is 10 s. The average value of Vickers hardness is obtained at five points in the sintered alloy section.
The wear resistance of the alloy was tested on the ML-100 abrasive wear machine (Jinan Yihua Tribological Testing Technology Co., Ltd., Suzhou, China). The abrasive particle size of the abrasive paper used in the experiment was 25 μm, the load was 20 N, and the wear time was 5 min. The wear rate of the alloy was calculated. After testing, the sample was cleaned with acetone. After drying, FA1004N electronic balance (accuracy is 0.1 mg) is used to call the mass of the sample after wear m, and the mass of the sample before wear m
0. The wear rate of the coating surface is calculated according to Equation (2). The wear rate of the coating surface was calculated:
Formula: ω is wear rate, mg/mm2; m is the mass after wear, mg; m0 is the pre-wear mass, mg; S is wear area, mm2.
Author Contributions
In this article, each author makes a different contribution; Conceptualization, Y.-J.A. and L.Z.; methodology, J.-J.L.; software, L.Z.; validation, Y.-J.A., S.-H.J. and X.-Y.L.; formal analysis, S.-H.J.; investigation, Y.-J.A.; resources, L.Z.; data curation, L.Z. and X.-Y.L.; writing—original draft preparation, J.-J.L.; writing—review and editing, L.Z.; visualization, L.Z. and J.-J.L.; project administration, Y.-J.A.; funding acquisition, Y.-J.A.
Funding
Science and Technology Research Project of Liaoning Education Department (Lj2017faL010).
Acknowledgments
Administrative and technical support.
Conflicts of Interest
The authors declare no conflict of interest.
References
- Zhang, Y. Science and technology in high-entropy alloys. Sci. China Mater. 2018, 61, 2–22. [Google Scholar] [CrossRef] [Green Version]
- Xiang, S.; Zhang, L.; Liu, X.; Le, G.-M. Effect of Laser Melting Deposition on Microstructure and Properties of CrMnFeCoNi High Entropy Alloy. Trans. Mater. Heat Treat. 2018, 39, 29–35. [Google Scholar]
- Zhang, Y.; Lu, Z.P.; Ma, S.G.; Liaw, P.K.; Tang, Z.; Cheng, Y.Q.; Gao, M.C. Guidelines in predicting phase formation of high-entropy alloys. Mrs Commun. 2014, 4, 57–62. [Google Scholar] [CrossRef]
- Roy, U.; Roy, H.; Daoud, H.; Glatzel, U.; Ray, K.K. Fracture toughness and fracture micromechanism in a cast AlCoCrCuFeNi high-entropy alloy system. Mater. Lett. 2014, 132, 186–189. [Google Scholar] [CrossRef]
- Wu, Z.; Yang, X.; Zhang, Z.; An, T.; Shi, J. Preparation methods and energy absorption properties of 2024 foamed aluminum alloy. Nonferrous Met. Eng. 2018, 8, 16–21. [Google Scholar]
- Wang, J.; Xu, Z.; Ao, Q.; Zhi, H.; Li, A.; Ma, J.; Tang, H. Research status of mechanical properties of metal fiber porous materials. Rare Met. Mater. Eng. 2016, 45, 1636–1640. [Google Scholar]
- Sui, Y.; Chen, X.; Qi, J.; He, Y.; Sun, Z. Research status and application prospect of multi principal element high entropy alloy. Funct. Mater. 2016, 47, 5050–5054. [Google Scholar]
- Wang, J.; Li, J.; Wang, J.; Bu, F.; Kou, H.; Li, C.; Zhang, P.; Beaugnon, E. Effect of Solidification on Microstructure and Properties of FeCoNi(AlSi)0.2 High-Entropy Alloy Under Strong Static Magnetic Field. 2018, 20, 275. [Google Scholar] [CrossRef] [Green Version]
- Kumar, A.; Dhekne, P.; Swarnakar, A.K.; Chopkar, M. Phase evolution of CoCrCuFeNiSix high-entropy alloys prepared by mechanical alloying and spark plasma sintering. Mater. Res. Express 2018, 6, 026532. [Google Scholar] [CrossRef]
- Liu, Y.; Ma, S.-G.; Liu, Y.-J.; Zhang, T. Friction and Wear Properties of AlxCrCuFeNi2 High-entropy Alloys with Multi-principal-elements. J. Mater. Eng. 2018, 46, 99–104. [Google Scholar]
- Sriharitha, R.; Murty, B.S.; Kottada, R.S. Phase formation in mechanically alloyed AlxCoCrCuFeNi (x = 0.45, 1, 2.5, 5 mol) high entropy alloys. Intermetallics 2013, 32, 119–126. [Google Scholar]
- Huang, P.K.; Yeh, J.W.; Shun, T.T.; Chen, S.K. Multi-Principal-Element Alloys with Improved Oxidation and Wear Resistance for Thermal Spray Coating. Adv. Eng. Mater. 2004, 6, 74–78. [Google Scholar] [CrossRef]
- Sharma, A.; Deshmukh, S.A.; Liaw, P.K.; Balasubramanian, G. Crystallization kinetics in AlxCrCoFeNi (0 ≤ x ≤ 40) high-entropy alloys. Scr. Mater. 2017, 141, 54–57. [Google Scholar] [CrossRef]
- Zang, C.; Tang, H.; Wang, J. Research progress in mechanical properties of sintered metal porous materials. Rare Met. Mater. Eng. 2009, 38, 437–442. [Google Scholar]
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