Synergistic strengthening mechanism of copper matrix composite reinforced with nano-Al 2 O 3 particles and micro-SiC whiskers

Although Cu–Al2O3 composites have good comprehensive performance, higher mechanical properties and arc erosion resistance are still required to meet heavy-duty applications such as electromagnetic railguns. In this work, a novel hybrid SiCw/Cu–Al2O3 composite was successfully prepared by combining powder metallurgy and internal oxidation. The microstructure and mechanical behavior of the SiCw/Cu–Al2O3 composite were studied. The results show that nano-Al2O3 particles and micro-SiCw are introduced into the copper matrix simultaneously. Well-bonded interfaces between copper matrix and Al2O3 particles or SiCw are obtained with improved mechanical and arc erosion resistance of SiCw/Cu–Al2O3 composite. The ultimate tensile strength of the SiCw/Cu–Al2O3 composite is 508.9 MPa, which is 7.9 and 56.1% higher than that of the Cu–Al2O3 composite and SiCw/Cu composite, respectively. The strengthening mechanism calculation shows that Orowan strengthening is the main strengthening mechanism of the SiCw/Cu– Al2O3 composite. Compared with Cu–Al2O3 composite, the hybrid SiCw/Cu–Al2O3 composite has lower arc time and energy and better arc stability.


Introduction
Cu-Al 2 O 3 composites have a wide range of applications in electrical contact due to their excellent conductivity [1] and high strength at elevated temperatures. For example, in the electromagnetic railguns system, the guide rail is seriously worn and partly melted due to the high projectile launching speed and driving current of the armature/ guide rail system [2]. This kind of equipment puts forward higher requirements on mechanical properties [3], current-carrying friction and wear resistance, and arc erosion resistance of materials [4]. However, the commercial Cu-Al 2 O 3 composites cannot meet the above requirements, and copper matrix materials used in the guide rail with better mechanical and electrical erosion resistance are still not developed.
Recently, hybrid copper matrix composites with two or more reinforcing phases have present better performance compared with the copper matrix composites reinforced by a single reinforcement [5]. And, the reported papers on synergistic reinforcements are mainly focused on CNTs [6]; Cr [7] and WC particles [8] strengthened the Cu-Al 2 O 3 composite [9]. For example, Zhang et al. [10] reported that the ultimate tensile strength of a Cu-Al 2 O 3 -La composite was 309 MPa. Pan et al. [11] found that the ultimate tensile strength (345 MPa) of Cu-6.1 vol% CNTs-1.2 vol% Al 2 O 3 composite was 41.1% higher than that of Cu-CNTs composite. Singh and Gautam [12] discovered that the ultimate tensile strength of Cu-0.5 vol% WC-5.0 vol% Al 2 O 3 -2.4 vol% Cr composite (385 MPa) was 71.9% higher than that of pure copper (224 MPa). Although the addition of particles or whiskers is beneficial to improve the mechanical properties of Cu-Al 2 O 3 composites, their ultimate tensile strength are still under 400 MPa [13]. The reason for the low mechanical property is the poor interface bonding between reinforcements and copper matrix, in which the reinforced phases are introduced into the copper matrix by a common powder metallurgy process [14]. However, hybrid copper-based materials with high mechanical properties fabricated by in-situ synthesis have not yet been reported.
Currently, SiC w has become a potential reinforcement phase in copper-based materials, which has the advantages of low density [15], high strength [16], corrosion resistance [17], and high-temperature stability [18]. The object of this work is to develop a novel SiC w /Cu-Al 2 O 3 composite with well-bonded interfaces between copper matrix and reinforcements. And, the preparation procedure combined powder metallurgy and internal oxidation, and the nano-Al 2 O 3 particles and micro-SiC w were introduced into the copper matrix simultaneously. The well-bonded interfaces between copper matrix and Al 2 O 3 particles or SiC w are obtained with improved mechanical and arc erosion resistance. The strengthening mechanisms of the SiC w /Cu-Al 2 O 3 composite were calculated and discussed. The fabrication procedures and experimental results can provide a reference for the design and preparation of high-performance copper matrix composites.
The experimental procedures were as follows: first, mixing of powders was performed. Cu-0.2 wt% Al alloy powder, Cu 2 O powder, and SiC w were mixed in appropriate proportions. The powders were blended by a QQM/B light ball mill for 16 h at 35 rpm and a ball-to-powder ratio of 5:1. Note that to realize the complete internal oxidation of Al to produce Al 2 O 3 in the subsequent internal oxidation process, the mass ratio of Cu 2 O powder to Cu 2 O powder is calculated to be 1.1:1 [19] (M(Cu-0.2 wt% Al):M(Cu 2 O) = 56.8:1). Second, cold isostatic pressing was performed. The mixed powders were pressed into cylindrical samples (Φ50 × 40 mm) in LDJ200/600-300 cold isostatic press at a pressure of 210 MPa and held for 10 min. Third, sintering was performed. The obtained cylindrical samples were sintered in a tube furnace protected by argon at 950℃ for 3 h. (Nano-Al 2 O 3 particles were formed in the chemical reaction of internal oxidation, and the specific process has been reported in ref. [20]). Finally, the sintered samples were put into a ZT-200-22Y vacuum sintering furnace under a hydrogen-argon atmosphere and kept at 900℃ for 2 h for reduction. To further improve the relative density of the copper matrix composites, the sintered samples were put into a YA32-315 four-column hydraulic press for hot extrusion, the extrusion ratio was 10:1, and bars with a diameter of 15 mm were obtained. For comparison, the Cu-Al 2 O 3 composite, the SiC w /Cu composite, and the pure copper were prepared under the same conditions. The specific composition is listed in Table 1.

Material characterization
The microstructures of composites were observed by inverted metallurgical microscopy (DMi8C, Leica Microsystems Inc). The transmission electron microscopy (TEM) samples were polished by a precision polishing system (GATAN691, USA), and the interface between the matrix and reinforcing phase was characterized by high-resolution transmission electron microscopy (HR-TEM; JEM-2100, JEOL, Japan). The grain size of the copper matrix composites was analyzed by field-emission scanning electron microscopy (FE-SEM; JSM-7800F, JEOL, Japan).

Performance tests
To evaluate the tensile strength of the composites, the samples were processed into rod-shaped tensile samples with a diameter of 5 mm and a gauge length of 25 mm. The tensile tests were performed by a tensile testing machine (AUTOGRAPH AG-I 250KN, Japan) by stretching along the extrusion direction with a tensile strain rate of 0.5 mm/min. Tensile fractures were analyzed by scanning electron microscopy (SEM; JSM-IT100, JEOL, Japan). Electrical contact tests were carried out by an electrical contact tester (JF04C, Kunming Institute of Precious Metals, China) shown in Figure 1. As-extruded samples were processed into electrical contact samples with a diameter of 3.8 mm and a length of 10 mm. Each contact pair was tested for 5,000 operation times under resistance load, with a voltage of 24 V DC, a current of 15 A, an average contact force of 50 cN, and an electrode spacing of 2 mm. Data on arc energy and arcing duration were automatically recorded by the JF04C test system. The arc erosion morphologies of the cathode and anode contacts were observed by SEM (JSM-IT100, JEOL, Japan).

The microstructure of the SiC w /Cu-Al 2 O 3 composite
The metallographic structures of pure copper, the Cu-Al 2 O 3 composite, and the SiC w /Cu-Al 2 O 3 composite are presented in Figure 2. Comparing Figure 2a with Figure 2b shows that the grain size of the Cu-Al 2 O 3 composite is smaller than that of pure copper, which indicates that Al 2 O 3 particles are formed in the process of internal oxidation and can inhibit grain growth. Figure  2b and c show that compared with the Cu-Al 2 O 3 composite, the grain size of the SiC w /Cu-Al 2 O 3 composite has no obvious decrease, which indicates that the addition of SiC w has a certain influence on the grain size change but has little effect. Figure 2d shows that the SiC whiskers are arranged in an orderly manner and distributed along the extrusion direction without large-area agglomeration. Figure 3 shows the high-resolution transmission electron microscopy (HR-TEM) images of the SiC w /Cu-Al 2 O 3 composite. Figure 3a shows that γ-Al 2 O 3 particles with a particle size of 3-8 nm are uniformly dispersed in the copper matrix, with an average particle spacing of 50 nm. Fine nano-Al 2 O 3 particles are more conducive to pinning dislocations, hindering dislocation movement and grain boundary slip and thus improving the strength of composites [21]. Figure 3b is an Inverse Fast Fourier Transform (IFFT) image of the area in the yellow dotted box in Figure 3a. The interface between γ-Al 2 O 3 and the copper matrix is flat, and there are no physical gaps at the interface and no compounds produced by adverse reactions. The HR-TEM image of the interface between SiC w and the copper matrix in Figure 3d shows that there is an obvious amorphous transition layer at the interface between the copper matrix and SiC w . This is because Si atoms in SiC diffuse into the copper matrix, leaving only a layer of carbon at the interface between the copper matrix and SiC w [22]. This can also be proven by the existence of an amorphous diffuse scattering halo in the FFT diagram in the lower-left corner of Figure 3d. Meanwhile, the SAD pattern in the upper right corner Figure 3d shows the SiC diffraction spots, proving that the whiskers used are β-SiC. Compared with α-SiC, β-SiC has better hardness, tensile strength, elastic modulus, and high-temperature resistance, which is more conducive to improve the tensile strength of composites [23]. It is generally believed that the rough or bamboo-like surface of whiskers can increase the friction between whiskers and the matrix, thus improving the interfacial bonding strength [24]. Excessive interfacial strength leads to the fracture of whiskers and copper matrix composites during stretching, which limits the contribution of the whisker pull-out effect to the toughness of copper matrix composites [25]. The amorphous ribbon transition layer formed at the interface between the copper matrix and SiC w can reduce the interfacial bonding strength, reduce the shear stress that needs to be overcome when whiskers are pulled out,   facilitate whisker pulling out, and play an obvious role in improving the tensile strength and toughness of the composite [26].

Mechanical properties and fracture
morphology of the SiC w /Cu-Al 2 O 3 composite Figure 4 shows the engineering tensile stress-strain curves of pure copper and its composites. In summary, compared with pure copper, the strength of the SiC w /Cu composite is not greatly improved by adding a single reinforcing phase of SiC w , but the tensile strength of the SiC w /Cu-Al 2 O 3 composite is greatly improved. The addition of the reinforcing phase inevitably led to a decrease in the ductility of the composites [27]. Hybrid reinforcement can improve the spatial configuration of each reinforcing phase, which is beneficial to fully exploiting the advantages of SiC w and nano-Al 2 O 3 particles and then improve the strength of copper matrix composites [29]. Figure 5 shows the tensile fracture morphology of pure copper and copper matrix composites. The pure copper sample has obvious plastic deformation before fracture, the fracture surface is mainly composed of large and deep dimples and torn edges, and the fracture surface is characterized by a typical micropore aggregation fracture surface, as shown in Figure 5a. Compared with the fracture morphology of pure copper, the dimples of the Cu-Al 2 O 3 composite become smaller and shallower, which worsens the plasticity of the composite (Figure 5b). Figure 5c shows the fracture morphology of the SiC w /Cu composite, and there is a large amount of agglomerated SiC w at the fracture. Agglomerated SiC w cannot effectively contact the copper matrix, which reduces the interfacial bonding strength and makes it easier to fall off under the action of an external force, resulting in the lower strength of the SiC w /Cu composite [30]. The fracture morphology of the SiC w /Cu-Al 2 O 3 composite (Figure 5d) shows that the axial direction of SiC w is the same as the tensile direction, which hinders grain boundary slip, and there are "SiC w bridges" and SiC w pulling out at the crack. The bridging effect of SiC w can hinder crack propagation and promote crack deflection. Nano-Al 2 O 3 particles can inhibit grain growth and promote crack deflection, which is conducive to improve the strength of composites, and dispersed nano-Al 2 O 3 particles are beneficial to improve the plasticity of materials [31].

Analysis of strengthening mechanism of the SiC w /Cu-Al 2 O 3 composite
There are many strengthening mechanisms in composites, such as grain refinement (GR) [32], Orowan strengthening (OS) [33], load transfer (LT) [34], and thermal mismatch (TM) [35]. Due to the different thermal expansion coefficients of the reinforcing phase and copper matrix, thermal mismatch strengthening occurs in the cooling process of the composite material, but the samples will play a considerable role in thermal mismatch strengthening only during rapid cooling [36]. As the cooling process of this test is furnace cooling or slow cooling, the contribution of thermal mismatch strengthening to the strength of materials is temporarily ignored. Therefore, the strength contribution of the SiC w /Cu-Al 2 O 3 composite mainly comes from grain refinement, Orowan strengthening, and load transfer strengthening.

Orowan strengthening
Research shows that long whiskers with large aspect ratios cannot be effectively surrounded by dislocation loops [37]. Therefore, the contribution of Orowan strengthening to the strength of the SiC w /Cu-Al 2 O 3 composite mainly comes from dispersed nano-Al 2 O 3 particles. Dislocation lines cannot cut through hard Al 2 O 3 particles directly, but bend gradually under the action of external stress until the dislocation lines at both ends meet to form a closed dislocation loop, and the remaining dislocation lines continue to move forward under the action of external force. The Orowan strengthening equation is as follows [38]: where G is the shear modulus (42.1 GPa for Cu [39]), v is the Poisson's ratio (0.326 for Cu [39]), b is the Burger's vector (0.256 nm for Cu [39]), L is the particle spacing, and D is the average particle size. According to the calculation, the contribution of the Orowan strengthening mechanism to the strength of the SiC w /Cu-Al 2 O 3 composite is 145.2 MPa. Equation (1) shows that the smaller the distance between particles is the greater the critical shear stress required for dislocation motion. Dislocation loops around particles also hinder the movement of dislocation sources and other dislocations, and further strengthen the copper matrix. Figure 2b and c show that the addition of SiC w and nano-Al 2 O 3 particles can obviously refine the grains. Theoretically, SiC w also hinders the movement of grain boundaries, but in this experiment, the addition of silicon carbide whiskers has little effect on grain size, and nano- Al 2 O 3 particles play a major role in grain refinement. To further determine the influence of nano-Al 2 O 3 particles and SiC w on grain size, EBSD analysis was carried out, as shown in Figure 6. The movement of nano-Al 2 O 3 particles on the grain boundary of the copper matrix produces Zener resistance, inhibits grain growth, and produces Hall-patch fine grain strengthening, according to the following equation [38]:

Grain refinement
where k is a constant (Cu = 0.07 MPa × m 0.5 [40]), d c is the grain size of the SiC w /Cu-Al 2 O 3 composite (d c = 1.43 μm), and d m is the average size of pure Cu. The σ GR value provided by nano-Al 2 O 3 particles was calculated to be 35.6 MPa.

Load transfer
When the load is fully applied and the whiskers arranged along the tensile direction are evenly distributed in the copper matrix, load transfer strengthening can be effectively realized [41]. Whiskers or short fibers are pulled out and broken during stretching. When the length is less than a certain critical length will be drawn out. When the length of SiC w is less than a certain critical length l c , the whiskers or short fibers will be pulled out; otherwise, it will break. The critical length can be obtained according to the equilibrium condition of force [42]: where σ SiCw is the tensile strength (20.8 GPa) of SiC w , D SiCw is the diameter of SiC w (0.5 µm), τ y is the matrix shear stress, and σ my is the yield strength of the Cu matrix (200.1 MPa). According to the calculation, the critical length is 52 μm, which is far greater than the actual length of whiskers; so, SiC w fails in the form of pulling out during the drawing process, which can be verified from Figure 5d. The strength increase provided by the load transfer strengthening of silicon carbide whiskers can be calculated by the following equation: where f SiCw is the volume of the reinforcing phase (4.7 vol%). According to equation (4), the contribution of load transfer enhancement to strength is 61.1 MPa. There is a positive correlation between the strength of the composites and the volume fraction of the reinforcing phase; that is, with increasing volume fraction, the strength of the composites increases gradually, but if the volume fraction of the reinforcing phase is too high, the strength of the composites will decrease.

Synergistic strengthening
The total contribution of various strengthening mechanisms in the Cu-Al 2 O 3 composite is 370.9 MPa, which is 68.8 MPa less than the actual tensile yield strength Liu et al. [43] studied the strengthening mechanism when SiC w and nanoparticles were added alone or in combination and found that the combination of SiC w and nanoparticles can change the microstructure of materials, and thus the mechanical properties of composites. Particle-whisker hybrid-reinforced copper matrix composites exert the advantages of each reinforced phase, and at the same time, each component synergistically strengthens copper matrix composites [37]. As seen in Figure 5c, agglomerated SiC w can be seen in the fracture morphology of the SiC w /Cu composite, but there is no large area of agglomerated SiC w in the fracture morphology of the SiC w /Cu-Al 2 O 3 composite in Figure 5d.
The synergistic effect is that nano-Al 2 O 3 particles can adjust the spatial distribution of SiC w in the copper matrix and reduce the agglomeration of SiC w . With the addition of SiC w , the distribution of nano-Al 2 O 3 particles is more uniform, and the spatial configuration is better, as shown in Figure 7. The contact area between dispersed SiC w and the copper matrix increases, which is beneficial for improving the interfacial bonding strength and reducing the shedding phenomenon caused by the agglomeration of SiC w during stretching. Nano-Al 2 O 3 particles further restrict the shedding and pulling-off of SiC w and further improve the tensile strength of the SiC w /Cu-Al 2 O 3 composite. The above results show that SiC w and nano-Al 2 O 3 particles have synergistic effects on strengthening copper matrix composites. The difference between the calculated value and the experimental value can be regarded as the contribution value of thermal mismatch and synergistic strengthening mechanism to the strength of the SiC w / Cu-Al 2 O 3 composite, and the contribution ratio to the strength is 7.1%.

Arc erosion resistance of the SiC w /Cu-Al 2 O 3 composite
The distributions of arc duration and arc energy data of Cu-Al 2 O 3 composite and SiC w /Cu-Al 2 O 3 composite are shown in Figure 8a and b. Arc duration and arc energy have the same fluctuation tendency. The electrical contact test of the Cu-Al 2 O 3 composite cannot be continued after 1,500 operations due to the serious oxidation of the contact surface. The SiC w /Cu-Al 2 O 3 composite has good arc stability in the first 2,700 operation times, and the arc duration and arc energy increase sharply in the later stage, but 5,000 operations have been successfully completed. Comparing the arc duration and arc energy of the first 1,500 operation times, it is found that the average arc duration and arc energy of the SiC w /Cu-Al 2 O 3 composite are lower than that of the Cu-Al 2 O 3 composite, and the SiC w /Cu-Al 2 O 3 composite has a better arc stability. Therefore, the addition of SiC w has a more significant effect on reducing the arc duration and energy of the Cu-Al 2 O 3 composite. Figure 9 shows the surface morphologies of anode and cathode contact materials after arc erosion. Compared with the erosion morphologies of the SiC/Cu-Al 2 O 3 composite, the erosion area of the Cu-Al 2 O 3 composite is larger. It is found that a large number of molten droplets adhere to the contact surface, which seriously damages the connectivity of the circuit and leads to the failure of the Cu-Al 2 O 3 contact pairs. The electrical contact experiment shows that the arc erosion resistance of the  Cu-Al 2 O 3 composite is far less than that of the SiC w / Cu-Al 2 O 3 composite.

Conclusions
(1) Nano-Al 2 O 3 particles and SiC w can adjust the spatial distribution of each other in the copper matrix, which is more conducive to fully exploiting the advantages of each reinforcing phase. The introduction of highstrength SiC w can bridge the copper matrix, promote crack deflection, and further improve the tensile strength of copper matrix composites. Author contributions: All authors have accepted responsibility for the entire content of this manuscript and approved its submission.