The superior ductility of fine SiC_p/Al2014 composites after extrusion

1 Introduction

SiC particle-reinforced aluminum alloy (SiC_p/Al) composites are the candidate materials for engineering applications in aerospace and automobile industries due to their high specific strength and modulus, good fatigue resistance, and outstanding wear resistance over the corresponding aluminum alloys [1], [2], [3].

Several processing techniques were developed for the fabrication of SiC_p/Al composites, such as powder metallurgy, stir casting, and pressure infiltration [4], [5], [6], [7]. Stir casting is one of the most ideal techniques for commercial production. However, stir casting also includes some disadvantages, such as nonuniform particle distribution and poor bonding strength of the SiC/Al interface, which severely degrade the mechanical properties of the composites [8]. It has been shown that the application of plastic deformation processes, such as extrusion, rolling, and forging, to SiC_p/Al composites can improve the particle distribution and bonding strength of the SiC/Al interface [9], [10], [11]. Currently, there are a lot of researches on the plastic behaviors of SiC_p/Al composites fabricated by stir casting [12], [13], [14], [15], [16], [17]. However, in such cases, the size of SiC particles is large (>10 μm). In addition, it is reported that the large-sized SiC particles are easily fractured during the plastic deformation, which will generate cracks and then decrease the ductility of composites [18], [19].

El-Kady and Fathy [20] investigated the effect of SiC particle size (70 nm, 10 μm, and 40 μm) on the mechanical properties of Al matrix composites. They found that, as the SiC particle size decreased, the hardness and compressive strength of the composites increased. In addition, the effect of reinforcement particle size (3 and 14 μm) on the microstructure and mechanical properties of SiC_p/Al-6Cu-0.4Mn composites was studied by Slipenyuk et al. [21]. The obtained results showed that using reinforcements with small size (3 μm) provided higher strength and elongation to fracture. It is of interest to apply plastic deformation to the small-sized SiC_p/Al composites to simultaneously improve the strength and ductility of the composites. However, the studies on the plastic deformation of small-sized (<10 μm) SiC_p/Al composites fabricated by stir casting are limited, because it is difficult for stir casting, due to their large surface-to-volume ratios and low wettability in Al alloy melts, to uniformly distribute small-sized (<10 μm) SiC particles in Al alloy melt.

Therefore, in the present paper, the small-sized (5 μm) 4 vol.% SiC_p/Al2014 composites were successfully fabricated by stirring the specially made composite particles. The effects of hot extrusion on the microstructures and tensile properties of small-sized (5 μm) 4 vol.% SiC_p/Al2014 composites were investigated.

2 Materials and methods

3 Results and discussion

Figure 1A and B presents the morphology and corresponding XRD results of oxidized SiC_p at 1373 K for 3 h. From Figure 1B, it can be seen that the phases consist of SiC (6H) and SiO₂, indicating that SiO₂ is created on the surface of the SiC particle after being oxidized at 1373 K for 3 h. Figure 1C shows the morphology of (Al2014-30SiC_p)_cp produced by low-energy ball milling and Figure 1D is the corresponding high-magnification image. It is evident that most of the SiC particles are pressed into the soft Al2014 alloy particles and display a relatively homogeneous distribution in each individual composite particle surface. When such composite particles are added into the melt, Al2014 particles start to dissolve, gradually releasing the SiC particles into the melt. The mechanical bonding of Al2014 alloy particles and SiC particles would provide adequate wettability with Al2014 alloy melt and ensure a good distribution of SiC particles in the melt.

Figure 1:

(A) SiC particles and (B) corresponding XRD pattern and the SEM micrographs of (Al2014-SiC_p) composite particles milled by 96 h at (C) low and (D) high magnification.

In the as-cast composites, as shown in Figure 2A and D, the average size of α-Al grains is approximately 112 μm. The SiC particle distribution is relatively uniform in the matrix, most SiC particles are distributed along the grain boundaries, and only a few particles are in the interior of the grains. After the hot extrusion process, the α-Al grain is obviously refined to approximately 50 μm. The distribution of the particles is found to be more uniform, although the particle distribution along the extrusion direction presents banded structure characteristics (see Figure 2B and C). Large radial compressive stress generated severe plastic deformation of the Al2014 matrix alloy during extrusion, whereas the rigid and nondeformable SiC particles tend to flow with the plastic deformation of Al2014 matrix alloy, leading to the more uniform particle distribution in the extruded composites. Figure 2F is the high magnification of Figure 2C, from which it can be seen that long axis of the SiC particle tends to be parallel to the extrusion direction. This is because the SiC particles adjust their position to be beneficial to the plastic deformation of the composites.

Figure 2:

OM micrographs of SiC_p/Al2014 composites before and after extrusion: (A) as-cast composites, (B) cross-section, (C) longitudinal section of extruded composites, and (D)–(F) images of (A)–(C) at high magnification, respectively.

A careful observation of Figure 3 indicates that no SiC particles fracture after extrusion. This is very different from what is observed in the composites reinforced with large-sized reinforcement [19]. The hard and brittle ceramic particles will fracture only when the stress concentration is beyond the fracture strength of the ceramic particles [22]. In general, larger-sized particles fracture more easily than smaller ones during extrusion. First, under the same condition, the larger-sized particles would share higher stress due to the less interfacial area. Second, the larger-sized particles have a greater probability of containing flaws [11].

Figure 3:

SEM microstructures of the extruded 4 vol.% 5 μm SiC_p/Al2014 composites.

The density value and porosity of the SiC_p/Al2014 composites before and after extrusion are summarized in Table 1. From Table 1, it can be seen that the measured density value of as-cast SiC_p/Al2014 composites is less than the theoretical density, and the corresponding porosity is 1.7 vol.%. After extrusion, the density value of the composites is close to the theoretical density with a much lower porosity of 1.0 vol.%. During extrusion, large compressive stress makes microcavities in the matrix and pores at the SiC/Al interface compress and even close and then decrease the porosity of the composites.

Figure 4 shows the TEM image of the interface between the SiC particle and α-Al in the SiC_p/Al2014 composites before and after extrusion. It is found that a thin and closely bonded interface exists between the SiC particle and α-Al in the as-cast SiC_p/Al2014 composites. The TEM-energy-dispersive X-ray spectroscopy (EDS) analysis of the SiC/Al interface shows the Al-O compositions (see Figure 4C), and this layer can be identified as Al₂O₃ phase formed by interfacial reaction according to Ref. [23]. The oxidation process creates an SiO₂ layer on SiC particle surfaces. The SiO₂ layer reacts with Al during mechanical stirring, which is described as follows:

Figure 4:

(A) TEM micrograph, (B) HRTEM image, and (C) TEM-EDS spectra of the interface in the composites before extrusion, (D) TEM micrograph and (E) HRTEM image of the interface in the composites after extrusion, and (F) selected area electron diffraction pattern of the SiC particles.

4Al(L)+3SiO2(S)→2Al2O3(S)+3Si(L)

Therefore, the Al₂O₃ phase is found at the interface of SiC_p and Al matrix in the composites.

However, in the extruded composites as shown in Figure 4D and E, part of the SiC particle bonds directly with the matrix alloy. This may be because the raw Al₂O₃ interface is locally destroyed by the large shear stress between the SiC particle and matrix generated during extrusion. This is considered to be more effective for tensile load transferring from matrix to ceramic particles and then enhancing the tensile strength of the composites.

Figure 5 is the TEM micrograph of the as-extruded SiC_p/Al2014 composites after solution treatment at 773 K for 2 h plus artificial aging at 433 K for 18 h. It can be observed that the precipitated phase (θ′phase) is uniformly dispersed in the α-Al matrix, and its length is approximately 55 nm and the width is approximately 8 nm. It is suggested that precipitation strengthening, which results from the ability of the nanoscale secondary phase precipitates to restrict and impede the dislocation actuation and movement by forcing dislocations to circumvent the nanoscale precipitates, makes a significant contribution to the strength enhancement.

Figure 5:

TEM micrograph of the as-extruded SiC_p/Al2014 composites after solution treatment at 773 K for 2 h plus artificial aging at 433 K for 18 h.

Figure 6A shows the fractograph of the 4 vol.% 5 μm SiC_p/Al2014 composites before extrusion. It can be seen that no obvious ductile dimples on the matrix and interfacial debonding between particles and matrix exist. It indicates that the bonding strength of SiC/Al interface is inferior to that of the matrix before extrusion. Figure 6B is the fractograph of the composites after extrusion. The fracture surfaces consist of numerous ductile dimples but few decohesion of SiC particles from the matrix. It suggests that the bonding strength of SiC/Al interface is enhanced after extrusion.

Figure 6:

SEM fracture surfaces of the 4 vol.% SiC_p/Al2014 composites (A) before and (B) after extrusion.

Figure 7 shows the engineering stress-strain curves of the extruded Al2014 alloy and 4 vol.% SiC_p/Al2014 composites before and after extrusion, and their yield strength (σ_0.2), ultimate tensile strength (σ_UTS), and fracture strain (ε_f) are summarized in Table 2. The tensile properties, especially the ductility, of the fine SiC_p/Al2014 composites were significantly improved by extrusion. Compared with the as-cast 4 vol.% SiC_p/Al2014 composites, the σ_0.2, σ_UTS, and ε_f of the extruded composites increased from 242 MPa, 367 MPa, and 3.8% to 304 MPa, 530 MPa, and 11.2%, which are increased by 25.6%, 44.4%, and 195%, respectively. As stated above, the enhanced tensile properties of the extruded 4 vol.% 5 μm SiC_p/Al2014 composites are attributed to the refined α-Al grains, increased distributed homogeneity of SiC particles, decreased porosity of the composites, improved interfacial bonding strength, and θ′ phase precipitation strengthening. Moreover, from Table 2, it can be seen that the 5 μm SiC particles significantly improve the tensile strength of Al2014 alloy. The σ_0.2 and σ_UTS of the extruded 4 vol.% 5 μm SiC_p/Al2014 composites were 25.6% and 15.2% higher than those of the extruded Al2014 alloy, respectively.

Figure 7:

Tensile engineering stress-strain curves of the extruded Al2014 and SiC_p/Al2014 composites before and after extrusion.

The 4 vol.% 5 μm SiC_p/Al2014 composites were fabricated by stirring the specially made composite particles combined with hot extrusion. Compared with the conventional powder metallurgy and stir casting method, the special fabrication method could ensure higher densification of the composites and better distribution of the small-sized SiC particles in the Al2014 matrix, respectively. Meanwhile, the small-sized SiC particles in the composites after extrusion would not fracture. This is very different from the large-sized (>10 μm) SiC_p/Al composites, which will fracture and generate cracks and then decrease the ductility of composites after plastic deformation. In addition, fine (5 μm) SiC particles are easier to flow in coordination with the deformation of Al2014 matrix than the coarse (>10 μm) SiC particles. As a result, the extruded 4 vol.% 5 μm SiC_p/Al2014 composites exhibit superior elongation.

4 Conclusion

The tensile properties, especially the ductility, of the composites were significantly improved by extrusion. Compared with the as-cast 4 vol.% SiC_p/Al2014 composites, the yield strength, ultimate tensile strength, and fracture strain of the extruded composites increased from 242 MPa, 367 MPa, and 3.8% to 304 MPa, 530 MPa, and 11.2%, which are increased by 25.6%, 44.4%, and 195%, respectively. The enhanced tensile strengths are attributed to the refined α-Al grains, increased distributed homogeneity of SiC particles, decreased porosity of the composites, and improved interfacial bonding strength. The superior elongation is due to the special fabrication method.