Effect of Sintering Temperature on High-Entropy Alloy Particle Reinforced Aluminum Matrix Composites via Vacuum Hot-Pressing Sintering

Abstract

In this paper, Al_0.6CoCrFeNi/5052 aluminum matrix composites were prepared at different sintering temperatures (550–700 °C) by vacuum hot-pressing sintering. The effects of sintering temperature on composites were studied by testing the morphology, phase composition and mechanical properties of the composites, which were characterized by uniaxial compression experiments, X-ray diffraction, scanning electron microscope, electron probe and nanoindentation. The results show that the prepared composites have high strength and very good ductility with sintering temperature lower than 700 °C. When the sintering temperature is above 550 °C, the interface layer is formed in the composite material. As the sintering temperature increases, the formation method of the interface layer changes. The generation of the interface layer has a significant effect on the mechanical properties of the composites.

1. Introduction

At present, particle-reinforced aluminum matrix composite is one of the most widely used and most important metal matrix composites. However, due to the great difference between the properties of the traditional particle reinforcement and the aluminum matrix, there is no good wettability between the reinforcement and the matrix, which makes the composites lack plasticity and toughness [1,2]. High-entropy alloys (HEAs), which not only have excellent high-temperature properties, mechanical properties and corrosion resistance, but can also form a good interface with aluminum matrix, have been used as a reinforcement material [3,4,5,6]. As of now, these alloys have made some achievements in the reinforcement of composites [7,8,9,10,11,12].

Wang et al. [13] have reported that 2024 aluminum alloy reinforced by FeNiCrCoAl₃ high-entropy alloy was consolidated using the hot extrusion method, whose compressive strength even reaches 710 MPa. AlCoNiCrFe-high-entropy-alloy-reinforced Cu matrix composites were fabricated by Chen et al. [14]. The yield strengths of the composite increased by more than 160% compared with that of the Cu matrix without the AlCoNiCrFe HEA. Karthik et al. [8] have prepared AA5083 matrix composite with nanocrystalline CoCrFeNi high-entropy alloy reinforcement particles via friction deposited in multiple layers, which has significantly higher tensile and compressive strengths. Chen et al. [15] synthesized 6061Al matrix composites reinforced by 7.5 vol.% nanocrystalline CoNiFeAl_0.4Ti_0.6Cr_0.5 high-entropy alloy (HEA) particles. It showed better ultimate tensile strength and fracture strain. Tungsten heavy alloy composite was developed by using novel CoCrFeMnNi high-entropy alloy by spark plasma sintering, with a higher heating rate and shorter sintering time that led to a higher compressive strength for the composite [16].

Moreover, the interface between high-entropy alloy particles and aluminum matrix is mostly produced by spark plasma sintering. Tan et al. [17] prepared Al_0.6CoCrFeNi-particle-reinforced aluminum matrix amorphous composites by spark plasma sintering, and there is a transition layer between the matrix and the reinforced phase. There is an obvious interdiffusion layer in CoCrFeMnNi-particle-reinforced aluminum matrix composites prepared by spark plasma sintering from our previous study [18]. Liu et al. [3] used spark plasma technology to prepare AlCoCrFeNi-particle-reinforced aluminum matrix composites. The effect of sintering temperature on the interface layer is investigated, and it is found that the thickness of the interface layer increases with the increase in sintering temperature. This is considered to be related to the working mode of spark plasma sintering. Compared with traditional sintering methods, spark plasma sintering as a rapid consolidation mode, its pulse direct current (DC) can produce a high temperature on the surface of metal particles and change the bonding mode between reinforcement and matrix [19,20]. Yang et al. have analyzed the formation of intermetallic compounds of Al composites reinforced by AlCoCrFeNi high-entropy alloy particles [21]. However, these studies did not reveal the formation mechanism of the interfacial layer and its role in high-entropy alloy reinforced aluminum matrix composites.

In this paper, Al_0.6CoCrFeNi-particle-reinforced aluminum matrix composites were prepared by vacuum hot-pressing sintering at different sintering temperatures, to explore the mechanism of interface layer and its effect on the mechanical properties of the composites.

2. Experiment

Using commercially available 5052Al alloy powder (~50 μm) and Al_0.6CoCrFeNi spherical powder (<25 μm) as starting materials, the volume fraction of HEA powder was determined to be 7%. In a planetary ball mill, the starting powder was uniformly mixed in vacuum atmosphere at a ball-to-powder ratio of 10:1 and a rotational speed of 200 r/min for 1 h. The composite powder was sintered in a vacuum hot-press sintering furnace under the pressure of 30 MPa for 1 h. The sintering temperatures were 550 °C, 600 °C, 650 °C and 700 °C, respectively. X-ray diffraction (XRD) was used to determine the phase composition of powders and composites. The morphology and microstructure of the composites were characterized using a metallographic microscope and scanning electron microscope (SEM), and the diffusion layer was quantitatively analyzed using an Electron Probe Micro-analyzer (EPMA). The nanoindentation experiment was carried out on the produced interface of the composite. The indentation moved from the particle through the interface to the matrix, with an interval of 1.5 μm between each indentation point, and the maximum load was 4000 μN. The samples with a size of φ 5 × 10 mm were compressed at room temperature with a loading rate of 0.2 mm/min.

3. Results and Discussion

3.1. Mechanical Properties of Composite at Different Sintering Temperatures

Figure 1 shows the microstructure image of aluminum matrix composites at different sintering temperatures. Obvious voids and defects are not observed on the surface of the sintered samples. It can be observed from the figure that with the increase in sintering temperature, a transition layer appears between the reinforcement particles and the matrix. At the same time, the new phase precipitated from the matrix 5052Al alloy gradually increases during the sintering process. In Figure 1a, the sintering temperature is 550 °C. The high-entropy alloy particles are tightly bonded with the matrix, which is mainly mechanically bonded, showing an obvious boundary. There is no obvious transition layer. Compared with Figure 1b–d, all the composites sintered above 600 °C have an annular transition layer. It is found that the morphology of the high-entropy alloy changes obviously with the increase in the thickness of the transition layer. It can be seen from the figure that, with the increase in sintering temperature, the thickness of the transition layer of the composite increased, and the thickness of the interface layer is 0 μm at 550 °C, and the maximum value is about 7 μm at 650 °C. There is a correlation between the thickness of the transition layer and the sintering temperatures. This is mainly due to different sintering temperatures that lead to different degrees of decomposition and diffusion of high-entropy alloy in the aluminum matrix. For hot-pressing sintering, the high sintering temperature will promote the diffusion behavior of high-entropy alloy elements in composite. Interestingly, the microstructure and morphology of composites at 700 °C has changed greatly, showing very different characteristics from the above three. It has shown a severe overburdening due to the high sintering temperature, which is already beyond the melting point of aluminum (660 °C).

Table 1. Density table of aluminum matrix composites with different sintering temperatures.

Sintering Temperature (°C)	Actual Density (g/cm3)	Theoretical Density (g/cm3)	Relative Density (%)
550	2.9086	3.0951	90.0
600	2.9937	3.0951	96.7
650	3.0019	3.0951	97.0
700	2.9398	3.0951	95.0

shows the density of aluminum matrix composites with different sintering temperatures. It can be seen from the table that with the increase in sintering temperature, the relative density of composites increases at first and then decreases. When the temperature increases from 550 °C to 650 °C, the relative density of composites increases from 94% to 97%, while when the temperature rises to 700 °C, the density of composites decreases to 95%. This is mainly because the increase in sintering temperature provides more driving force for the diffusion of atoms, promotes the bonding and growth of grains, expels the gas between the particles, and reduces the porosity of the composites. Moreover, the transition layer formed by the diffusion of high-entropy alloy elements at a higher sintering temperature reduces the contact pores caused by the mechanical contact between the reinforcement particles and the matrix, and increases the density of the composites. However, too high a temperature will make the aluminum alloy powder melt. In the process of re-solidification, the gas will form pores in the grains, which will lead to a decrease in the density of the composites.

To explore the influence of different sintering temperatures on composite, the macro-mechanical properties of composite were tested. Figure 2 shows the hardness distribution of aluminum matrix composites at different sintering temperatures. It can be seen from the figure that the hardness of the composites sintered at 550 °C is the smallest, about 77.1 HV. Compared with the composites with an obvious transition layer, the hardness of the composites is greatly improved, and the hardness of the composites sintered at 650 °C has a maximum hardness value of 105.9 HV. The hardness of the composites increases with the increase in sintering temperature below 700 °C, which is mainly due to the following reasons: (1) In the process of hot-press sintering, some new phases will be precipitated in the matrix, and the precipitated phases will be dispersed in the matrix. At the same time, the increase in temperature will increase the precipitated phase, which will add to the hardness of the composites. (2) In addition, the formation of the transition layer also greatly improves the hardness of the composites, and the increase in the thickness of the transition layer also greatly improves the hardness of the composites. This is mainly due to the serious element diffusion and interfacial reaction between the particles and the matrix at the transition layer. The hard phase and solid solution in interface will improve the hardness of the composites.

Figure 3 shows the true stress–strain curve of uniaxial compression experiments of composites with different sintering temperatures. It can be seen from the diagram that the yield strength and compressive strength of composites increase at first and then decrease with the increase in sintering temperature; the plasticity decreases overall. It can be seen from

Table 2. Compressive mechanical properties of composite at different sintering temperatures.

Sintering Temperature (°C)	Yield Strength (MPa)	Compressive Strength (MPa)	Ductility (%)
550	86.2	200	60.8
600	97.5	215	55.0
650	117.2	226	34.6
700	80.5	138	8.7

that the yield strength and compressive strength of the composites have the maximum values of 117.3 MPa and 226.6 MPa at 650 °C, which are 35.7% and 13% higher than those of 86.2 MPa and 200 MPa at 550 °C, respectively. And the prepared composites have a very good ductility below 700 °C. The maximum ductility exceeds 60%. This is mainly due to the formation of the transition layer and high relative density. The transition layer formed by the diffusion of high-entropy alloy elements changes the bonding mode between the reinforced particles and the matrix from weak mechanical bonding to diffusion bonding. The change of the bonding mode will improve the bonding ability between the reinforced particles and the matrix. At the same time, the high-entropy alloy particles in the core still maintain good strength and plasticity. When the interface is bonded well, the transition layer can hinder and deflect the crack propagation in the composite and maintain the completeness of the interface before the particle fracture, which will greatly improve the strength of the composite. Also, the high sintering temperature increases the density of the composites, reduces the porosity and improves the strength of the materials [22]. In addition, the formation of the transition layer means the formation of a large number of brittle phases, which will evolve into defects during deformation. The “second” defects will lead to a large amount of stress concentration, accelerate the crack propagation, and adversely affect the properties of the composites. When the sintering temperature is 700 °C, the strength and plasticity of the composites decrease greatly, which is mainly due to the relative increase in the content of defects in the composites and a large amount of agglomeration in the interface layer of the composites. As a result, the uniformly distributed small particle reinforcements in the composites become “large inclusions” with an irregular shape, and a large amount of stress concentration appears in the agglomeration area, which seriously destroys the mechanical properties of the composites.

3.2. Interface Composition Analysis

To detect the composition of the transition layer, a region of interface between reinforcements and matrix was selected for EPMA testing. Figure 4 shows the backscattered electron image and element distribution of the composite sintered at 600 °C. It can be seen that the matrix, interface layer and particles have a different phase composition. The elements in the transition layer are mainly composed of high-entropy alloy elements and Al and Mg elements from the matrix. And the content of Al and Mg elements in the transition layer decreases gradually from the matrix to the particles. On the contrary, few matrix elements gradually spread into the high-entropy alloy particles. In order to explore the diffusion process of elements in the sintering process, the red box in Figure 4 is selected for line scanning, and the results are shown in Figure 5. It can be seen from the figure that the closer the relative position of the high-entropy particles is, the lower the content of Al and Mg elements is. The high-entropy alloy elements show the same distribution trend, and the content keeps the same level. And the element diffusion behavior is basically the same at different sintering temperatures. Figure 6 shows the composites prepared at 650 °C. Compared to the composite material prepared at 600 °C, except for the distribution of Mg elements, other elements have the same distribution form. This is related to the thickness of the interface layer.

Figure 7 shows the backscattering image of the composite sintered at 700 °C. It can be seen from the figure that, compared with the microstructure morphology of the composite obtained at 600 °C and 650 °C, the interface layer of the composite is divided into three layers and the phases are different at this temperature. Magnifying a single particle, such as Figure 7, it can be seen that the innermost interface layer has a regular annular distribution around the high-entropy alloy particles. The other two layers show irregular radiation morphology, and the dividing line can be clearly seen from the distribution map of Mg elements. This is due to the liquefaction of some aluminum alloy powders during sintering due to the high sintering temperature, which is different from the solid nucleation in the lower sintering temperature. In this sintering process, the liquid phase is re-solidified into solid in the cooling process, which makes the behavior of the composite quite different from that of conventional sintering. In order to clarify the behavior in the sintering process and explore the detailed distribution of elements in different interface layers, electron probe tests were carried out at different positions of the interface layer and particles. After normalization, the element composition is shown in

Table 3. The element content of the interfacial layer of composites (at.%).

Regions	Al	Fe	Mg	Si	Cr	Ni	Co
A	74.25	5.41	3.1	/	3.55	7.24	6.45
B	74.85	5.57	2.32	/	3.97	6.58	6.71
C	72.89	6.79	/	0.89	7.62	5.16	6.65
D	72.77	7.09	/	0.79	7.20	5.10	7.05
E	37.44	15.59	/	/	16.43	14.46	16.08
F	12.56	20.73	/	/	22.39	21.58	22.74

. It can be seen in Figure 8 that in the outer layer of the interface layer, the existence of Al and Mg elements ensures that the main matrix in this area is aluminum alloy, and the diffusion phenomenon of high-entropy alloy elements occurs in the aluminum matrix, while the contents of Fe, Co and Ni elements are higher than that of Cr elements, mainly because Fe, Co and Ni elements, as stabilizers of FCC, can stabilize the FCC structure of aluminum alloy during solidification, and the lower mixing enthalpy between elements makes Ni element more inclined to combine with Al element [23,24]. Comparing the blue marked part and the distribution of other interface layers in Figure 7, it can be inferred that all the high-entropy alloys here “dissolve” to form an interface layer. Therefore, it is judged that the formation of the intermediate layer and the inner layer of the interface layer is due to the higher temperature and the larger element concentration gradient (entropy gradient), which causes the aluminum element in the liquid alloy to diffuse to the high-entropy alloy particles. At the same time, Al and Cr elements are used as stabilizers for the BCC structure of high-entropy alloys, and the content in the middle layer is significantly higher, and the content of other high-entropy elements tends to be consistent [16].

Figure 9 shows the XRD spectra of the composites with different sintering temperatures. Form the XRD map, it can be seen that there is no new diffraction peak in the composites, and no new phase is formed in the formation of the interface layer compared to at 550 °C. However, the existence of the interface layer increases the content of Al (Co, Fe) compounds in the composites, and the higher sintering temperature promotes the precipitation of the Al₁₂Mg₁₇ phase in the aluminum alloy solid solution. With the increase in sintering temperature, the peak height of the HEA diffraction peak first increases and then decreases, and the peak height is the highest in the composites sintered at 650 °C, which may also affect the mechanical properties of the composites. Compared with the XRD pattern of the composite sintered at 700 °C, because it is different from at other temperatures, it has more full reaction and interaction with high-entropy alloy elements, forming more Al (Co, Fe) compounds [21]. Among them, the Al0.6CoCrFeNi high-entropy alloy is a biphasic crystal structure of BCC + FCC.

3.3. Interface Micromechanical Properties

In order to explore the micromechanical properties of interface features, nanoindentation was used to test the interface at different sintering temperatures. The p-h curve obtained from the indentation experiment is shown in Figure 10, in which the loading stage of the p-h curve is uniform and continuous. It can be seen from the figure that when the sintering temperature is 650 °C and 700 °C, there is a non-uniform plastic deformation region on the loading curve at the interface, that is, serrated flow. According to the conclusion of Niu et al. [25], this is mainly due to the dynamic strain aging of the alloy, which is the important role in the interaction between solute atoms and dislocations. In this experiment, due to the interface layer formed by the diffusion of high-entropy alloy elements, there are a large number of solute elements in the Al matrix. Hence, there are a large number of mismatches between atoms due to the different atomic sizes. In the process of loading, the increase in dislocation density in the plastic deformation process under the indenter increases the diffusion efficiency of solute elements, interacts with dislocations and produces serrated flow. The reason why there is no serrated flow at 600 °C may be because the diffusion of high-entropy alloying elements in the interface layer is lower at a lower sintering temperature, and the thinner interface layer does not produce more size mismatch.

According to the method of Oliver and Pharr [26,27], the hardness and Young’s modulus can be obtained through the load–depth (p-h) curve. Figure 11 shows the Young’s modulus and hardness of the composite material with their confidence intervals (95%). It can be seen from the figure that the Young’s modulus and hardness show a gradual decline from the particle to the matrix, which is related to the solid solubility of the high-entropy alloy elements at the interface. The Young’s modulus and hardness of HEA particles at 600 °C and 700 °C are 145.7 GPa, 160.7 GPa and 7.9 GPa, 8.7 GPa, respectively. They are lower than 192 GPa and 11.9 GPa at 650 °C, which are consistent with the crystal structure of high-entropy alloys. Relatedly, the Al_0.6CoCrFeNi high-entropy alloy has an FCC + BCC structure, and the FCC structure has better plasticity than the BCC structure. At the same time, the size effect of indentation in nanoindentation magnifies the impact on the indentation result when the indentation depth is shallow. Comparing Figure 11a,b, it can be found that the hardness and Young’s modulus of the interface layer are between 4–5 GPa and 120–130 GPa, respectively. And there is no big difference in the interface properties between 600 °C and 650 °C sintering temperature. When the sintering temperature is 700 °C, the hardness at the interface fluctuates significantly, and it decreases from 5.6 GPa to 3.1 GPa, which is related to two reasons. One is the decrease in solute element content, the other is related to the solvent matrix. When the solvent matrix changes from high-entropy alloy to aluminum matrix, the hardness decreases greatly. This may be due to element concentration gradient, which makes an entropy gradient alloy in the transitional interface. The higher the entropy, the more mismatches exist, which cause high hardness (or Young’s modulus).

4. Conclusions

(1)

With the increase in sintering temperature, the density, strength and hardness of the composites increased at first and then decreased, while the plasticity decreased. The yield strength and compressive strength of the composites have maximum values of 117.3 MPa and 226.6 MPa at 650 °C. When the sintering temperature is lower than 700 °C, the prepared composites have a very good ductility, with the maximum values exceeding 60%.

(2)

With the increase in sintering temperature, there is an obvious transition layer in the composites, and the elements of the transition layer are mainly aluminum and high-entropy alloy. When the sintering temperature is below 700 °C, the thickness of the transition layer increases with the increase in temperature. The maximum thickness of the transition layer is ~6 μm. When the sintering temperature is 700 °C, the liquefaction phenomenon occurs in the sintering process of the composites, and the interface layer changes greatly.

(3)

The bonding mode between matrix and particles in the composites is related to the sintering temperature. Below 600 °C, the bonding mode is mainly mechanical bonding, and above 600 °C, the formation of the transition layer makes the bonding mode change to diffusion bonding. This has a significant effect on the mechanical properties of the composites.