Effects of Reinforcement Volume Fraction on Mechanical Properties and Microstructures of 7075Al Matrix Composites Reinforced by FeCoCrNiAl High-Entropy Alloy Particles

Abstract

High-entropy alloy particles reinforcements improve the mechanical properties of aluminum matrix composites while increasing its density. Investigating the effects of the reinforcement content is conducive to the fabrication of lightweight and high strength composites. FeCoCrNiAl high-entropy alloy particles (HEA_p) reinforced 7075Al matrix composites (HEA_p/Al) with different HEA_p volume fractions of 5, 10, 15, and 20 vol% were fabricated via a vacuum ball milling and vacuum hot pressing sintering, and then the microstructures, interface, density, and mechanical properties of the HEA_p/Al composites were characterized. The results indicated that composites with high relative density can be obtained with a holding time of 10 min at 580 °C, 30 MPa; the HEA_p distribution was homogeneous as the increase of reinforcement volume fraction decreases and forms agglomeration, especially when the volume fraction is up to 15 vol% and 20 vol%; the density and hardness of HEA_p/Al composites improved with the increase of reinforcements’ volume fraction, whereas the flexural strength and fracture toughness of HEA_p/Al composites increased at first and then decreased; the HEA_p/Al composites reinforced by 10 vol% HEA_p showed the maximum flexural strength and fracture toughness, which were increased by 124.6% and 107% compared with 7075Al, respectively; the improvement of mechanical properties was attributed to the dislocation strengthening behavior and load transfer effect of the HEA_p reinforcement.

1. Introduction

Owing to their excellent characteristics, such as low density, high specific strength and modulus, good dimensional stability, and wear resistance, aluminium matrix composites have a wide application prospect in aerospace engineering, military area, and automotive fields [1,2]. According to the reinforcement types, Al matrix composites can be divided into two kinds, such as discontinuous reinforcement reinforced Al matrix composites and continuous reinforcement reinforced Al matrix composites. Interestingly, Al matrix composites reinforced by discontinuous reinforcement, having better machinability and isotropic properties compared with continuous fiber reinforced aluminum matrix composites, is one kind of the most widely used composites in engineering practice. The traditional discontinuous reinforcements, including Al₂O₃ [3], SiC [4], BN [5] and other ceramic particles, have obvious thermal expansion mismatch with Al matrix, which can lead to the generation of microcracks and defects in composites and weaken the strengthening effect of reinforcements. As one rare species of metal materials, high-entropy alloys have excellent corrosion properties and physical properties [6,7,8,9], including high strength, high hardness, high modulus, and high thermal stability, and can form a good metallurgical bonding interface with Al [10]; these characteristics illustrate that high-entropy alloys can meet the performance requirements of ideal reinforcements used for Al matrix composites.

Currently, research on high-entropy alloys reinforced Al matrix composites have been reported. Zhu et al. [11] studied the effect of the particle shapes of Al_0.23Cu_0.75FeCoNi high-entropy alloy reinforcements on the properties of Al matrix composites and found that flake particles were easy to agglomerate in the matrix, which would cause the initiation of microvoids so that the failure mode of composites was dominated by the rupture of agglomeration. Conversely, ellipsoidal particles were not only beneficial to the uniform distribution of reinforcements, but also effectively inhibited stress concentration between reinforcement and matrix—Yuan et al. [12] studied the interface microstructure on mechanical properties of 5052Al matrix composites reinforced with 7 vol% content of Al_0.6CoCrFeNi, and found that core-shell structure forming at interface without new phase generated between reinforcements and matrix improved Young’s modulus and hardness of Al matrix composites. The compressive properties of FeNiCrCoAl₃ reinforced 2024Al composites fabricated by two-stage method consisting of ball milling and hot extrusion were studied in ref. [13], in which composites showed excellent compressive strength with the values of 700 MPa compared with the 2024Al matrix. Karthik et al. [14] realized that the tensile and compressive properties of AA5083 matrix composites can be improved by the addition of CoCrFeNi nanocrystalline high-entropy alloy particles, and the composites possessed much better combination of strength and ductility compared with conventional Al matrix composites reinforced by ceramic particles. Also, the high-entropy alloy could improve the wear resistance, and macro- and micro-hardness [15,16] of metal matrix composites. Especially, thermal expansion coefficient of high-entropy alloy particle reinforced Al matrix composites decreased as high-entropy alloy volume fraction increased [17], and even could be compared favourably with those of ceramic particles reinforced Al matrix composites [18,19]. Nevertheless, the addition of high-entropy alloy particles with high density (>6 g/cm³) would weaken the lightweight advantages of Al matrix composites with the improving of mechanical properties. Controlling the content of high-entropy alloy reinforcement in composites is significant for the fabrication of lightweight composites on the premise of maximally optimizing the properties of the alloy matrix. Unfortunately, there were few reports about the effect of reinforcement on the combination of density and mechanical properties.

With a high microhardness [20], specific strength, Young’s modulus [21], certain ductility [22], and excellent thermal stability [23], FeCoCrNiAl high-entropy alloy particles (HEA_p) with face-centered cubic structure, can be obtained using powder metallurgy method with low energy consumption [24], and used in Al matrix composites as metal particle reinforcement with ideal performance. In this work, 7075Al powders and FeCoCrNiAl high-entropy alloy particles powders were used for the matrix and reinforcement, respectively. FeCoCrNiAl HEA_p reinforced 7075Al composites (HEA_p/Al) with different volume fractions of reinforcement were prepared by vacuum ball milling firstly and then a hot pressing sintering process. By characterization of microstructure, density, and mechanical properties of composites, the effects of volume fraction of reinforcements on mechanical performances of HEA_p/Al were revealed so that a theoretical reference could be provided for the fabrication of lightweight high performances aluminum matrix composites.

2. Experimental

2.1. Fabrication of HEA_p/Al Composites

FeCoCrNiAl high-entropy alloy particle reinforcements (HEA_p) were provided by Beijing Gaoke New Materials Co., Ltd., Beijing, China, and particles with size of about 13 μm are regular spherical. 7075Al spherical powders (provided by Hebei Qinbang New Material Co., Ltd., Shijiazhuang, China) with particle diameter of about 25 μm were used for aluminum matrix.

The fabrication process of FeCoCrNiAl HEA particles reinforced 7075Al composites (HEA_p/Al) is illustrated in Figure 1; its procedure proceeded as follows: Firstly, the mass of HEA_p was calculated on the scales of 5, 10, 15, and 20 vol% of composites respectively, and then weighed using an electronic analytical balance (AUW120D, Shimadzu Co., Tokyo, Japan) to obtain the scheduled mass, while the scheduled mass of 7075Al powder was weighed in a similar way with the volume fraction of 95, 90, 85 and 80 vol% respectively; secondly, the scheduled HEA_p and 7075Al were put into sealed ball milling jars, and then the jars were filled with high purity Ar with 0.04 MPa for oxidation protection. The mixing powders were milled for 6 h with a milling rate of 300 r/min under a certain ball to powders weight ratio of 10:1; lastly, the milled powders were packed into graphite mold, and successively vacuum sintered using program control vacuum sintering instrument (RYJ-2000Z, Zhengzhou abrasive grinding Research Institute Co., Zhengzhou, China) at 580 °C, 30 MPa for 10 min. In order to reduce cavity defects in composites, the heating temperature was held for 5 min at 420 °C. After cooling with furnace, the HEA_p/Al composites with different HEA_p volumes were prepared.

2.2. Testing and Characterization

According to the YB/T 5349-2014 and ASTM E1304-97 (2020) standards, three-point bending samples with 3 mm × 4 mm × 30 mm long stripe and fracture toughness test samples with the size of 3 mm × 4 mm × 30 mm (integral notch, depth 2 mm) were processed by the cutting of HEA_p/Al composites using wire electro-discharge machining, and successively polished the surface of the samples through 240, 400, 600, 800, 1000, 1500# metallographic water-resistant sandpaper. The tests on flexural strength and fracture toughness of HEA_p/Al composites were carried out on an electronic universal testing machine (MTS E44.304, MTS Systems Co., Eden Prairie, MN, USA) with a constant movement rate of 0.3 and 0.1 mm/min respectively at ambient temperature. Each final value represents the average result of three measurements to ensure the accuracy and repetitiveness of measured results.

The phase components of HEA_p/Al composites were characterized from 20° to 80° with a scanning rate of 6°/min using x-ray diffractometer (D8 ADVANCE, Bruker AXS Co., Karlsruhe, Germany). The densities of HEA_p/Al composites with different reinforcement contents were tested using the automatic density measuring instrument (MH-300A, Xiamen east equipment Co., Ltd., Xiamen, China). Rockwell hardness of HEA_p/Al composites was characterized using an electric Rockwell hardness tester (HRD-150, Lailuote experiment instrument Co., Ltd., Yantai, China), the average value of seven points was selected as final value. Lastly, the microstructures, interface element distribution, and fracture morphologies were investigated using a scanning electron microscope with an energy spectrometer (JSM-6510SEM, JEOL Ltd., Tokyo, Japan) and transmission electron microscope (FEI Talos F200X TEM, FEI Co., Hillsboro, OR, USA).

3. Results and Discussion

3.1. X-ray Diffraction Patterns of HEA_p/Al Composites

The X-ray diffraction patterns of unreinforced 7075Al and HEA_p/Al composites with 10 vol% HEA_p content are shown in Figure 2. Visibly, crystal diffraction peaks with high diffraction intensity of Al occur at 38.5° (2θ), 44.7° (2θ), 65.1° (2θ), and 78.2° (2θ), which were corresponding to (111), (200), (220) and (311) crystal planes respectively. In addition, the phase diffraction peaks of Al₁₂Mg₁₇, AlCuMg, and AlZn Al₀.₄₀₃Zn₀.₅₉₇ with weak diffraction intensity present in the spectrum, indicate that the elements Mg, Cu, and Zn reacted with Al to form intermetallic compounds. After the addition of HEA_p, the crystal diffraction peaks of HEA_p and Al₉Co₂ occur at the HEA_p/Al patterns with the exception of Al and its compounds. Notably, Al₀.₄₀₃Zn₀.₅₉₇ (PDF# 52-0856) and Al diffraction angles (PDF#89-4037) were similar; they were represented by the same symbol. These results prove that the HEA_p still maintained a complete structure, while the metallurgical bonding was formed at the interface between reinforcements and matrix.

3.2. Microstructure of HEA_p/Al Composites

Figure 3 shows microstructure morphologies of HEA_p/Al composites reinforced with 5, 10, 15 and 20 vol% HEA_p respectively. As shown in Figure 3a,b, HEA_p reinforcements are distributed homogeneously in the matrix when the HEA_p volume fraction did not exceed 10 vol%. However, the distribution uniformity of HEA_p become worse as the HEA_p content was up to 15 vol% and 20 vol% respectively, and there are a small amount of particle agglomeration occurring in the 7075Al matrix (shown in Figure 3c,d). Besides, the accompanying diagram of Figure 3b illustrates that element line scanning obviously varies from Al matrix to HEA_p, which indicates that HEA_p still maintained the original regular spherical without obvious melting and crushing, which was corresponding to the presence of HEA_p diffraction apex in X-ray diffraction analysis of HEA_p/Al composites.

The distribution uniformity of reinforcements in the matrix plays an important role in the properties of composites, and is mainly affected by the volume fraction and particle diameter of reinforcements and matrix. The critical volume fraction W_crit of reinforcements with a homogeneous distribution is calculated by the formula of Equation (1) shown as follows [17]:

$$W_{\mathrm{crit}}=\mathsf{\alpha }\times \frac{V_r}{V_r+V_m}=\mathsf{\alpha }\times \left\{1-{\left[1+{\left(\frac{\mathrm{d}}{\mathrm{D}}\right)}^3+\left(\frac{2}{\sqrt{\mathsf{\lambda }}}+\mathsf{\lambda }\right)\times {\left(\frac{\mathrm{d}}{\mathrm{D}}\right)}^2+\left(\frac{1}{\mathsf{\lambda }}+2\sqrt{\mathsf{\lambda }}\right)\times \frac{\mathrm{d}}{\mathrm{D}}\right]}^{-1}\right\}$$

(1)

where α represents the fixed parameter with the value of 0.18, d and D refers to the particle size medium diameter of HEA_p (d = −13 μm) and 705Al matrix (D = −25 μm) respectively, and λ indicates the extrusion ratio (the value of λ without extrusion is 1). The values of W_crit of HEA_p calculated using Equation (1) is about 12.87 vol%, which means particle agglomeration became serious in the matrix when the reinforcement content exceeded 12.87 vol%. Thus, the particle agglomeration of HEA_p/Al composites with 20 vol% HEA_p is more serious than that of HEA_p/Al composites with 15 vol% HEA_p.

3.3. Interface Microstructure of HEA_p/Al Composites

The energy-dispersive X-ray spectroscopy (EDS) mappings of elements Al, Co, Cr, Ni, Fe and Mg at the interface between HEA_p and 7075Al matrix are shown in Figure 4. Clearly, there is an obvious boundary formed at the interface caused by the heterogeneity of elemental distribution, which was similar to the line distribution of elements. Notably, the element Mg is accumulated around the HEA_p, which was caused by the difference of bonding energy between elements. The enthalpy of mixing (∆H_mix) between elements Al and Mg is −2 Kj/mol; its absolute value is lower than that of Mg-Ni (−4 Kj/mol) [25], and there is an interdiffusion formed between Ni and Mg. However, the interdiffusion between Mg and Ni is suppressed by the high stability of HEA_p so that the Mg elements enrichment area was formed at the interface between HEA_p and 7075Al matrix.

Interface microstructure between HEA_p and 7075Al matrix is shown in Figure 5. As shown in Figure 5a,b, an obvious boundary with incoherent lattice is formed between HEA_p and 7075Al. Besides, many dislocations can be found around the HEA_p in the 7075Al matrix, which are generated and stored because of thermal expansion coefficient mismatch.

The density parameters reveal indirectly the defect content of composites and evaluate effectively the reasonability of the sintering process. The measured density and relative density of HEA_p/Al composites with various HEA_p volume fractions are shown in Figure 6. Clearly, the measured density of HEA_p/Al composites increase as the HEA_p volume fraction improved from 0 vol% to 20 vol%. The HEA_p/Al composites with 20 vol% HEA_p content showed the maximum value with 3.57 g/cm³, which have an improvement of about 27% compared with unreinforced 7075Al matrix (about 2.81 g/cm³). Besides, the relative density has no obvious variation as to the improvement of HEA_p content. In addition, all the HEA_p/Al samples had a high relative density exceeding 99%.

Theoretically, the density of HEA_p/Al composites follow the rule of mixture, which means that the theoretical density was decided by the volume fraction of reinforcement and matrix, the relation is shown as Equation (2):

p_c = V_rp_r + V_mp_m

(2)

where, V and p refer to the volume fraction and density, subscript m, c, and r represent the matrix, composites, and reinforcement respectively. The theoretical density of HEA_p and 7075Al is calculated by Equation (2) with the p_HEA= 6.72 g/cm³ and p_Al = 2.82 g/cm³, the theoretical values of HEA_p/Al with 0, 5, 10, 15 and 20 vol% are 2.82, 3.04, 3.21, 3.41 and 3.59 g/cm³, which means the values were similar to the measured density tested by the Archimedes method. According to relevant reference [26], breaking of the oxide layer of particles and decrease of pores attributed to the mechanical and thermal effects during the sintering process can obtain a high relative density exceeding 96%. In this work, the high relative density larger than the former can demonstrate that the sintering process was reasonable and the carve defects were absent in the composites.

3.4. Hardness of HEA_p/Al Composites

As one of the significant parameters of mechanical properties, hardness can well reflect the magnitude of plastic deformation resistance of composites. The Rockwell hardness values of HEA_p/Al composites with different HEA_p contents are shown in Figure 7. The hardness increases with the improvement of HEA_p volume fraction, and reaches a maximum with the value of 84HRC when the volume fraction was up to 20 vol%. Compared with the unreinforced Al matrix (74HRC), the hardness of HEA_p/Al composites with 5, 10, 15, and 20 vol% HEA_p is improved by about 4%, 6.7%, 9.5%, and 13.5%, and the value is 77, 79, 81 and 84HRC, respectively. The improvement of Rockwell hardness of HEA_p/Al is mainly attributed to the following reasons: (1) the hardness of HEA_p is about 500HV [17], which is considerably larger than that of the 7075Al matrix (74HRC ≈ 80HV). The addition of the hardening phase not only bears mass load but also restrains the plastic deformation of the Al matrix, which means that the higher the reinforcement content, the higher the hardness; (2) the semi-coherent or incoherent interface between reinforcement and matrix inhibit the movement of dislocations in aluminum matrix; (3) increase of dislocation density at interface caused by the thermal expansion mismatch between HEA_p and 7075Al matrix lead to difficulty in dislocation movement. Hence, the HEA_p/Al composites have a better ability to resist plastic deformation than unreinforced Al matrix.

3.5. Mechanical Properties and Failure of HEA_p/Al Composites

The flexural strength and fracture toughness of HEA_p/Al composites with different HEA_p volume fractions of 0, 5, 10, 15, and 20 vol% respectively are shown in Figure 8. It can be seen that the values of flexural strength and fracture toughness have a variation of increase at first and then decrease, and are higher than those of 7075Al matrix, which indicates that the HEA_p possessed excellent strengthening and toughening effect on the mechanical properties of 7075Al matrix. The HEA_p/Al composites with 10 vol% HEA_p have the best flexural strength of 640 MPa and fracture toughness of 13.67 MPa·m^1/2 compared with other HEA_p/Al composites, which is improved by about 124.2% and 107.8% respectively compared with 7075Al matrix. Interestingly, the flexural strength and fracture toughness of HEA_p/Al composites with 15 vol% HEA_p is up to 582 MPa and 13.45 MPa·m^1/2, which are slightly less than those of HEA_p/Al composites with 10 vol% HEA_p.

The fracture morphologies of HEA_p/Al composites can reflect their failure mode variation with the improvement of HEA_p content. As shown in Figure 9a, a large amount of dimples appears on the fracture surface with small-size appearing on fracture of 7075Al, this indicates that failure mode was essential of obvious ductile fracture characteristics. With the addition of HEA_p reinforcements, the fractures become more uneven as the increase of HEA_p content, as shown in Figure 9b–d. Moreover, two varieties of dimples with obviously different sizes appear on the fracture. The dimples with large size are formed because of the interfacial delamination of HEA_p while the small-sized dimples are generated by the ductile fracture of 7075Al matrix. In addition to the formation of different size dimples, particle agglomeration can be observed on the fracture of HEA_p/Al composites with 20% HEA_p content (Figure 9d), which caused the matrix deficiency between HEA_p.

The strengthening mechanisms of particles reinforced metals matrix composites mainly include dislocation strengthening behavior and load transfer effect [27]. Because of the rapid cooling rate of HEA_p/Al from sintering temperature to room temperature, mismatch strain ε_CTE will be induced owing to the significant difference in thermal expansion coefficient between HEA_p and 7075Al matrix. Then, geometrically necessary dislocation loops will be imposed around the surface of reinforcement to accommodate the thermal mismatch deformation. Hence, the number of dislocations loop increases as the improvement of HEA_p volume fraction, in other words, high-density dislocation loops can restrain the crack initiation and propagation of microcracks. Besides, the metallurgical bonding interface between reinforcements and matrix improves the load transfer efficiency from 7075Al matrix to HEA_p, and constrains the deformation of 7075Al matrix. These are reasons for the performance improvement of HEA_p/Al composites. In comparison, the load transfer effect plays a greater role compared with dislocation strengthening behavior due to the incomparable size between HEA_p and dislocation loops. Interestingly, Excessive content of reinforcement (20 vol% HEA_p) lead to aggravating of stress concentration and microcrack initiation and propagation, while decrease dislocation density and load transfer efficiency, these means strengthening effect would be weakened, so that the mechanical properties of HEA_p/Al composites with 20 vol% is lower than those of HEA_p/Al composites with 10 vol% and 15 vol%.

4. Conclusions

• FeCoCrNiAl high-entropy alloy particles (HEA_p) reinforced 7075Al matrix composites (HEA_p/Al) with dense microstructure and homogeneous distribution of reinforcement were fabricated by vacuum hot pressing sintering process at 580 °C, 30 MPa with 10 min holding time;
• With the increase of HEA_p volume fractions, the hardness of HEA_p/Al composites increase and have a larger value compared with the 7075Al matrix. While flexural strength and fracture toughness of HEA_p/Al composites increase first and then decrease. the HEA_p/Al composites with HEA_p volume fraction of 10 vol% show the highest maximum value of ~640 MPa and ~13.67 MPa·m^1/2 and temperate density with the value of 3.19 g/cm³, respectively. The strengthening effect including load transfer effect and dislocation strengthening behavior increased with the improvement of HEA_p content from 5 vol% to 15 vol%, while it was weakened when HEA_p was up to 20 vol%.