Effect of Al₂O₃ Nanoparticles as Reinforcement on the Tensile Behavior of Al-12Si Composites

Abstract

Al₂O₃ nanoparticle-reinforced Al-12Si matrix composites were successfully fabricated by hot pressing and subsequent hot extrusion. The influence of weight fraction of Al₂O₃ particles on the microstructure, mechanical properties, and the corresponding strengthening mechanisms were investigated in detail. The Al₂O₃ particles are uniformly distributed in the matrix, when 2 and 5 wt. % of Al₂O₃ particles were added to the Al-12Si matrix. Significant agglomeration can be found in composites with 10 wt. % addition of Al₂O₃ nanoparticles. The maximum hardness, the yield strength, and tensile strength were obtained for the composite with 5 wt. % Al₂O₃ addition, which showed an increase of about ~11%, 23%, and 26%, respectively, compared with the Al-12Si matrix. Meanwhile, the elongation increased to about ~30%. The contribution of different mechanisms including Orowan strengthening, thermal mismatch strengthening, and load transfer strengthening were analyzed. It was shown that the thermal mismatch strengthening has a more significant contribution to strengthening these composites than the Orowan and load transfer strengthening mechanisms.

1. Introduction

Al metal matrix composites (MMCs) have gained considerable attention as the ideal candidates for potential lightweight materials used in automobiles and other structural applications because of their attractive properties, such as low density, high specific stiffness, and superior wear resistance [1,2,3,4,5,6]. Recently, nano-sized reinforcements have been examined for the fabrication of Al-based composites. It has been shown that the addition of a small amount of fine ceramic particles improves the strength significantly [7,8]. Al₂O₃ particles with high specific stiffness and superior high temperature properties are used as inert ceramic reinforcement phases in MMCs [9,10]. A large number of studies have been performed on the fabrication and characterization of nano-sized Al₂O₃-reinforced Al matrix composites. Karbalaei Akbari et al. [11] studied nano-sized Al₂O₃-reinforced A356 alloy with stirring casting, and agglomerated nanoparticles were observed on dendrites in the fracture surface of the Al-Al₂O₃ reinforcement samples. Su et al. [12] fabricated Al₂O₃ nanocomposites by solid-liquid mixed casting combined with ultrasonic treatment and demonstrated that the ultrasonic vibration during the solidification was beneficial to refine the grain structure, and to improve the resulting distribution of Al₂O₃ nanoparticle in the matrix. Sajjadi et al. [13,14] showed that decreasing the alumina particle size combined with compo-casting process can yield the best mechanical properties for Al-Al₂O₃ composites. The addition of alumina led to the improvement in yield strength (YS), ultimate tensile strength (UTS), compression strength, and hardness. However, the poor wettability of Al₂O₃ with molten aluminum makes it difficult to achieve a uniform distribution of Al₂O₃ nanoparticles in the composite [15].

The results demonstrate that the solidification processes have some advantages such as a wide selection of materials and range of materials that can be processed. Good matrix-particle bonding, ease in controlling the microstructure of the matrix, and flexibility in processing are some attractive features. However, the solidification processes are not suited for the dispersion of fine-sized particles due to the possibility of the formation of agglomeration. Agglomerate formation is due to the van der Waals force of attraction, non-uniform particle distribution, and poor wettability at the interface between metallic and ceramic reinforcements [16,17,18,19,20,21,22]. Powder metallurgy (PM) is an alternative fabrication technique the can yield a uniform distribution of reinforcement and flexibility in reinforcement composition and design [18,19,20,21,22]. Al matrix composites reinforced with nano- and submicron-sized Al₂O₃ particles were prepared by wet attrition milling and subsequent hot extrusion processing by Tabandeh Khorshid et al. [23]. It has been observed that both the hardness and strength of the composites increase and reach a saturation before showing a decline upon increasing the volume fraction of the nanoparticles. Razavi et al. [24] addressed the effect of mechanical milling and reinforcement nanoparticles on the densification response and the compressibility of aluminum powder. However, the effect of reinforcement on the strengthening mechanisms are not well illustrated. To facilitate the development of such composites, it is necessary to fully understand the strengthening mechanism of nano-sized particle reinforcements in the Al matrix composite. In this paper, the nano Al₂O₃ particle-reinforced Al-12Si matrix composites were prepared by powder metallurgy. Al-12Si alloy was chosen for this study because it is a near eutectic composition with good fluidity that has been widely used for a variety of industrial applications [25,26,27,28]. The influence of the weight fraction of Al₂O₃ particles on the microstructure and mechanical properties of Al alloy matrix was studied in detail. Additionally, the strengthening mechanism operating in these materials was investigated.

2. Materials and Methods

The Al-12Si gas atomized alloy powder (nominal composition of Al-88 wt. % and Si-12 wt. %) was selected as the matrix material. Al₂O₃ nanoparticles (d50 ~50 nm) of 2%, 5%, and 10% wt. % were used as reinforcement. The powders were milled to have a uniform mixture of reinforcement in the Al-12Si matrix. Milling was performed in a planetary ball mill (Retsch, Haan, Germany) under an argon atmosphere at room temperature for 2 h. The milling speed and ball to powder ratio were 150 rpm and 10:1, respectively. Consolidation (hot pressing) of the homogeneously mixed powders were carried out using hardened steel dies. Samples with a diameter of 10 mm and a height of 10 mm were hot pressed using the following parameters: load—600 MPa, temperature—673 K, and time—15 min. The samples were subsequently hot extruded with a pressure of ~800 MPa at 673 K, and the extrusion ratio was 6:1. As a result, 4-mm diameter rods were obtained after hot extrusion. For comparison, a monolithic Al-12Si matrix alloy (without reinforcement) was also fabricated by the same method.

For metallographic observation, the samples were prepared using conventional grinding and polishing techniques (from 200 grit to 4000 grit papers and then cloth polishing using diamond paste). The samples were then etched in 0.5% Hydroflouricacid (HF) solution. The microstructure of the composites were characterized by optical microscope (OM) and scanning electron microscope (SEM, Gemini1530 Zeiss) - (Carl Zeiss AG, Oberkochen, Germany). The structural characterization was performed by X-ray diffraction using D3290 PANalytical X’pert PRO (Almelo, The Netherlands) equipped with Cu-Kα radiation. Hardness measurements were carried out using a Vickers micro-hardness (Schimadzu, Dusiburg, Germany) tester at a constant load of 200 gf for a dwell time of 15 s. Tensile tests were carried out using an Instron-5869 device (INSTRON GmbH, Darmstadt, Germany) at a rate of 0.5 mm/min, and the strain was calculated using a laser extensometer. The tensile test samples (according to American Society of Testing and Materials (ASTM) standard: ASTM: E8/E8M–13a) were cylindrical tensile bars with a total length of 52 mm that were machined from the extruded samples. The dimensions along the gauge length of the tensile bars were: length—17.5 mm, diameter—3.5 mm, and 4 mm around the grips. Three nominally identical specimens were tested to obtain the average values.

3. Results

3.1. Microstructure Analysis and Phase Identification

SEM micrographs of monolithic Al-12Si and Al-12Si/Al₂O₃ nanocomposites are shown in Figure 1. The extruded sample in Figure 1a reveals that the eutectic Si (~20 µm) is uniformly distributed in the Al matrix. Figure 1b,c show the microstructure of the composites as a function of the increasing content of Al₂O₃ particles. It can be observed from Figure 1b,c that the Al₂O₃ particles have a relative homogeneous distribution without significant agglomeration of the particles. When the reinforcement content reaches 10 wt. %, the agglomeration of the Al₂O₃ particles is quite evident (Figure 1d). The degree of Al₂O₃ agglomeration varies randomly throughout the samples, as marked in the Figure 1d, which may lead to the formation of defects such as pores and may act as stress concentrators, thereby hampering the overall properties of the composites. The high magnification image in Figure 2a shows the presence of agglomeration in the Al-12Si/Al₂O₃ (10 wt. %) nanocomposite. The energy dispersive spectrum analysis (Figure 2b) shows that the small particles within the agglomeration consist mainly of Al and O and the ratio is close to that of Al₂O₃. Since the nanoparticles have high surface energy due to the high surface to volume ratio, the nanoparticles tend to stick with each other and form agglomerates. Hence, the agglomerates increase with greater additions of the nanoparticles (in excess), resulting in an uneven distribution of the reinforcement in the matrix. Nevertheless, no visible flaws or pores were observed at the Al₂O₃/Al interface.

Figure 3 shows the XRD pattern of the composites reinforced with different contents of Al2O3. As observed, the samples mainly consist of three phases: (1) the Al matrix phase; (2) the Si phase from the matrix; and (3) the Al₂O₃ phase from the reinforcement. The diffraction peaks of the Al phase show the highest intensity because of their weight fraction in the composite. At 2 wt. % Al₂O₃ content, the diffraction peaks of v are hard to observe in the XRD pattern due to their low intensity. When the Al₂O₃ content increases to 5 wt. %, the small diffraction peaks of the Al₂O₃ phase are visible, and their intensity increases when the Al₂O₃ content is increased to 10 wt. %. No other visible peaks were observed, suggesting that there is no additional phase formed between the (Al-12Si) matrix and the (Al₂O₃) reinforcement.

3.2. Physical and Mechanical Properties

Both the theoretical and experimental densities for all composite samples are listed in

Table 1. Density of the as-extruded Al-12Si/Al₂O₃ nanocomposites.

Weight Percent of Al2O3 (%)	Theoretical Density (g/cc)	Measured Density (g/cc)	Relative Density (%)
0	2.61	2.60	99.6
2	2.63	2.61	99.2
5	2.66	2.64	99.5
10	2.70	2.65	98.1

. The theoretical density of the composites are calculated using the rule of mixtures. High relative densities are achieved for all composites, indicating a good interface between the Al-Si matrix and the Al₂O₃ reinforcement. The relative density for the composite with 10 wt. % Al₂O₃ decreases slightly due to the agglomeration of Al₂O₃ particles, which leads to the presence of some pores.

Figure 4 shows the Vickers hardness data for the Al-12Si/Al₂O₃ nanocomposites. The matrix shows a hardness value of 72 ± 1 HV_0.2, which increases to 77 ± 1 HV_0.2 (a change of ~5 HV_0.2) when 2 wt. % of Al₂O₃ reinforcement is added to the matrix. The composite reinforced with 5 wt. % Al₂O₃ nanoparticles shows a higher hardness (80 ± 1 HV_0.2) compared to the other systems. With a further increase in the weight percentage of Al₂O₃ nanoparticles (to 10 wt. %), the composites show a decrease in the hardness value compared to the former ones due to the porosity and irregularities generated by the agglomeration of nanoparticles. Al₂O₃ nanoparticles dispersed uniformly in the Al-12Si alloy may significantly improve their hardness properties compared to the base alloy. The hard Al₂O₃ nanoparticles in the Al matrix act as barriers to the motion of dislocations generated in the matrix and the higher particle density causes Orowan mechanism. However, an increased addition of nanoparticles leads to agglomeration of the reinforcement, resulting in an increased level of porosity in the material and a subsequent degradation of the hardness values.

Figure 5 shows the room temperature tensile stress-strain curves of the nanocomposite, and their properties are summarized in

Table 2. Tensile properties of the as-extruded Al-12Si/Al₂O₃ nanoparticle-reinforced composites.

Weight Percent of Al2O3 (%)	Yield Strength (MPa)	Ultimate Tensile Strength (MPa)	Elongation (%)
0	198 ± 2	228 ± 2	23 ± 2
2	228 ± 2	256 ± 3	24 ± 2
5	245 ± 2	286 ± 3	30 ± 3
10	195 ± 2	221 ± 2	19 ± 1

. It can be observed that the addition of Al₂O₃ significantly improves the yield strength (YS), ultimate tensile strength (UTS), and elongation of the Al-12Si matrix up to 5 wt. % Al₂O₃ addition. This is due to the good dispersion and strong interfacial bonding between the Al-12Si matrix and Al₂O₃ reinforcement. The YS, UTS, and elongation of the Al-12Si matrix in the as-extruded condition is found to be 198 MPa, 228 MPa, and 23%, respectively. Adding 2 wt. % of Al₂O₃ changes the YS, UTS, and elongation to 228 MPa, 256 MPa, and 24%. With further increasing the Al₂O₃ addition to 5 wt. % the YS increases to 245 MPa, UTS to 286 MPa, and ductility to 30%. The YS and UTS of the composite drops (to 195 MPa and 221 MPa, respectively) when the reinforcement content reaches 10 wt. %. In summary, the composite reinforced with 5 wt. % Al₂O₃ nanoparticles shows higher YS and UTS compared to the base alloy and composites reinforced with varying percentages of Al₂O₃ nanoparticles considered in this study.

Figure 6 shows the tensile fracture surfaces of Al-12Si/Al₂O₃ nanocomposites. It can be observed (Figure 6a) that the fracture mode of the matrix Al-12Si alloy is predominantly ductile with several dimples all over the surface. As shown in Figure 6b, 2 wt. % nano Al₂O₃-reinforced composite also displays dimples like the matrix Al-12Si alloy, except that the length scale of the dimples are different. Similarly, composites with 5 wt. % Al₂O₃ also show the presence of dimples again at different length scales than the 2 wt. % Al₂O₃ composite and Al-12Si-based alloy. When the Al₂O₃ content is increased to 10 wt. % (Figure 6d), both the number and size of the dimples decrease significantly, leading to a relatively brittle type of failure. Some defects such as cracks and segregation are observed on the fracture surface (along the neighborhood of the Al₂O₃ reinforcement particles), which also limits both the strength and ductility of the composites. The segregation/agglomeration will prevent an effective bonding between the reinforcement and the matrix and may lead to further defects such as cracks in the composites, and these cracks will inevitably leads to a premature failure of the composites. In addition, the agglomeration/clustering of the reinforcement particles can cause the presence of increased porosity in the composites [29].

3.3. Strengthening Mechanism

Unlike tensile strength, yield strength usually shows less dependency on the presence of the stress concentrations in the composites. Therefore, yield strength is more representative of the discussion of the strengthening mechanisms [30]. The strengthening mechanisms operating in the Al-12Si/Al₂O₃ nanocomposites mainly include the load transfer mechanism, Orowan strengthening mechanism, and thermal mismatch enhancement mechanism. These three mechanisms operate simultaneously [31,32,33]. The most accepted strengthening mechanism is the direct strengthening mechanism, also known as load transfer mechanism. The strengthening of the composites takes place due to the transfer of load from the weaker matrix to the harder reinforcement through their interface, as proposed by Nardon and Prewo [34].

$$\mathsf{\Delta }{\sigma }_{load}=0.5f{\sigma }_m$$

(1)

where ${\sigma }_m$ is the yield strength of the matrix (MPa), and f is the volume fraction of Al₂O₃ particles. According to Equation (1), the effect of load transfer is calculated and is given in

Table 3. Contribution of different strengthening mechanisms on the yield stress of Al-12Si/Al₂O₃ extruded nanocomposites.

Weight Percent of Al2O3 (%)	△σLoad (MPa)	△σOrowan (MPa)	△σCTE (MPa)	△σC (MPa)
2	1.50	3.85	27.94	33.19
5	4.14	6.13	42.47	52.74
10	6.65	8.75	53.49	68.89

. The Orowan strengthening mechanism is very important in metal matrix composites, if the reinforcement particles are fine and the inter-particle spacing is not large. Incremental yield strength of the composite caused by Orowan stress of the Al₂O₃ particles may be expressed as [35,36]:

$$\mathsf{\Delta }{\sigma }_{orowan}=\frac{0.538G_{m}b\sqrt{f}}{d}\ln \frac{d}{2b}$$

(2)

where Gm is the shear modulus of the matrix.

$$G_m=G_{Al}{V}_{Al}+G_{Si}{V}_{Si}$$

(3)

where G_Al=27 GPa, G_Si= 66.8 GPa, B is the Burgers vector of the matrix, b = ((1.414/2) × 0.286 nm) = 0.202 nm, and d and f are the diameter and volume fraction of Al₂O₃ particle, respectively. This effect is calculated and the results are given in Table 3. As shown in Table 3, the higher the Al₂O₃ content, the greater the effect of the Orowan mechanism. When a composite is subjected to thermal changes, then the difference in their coefficients of thermal expansion (CTE) between the matrix and reinforced particles will produce a thermal strain and internal stress state changes. In order to accommodate this thermal mismatch effect, dislocations are generated around reinforced particles within the matrix to reduce the stored energy. The magnitude of thermal strain determines the density of dislocations generated within the composites. The increased dislocation density within the matrix leads to the strengthening of the composite. In this study, the large discrepancy of thermal expansion coefficients between the Al₂O₃ (7.85 × 10⁻⁶/K) and the Al-12Si matrix (20.96 × 10⁻⁶/K) generate dislocations in the Al matrix. The improvement in yield strength from thermal mismatch can be calculated using Equation (4) [37,38,39]:

$$\mathsf{\Delta }{\sigma }_{CTE}=\sqrt{3}aGb\sqrt{{\rho }_{CTE}}$$

(4)

where a is a constant, a = 1. ${\rho }_{CTE}$ is the dislocation density caused by CTE differences, and can be described as follows:

$${\rho }_{CTE}=\frac{12\mathsf{\Delta }\alpha \mathsf{\Delta }Tf}{bd(1-f)}$$

(5)

where △α is the CTE difference between the matrix and the reinforcement, △T is the temperature difference between the testing temperature and room temperature. This strengthening effect is also shown in Table 3. These strengthening mechanisms are additive, and the total yield strength of the composite could be considered as the sum of different mechanisms rather than the contribution of a single one. In its simplest form, the superposition of the individual strengthening mechanisms may be carried out linearly.

$$\mathsf{\Delta }{\sigma }_C=\mathsf{\Delta }{\sigma }_{Load}+\mathsf{\Delta }{\sigma }_{Orowan}+\mathsf{\Delta }{\sigma }_{CTE}$$

(6)

The individual contributions of the described three mechanisms to the yield strength of the composite calculated from Equations (1)–(4) are listed in Table 3 as a function of the weight percentage of Al₂O₃ nanoparticles. The presented data show that the effect induced by thermal mismatch strengthening (△σ_CTE) makes the largest contribution to the yield strength improvement. The strength increments resulting from the load transfer (△σ_Load) and Orowan strengthening (△σ_Orowan) are very similar and are much lower. Figure 7 shows the variation of experimental YS and the theoretical values calculated using Equation (6). The total improvement from the multiple strengthening mechanisms for yield strength is ~30 MPa and 45 MPa, and the predicted values of the yield strength for the composites are ~33 MPa and 53 MPa, respectively, for the composites with 2 and 5 wt. % Al₂O₃ addition. This shows that the experimental and theoretical values are very close and hence the strengthening mechanisms based on load sharing, Orowan mechanism, and thermal mismatch strengthening are expected to play a role in these composites. The small discrepancy between the experimental and theoretical values may arise from the defects (such as porosity, agglomeration, and impurities) present in the samples. A large discrepancy was observed for the composite with 10 wt. % Al₂O₃ as reinforcement, which was mainly caused by the profound agglomeration of the reinforcement in the composite.

4. Summary

In this study, Al₂O₃ nanoparticle-reinforced Al-12Si composites were prepared by hot pressing and subsequent hot extrusion. The microstructure, mechanical properties, and the strengthening mechanisms were investigated in detail. The conclusions are as follows:

(1) The composites reinforced with 2 and 5 wt. % Al₂O₃ nanoparticles exhibited relatively homogeneous distribution, while the microstructure of the composites with 10 wt. % reinforcement exhibited agglomeration of the Al₂O₃ particles in the matrix.

(2) The hardness of the composites increased first (for 2 and 5 wt. % reinforcement) and then decreased (for 10 wt. % reinforcement). The strength of the composites also displayed similar trends to that of the hardness. The maximum values were obtained for the composites with 5 wt. % Al₂O₃. The YS, UTS, and the elongation were 244 MPa, 286 MPa, and ~30%, respectively, with 5 wt. % Al₂O₃ addition, which showed an increase to about ~23%, 26%, and 30%, respectively, compared with the Al-12Si matrix alloy.

(3) The strengthening mechanism analysis shows that the yield strength of the composites increased mostly because of the thermal mismatch enhancement mechanism compared to the other two strengthening mechanisms (load transfer and Orowan strengthening).