Microstructure evolution and strengthening mechanism of A356 composites reinforced with micron and nano SiCp

According to the previous work progress of the research group [18], the dual-scale SiC_p/A356 composites with volume fractions of 15, 20, 25 and 30 vol.% were prepared. The volume fractions of nano sized SiCp (80 nm) and micro sized SiC_p (10 μm) are 2 vol.% and 13, 18, 23 and 28 vol.%, respectively. The size of the base material is 7 μm. The chemical compositions of the A356 alloy used are given in table 1. The microstructure of the original powder used for the work is shown in figure 1.

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The preparation process of dual-scale SiC_p/A356 composites was as follows: Firstly, the micron A356 powder was mixed with nano sized SiC_p by the high energy ball milling (QM-BP type, Zhuo De Instrument (Shanghai) Co., LTD., Shanghai, China) for 12 h with a rotation speed of 150 rpm and ball-to-powder weigh ratio of 8:1, the microstructure of the composite powders was shown in figure 2(a); and then the pre-prepared composite powders and micron SiC_p were mixed by the same process to obtain the mixed powder(figure2(b)). Secondly, LDJ 200/600–300 YS cold isostatic press was used to press the mixed powders under the pressure of 240 MPa for 15 s. Thirdly, the cold-pressed specimens were vacuum sintering using a VAF-7720 vacuum furnace (Shenyang Shenzhen Vacuum Technology Co. LTD., Shenyang, China) at 550 °C for 4 h. And then the sample was extruded into composite bar by an XJ-500 extruder (Wuxi Jishun Air Separation Equipment Co. LTD., Wuxi, China) with an extrusion ratio of 15:1 and a speed of 1 mm s⁻¹. Finally, the bars were annealed at 300 °C for 2 h.

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The composites samples were corroded with Keller reagent after grinding and polishing. The microstructure of the composites was observed by ZEISS 200MAT metallographic microscope (OM, Carl Zeiss AG, Jena, Germany), ZEISS EVO18 scanning electron microscope (SEM, Carl Zeiss AG, Jena, Germany) and JEOL JEM-2100 transmission electron microscope (TEM, Japan Electronics Co. LTD., Tokyo, Japan). The samples for TEM observation were thin foils with the diameter of 3 mm and about 50 μm, and then the ion beam thinned.

Archimede's drainage method was used to measure and calculate the density of composite samples. The hardness of the AMCs was performed by the HB-3000 C Brinell hardness tester (Laizhou Huayin Test Instrument Co., Ltd., Shandong, China), and its average value was obtained by measuring 5 points. The tensile properties of the AMCs were carried out using DDL200 tensile testing machine (Sinotest Equipment Co., Ltd., Changchun, China), with a tensile rate of 1.0 mm min⁻¹. Each material was cut into three samples used for repeat tensile tests. The dimensions of the tensile specimens were shown in figure 3. Besides, the fracture morphology was observed by SEM.

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Figure 4 shows the metallographic structure of dual-scale SiC_p/A356 composites with different volume fractions. As can be seen from figure 4(a) that Si particles uniformly distribute on the aluminum matrix. With the increase of dual-scale SiC_p content, the size of Si particles does not change significantly. Figures 4(b)–(d) shows that the micron SiC_p in the aluminum matrix is evenly distributed without particle agglomeration. When the content of dual-scale SiC_p is 30 vol.%, micron SiC_p appears agglomeration, and several micron hole defects occur in the agglomeration area, as shown in figure 4(e). Therefore, as the content of dual-scale SiC_p increases, the uniformity of SiC_p distribution first increases and then decreases. When the content of dual-scale SiC_p is 25%, the uniformity reaches the maximum. Nano SiC_p was not observed in the metallographic structure due to the low magnification of the metallographic microscope. The distribution of nano SiC_p in the composite was subsequently observed by TEM.

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Figure 5 presents the SEM images of dual-scale SiC_p/A356 composites with various volume fractions. When the content of dual-scale SiC_p is less than 25%, the microstructure of the composite is compact, and no hole defects are found. The distribution of micron SiC_p is relatively uniform, as shown in figures 5(a)–(c). When the content of dual-scale SiC_p is 30%, SiC_p agglomeration and some holes can be observed in the composites (figure 5(d)). When the dual-scale SiC_p content is too high, the local SiCp will aggregate together, resulting in leaving holes in the composite. It is consistent with the metallographic analysis of the composites.

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Figure 6 shows the EDS spectrum of dual-scale SiC_p/A356 composites. The second phases of SiC and Si exist in the matrix, and Al, Si, Mg and C elements in the matrix are evenly distributed, as shown in figure 6(a). Figures 6(b)–(d) correspond to EDS data at point A, B and C in figure 6(a), respectively. The results show the irregular large-size particles in the matrix are micron SiC_p (point A). The dark gray particles with regular shapes, round, oval and smooth edges are Si particles (point B), while the gray matrix phase is the α-Al phase (point C).

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The TEM images of the dual-scale SiC_p/A356 composites are shown in figure 7. As shown in figure 7(a), the nano SiC_p is evenly distributed in the Al matrix, and no nano SiC_p agglomeration is found. Figures 7(b)–(c) shows the electron diffraction spots of micron SiC_p and nano SiC_p, respectively. It is determined that the micron SiC_p is of dense hexagonal structure α-SiC_p, nano SiC_p with face-centered cubic structure β-SiC_p. Figures 7(d)–(e) shows HRTEM images of the interface between dual-scale SiC_p and the Al matrix. It can be seen that micron and nano SiC_p are closely bonding with the matrix, and no reaction products are found at the interface. The results show that the micron and nano SiC_p in the composite form a good interface with the matrix, which provides a favorable structural basis for load transfer and contributes to the improvement of the strength of the composite [19].

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Figure 8 shows the microstructure and dislocation morphology at the interface of dual-scale SiC_p/A356 composites. As shown in figure 8(a), there are high-density dislocations around micron SiC_p. Figure 8(b) shows the sub-grain morphology near micron-SiC_p, with low dislocation density in the grain and its size of about 200 ∼ 400 nm. Figure 8(c) shows that the sub-grain near the nano SiC_p is fine, and the grain size is about 100 ∼ 300 nm. It is mainly because that the nanoparticles pin the grain boundary and inhibit the grain growth. Figure 8(d) shows the sub-grain morphology in the matrix away from SiC_p, and the grain size is about 400 ∼1000 nm. The results show that the grain size of the matrix near micron and nano SiC_p is smaller than that of the blank matrix, resulting in fine grain size strengthening. It is consistent with the results of Bagherpour et al [20]. It can be seen from figure 8(e) that nano SiC_p will be encountered during dislocation movement. Then Orowan strengthening will occur when dislocations are difficult to cut and bypass nano SiC_p, thereby improving the properties of the composite [21, 22].

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Figure 9 exhibits the properties of dual-scale SiC_p/A356 composites with different volume fractions. It can be seen from figure 9(a) that the density of 15 ∼ 25 vol.% dual-scale SiC_p/A356 composites is more than 98.1%. However, the content of SiC_p is 30%, and its density decreases significantly due to the SiC_p accumulation and the more gas adsorbed in the process of sintering and hot extrusion, which results in a significant reduction in density. These results are consistent with microstructure analysis.

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The hardness of composites increased first and then decreased. When the SiC_p reached 30%, its hardness decreases slightly, as shown in figure 9(b). It is because the content of SiC_p is within 25%, and the SiC_p is evenly distributed, which can effectively restrict the deformation of the matrix, thereby improving the hardness (as high as 112.3 HBW) of the composites. When the content of SiC_p exceeds 25%, SiC_p agglomeration and micron holes appear in the composites, which reduces the number of effective SiC_p that bears its strengthening effect and the density of the composites, resulting in decreased hardness.

Figure 10 confirms the tensile properties of dual-scale SiCp/A356 composites with different volume fractions. It can be seen that the strength of the composites increases with the increasing SiC_p volume fraction, and when the content of SiC_p is 25%, it reaches the maximum value with a yield strength of 228 MPa and tensile strength of 310 MPa, respectively. This is because the content of double-scale SiC_p is within 25 vol.% and the distribution of SiC_p is uniform. It can not only hinder the movement of dislocations, but also induce the initiation of many dislocations in the matrix due to the difference of thermal expansion coefficient, thus significantly enhancing the strength of the composite. However, when the dual-scale SiC_p content reaches 30 vol.%, the SiC_p distribution is not uniform, and it is easy to agglomerate. The amount of SiC_p undertaking the strengthening effect decreases, resulting in the reduction of the strength of the composite. In addition, with the increase of SiC_p content, the elongation of the composites gradually decreased from 5.2% to 4.6%, 3.1% and 1.8%, which is much lower than 11.6% of the matrix. According to reference [23], the mechanical properties of 15% dual-scale SiC_p/A356 composite, such as yield strength, tensile strength and elongation, are much higher than other research results.

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Figure 11 displays the SEM images of the tensile fracture of dual-scale SiC_p/A356 composites with different volume fractions. It can be seen from figure 11(a) that the fracture morphology of A356 alloy is mainly composed of dimples and tearing edges, which belong to the typical plastic fracture characteristics. The larger dimples are formed by separating Si particles, and the smaller dimples are formed by the plastic tearing of the matrix. Compared with figure 11(a), the tensile fracture morphologies displayed in figures 11(b)–(d) are changed from plastic fracture to brittle fracture due to the addition of dual-scale SiC_p. There are not only dimples and tearing edges, but also exposed SiC_p in the fracture of the composite. With the increase in SiC_p content, there are more exposed SiC_p. There is a significant difference in the coordinated deformation ability between SiC_p and the matrix during the tensile deformation, which results in the deterioration of the plasticity and the reduction of the elongation. When the content of dual-scale SiC_p is 30%, it can be seen from figure 11(e) that there are a large number of exposed and aggregated SiC_p in the tensile fracture of the composites, the bonding force between these SiC_p is inferior, which is easy to cause initial cracks. Therefore, the strength and elongation of 30 vol.% dual-scale SiC_p/A356 composite are significantly reduced.

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Figure 12(a) shows the Al element content on the SiC_p surface is high, and its fracture mode is mainly the tearing of the matrix. The results show that the interface between SiC_p and Al matrix in the composite is well bonded. It can be seen from figure 12(b) that the Al element is basically invisible on the surface of SiC_p, which indicates that cleavage cracking occurs in the SiC_p of the composites. The results show that SiC_p in the composite is well bonded to the matrix, and the bonding strength of the interface is high. Therefore, the fracture mode of the composites is mainly matrix alloy tearing and SiC_p fracture.

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3.4.1. Load transfer mechanism

For particle-reinforced AMCs, when the particles have a good interface with the matrix, the load can be effectively transferred from the matrix to the reinforcement through the interface [24]. The yield strength increment can be expressed by formula (1) [25]:

$$\begin{eqnarray}&&{\rm{\Delta }}{\sigma }_{{\rm{Load}}}=0.5{v}_{{\rm{p}}}{\sigma }_{{\rm{m}}}\end{eqnarray} \tag{ 1 }$$

where, Δσ_load, v_p and σ_m are the yield strength increment (MPa), particle volume fraction (vol.%) and matrix yield strength (MPa), respectively. It can be seen from formula (1), the Δσ_load increases with the increase of the particle volume fraction and matrix yield strength. Therefore, the load transfer mechanism is one of the main strengthening mechanisms for the dual-scale SiC_p/A356 composites.

3.4.2. Orowan strengthening mechanism

The small hard particle (≤1 μm) in particle-reinforced AMCs can not only transfer load, but also hinder the dislocation movement, thus causing the Orowan strengthening. The increment of yield strength can be expressed by formula (2) [26, 27]:

$$\begin{eqnarray}&&{\rm{\Delta }}{\sigma }_{{\rm{Orowan}}}=\displaystyle \frac{0.81Gb}{2\pi {(1-\nu )}^{1/2}}\displaystyle \frac{1}{\lambda }\,\mathrm{ln}({d}_{{\rm{p}}}/b)\end{eqnarray} \tag{ 2 }$$

in which, G, b, ν represent the shear modulus (2.64 × 10⁴ MPa), Burgers vector (0.286 nm) and Poisson's ratio (0.33), respectively. Δσ_Orowan is the yield strength increment (MPa), λ is particle spacing (μm), d_p represents average particle size (μm). Particle spacing λ can be calculated by v_p and d_p [28]:

$$\begin{eqnarray}&&\lambda ={d}_{{\rm{p}}}{\left(\displaystyle \frac{\pi }{6{V}_{{\rm{p}}}}\right)}^{1/2}-\displaystyle \frac{\pi }{4}{d}_{{\rm{p}}}\end{eqnarray} \tag{ 3 }$$

According to formulas (2) and (3), the smaller the average particle size, the larger the volume fraction is, and the more significant the Orowan strengthening effect is. For the dual-scale SiC_p/A356 composites, although the Orowan strengthening effect of micron SiC_p is minimal, however, the nano SiC_p Orowan strengthening can play a significant role in improving the yield strength of the composites.

3.4.3. Hot mismatch strengthening mechanism

Due to the difference in thermal expansion coefficient between the particles and matrix, high-density dislocations are generated in the composite, which is the thermal mismatch (CTE) strengthening mechanism [29, 30]. Its contribution to yield strength can be expressed by formula (4) [31]:

$$\begin{eqnarray}&&{\rm{\Delta }}{\sigma }_{{\rm{CTE}}}=\alpha Gb\sqrt{\displaystyle \frac{12{V}_{{\rm{p}}}{\rm{\Delta }}\alpha {\rm{\Delta }}{\rm T}}{b{d}_{{\rm{p}}}}}\end{eqnarray} \tag{ 4 }$$

where, G, b, α, d_p, v_p are shear modulus, Burger's vector, dislocation strengthening coefficient (taking 1.4), average particle size and particle volume fraction, respectively; Δσ_CTE is yield strength increment (MPa), Δα is the difference of thermal expansion coefficient (23.7 × 10^–6 /K), ΔT is the temperature difference between the preparation temperature and room temperature (°C).

It can be seen from formula (4) that when the particle size is smaller, the volume fraction is higher, and the temperature difference is larger, the contribution of thermal mismatch to the yield strength of the composite will be more significant. However, formula (4) is mainly proposed for micron-particle reinforced composites. As for nanoparticle reinforced composites, the contribution of dislocation proliferation strengthening to yield strength is ignored. So thermal mismatch strengthening of micron SiC_p is one of the main strengthening mechanisms of dual-scale SiC_p/A356 composites.

3.4.4. Fine grain strengthening mechanism

The dispersed reinforced particles in the composite matrix will reduce the size of the sub-grain and refine the matrix grain. At room temperature, grain boundaries hinder dislocation movement, resulting in grain boundary strengthening. The increment of yield strength caused by grain refinement can be described by the Hall Petch formula [32, 33]:

$$\begin{eqnarray}&&{\rm{\Delta }}{\sigma }_{{\rm{HP}}}=K{\rm{\Delta }}{d}^{-\displaystyle \frac{1}{2}}\end{eqnarray} \tag{ 5 }$$

in which, Δσ_HP, K, ${{\rm{\Delta }}}^{-\displaystyle \frac{1}{2}}$ are the yield strength increment (MPa), strengthening coefficient (0.1 MPa$\sqrt{m}$) and variation of grain size (μm), respectively. From formula (5), it can be seen that the greater variety of grain size is, the more pronounced the effect of fine grain strengthening will be. Due to the addition of dual-scale SiC_p, the matrix grains are refined, so fine grain size strengthening is also one of the strengthening mechanisms of the composites.

3.4.5. Composite strengthening mechanism

To sum up, the strengthening mechanisms of AMCs mainly include Orowan strengthening, thermal mismatch strengthening, load transfer strengthening and fine grain strengthening. For the (10 μm + 80 nm) dual-scale SiC_p/A356 composites, the strengthening mechanism of micro and nano SiC_p reinforcing phase can be summarized as follows: (1) Thermal mismatch strengthening of micron SiC_p (∆σ_M-CTE);(2) Orowan strengthening of nano SiC_p (∆σ_N-Or);(3) Fine grain strengthening due to the combined action of micron and nano SiC_p(∆σ_HP) and load transfer strengthening (∆σ_load) Based on the above analysis, the total contribution of dual-scale SiC_p to the yield strength of A356 can be expressed as [34]:

$$\begin{eqnarray}&&{\rm{\Delta }}{\sigma }_{0.2}={\rm{\Delta }}{\sigma }_{{\rm{load}}}+{\rm{\Delta }}{\sigma }_{{\rm{HP}}}+{\rm{\Delta }}{\sigma }_{{\rm{M}}-{\rm{CTE}}}+{\rm{\Delta }}{\sigma }_{{\rm{N}}-{\rm{Or}}}\end{eqnarray} \tag{ 6 }$$

Figure 13 shows the theoretical and experimental values of the yield strength increment of the dual-scale SiC_p/A356 composites. The results show that the theoretical value of the composite strength increment caused by the superposition of different strengthening mechanisms is higher than the experimental value. It is because the theoretical value is obtained under the ideal condition, such as the dual-scale SiCp is entirely uniformly distributed in the matrix, and there are no defects in the composites.

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Taking the 25 vol.% dual-scale SiCp/A356 composites as an example, the yield strength increment (123.7 MPa) of the composite with different strengthening mechanisms is higher than the experimental value (102 MPa). The specific calculation results of the theoretical value increment are as follows: the Orowan strengthening of nano SiC_p of 59.2 MPa; Micron SiC_p thermal mismatch strengthening of 35.0 MPa; Load transfer strengthening of 15.8 MPa; Fine-grain strengthening of 13.7 MPa. The above analysis shows that the room temperature strengthening mechanism of dual-scale SiC_p/A356 composites is mainly Orowan strengthening along with thermal mismatch strengthening, load transfer strengthening and fine grain strengthening.