Laser-Ignited Self-Propagating Sintering of AlCrFeNiSi High-Entropy Alloys: An Improved Technique for Preparing High-Entropy Alloys

Abstract

AlCrFeNiSi system porous high-entropy alloy material is manufactured by laser ignition and self-propagating sintering of natural chromite powder, which provides the idea of breaking the traditional synthesis procedure of high-entropy alloy compound material. The raw material powder obtained by ball milling is compacted into cylindrical compacts, and the self-propagating reaction comes from the ignition caused by the laser on the surface of compacts, the high-entropy alloy composite of chrome iron powder synthesized by laser sintering, is obtained as well. The raw material is prepared from Al, Cr, Fe, Ni and Si elements with similar effective components of natural chromite powder. The selected chromite powder is energy-saving and environment-friendly, so the preparation of high-entropy alloy by the low-cost short-process can be made for processing for pre-theoretical reserve and process design. The effect of Si content on microstructure and properties of AlCrFeNiSi high-entropy alloy is investigated.

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 AlCrFeNiSi_x 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.

2. Selection of Experimental Materials

The high-entropy alloy material in this paper mainly selects Cr, Fe, Ni and Al elements that are similar to the composition of natural chromite to form an AlCrFeNiSi_x series high entropy alloy to simulate natural chromite powder to be the preparation high-entropy alloy.

AlCrFeNiSi_x high-entropy alloys are prepared from high-purity powders of Cr, Fe, Ni, Al, Si and high-carbon ferrochromium with particle size of 75 μm. Among them, x = 0.2, 0.4, 0.6, 0.8 and 1.0. The AlCrFeNiSi_x high-entropy alloy in this paper is allocated as shown in

Table 1. Design of AlCrFeNiSi_x high-entropy alloy (mole ratio/%).

Alloy	Cr	Fe	Ni	Al	Si
AlCrFeNiSi0.2	23.8	23.8	23.8	23.8	4.8
AlCrFeNiSi0.4	22.7	22.7	22.7	22.7	9.1
AlCrFeNiSi0.6	21.7	21.7	21.7	21.7	13.0
AlCrFeNiSi0.8	20.8	20.8	20.8	20.8	16.7
AlCrFeNiSi1.0	20.0	20.0	20.0	20.0	20.0

. The composition (mass fraction wt %) of high carbon ferrochrome powder with particle size of 75 um is 65.0%Cr + 23.45%Fe + 9.0%C + 2.0%Si + 0.5%Mn + 0.05%S.

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 CO₂ 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):

$$\eta =(1-\frac{\rho }{{\rho }_{\mathrm{th}}})\times 100\%.$$

(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 ρ_th 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 m₀. The wear rate of the coating surface is calculated according to Equation (2). The wear rate of the coating surface was calculated:

$$\omega =\frac{m-m_0}{S}.$$

(2)

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.

4. Performance Results and Analysis of Alloy Materials

4.1. XRD Test and Microstructure

Figure 2 shows the X-ray diffraction pattern of AlCrFeNiSi_x high-entropy alloy, in which x = 0.2, 0.4, 0.6, 0.8 and 1.0. AlCrFeNiSi_0.2 is sintered to form a high-entropy alloy with a single BCC phase structure. With the increase of silicon content, a small amount of FCC phase starts to appear in the single BCC phase structure when x = 0.6, especially when x = 1.0.

According to Gibbs phase law, the number of equilibrium phases produced by N-element mixed alloy system is p = n + 1, and the number of phases produced by non-equilibrium solidification is p > n + 1. However, in a high-entropy alloy system, the high-entropy effect makes the alloy tend to form a simple structure of BCC and FCC instead of complex intermetallic compounds. According to the Boltzmann formula and the additivity of entropy, calculated by the entropy value equation (Equation (3)):

$$\Delta S_{conf}=-R{\displaystyle {\sum }_{i=1}^n{c}_i\ln c_i},$$

(3)

where R is the gas constant (R = 8.3145 J/K·mol), and c_i is the molar fraction of element i:${\sum }_{i=1}^n{c}_i=1$.The high-entropy effect produced by the high-entropy alloys in this paper results in the formation of simple bulk BCC and FCC phases in the microstructure of AlCrFeNiSi alloys, which do not tend to produce brittle intermetallic compounds, and the phase number P is much less than 6. The high mixing entropy of alloys makes the atoms in alloys very chaotic. Diffusion in alloys requires the co-diffusion of different kinds of atoms. The energy required for diffusion and the barrier crossed is much larger than those in traditional alloys. Therefore, the atom diffusion in alloys becomes very difficult, which greatly increases the mutual solid solubility of atoms of different elements in alloys. Finally, the alloys are solidified. A simple BCC or FCC solid solution is formed during solidification.

Figure 3 is an SEM image of the AlCrFeNiSi high-entropy alloy structure, where Figure 3a shows the microstructure of the AlCrFeNiSi_0.4 high-entropy alloy. It can be seen that the microstructure of the alloy is simple and consists mainly of grayish white microstrusture (A region) and gray tissue (B region). Figure 3b shows the microstructure of AlCrFeNiSi_0.6 high-entropy alloy. The microstructure of the alloy consists mainly of grayish white structure (A region), gray structure (B region) and dish scale structure (C, D, E regions). The fish scales in the C, D, and E regions are newly emerging FCC phases in the alloy according to the results of XRD analysis. The energy spectrum analysis is performed on the regions where A, B and C in Figure 3b are located and the results are shown in

Table 2. EDS analysis results of AlCrFeNiSi_0.6 high-entropy alloy.

Point	Crystal Structure	Cr	Fe	Al	Ni	Si
Original ratio	-	21.7	21.7	21.7	21.7	13.0
A	BCC	30.69	8.30	30.20	20.48	10.33
B	BCC	31.84	8.85	29.77	18.02	11.52
C	FCC	26.78	7.16	30.28	21.79	12.86

. The EDS spectrum analysis showed that the grayish white microstrusture and gray microstructure mainly contained Cr, Al and Ni; the fish scaled structure mainly contained Al, Cr, Ni and Si elements.

4.2. Alloy Morphoiogy

Figure 4 is a metallographic photograph taken by a confocal laser metallographic microscope of a porous AlCrFeNiSi_x alloy with different Si content. It can be seen from the picture that pore morphology changes obviously with the increase of Si content. The pores in the multicomponent alloys of Figure 4a,b are small and uniformly distributed. The pores in the multicomponent alloys of Figure 4c,d gradually aggregate and become coarse and uneven. The multicomponent alloy matrix is joined together over a large area. However, no obvious change is found in the alloy structure. The main reason is that the laser ignition self-propagating sintering has the characteristics of fast heating and rapid cooling and extremely high temperature gradient. This makes the multicomponent alloys evenly distributed, the grain size is extremely small, and there is no time for component segregation. The long-range migration of the same species gathers together and cools and solidifies. The pores of the multicomponent alloy are mainly from two sources. The larger macroscopic pores are formed by the expansion of the air in the compact during the sintering process. The microscopic pores in the matrix are caused by the oxidation of the C element of the high carbin ferrochrome powder in the raw material at high temperature. Pores in multicomponent alloys are mainly derived from two sources, the larger macropore is formed by the expansion of air in the compact during sintering, and the micro-pore in the matrix is caused by CO₂ produced by oxidation of C element of high carbon ferrochromium powder in raw material at high temperature.

4.3. Alloy Density and Porosity

The size of the voids of the multicomponent alloy block sintered by the self-propagating reaction mainly depends on the power parameters of the sintering, the loading pressure of the billet and the severity of the reaction. During the self-propagation process, the porosity of the samples are also affected by the severity of the sintering reaction. The more severe the reaction, the greater the porosity of the sample, and vice versa.

With the increase of atomic ratio of silicon, the density and porosity of the sintered AlCrFeNiSi_x high entropy alloy are shown in Figure 5. The density of the high-entropy alloy first rises and then decreases. Its standard deviation is 0.283. When x = 0.6, the alloy density reaches the maximum value of 4.657 g·cm⁻³, and then decreases. The variation of the porosity of the alloy with the increase of Si content is opposite to that of the density change, which is characterized by first decreasing and then increasing. Its standard deviation is 0.401. When x = 0.6, the alloy porosity is the smallest, only 17.3% and when x < 0.6, the density of the alloy increases rapidly as the Si element increases, and the porosity decreases sharply. When x ≥ 0.6, the FCC structure begins to appear in the original single BCC structure. Due to the appearance of different phases, the diffusion of metal is inhibited, the density of the alloy begins to decrease and the porosity increases slightly. The porous high-entropy alloy with x = 0.2 has the largest porosity of 43.9%, and the alloy density at this time is 3.396 g·cm⁻³.

4.4. Alloy Hardness and Wear Resistance

The Vickers hardness and wear rate of high-entropy alloys under different Si Contents is shown in

Table 3. Alloy hardness and rate of wear.

Alloy	Microhardness/(HV)	Wear Rate/(mg·cm−2)
AlCrFeNiSi0.2	167.2	84.8
AlCrFeNiSi0.4	368.6	79.6
AlCrFeNiSi0.6	533.6	75.6
AlCrFeNiSi0.8	508.6	77.8
AlCrFeNiSi1.0	491.8	81.6

. The standard deviation of the Vickers hardness and wear rate of high-entropy alloys is 148.61 and 4.11. It can be seen that, with the increases of Si content, the hardness of the alloy first rises rapidly and then decreases slightly. The wear rate of the alloy shows a slight increase after a rapid decline. With the addition of silicon, the hardness and wear resistance of silicon are gradually enhanced due to the difference of radius between atoms of each element and the increase of lattice distortion. The micro-Vickers hardness of the sintered high-entropy alloy reaches a maximum of 533.6 HV when x = 0.6 and the alloy wear rate is at least 75.6 mg·cm⁻². However, when x ≥ 0.6, the FCC phase begins to appear in the alloy. Since the slip surface of the FCC phase is more stable than BCC, the hardness and wear resistance of the alloy are slightly reduced. In addition, when x = 1.0, the hardness value is 491.8 HV, and the wear rate is 81.6 mg·cm⁻².

Figure 6 is a picture of the worn surface of the contact surface after the abrasive wear is carried out by the means of SEM. The abrasive wear test is carried out with corundum abrasive cloth, the Mohs hardness of corundum is 9, and the high hardness of the steel has a Mohs hardness of 5–6, which is a hard abrasive wear. It can be seen clearly from the figure that there are grooves on the friction surface. Generally, the wear resistance of a material is directly proportional to its hardness. Considering the combination with the hardness and wear resistance analysis in this paper, it can be seen from the wear profile that the groove on the friction surface is shallow when x = 0.2 because the surface hardness of the test piece is the highest, the number of abrasive grains embedded in the surface of the test piece is small, and the groove is the shallowest. Less hard particles on the surface, less abrasive wear and best wear resistance. Because of the depth of the groove caused by the plows due to their low hardness when x = 0.4 and x = 0.8, it can be seen from the figure that the groove depth caused by plough wrinkles, the surface of the groove is also severely peeled off, the wear is the most serious, and the wear debris under abrasive cutting is the most, resulting in the worst wear resistance of these two kinds of wear. The hardness of the specimen with x = 0.6 is in the middle position, the groove formed by friction is shallow, the hard particles falling off are less, and the wear resistance is better. It can be seen from above that the wear morphology is consistent with the change of hardness and wear rate.

5. Conclusions

The research on porous high-entropy alloy material prepared by the method of laser ignition self-propagating sintered natural chromite ore powder is the first time at home and abroad. The macroscopic morphology is honeycomb-shaped, the structure is simple, and there is no obvious component segregation and the alloy consists of a single BCC phase or BCC + FCC phase. The prepared AlCrFeNiSix porous high-entropy alloy material has excellent performance; it is feasible to use natural ferrochrome ore powder to prepare a high-entropy alloy. The porosity of AlCrFeNiSi_x porous high-entropy alloy is finer and distributes uniformly in the alloy matrix. The porosity is up to 43.9% when x = 0.2, and the density is only 3.396 g·cm⁻³; when x = 0.6, the porosity (17.3%) is the smallest and the density (4.657 g·cm⁻³) is the largest. The wear rate of the alloy is the lowest, only 73.41 mg·cm⁻².