Nano-SiC_p/Al2014 composites with high strength and good ductility

1 Introduction

Engineering and scientific interests in ceramic particle-reinforced aluminum matrix composites have grown remarkably in the past decade because of lightweight and such attractive mechanical properties as high specific strength, high specific modulus, excellent wear, and elevated temperature resistance [1], [2].

Normally micro-sized particles were used to improve the yield and ultimate strength, but significantly reduced ductility [3], [4]. It was reported that the elongation, ultimate strength, and yield strength of the composites reinforced with small amount of nano-sized ceramic particles were enhanced more markedly than those reinforced with micro-sized particles [5]. Ma et al. showed that the tensile strength of 1 vol.% 10-nm Si₃N₄/Al composites was comparable to that of the 15 vol.% 3.5-μm SiC_p/Al composites, whereas the yield strength of the former was much higher than that of the latter [6]. Yang et al. reported that more than 50% improvement in yield strength of the A356 alloy was observed with 2.0 wt.% nano-sized SiC particles [3]. Nie et al. found that the yield strength, ultimate tensile strength, and elongation to fracture of the 1 vol.% SiC_p/AZ91 nano-composites were simultaneously improved compared with the as-cast AZ91 alloy [7].

Currently, there are numerous fabrication methods for nano-sized ceramic particle-reinforced metal matrix composites, including high-energy milling, powder metallurgy, nano-sintering, and stir casting [8], [9], [10], [11], [12], [13]. Among these methods, stir casting is attractive for its capability in producing complex shapes at a high production rate, low cost, and near net shaping. However, it is very difficult for the conventional stir casting method to disperse nano-particles uniformly in metal melts due to their large surface-to-volume ratio, attractive Van der Waals interactions, and their poor wettability in metal melts [3], [9]. In this study, the nano-SiC_p/Al2014 composites with relatively uniform reinforcement distribution were successfully fabricated by the following processes: ball milling+semi-solid stir casting+hot extrusion treatment.

In the present paper, the microstructures and tensile properties of nano-sized SiC particle-reinforced Al2014 matrix composites were studied, and the strengthening mechanisms were also discussed.

2 Materials and methods

2.2 Formation of the composite slurry by semi-solid stir casting process

Schematic of the graphitic stirrer used in the production of the composites is shown in Figure 1. First, the Al2014 alloy matrix was melted at the temperature of 1023 K. The designed amount of Al2014-SiC composite particles was added into the melt by stirring at a speed of 300 rpm. Second, after the SiC particles were all added into the melt, the melt was cooled to 873 K at which the alloy was in semi-solid condition, and then the composite slurry was stirred for 30 minutes at a speed of 500 rpm. Finally, the composite slurry was rapidly heated to 1023 K and cast into a preheated (473 K) cylindrical steel mould of σ38×100 mm as soon as possible. The solid and liquid phase lines of the Al2014 alloy matrix are 811–922 K. The volume content of the nano-SiC particles in composites were 0.5, 1, and 1.5%, respectively.

Figure 1:

The schematic diagrams of graphitic agitator blade.

3 Results and discussion

Figure 2A and B shows the SEM images of as-received Al2014 powders and nano-SiC powders with average sizes of about 13 and 40 nm, respectively. It can be clearly observed that the raw nano-SiC particles are agglomerated. When such nano-SiC powders are added into the melt without any pre-dispersion treatment, it is extremely difficult for mechanical stirring to break up the agglomeration in the melt [14]. Figure 2C presents the FESEM micrograph of Al2014-SiC composite particles and the corresponding high-magnification image (Figure 2D). It is evident that most of the nano-SiC particles are pressed into the soft Al2014 particles and display a uniform distribution in each individual composite particle surface. It suggests that the nano-SiC particle agglomerations are effectively broken up due to the repeated collisions between the balls and the powders during ball milling. When such composites particles are added into the melt, Al2014 particles start to dissolve in the melt, gradually releasing nano-particles into the melt. The mechanical bonding between Al2014 particles and nano-SiC particles would provide adequate wettability with the molten Al2014, ensuring a good distribution in the melt [15], [16].

Figure 2:

The SEM micrographs of (A) Al2014 powder (13 μm), (B) SiC particles (40 nm), 5 vol.% nano-SiC_p/Al2014 composites particles milled by 48 h: (C) low magnification and (D) high magnification.

Figure 3 shows the optical images of as-cast Al2014 alloy and nano-SiC_p/Al2014 composites with different particle contents. With the addition of nano-SiC particles, the α-Al grains in the casting composites are refined as shown in Figure 3B–D. In the Al2014 alloy, the average size of the α-Al grains is 122 μm (Figure 3A), whereas in the 0.5, 1.0, and 1.5 vol.% nano-SiC_p/Al2014 composites (Figure 3B–D), the average sizes of the α-Al grains are 53, 67, and 88 μm, respectively. The lattice misfits between the (0 0 1) of the SiC and the (1 1 1) of α-Al are 7.4%. When the lattice misfit of the nucleus and crystal phase is between 5 and 15%, the nucleus can act as the effective heterogeneous nuclei for the crystal phase [17]. Therefore, the nano-SiC particles may act as the heterogeneous nucleation sites during solidification, resulting in the refinement of α-Al grains. In addition, the number of crystal nucleus increased as the SiC particles contents increased. However, the aluminum alloy melt cannot provide enough energy ups and downs and makes all of the crystal nucleuses grow up. So, the grains of the as-cast nanocomposites would not be further refined. Moreover, as the SiC particle contents increased, the SiC particle agglomerations would appear, and then the number of effective crystal nucleus was reduced. Therefore, under the functions of these two factors, the grain sizes of as-cast nanocomposites increased as the SiC particle contents increased.

Figure 3:

The OM micrographs of casting nano-SiC_p/Al2014 composites with different particle contents: (A) 0, (B) 0.5 vol.%, (C) 1 vol.%, and (D) 1.5 vol.%.

Figure 4 shows the optical images of the Al2014 alloy and nano-SiC_p/Al2014 composites after hot extrusion. It can be seen that their grain sizes decreased significantly after hot extrusion. In the Al2014 alloy, 0.5, 1.0, and 1.5 vol.% nano-SiC_p/Al2014 composites, the average sizes of the α-Al grains are 42, 18, 25, and 31 μm, respectively.

Figure 4:

The optical images of (A) Al2014 alloy and (B) 0.5 vol.%, (C) 1 vol.%, and (D) 1.5 vol.% nano-SiC_p/Al2014 composites after hot extrusion.

The SEM morphology and SEM-energy-dispersive spectroscopy (EDS) map analysis of the as-cast nano-SiC_p/Al2014 composites with different particles content are shown in Figure 5. SEM-EDS map analyses of C and Si demonstrate that the particles are nano-SiC particles. The SEM-EDS map analyses of C and Si K are relatively homogeneous in the 0.5 and 1.0 vol.% nano-SiC_p/Al2014 composites, which indicate that the distribution of nano-SiC particles in the two composites is relatively uniform. However, some nano-SiC particle agglomerations can be seen in the grain boundary of 1.5 vol.% nano-SiC_p/Al2014 composites, as shown in Figure 5C.

Figure 5:

The SEM morphology and SEM-EDS map analysis of the as-cast nano-SiC_p/Al2014 composites with different particle contents: (A) 0.5 vol.%, (B) 1 vol.%, and (C) 1.5 vol.%.

Figure 6A–C shows the FESEM micrographs of extruded 0.5, 1.0, and 1.5 vol.% nano-SiC_p/Al2014 composites. In Figure 6A and B, the nano-SiC particles distribute uniformly in the composites. In Figure 6C, most of the nano-SiC particles are uniformly distributed in the composites and only small nano-SiC particle agglomerations can be observed. The conclusion that the extrusion further improves the distribution of the nano-SiC particles in the composites can be drawn.

Figure 6:

The FESEM micrographs of extruded nano-SiC_p/Al2014 composites reinforced with different contents of SiC particles: (A) 0.5 vol.%, (B) 1 vol.%, and (C) 1.5 vol.%.

Figure 7 gives the engineering stress-strain curves of extruded Al2014 alloy and nano-SiC_p/Al2014 composites. The yield strength (σ_0.2), ultimate tensile strength (σ_UTS), and fracture strain (ε_f) of them are summarized in Table 1. It can be observed that the σ_0.2, σ_UTS, and ε_f of the composites start to decrease slightly with increasing the content of nano-SiC particles. These may be the result of greater nanoparticle agglomerations present in the composites with higher SiC content. Besides, the increased content of the nanoparticles would decrease the effective slip distance of dislocations during the tensile test, which leads to the decrease in the fracture strain [18], [19]. The addition of 0.5 vol.% nano-sized SiC particles significantly improved the tensile strengths of the Al2014 alloy without sacrificing the ductility. Compared with the Al2014 alloy, the σ_0.2, σ_UTS, and ε_f of 0.5 vol.% nano-SiC_p/Al2014 composites changed from 242 MPa, 460 MPa, and 17.1% to 306 MPa, 532 MPa, and 17.7%, respectively.

Figure 7:

The tensile engineering stress-strain curves of the extruded Al2014 and nano-SiC_p/Al2014 composites reinforced by different contents of SiC particles.

According to the classic Hall-Petch equation: σ_y=σ₀+K_yd^-1/2, where σ_y is the yield strength, σ₀ and K_y are material constants, and d is the mean grain size. With the addition of nano-SiC particles, the size of α-Al grains in the extruded 0.5 vol.% nano-SiC_p/Al2014 composites significantly decreases, which would obviously result in the improvement of yield strength. Moreover, the refined α-Al grains could increase the boundary concentration and extend the crack propagation path, which is helpful to improve the ductility of the composites.

On the other hand, the uniformly distributed nano-SiC particles (see Figure 6A) in the extruded 0.5 vol.% nano-SiC_p/Al2014 composites can considerably increase the yield strength due to the resistance of the closed nano-SiC particles to the passing of dislocations [20]. Therefore, the uniformly distributed nano-SiC particles are important to the improvement of the yield tensile strength. On another hand, the closely bonded interfaces without cavities in the extruded 0.5 vol.% nano-SiC_p/Al2014 composites can be seen by HRTEM, as shown in Figure 8A. The well-bonded interfaces can benefit from the effective transfer of tensile load from the matrix to the particles, thus improving the tensile strength of the composites.

Figure 8:

(A) The HRTEM image of the interface between the nano-SiC particle and the matrix in the 0.5 vol.% nano-SiC_p/Al2014 composites, (B) the selected area electron diffraction pattern at the area B, (C) SAED pattern at the area C, and (D) IFFT image of the zone D.

A high density of the dislocations in the 0.5 vol.% nano-SiC_p/Al2014 composites is easily observed by TEM (see Figure 8B). According to the Bailey-Hirsch relationship in the current study: σ_y=MαGbρ^1/2, where σ_y is the yield strength, M is the mean orientation factor, α is a constant, G is the shear modulus, b is the magnitude of the Burgers vector, and ρ is the dislocation density. It can be seen that with the increase of the dislocation density, the yield strength increases.

As stated above, the superior tensile properties of the nano-SiC_p/Al2014 composites are attributed to the refined α-Al grains, uniformly distribution of reinforcement, good interface bonding, and high dislocation density.

4 Conclusions

In this study, Al2014 matrix composites reinforced with nano-SiC particles were successfully fabricated by semi-solid stirring combined with hot extrusion. The obtained composites exhibited refined α-Al grains, reasonable nano-SiC particles distribution, good interface bonding, and high dislocation density. Compared with the Al2014 alloy, the yield strength, ultimate tensile strength, and fracture strain of 0.5 vol.% nano-SiC_p/Al2014 composites changed from 242 MPa, 460 MPa, and 17.1% to 306 MPa, 532 MPa, and 17.7%, respectively. The superior tensile properties of the composites were attributed to the refined α-Al grains, uniform distribution of reinforcement, good interface bonding, and high dislocation density.