Microstructure and corrosion properties of SiC/Al-Mg-Cu-Si-Sn composites

1 Introduction

SiC-particle-reinforced aluminum matrix composites (AMCs) are considered excellent structural materials suitable for use in the aeronautic-aerospace transportation and automotive industries because of their excellent combination of qualities, specifically their low density, high thermal conductivity, and high specific strength, specific stiffness, and heat resistance [1–4]. The key to improving these properties lies in the structure, chemistry, and the bonding nature of aluminum (Al)-silicon carbide (SiC) interfaces. In this way, improving the combined properties of AMCs, age hardenable aluminum alloys such as those based on Al-Cu [5–7], Al-Mg-Si [8–12], and Al-Zn-Mg-Cu [13, 14] have been used as matrix alloys. Many methods have also been developed to fabricate the SiC-particle-reinforced AMCs, including casting, pressure infiltration, and powder metallurgy (PM). Among all these fabrication techniques, PM is the most attractive because it offers uniform distribution of the reinforcements, fine-grained structures, and easy control of the microstructures [15].

Reports of the effects of aging treatment and SiC content on age-hardening and corrosion behavior have not produced consistent results. Rack reported that age hardening is slower in a 20 vol.% SiC_w/Al-Mg-Si composite than in a monolithic alloy [16]. Ahn and Yu found that the precipitation sequence and the characteristics of the precipitates in the Al-Mg-Si-Cu alloy were not considerably affected by the presence of SiC particulate and the volume fraction of SiC particulate [17]. Mohmad thought that the degree of corrosion of T6-treated SiC_p/Al-Mg-Si composites in chloride solutions increased with increasing SiC_p content [18]. However, the study published by Naya and Hebbar showed that the corrosion inhibition efficiency of T6-treated Al-Mg-Cu-SiC composite was found to be poorer than that of the non-heat-treated composite [19].

The objective of this study was to investigate the age-hardening and corrosion behavior of Al-0.64Mg-0.60Cu-0.2Si-0.15Sn (mass.%) matrix composites with different volume fractions of SiC_p and to discuss how the SiC particles affected the age-hardening and corrosion behaviors using hardness measurement, corrosion testing, and microscopy.

2 Materials and methods

The raw materials included the following: pre-alloying Al-0.64Mg-0.60Cu-0.2Si-0.15Sn powder (Angang Group Aluminium Powder Co., China) and α-SiC particles (99.5%) (Xuzhou Jiechuang New Material Technology Co., China). Figure 1 shows the scanning electron microscope (SEM) images of SiC particles and pre-alloying powders. The pre-alloying powder was composed of almost spherical particles (Figure 1A) with an average size of 10 μm. The SiC particles with the average size of 30 μm are angular and bulky (Figure 1B). The 0 vol.% (sample 1), 5 vol.% (sample 2), 10 vol.% (sample 3), 20 vol.% (sample 4), or 30 vol.% SiC (sample 5) powder was added to Al-0.64Mg-0.60Cu-0.2Si-0.15Sn pre-alloying powders before milling. Mechanical milling was performed for 24 h in a planetary ball milling system (Nanjin Kexi Instrument, Nanjin, China) in alcohol under argon atmosphere using WC-5Co balls with a diameter of 10 mm as the milling bodies. The ball-to-powder weight ratio was 3:1, and the milling speed was 8.5 g. After milling, the pulp was dried in a vacuum oven at 70°C for 12 h. The mixed reactants were granulated and die-pressed to compact discs with 50 mm in diameter and 50 mm in thickness under a load of 300 MPa. Because the liquid phase sintering temperature of Al-Mg-Si alloy was 649°C, the compacts were heated to 600°C with a heating rate of 10°C/min in vacuum [20]. At 600°C, the compacts were sintered for 2 h with an applied load of 25 MPa. After sintering, the samples were cooled to room temperature in the furnace. The samples were treated with solution at 500°C for 2 h, followed by water quenching, and finally aged for different times at 175°C.

Figure 1:

SEM graphs of (A) pre-alloying powder and (B) SiC particles.

The hardness of the samples was measured using an HD9-45 optical surface Vickers hardness tester with a load of 100 g (Sh-lianer Testing Equipment, Shanghai, China). For each aging time, an average of five indentations was performed. The microstructural observation of the samples was conducted using an optical microscope (Olympus-bx51m; Olympus America) and a FEI Nano230 SEM (FEI Company, OR, USA).

To determine the effect of SiC particle on the corrosion behavior of Al-Mg-Cu-Si-Sn alloy, the electrochemical experiment was carried out on the CHI660C electrochemical test equipment (CH Instruments Inc., Shanghai, China) at 25°C using the typical three-electrode electrochemical system [21], which consists of a platinum electrode as the counter electrode, a saturated calomel electrode as the reference electrode, and samples as the working electrode with a surface of about 0.5 cm² exposed to the 3.5% NaCl solution. The polarization test was carried out by sweeping the potential from cathode to anodic direction at a scan rate of 1 mV/s. Each polarization experiment was run after holding the electrode for 1 min at the open circuit potential to allow steady state to be achieved. All the tests were executed in triplicate.

3 Results and discussion

Figure 2 shows the SEM images of the sintered composites. When the SiC_p fraction was <5 vol.%, the reinforcement particles were located in the vacant sites between the coarser matrix grains, as shown in Figure 2a. The grain surfaces of sample 3 were almost decorated by the reinforcement particles at 10 vol.% SiC (Figure 2c), although some of the reinforcement particles formed clusters in some limited positions. At higher volume fractions, however, non-uniform arrangement of reinforcement particles and clusters of SiC_p were clearly observed (Figure 2d). The clustering became more severe as the SiC particle fraction increased to 30 vol.% (Figure 2e). It is pertinent to point out that the pores formed inside the necklace structure of SiC particles were partly attributable to the removal of the reinforcement particles during sample preparation (Figure 2e).

Figure 2:

Microstructure graphs of the samples after hot sintering at 600°C. (a) Sample 1; (b) sample 2; (c) sample 3; (d) sample 4; (e) sample 5.

Figure 3 shows the variation in hardness with aging time at 175°C for different samples. Different behaviors have been observed in the effects of SiC contents on the aging response. In the unreinforced sample 1, the increase in the hardness with aging time was not obvious, and the hardness reached a peak at 63 HV at 10 h. However, the hardness of sample 2 increased remarkably with the aging time, peaking at 92 HV at 10 h. Similarly, in samples 3 and 4, the hardness gradually increased with the aging time up to the peak hardness, with the hardness of 101 HV and 112 HV at 5 h aging, respectively. However, the maximum hardness has no obviously increase (with a value of about 112 HV), when the SiC contents were further increased to 30%. Results have shown that the age-hardening accelerated in that the times required to reach peak hardness were shortened by about 5 h by the presence of reinforcement in the SiC/Al-Mg-Cu-Si-Sn composites.

Figure 3:

Aging-hardness curves of the samples aging at 175°C.

Figure 4 shows the transmission electron microscope (TEM) micrographs of samples 1 and 3 observed after aging for 10 h at 175°C. This shows that the volume fraction of precipitate phases in the composite is significantly greater than those of sample 1. Ahn and Yu showed that the diffusion of alloy elements is required in order to produce precipitation during the aging treatment [17]. Many dislocations were introduced by severe deformation at the interface between SiC and Al matrix during hot pressing sintering in the composites. These high-density dislocations offer not only a heterogeneous nucleation site for the precipitation but also a high-diffusivity path for the alloy elements [22]. In this way, the age hardening of the composites was accelerated. Also, sample 3 required less time to reach peak hardness than sample 2 because the number of dislocations increased with increase in the volume fraction of the reinforcements.

Figure 4:

TEM graphs of (A) sample 1 and (B) sample 3 after aging for 10 h at 175°C.

Figure 5 shows the potentiodynamic polarization curves for the materials in 3.5% NaCl solution at 25°C. This showed that the different cathodic polarization currents were caused by the different compositions of the samples. The galvanic corrosion current density decreased as the SiC content increased. This finding is inconsistent with published results concerning galvanic corrosion of SiC wafer reinforced by Al-1.0Mg-0.6Si [23]. As the volume fraction of SiC in the composite increased, the dissolution rate of the aluminum matrix decreased. It was therefore concluded that the principal corrosion process for SiC_p/Al-Mg-Cu-Si-Sn composites was galvanic coupling of the matrix and reinforcement [18]. Meanwhile, Table 1 shows E_corr, and Figure 5 shows i_corr. It was observed that none of the E_corr of any of the samples showed more than moderate variation or any definite trends in relation to the amounts of SiC particulate content. However, the i_corr in the samples containing SiC was lower than that of sample 1.

Figure 5:

Polarization curves of the samples after aging for 10 h at 175°C.

The corrosion resistance of SiC/Al-Mg-Cu-Si-Sn composites depends on many factors, such as the SiC contents and size [2, 24], Al-Mg-Cu-Si-Sn alloy composition, Si/Mg molar ratios [4], microstructure, environment, and properties of the Al₂O₃ film covering the alloy matrix surface in the aggressive solution. The silicon carbide particles are conductive and act as local cathodes for the reduction of oxygen in the composites [2]. The study by Zakaria showed that reducing the SiC particles size and/or increasing the volume fraction of the SiC particulates reduces the corrosion rate of the Al/SiC composites [24]. Meanwhile, Mohmad inferred that the degree of corrosion changed with increasing SiC content in the SiC/Al-Mg-Si composites probably mainly due to galvanic couple [18].

4 Conclusions

The time required to reach peak hardness in the composites with 10–30% SiC was shorter than that of the unreinforced Al-0.64Mg-0.60Cu-0.2Si-0.15Sn alloy. The times required to reach peak hardness were shortened by about 5 h by the presence of reinforcement in the SiC/Al-Mg-Cu-Si-Sn composites. The age hardening was accelerated in most of the samples that contained composites with 20% and 30% SiC. The galvanic corrosion current density decreased as the SiC contents increased, and the dissolution rate of the aluminum matrix decreased.