Influence mechanisms of Zr and Fe particle additions on the microstructure and mechanical behavior of squeeze-cast 7075Al hybrid composites

Journal of Alloys and Compounds 798 (2019) 587e596

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Influence mechanisms of Zr and Fe particle additions on the microstructure and mechanical behavior of squeeze-cast 7075Al hybrid composites Tiwen Lu a, b, *, Weiping Chen a, **, Bing Li a, Mengdi Mao a, Zixuan Li a, Yixiong Liu c, Sergio Scudino b a b c

Guangdong Key Laboratory for Advanced Metallic Materials Processing, South China University of Technology, Guangzhou, Guangdong 510640, China IFW Dresden, Institute for Complex Materials, Helmholtzstraße 20, D-01069, Dresden, Germany School of Mechanical and Electronic Engineering, Guangdong Polytechnic Normal University, Guangzhou, Guangdong 510665, China

a r t i c l e i n f o

a b s t r a c t

Article history: Received 27 January 2019 Received in revised form 4 May 2019 Accepted 27 May 2019 Available online 28 May 2019

The different influence of Zr and Fe particle additions on the microstructure and mechanical behavior of 7075Al matrix composites reinforced with 40 vol% SiCp has been systematically investigated. The results indicate that Zr particles are well retained and the interfaces between the Al matrix and Zr particles are clean following the preparing process. However, a hard and brittle Fe-Al layer with an average thickness of 12 mm is generated at the interfaces between the Al matrix and Fe particles. As a result, the as-cast AMC-Zr composite exhibits the best ultimate tensile strength, fracture strain and Young's modulus with values of 564 MPa, 1.12% and 151.3 GPa, respectively. On the contrary, the addition of Fe particles degrades the mechanical properties of the SiCp/7075Al composite. The difference in the mechanical properties results from interface structure and micro-zones surrounding metallic particles. The microzones formed by Zr particle additions have good damage tolerance and hinder the crack propagation in the AMC-Zr composite. However, the formation of hard and brittle Fe-Al interfacial layers result in the random development of the crack propagations in the AMC-Fe composite. In terms of fracture mechanics, the AMC-Zr composite shows a feature of mixed ductile and brittle fracture, and the Zr particles are rarely found in the fracture because of good interface bonding. However, most Fe particles in the fractured AMC-Fe composite exhibit interfacial debonding with the Al matrix. © 2019 Elsevier B.V. All rights reserved.

Keywords: 7075Al hybrid composites Metallic particles Interface structure Mechanical behavior

1. Introduction Aluminum matrix composites reinforced by high volume fraction of ceramic particles (PRAMCs) exhibit many advantages, such as high specific strength and modulus, wear-resistance and good thermo-physical properties [1e3]. Therefore, those aluminum matrix composites (AMCs) have been widely used in aerospace and automotive fields [4]. However, the hard and brittle ceramic particles, such as SiC and B4C and AlN, have a lower damage tolerance and degrade the plasticity of Al matrix [5e7], which lead to AMCs

* Corresponding author. Guangdong Key Laboratory for Advanced Metallic Materials Processing, South China University of Technology, Guangzhou, Guangdong, 510640, China. ** Corresponding author. E-mail addresses: [email protected], [email protected] (T. Lu), [email protected] (W. Chen). https://doi.org/10.1016/j.jallcom.2019.05.301 0925-8388/© 2019 Elsevier B.V. All rights reserved.

reinforced by high volume fraction of ceramic particles being sensitive to the defects and the decrease of mechanical properties, such as low plasticity and fracture toughness. Additionally, it is difficult to obtain proper interface strength between the Al matrix and these reinforcements because of their poor wettability and chemical reactions [8,9]. These factors have hindered the transition of Al matrix composites reinforced by a high volume fraction of ceramic particles into several structure applications. Fortunately, composites provide a high degree of freedom in the material design. Numerous efforts have been devoted to optimize the microstructure to improve the mechanical properties of PRAMCs [10e13]. Hybrid composites have been fabricated using different processing methods, and become an important method to improve their mechanical properties of composites. Multicomponent reinforcements significantly change the microstructure of the composites and can take full advantage of the reinforcement combinations [10]. Therefore, the hybrid composites

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may have different mechanical properties and fracture mechanisms in comparison to those of composites reinforced by a single reinforcement. The type of reinforcements in hybrid composites mainly include multi-component ceramic particles, multi-scale ceramic particles and the combination of particle and fiber ceramic reinforcements, such as (Al2O3p þ SiCp)/Al [11], (SiCnp þ SiCmp)/2014Al [12], (SiCw þ SiCp)/2024Al [13], and all achieve good mechanical properties. However, there are only a few reports on hybrid composites reinforced by the mixture of metallic particles and ceramic particles. It is well known that the mechanical properties of PRAMCs depend not only on the reinforcement size, distribution and volume fraction, but also on the proper interface structure between the reinforcement and matrix [8,14]. A good interface can effectively transfer load from the matrix to the reinforcements, leading to significant improvements in strength and stiffness [8]. In general, the interface bonding between the Al matrix and reinforcements can be classified as: mechanical bonding, diffusion bonding and chemical bonding, as well as coherent or semi-coherent bonding [9]. Metallic bonding is more ductile than the others and has a positive effect on the mechanical properties of metal matrix composites [13]. The inspection of the literature on this topic reveals that some metallic particulate reinforcements, such as Ni [15], TiNi [16], -Al3Mg2 [17], high-entropy alloys (HEAs) [18,19] and amorphous alloys [20] have been successfully used to improve mechanical properties. As we know, many researchers studied the influence of interface structure between the Al matrix and metallic particles on the microstructure and mechanical properties of Al matrix composites. Three typical composites have been widely reported: Firstly, in-situ Al composites, such as Al13Fe4/Al matrix composites [21], Al3Zr/Al composites [22], etc. Secondly, aluminum matrix composites are reinforced by core-shell structured metallic particles through the partial reaction between the Al matrix and metallic particles, such as Ti-(AlxTiy) core-shell structured particles reinforced 7075Al matrix composites [23], Fe-Al5Fe2 core-shell structured particles reinforced aluminum matrix composites [24], Ti@(AleSieTi) particulate-reinforced A356 composite [25], etc. Thirdly, Al matrix composites reinforced with metallic particle have the clean interface between the Al matrix and metallic particles, such as Al-based metal matrix composites reinforced with Zr-based glassy particles [20], Ale12Si matrix composites reinforced with TiAl-based particles [26], (Tip þ SiC)/Al matrix composites [14]. In our former studies [27e29], the slight addition of Cr or Ti particles have significantly improved the mechanical properties of Al matrix composites reinforced by high volume fraction of SiC particles. However, we do not know which kind of interface structure between the Al matrix and metallic particles is suitable for this composites. Both Zr and Fe particles was used as single reinforcements in Al matrix composites have been investigated, and both composites showed excellent mechanical properties. Additionally, there is a relatively big difference in the diffusion coefficient between the two particles with the liquid aluminum, which may result in different interface structure. For these reasons, in this work, we select the Zr and Fe particle additions as study targets to research the effect of different metallic particles on the microstructure and mechanical properties of hybrid composites. In view of the above discussion, the present work is motivated by the following factors. Firstly, the vast majority of published studies focuses on the hybrid composites reinforced by multi-scale ceramic particles or the combination of particle and fiber reinforcements, and hence fundamental information related to the hybrid composite reinforced by the mixture of ceramic particles and metallic particles is few. Secondly, there are no published studies that provide a direct comparison about the influence of different metallic particles on the microstructure and mechanical

properties of aluminum matrix composites reinforced by high volume fraction of ceramic reinforcements. To provide insight into two observations, we report on a systematic study about the microstructure and mechanical properties of 7075Al hybrid composites reinforced by 40 vol% SiCp and 5 vol% different metallic particles (Zr and Fe particles) fabricated by squeeze casting, and compare the results to 7075Al, the 40 vol% SiCp/7075Al and 45 vol% SiCp/7075Al composites. The comparative research is based on the as-cast composites, in order to avoid other factors' effects on the microstructure and mechanical properties. 2. Experimental details 2.1. Materials Commercial 7075Al (Al-6.06Zn-1.97Mg-1.67Cu alloy, wt.%) was used as the matrix alloy. Based on our former studies about the size of reinforcements [14], small SiC particles (99.9% purity, D50 ¼ 7.1 mm), big SiC particles (99.9% purity, D50 ¼ 29.6 mm), Zr particles (99.9% purity, D50 ¼ 30.8 mm) and Fe particles (99.9% purity, D50 ¼ 31.2 mm) were selected as the reinforcements. The size distribution and morphology of these particles, characterized using a Mastersizer 2000 (Malvern, UK) and a QUANTA 200 SEM, are shown in (Fig. 1(a)). The size and morphology of Zr and Fe particles used here are almost identical in order to get a better comparison of results. 2.2. Fabrication and characterization 7075Al hybrid composites with a size of Ø800 40 mm were fabricated using squeeze casting. 5 vol% metallic particles (Zr or Fe) and 40 vol% small SiC particles were first blended in a grinding mill for 10 h. Then, well-blended powder mixtures were compacted into a mold to form a billet (Ø800 40 mm). Subsequently, the billet was placed into a mold and preheated to 857 K. It is worth noting that oxide layers are produced on the surface of the metallic particles during the preheating process, XRD results shown in Fig. 1(b). Molten 7075Al alloy (1073 K) was poured into the mold with the inner diameter of 80 mm. The mold is preheated to 857 K, in order to molten aluminum better infiltrating into the preform. Then, molten aluminum was squeezed into the reinforcement billet with a pressure of 75 MPa. The detailed fabricating process is showed in Fig. 2. To investigate the influence of Zr and Fe particles, composites reinforced by 40 vol% SiCp and 40 vol% SiCp þ 5 vol% big SiCp were also fabricated under the same condition for comparison. Table 1 presents four different composites, which will henceforth be referred to as the AMC-40, AMC-45, AMC-Zr and AMC-Fe composites. The microstructure of the composite samples was characterized by scanning electron microscopy (NOVA NanoSEM 430, USA) and high-resolution transmission electron microscopy (HRTEM, G2 F20) equipped with an energy dispersive spectrometer (EDS). Phase analyses of the powders and composite samples were carried out using a Bruker D8 X-ray diffractometer (XRD) with Cu Ka radiation. The microhardness of the interface layers was measured using a SCTMC digital microhardness tester with a 100 gf applied load and a load time of 30 s. All nanoindentation tests were done using a Hysitron TI-950 nanoindenter in load-controlled mode with a nominally constant strain rate of 0.05 s1. There are 4 mm spacing between neighboring indents (indentation from the edge to the center of particles). The dog-bone-shaped tensile specimens, displaying a gauge length of 15 mm and a rectangular cross section of 5 mm’ 2 mm, were used in the tensile tests at a constant strain rate of 1  103 mm/s on a Zwick universal material testing machine (Zwick, Germany) at room temperature.

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Fig. 1. The characterization of particles: (a) the size distribution and morphology, (b) the XRD patterns of preheated powder mixtures.

Fig. 2. Schematic diagram of fabricating process for 7075Al hybrid composites.

Table 1 Four composites under the same preparation condition. Composites

Matrix

Reinforcement

AMC-40 AMC-45 AMC-Zr AMC-Fe

7075Al 7075Al 7075Al 7075Al

40 vol% SiCp 5 vol% big SiCp and 40 vol% SiCp 5 vol% Zr particle and 40 vol% SiCp 5 vol% Fe particle and 40 vol% SiCp

3. Results 3.1. Microstructure of the composites Fig. 3 shows that the reinforcements are homogeneously distributed in the 7075Al composites and no obvious defects can be observed. Fig. 3(a) and (b) present the microstructure of the AMC40 and AMC-45 composites. Generally, severe reactivity would take place between the metallic particles and Al matrix under the thermodynamic conditions of casting techniques [30,31]. Khvan [32] reported that 99.7 at.% Al (99.99 wt%, purity) and 0.30 at.% Zr (99.99 wt%, purity) were held for 2 h at 1273 K, and the Al3Zr phase was produced. However, Fig. 3(c) shows that Zr particles are well retained in the AMC-Zr composite and no obvious interfacial products (such as Al3Zr) can be observed at the interfaces between the 7075Al matrix and Zr particles. In contrast, the Fe particles are surrounded by a thick layer of interfacial products in the AMC-Fe

composite (Fig. 3(d)). Clearly, it reveals that the Fe and Zr metallic particles have formed the different interface with the Al matrix under the same preparation condition. In order to investigate the interfaces between the Al matrix and metallic particles, energy dispersive analysis (EDS) analysis was carried out to estimate the reaction products during the fabrication process. The line scanning analysis shows that Fe-Al diffusion layer can be observed, and the map scanning analysis further indicates that Zr particles are well retained in the AMC-Zr composite without obvious interface (Fig. 4(a)). According to the literature [30,33], the potential reaction products between liquid Al and solid Fe are Fe4Al13, Fe2Al5, FeAl2 and Fe3Al. In Fig. 4(b), the point analysis of the interfacial product reveals that the atomic ratio of Fe:Al is 46:16. Due to multiple elements of 7075Al alloy and the surface oxidation layer of the Fe particles, it is difficult to determine the phase of the Fe-Al interfacial layer. Furthermore, the average size of the Fe particles which react with Al matrix seems larger than original state. 3.2. TEM observation Fig. 5 presents the TEM microstructure of the as-cast hybrid composites. As shown in Fig. 5(a), (b) and (e), a high density of dislocation rings can be seen in the Al matrix nearby the SiC particles in the AMC-45 composite, while fewer dislocations can be seen nearby the Zr particles in the AMC-Zr composites. Therefore, it can be assumed that micro-zones formed by Zr particles can bear

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Fig. 3. SEM micrographs of the as-cast composites: (a) the AMC-40 composite, (b) the AMC-45 composite, (c) the AMC-Zr composite, (d) the AMC-Fe composite.

some residual stress and reduce the dislocation density around the Zr particles. The behavior can be ascribed not only to the larger difference in the expansion coefficient between Al matrix (23.30106/ C) and SiC (4.7010-6/ C) particles than that between Al matrix and Zr particles (5.80 106/ C) [34], but also to better deformability the Zr particles than SiC particles during cooling process. Different from the clean interface between the Al matrix and Zr particles, the interface is vague between Fe particles and the Al matrix because of severe diffusion (Fig. 5(f)). Surrounding the Fe particles, there are lots of dislocation walls distributed in the Al matrix, which might be attributed to bad deformability of the Fe-Al interfacial layer in the cooling process. According to the SEM and TEM observations, the average thickness of the Zr-Al interface layers in the AMC-Zr composites is 8 nm (Fig. 5(c)), while the thickness of Fe-Al interfacial layers in the AMC-Fe composite is about 12 mm (Fig. 3(d)). To further examine the configuration of the lattice planes at the interfacial layer in the AMC-Zr composite, an inverse Fourier transformed pattern (see Fig. 5(d)) is obtained from the selected area at the interface between the interfacial layer and Al matrix. It is seen that some lattice planes are terminated at the interface, and thus an array of dislocations are formed, as indicated by “T” along the interface. Overall, it indicates that the atomic configuration of the interfacial layer displays a well match with the Al matrix. It has been reported that there was a MgAl2O4 interface between SiCp and 6061Al matrix [22]. According to the results of EDS analysis (Fig. 5(g)), the interfacial products might be a similar phase in the AMC-Zr composite. However, it is still difficult to determine the phase of Fe-Al interfacial layers. In summary, the appropriate interface reaction can ensure that the AMC-Zr composite shows a better interface microstructure and structure than that of the AMCFe and AMC-45 composites.

3.3. Mechanical properties Fig. 6(a) shows the microhardness versus positions in the AMCZr and AMC-Fe composites. The microhardness of the edge in the Zr particles is similar to that of the center. On the contrary, the microhardness of the interfacial product between Al matrix and Fe particle is 231 HV, which is obviously higher than the center of the Fe particles. Nanoindentation tests were also performed in two metallic particles with the aim to further investigate mechanical properties of the Zr and Fe particles after reaction with the Al matrix (Fig. 6(b), (c) and (d)). There are 4 mm spacing between neighboring indents (indentation from the edge to the center of particles). The modulus and hardness of points 1e4 in the AMC-Zr composites are similar to each other (Fig. 6(b)), with average values of 2.92 GPa and 78.1 GPa, which correspond to those of the Zr [35]. The values of modulus and hardness of the Fe particles gradually increase from point 1 to point 4, and the values of point 1 are approximately equals to those of Fe. Therefore, the thick and hard interfacial layers with high modulus and hardness are produced between Fe particles and Al matrix in the AMC-Fe composite, whereas the Zr particles still have not shown evident reaction with the Al matrix in the AMC-Zr composite. The thick and hard interfacial layers are responsible for the high dislocation walls surrounding the Fe particles due to the bad deformability. Fig. 7 shows the representative tensile properties of the as-cast composites. The average tensile strength of the as-cast AMC-Zr composite is 564 MPa, significantly higher than that of the unreinforced 7075Al matrix. The low plasticity of as-cast 7075Al is related with the large grain size [14]. Because of the addition of high volume fraction of reinforcements, the grains in composites are significantly refined [36]. Compared with AMC-45 and AMC-40

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Fig. 4. Energy dispersive analysis (EDS) of the as-prepared composites: (a) the AMC-Zr composite, (b) the AMC-Fe composite.

Fig. 5. TEM images of the as-cast composites: (a) an overview TEM image of the SiC/Al interface in the AMC-Zr composite, (b) an overview TEM image of the Zr/Al interface in the AMC-Zr composite, (c) the HRTEM image of the interface in Fig. 5(b), (d) the inverse Fourier transformed pattern of the selected area in Fig. 5(c), (e) an overview TEM image of the AMC-Zr composite, (f) an overview TEM image of the Fe/Al interface in the AMC-Fe composite, (g) the EDS analysis result of the interface in Fig. 5(b) and (f).

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Fig. 6. Vickers microhardness and nanoindentation curves of as-cast composites: (a) average microhardness values versus positions in AMC-Zr and AMC-Fe composites, (b) variation of indentation curves of Zr particles in the AMC-Zr composite, (c) the comparison of hardness and Young's modulus as a function of position for the AMC-Zr and AMC-Fe composites, (d) variation of indentation curves of Fe particles in the AMC-Fe composite.

Fig. 7. Tensile properties of the as-cast composites: (a) typical engineering tensile stress-strain curves of 7075Al and four composites, (b) corresponding data for mechanical properties.

composites, the average tensile strength of the AMC-Zr composite is increased by 28% and 33%, respectively. What's more, the average fracture strain of the as-cast AMC-Zr composite is almost two times than that of the AMC-40 composite, suggesting the promising strengthening and toughing effects of Zr particles on the SiCp/ 7075Al composite. However, the strength and strain of the AMC-Fe composite are significantly lower than those of the AMC-45 and AMC-40 composites. As shown in Fig. 7(b), the Young's modulus of the as-cast AMC-Zr composite is still the best value. The above results indicate that the selection of metallic reinforcement is important for improving the mechanical properties of the 7075Al hybrid composites. The AMC-Zr composite presents good

mechanical properties in comparison to other squeeze-cast composites with high volume fraction of reinforcements [14,37e39],

Table 2 Comparisons of squeeze-cast composites. Composites

UTS

Fracture strain (%)

20 vol% SiCw/6061Al [37] 45 vol% SiCp/7075Al [38] 45 vol% Si3N4p/2024Al [39] 45 vol% SiCp/7075Al [14] AMC-Zr

165 475 360 526 564

1.0 0.58 0.23 0.5 1.12

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shown in Table 2. 3.4. Fracture mechanism The failure modes of reinforcements in PRAMCs mainly consist of the interfacial debonding and reinforcement fracture [40]. Our former research shows that the failure of the AMC-40 composite can be mainly ascribed to the fracture of SiC particles [14,29]. Fig. 8 shows the SEM tensile fracture morphology of the AMC-Zr, AMC-Fe and AMC-45 composites. Many torn edges can be observed in the fracture of the AMC-Zr composite, showing a feature of mixed ductile and brittle fracture. Besides, Zr particles are seldom seen in the fracture of the AMC-Zr composites. Only some fractured Zr particles are distributed at the bottom of the dimples (Fig. 8(a) and (d)), which implies that the Zr particles have a good bonding with the Al matrix. However, the fracture surface of the AMC-Fe composite is flat and many cracks can be easily observed (marked by blue arrows), revealing a typical characteristic of brittle fracture. Some Fe particles or pits are distributed on the fracture surface of the AMC-Fe composite (Fig. 8(b) and (e)). Besides, the interfaces between the Al matrix and Fe particles are filled with cracks. In combination with the EDS analysis, it can be concluded that fracture takes place at the interface layers between Fe particles and the Al matrix. The fracture of the AMC-45 composite also shows the absence of dimples and all big SiC particles are fractured. Small SiC particles also show interfacial debonding with the Al matrix (marked by red arrows). As discussed above, it comes to a conclusion that the fracture of the AMC-Zr composite shows better plastic characteristics than those of the AMC-Fe and AMC-45 composites. In order to investigate concrete decisive factors of fracture behaviors in the as-cast AMC-Zr and AMC-Fe composites, crack propagation paths were obtained and analyzed. Large SiC particles in the AMC-45 composite are nearly cracked and crushed in the crack propagation path, reported in our previous research [14]. Fig. 9 presents the SEM images of the crack propagation paths in the as-cast AMC-Zr and AMC-Fe composites. Evidently, the crack propagation paths exhibit significant difference in the two composites. Under the same load, the density of the cracks present in the AMC-Fe composite is obviously higher than that in the AMC-Zr composite. Besides, the crack propagation path in the AMC-Zr

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composite has a more winding shape compared to that in the AMC-Fe composites. The further observation reveals that the cracks encountering Zr particles in the AMC-Zr composite will change the path (Fig. 9 (a) and (c)), which indicates that the Zr particles can hinder the crack propagation and raise the damage tolerance of composites, consistent with the results reported in our former research [14,27,29]. Along the propagation path in the AMC-Fe composite, the cracks go across the interfaces between the Al matrix and Fe particles without obvious deflection. Furthermore, the cracks are mainly located at the Fe-Al interfacial layers (Fig. 9(b) and (d)). The results further validate that Fe-Al interfacial layers is a brittle and hard phase and has a negative effect on the mechanical properties of composites reinforced by high volume fraction of SiC particles. 4. Discussion As discussed above, the interface structure and micro-zones surrounding the metallic particles in two composites are completely different. In terms of their interfaces (Figs. 3 and 4), there are two main reasons for the formation of different interfaces: (І) Due to the cooling effect of the mold, the contact time between the metallic particles and the molten aluminum is shortened, and this prevents metallic particles from reacting completely with molten aluminum [41]. (ІІ) The different thickness of interfacial layer between the matrix and the two metallic particles may attribute to the diffusion coefficient D between liquid Al and solid Zr or Fe particles. The diffusion coefficient D is determined by free energy of diffusion Q and diffusion constant D0, as shown below.


Q D ¼ D0exp  RT

[1]

The diffusion coefficient of liquid Al into solid Zr is about (1.76e2.50)’1015 m2/s (1203 K) [42], while the coefficient of liquid Al into solid Fe is about 1.80 104 m2/s (1003 K-1673 K) [43]. The growth of the Fe particles during the squeeze casting also can be explained by thermodynamic principles, a pronounced Kendall effect arising from the large difference in the diffusivity between liquid Al and solid Fe [44]. It is attributable to the difference that the

Fig. 8. SEM fracture morphologies of the fabricated composites: (a) and (d) the AMC-Zr composite, (b) and (e) the AMC-Fe composite, (c) and (f) the AMC-45 composite.

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Fig. 9. Crack propagation paths in the two composites: (a) the AMC-Zr composite, (b) the AMC-Fe composite, (c) the representative region in the AMC-Zr composite, (d) the representative region in the AMC-Fe composite.

diffusion coefficient of solid Fe into liquid Al, which is about 530104 m2/s is significantly higher than that of liquid Al into solid Fe at the same temperature (about 1.80 104 m2/s) [43,45], and the specific values are showed in Table 3. Finally, the difference in the diffusion coefficient between liquid Al and solid metallic particles leads to the formation of two types of interface. Besides, the interfaces between the Al matrix and Zr particles in the AMC-Zr composite belong to metallic bonding, which is more ductile than other bonding, and is desirable in the MMCs [8,41]. However, due to the formation of the hard and brittle interfacial layer, the interfaces between the Al matrix and Fe particles in the AMC-Fe composite belong to diffusion bonding, and too thick diffusion layers does not favor good mechanical properties [46]. Therefore, the interface bonding strength in the AMC-Zr composite is superior to that of the AMC-Fe composite. The proper interface structure between the Al matrix and metallic particles is important for the mechanical properties of Al hybrid composites. The microstructure observed by TEM analysis can be further confirmed by the XRD analysis. To quantitatively analyze the dislocation density present in the as-cast composites, the XRD patterns (Fig. 10(a)), as well as the Williamson-Hall method are often used to investigate the dislocation density [47,48]. The grain size, lattice strain and dislocation density of the composite samples can be calculated by following equations [48]:

B cos q ¼

Kl þ 2ε sin q D

[2]

r¼

pffiffiffi 2 3ε Db

[3]

where D, ε and B are the grain size, the lattice strain and the peak width at half maximum, l is the wavelength of Cu Ka radiation (0.154 nm), K is the Scherrer constant (assumption K ¼ 0.9) and q is the Bragg angle. r is dislocation density and b is the Burgers vector (b ¼ 0.286 nm for aluminum). The calculated results for Al grain size, lattice strain and dislocation density in the Al matrix of the ascast AMC-Zr, AMC-Fe and AMC-45 composites are summarized in Fig. 10(b). It is worth pointing out that a high dislocation density could result in stress concentration surrounding the interfaces between the Al matrix and reinforcements, resulting in the degradation in the plasticity and fracture toughness of the composites [49]. Compared with the AMC-Fe and AMC-45 composites (1.6101013/m2 and 2.7301013/m2, respectively), the AMC-Zr composite possesses relatively low dislocation density (1.070 1013/m2), which indicates that some residual stress present in Al matrix is effectively transferred from the Al matrix to the Zr particles. The influence of the Zr and Fe particles on the microstructure and fracture mechanism of SiCp/7075Al hybrid composites is illustrated by the schematic diagrams shown in Fig. 11. During the preheating process of powder mixtures, the oxide layers are produced on the surface of the metallic particles. Compared with ceramic particles, metallic particles possess good plasticity. Additionally, because of the clean and strong interfaces between the Al matrix and Zr particles, the addition of Zr particles provide relaxation routes for residual stress present in Al matrix and ensure effective load transfer from the Al matrix to the Zr particles, which is helpful for the decrease of the high dislocation density of the Al

Table 3 Diffusion coefficient between liquid aluminum and solid metallic particles. Diffusion coefficient DZr-Al DFe-Al

Aluminum into metallic particles (m2/s) (1.76e2.50)’10 1.80 104 [43]

15

[42]

Metallic particles into aluminum (m2/s) e 530 104 [44]

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Fig. 10. XRD curves of three composites and calculated results of grain size, lattice strain and dislocation density.

Fig. 11. Schematic diagrams illustrating the formation of microstructure and fracture mechanisms in the AMC-Zr and AMC-Fe composites.

matrix in the AMC-Zr composite. By contrast, hard and brittle Fe-Al interfacial layers with an average thickness of 12 mm formed in the AMC-Fe composite. The Fe-Al interfacial layers have the bad deformation ability, which leads to high dislocation density surrounding the interface. Finally, during the tensile process, due to the good interface between Zr particles and Al matrix, the addition of Zr particles can hinder the crack propagation and raise the damage tolerance of the AMC-Zr composite. Therefore, the addition of Zr particles results in the improvement of tensile strength and plasticity. By contrast, the hard and brittle Fe-Al interfacial layers in the AMC-Fe composites tend to provide sites for crack nucleation and propagation, which is harmful for the mechanical properties of the composites.

5. Conclusions Two hybrid composites reinforced by the combination of 5 vol% Fe particle þ 40 vol% SiCp and 5 vol% Zr particle þ 40 vol% SiCp were successfully synthesized via squeeze casting, respectively. In terms of microstructure, the Zr particles are well retained during the

preparation and the interfaces between the Al matrix and Zr particles are clean and strong. However, the AMC-Fe composite can generate a Fe-Al interfacial layer with an average thickness of 12 mm, which has characteristics of high hardness and brittleness. The mechanical properties of the SiCp/7075Al hybrid composite can be improved significantly with the addition of Zr particles. The ultimate tensile strength, fracture failure and Young's modulus of the AMC-Zr composite are 564 MPa, 1.05% and 151.3 GPa, respectively. However, the addition of Fe particles have an opposite influence on the mechanical properties of the composites. The bonding strength between the Al matrix and Zr particles is higher than that of between the Al matrix and Fe particles by the analysis of tensile fracture of composites, which is ascribed to the results that the proper interface structure presents between the Al matrix and Zr particles, and the Fe-Al interfacial layer is sensitive to the crack. The above results suggest that the proper interface structure between the Al matrix and metallic particles is important for the mechanical properties of squeeze-cast 7075Al composites reinforced by a high volume fraction of SiC particles.

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