Introduction

The metal-matrix composites reinforced with micron- and nano-sized particulates (MMC and MMNC, respectively) are recognized as one of the attractive advanced materials for various structural applications. Hard ceramic reinforcement materials (oxides, carbides, borides, etc.) are routinely incorporated into these MMCs/MMNCs due to their high strength and stiffness/modulus at both the ambient and elevated temperatures [1,2,3]. Different from the classical MMC, MMNCs are generally referred to as the composites containing nano-sized reinforcements and/or nano-sized matrix grains. These MMNC materials have recently received a tremendous attention in the hopes of not only attaining superior strength, but also achieving higher ductility than have been obtained with micron-sized reinforcements. In the lightweight structural materials field, Al and its alloys are the most popular base metals in MMC/MMNC that can be widely used for high-performance applications including automotive, military, aerospace, and electricity industries. Al-based composites often exhibit improved physical and mechanical properties such as superior strength to weight ratio, good ductility, high strength and high modulus, low coefficients of thermal expansion (CTE), high wear resistance, and excellent corrosion resistance [4,5,6,7,8,9].

MMC/MMNC can be manufactured by several synthesis techniques including casting (i.e., solidification) and powder metallurgy (PM) [10,11,12,13]. These synthesis techniques can be generally divided into three categories depending on their processing states, (i) liquid-state processing that involves casting, liquid and pressure infiltration, dispersion methods, etc., (ii) solid-state processing which includes PM techniques with variations in the processing steps such as diffusion bonding, sinter forging, etc., and (iii) liquid–solid processing such as compo-casting and semi-solid forming. The details of these processing methods can be found elsewhere [14,15,16]. Generally, casting is the preferred synthesis method for bulk production with relatively inexpensive processing costs, while PM could be applied to easily control the matrix grain sizes of final products. One of the important advantages of PM is on its low processing temperature compared with the melting techniques, which may reduce the degree of undesired interactions between the Al matrix and reinforcement phases.

Increased strength in these ceramic particulate-reinforced MMCs/MMNCs could be achieved through activation of several strengthening mechanisms such as (i) load transfer [17, 18], (ii) Hall–Petch (i.e., grain refinement) [19, 20], (iii) Orowan [21, 22], (iv) coefficients of thermal expansion (CTE), and/or elastic modulus (EM) mismatch strengthening [23, 24]. Of these, the load transfer and the CTE/EM mismatch strengthening mechanisms can account for the strength increase in the MMC materials with micron-sized particulates, whereas the Orowan effect is supposed to describe the strength increase of MMNC with nano-sized reinforcements, respectively [17, 22, 25]. The Hall–Petch mechanism is one of the primary routes to enhance the strength of both MMC/MMNC systems [19, 24]. The resultant strength increase stems from complicated factors including processing methods/conditions, types of constituent materials, and subsequent microstructural features. Although a certain degree of strength improvement can be achieved in MMC/MMNC by activating some of the preceding strengthening mechanisms, a substantial decrease in energy absorbing abilities (e.g., ductility, toughness, or strain to failure) is commonly observed in these materials [26,27,28]. One of the contemporary goals of developing advanced MMNC is to minimize such reduction in the ductility of composites.

In the present work, we have comprehensively collected existing data for pure and alloyed Al MMC/MMNC to address various mechanical performances such as (i) yield strength (σ y), (ii) ultimate strength (σ us), and (iii) strain to failure (ɛ f) of the composites. Using the data sets currently available, we attempted to quantitatively analyze the impacts of the microstructural features of composites, (i) reinforcement volume fractions, (ii) reinforcement sizes, and (iii) matrix grain sizes on σ y, σ us, and ɛ f for pure and alloyed Al MMC/MMNC. In addition to the analysis of the microstructure–mechanical property relationship, we attempted to compare the mechanical performance between Al MMC and MMNC and to discuss the strengthening mechanisms. For alloyed Al systems, the trends of collective data sets were considered to examine the general mechanical property changes. In the following section, the features of particulate reinforcements are addressed. The next sections will then include the detailed microstructural and mechanical property information for pure and alloyed Al composites.

Features of particulate reinforcements

In Table 1, some important material properties of ceramic reinforcements commonly used in Al MMC/MMNC are summarized [22, 29, 30]. The thermophysical properties listed here are based on the measurements at room temperature. The CTE values for these materials were approximately linear over the experimental temperature range [31]. Because oxide ceramics are in general very susceptible to plastic deformation at elevated temperatures, they are not considered as structural materials at high-temperature applications [32]. A critical step in the processing of cast particulate-reinforced MMC/MMNC is the incorporation of hard ceramic particulates into the molten Al matrix. In a foundry fabrication method (i.e., casting), controlling the wettability of reinforcement particles is one of the major issues. In general, the ceramic particulates exhibit poor wettability to molten Al metals [33]; the contact angles of Al–Al2O3 and Al–SiC were calculated as 97° and 79° at 700 °C, respectively, and the contact angles were found to decrease with increasing temperature [34]. The CTE mismatch between reinforcements and Al matrix is also an essential consideration for composites that will be exposed to thermal cycling [35]. The choice of reinforcement types could be dictated by several factors including particle shapes, processing methods, processing costs, and any desired applications, etc. With these considerations in mind, two reinforcement compounds, SiC and Al2O3, have received the most attention in Al MMC/MMNC applications with their relative high modulus and high fracture toughness [36, 37].

Table 1 Typical material properties of ceramic reinforcements [22, 29, 30]
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Table 2 shows the typical ranges of the reinforcement sizes, volume fractions of reinforcements, and metal-matrix grain sizes that are often observed in Al MMC/MMNC. In the current work, we categorized Al MMNC as the samples with embedded particulate size of less than ~100 nm. Depending on the processing methods, different factors must be taken into consideration to produce high-quality composites. For example, with PM processing, the compositions of the matrix and the types of reinforcements might be controlled in a relatively independent way; however, in casting, they are intimately linked through the reactivity between reinforcements and matrix in the molten state. The distribution of reinforcement particulates is also a critical factor to determine the mechanical properties, and it is greatly influenced by the processing methods. Several advanced synthesis techniques such as stir mixing and ultrasonic mixing can be applied for the homogeneous distribution of particulates in casting. In the PM techniques, some secondary fabrication methods such as extrusion and rolling are essential in the composite processing, and these post-processing methods have been identified to improve the physical properties of composites by redistributing the reinforcement arrangements [38]. In PM-processed composite materials, the distribution aspects of reinforcement particles will depend on the blending and the consolidation procedures as well as the matrix-to-reinforcement particle size ratios (PSR). If the size of matrix powders is large relative to the size of reinforcements, the reinforcing particles tend to segregate in the interstices of the coarse-grained matrices. In composites processed by molten metal mixing methods (i.e., casting), the particulate distribution features could be more complicated because the distribution is influenced by numerous processing factors such as the particle distribution in the liquid and the particle redistribution as a result of solidification.

Table 2 Typical microstructural specifications for Al MMC and MMNC systems
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Pure Al systems

In Tables 3 and 4, we summarized the currently available data for the reinforcement types, processing methods/conditions, reinforcement fractions/sizes, matrix grain sizes, and resultant mechanical properties including the yield strength (σ y), ultimate strength (σ us), and strain to failure (ɛ f) for pure Al MMC (Table 3) and MMNC (Table 4) systems, respectively [14, 39,40,41,42,43,44,45,46,47,48,49,50]. Instead of ultimate tensile or compressive strength (i.e., σ uts or σ ucs, respectively), σ us was used because some of the existing ultimate strength data are based on the compression tests. In the tables throughout this document, (C) indicates the properties obtained through the compression tests. In the following subsections, we address the effects of the reinforcement volume fractions, reinforcement sizes, and matrix grain sizes on σ y, σ us, and ɛ f of pure Al-based composites. Note that although similar processing techniques (e.g., PM + extrusion) may have been applied to produce some of the collected examples shown in Tables 3 and 4, experimental conditions such as the sizes of starting powder/particle, extrusion temperatures, and extrusion ratios are vastly different.

Table 3 Reinforcement types, processing methods, microstructural features, and mechanical properties of pure Al MMC systems
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Table 4 Reinforcement types, processing methods, microstructural features, and mechanical properties of pure Al MMNC systems
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Particle volume fraction effects

MMCs

Figure 1 shows the selected examples of (a) σ y, (b) σ us, and (c) ɛ f variations along with the particulate volume fractions for pure Al MMC systems listed in Table 3. In general, as the volume fraction of reinforcements increases, σ y and σ us initially increase and then decrease, and ɛ f decreases, respectively. The decrease in σ y and σ us with high reinforcement vol% is generally observed in both MMC and MMNC and is commonly explained by the clustering of reinforcements and by the formation of weak regions in the composites [41]. Homogeneous and uniform distribution of reinforcement particles depends not only on the particle contents but also on the matrix-to-reinforcement PSR. Although a homogeneous distribution of embedded particulates is highly desired, it can be hardly achieved when the PSR value is large [51]. Due to the decay of these σ y and σ us beyond a certain amount of reinforcement addition, identification of the peak particulate vol% position to produce maximum σ y and/or σ us is one of the primary concerns in designing MMCs/MMNCs. The peak positions of reinforcement vol% to exhibit the best performance could be largely different. One of the reasons for such difference in the peak positions is correlated with the reinforcement particle size; there would be a high propensity for clustering when the particulate size is smaller, and this clustering of particles will decrease the strength of composites at a lower reinforcement vol%. This trend is clearly shown in Fig. 1a, b; as the particulate sizes increase from 0.47 [43] → 3–48 [42] → 70 [14] μm, the peak vol% values for σ y and σ us shift from 2% → 7% → none. In Al/SiC composites in Ref. [14], the decrease in σ y and σ us was not even observed, which implies that considerable clustering/segregation of reinforcement particulates did not occur in the composite structure up to 20 vol% of particulate addition. In the Al/Al2O3 composites [42] shown in Fig. 1b, there is a maximum σ us for the samples with particle sizes of 3 and 12 μm, while no prominent maximum is observed for the sample reinforced by 48 μm particle size. This indicates a lower tendency for clustering in the composites with larger particles, which could be attributed to a higher specific surface area of smaller particles. Also, the difference in the peak vol% position could be resulted from different processing methods. If a high degree of porosity is generated during synthesis, the peak vol% position may be significantly lowered.

Figure 1
figure 1

Effects of particle volume fractions on a σ y, b σ us, and c ɛ f for pure Al MMC systems

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Note that σ y and σ us of the monolithic pure Al (i.e., non-reinforced samples, 0% reinforcement fractions) show different strength values in Fig. 1a, b. As there is no alloying effect in the matrix for these pure Al matrix samples, such initial differences in σ y and σ us presumably come from the differences in their metal-matrix grain sizes and the initial dislocation densities derived from various processing methods and heat treatment histories. However, in many of the previous experiments, measurements for the matrix grain size have not been quantitatively performed. For example, in Refs. [14, 41, 43], information for the initial Al powder sizes is sometimes provided, but the grain sizes of monolithic unreinforced Al samples were not provided. The samples reinforced by SiC particles [14] showed relatively low strength even at the particulate vol% of 20. The reason is explained by the grain coarsening resulted from hot extrusion; the sintered specimens (at 575 °C for 10 h) were hot extruded to rods at 570 °C with an extrusion ratio of 9:1 to form tensile test samples. The difference in σ us of Al/SiC [14] and Al/Al2O3 [42] composites can be related to many factors such as the dissimilar particulate types (SiC and Al2O3), particulate sizes (70 and 3–48 μm), testing types (tensile and compression tests), and processing methods (with and without extrusion).

As shown in Fig. 1c, the variations of ɛ f with particle vol% clearly present the consistent impacts of reinforcement vol% on the ductility of Al MMCs; ɛ f decreases as the reinforcement vol% increases. The reduction of ɛ f in the samples with higher vol% of reinforcement particulates is explained by substantially lower ɛ f of brittle ceramic particles and by a higher degree of particle agglomeration/clustering. Figure 1c also shows the effects of Al2O3 sizes on ɛ f [42]; as the size of the Al2O3 reinforcement particles increases, the measured ɛ f value decreases. This decrease is even greater in the specimens with the particle sizes ranging from 12 to 48 μm due to a smaller density and a larger porosity in these specimens. Larger porosity could be originated from the decline in the pressing capability of samples with larger contents of Al2O3. Prohibitive effects of Al2O3 particulates on sintering mechanisms might be another reason for this phenomenon. Due to its high melting point, Al2O3 will be likely to form a weak bonding to pure Al, which can lead to a weaker network.

MMNCs

Analogous to the MMC case, it is generally expected that the strength (σ y and σ us) and the ductility (or strain to failure, ɛ f) of MMNC are increased and then decreased and decreased, respectively, with increasing the vol% of reinforcement particulates. Figure 2a–c provides the plots of selected data sets from Table 4 to display (a) σ y, (b) σ us, and (c) ɛ f variations with reinforcement vol% in pure Al MMNC systems. Compared with MMC materials, the reinforcement vol% incorporated in MMNC systems is typically much smaller, because of a much stronger propensity for agglomeration of nano-sized reinforcements during synthesis. From Fig. 2a, one can clearly see that Al/Al2O3 MMNC shows a wide range of σ y with various reinforcement vol% in the composites [39, 45, 48, 50]. As mentioned earlier, the reinforcement vol% is not the only parameter determining mechanical properties. One of the reasons for the difference in σ y shown in Fig. 2a is accounted for the sample fabrication history. Kang and Chan [39] employed the following sequential steps to prepare their tensile test samples: (i) cold isostatic processing (CIP) at 150 °C, (ii) sintering at 620 °C for 2 h, (iii) extrusion at 420 °C with a ratio of 36:1, and (iv) annealing at 350 °C for 2 h. On the other hand, the sample preparation procedure used in Ref. [48] is: (i) ball milling (BM), (ii) compaction at 140 MPa, and (iii) hot extrusion at 600 °C with a ratio of 20:1. Alizadeh [50] roll-bonded the billets at room temperature with a 50% ratio for 9 cycles. From these, it is appreciable that higher plastic deformation prior to the tensile testing can produce a sample with finer grains along with higher σ y and σ us (i.e., work-hardening effect). In Fig. 2a, different peak positions for σ y are observed in Refs. [39] and [48], where Al/Al2O3 MMNC was prepared using PM processes. Powder compaction step was followed by sintering at 620 °C for 2 h in Ref. [39], while a hot forward extrusion process at 600 °C was employed instead of sintering to obtain a highly dense product in Ref. [48]. As previously discussed, a lower porosity in the synthesized sample can be obtained at an elevated sintering temperature, and a higher degree of porosity may cause a significantly lower peak position [41]. It is notable that, sometimes, the increasing and decreasing trend for σ y with reinforcement vol% is not found in the experiments as exemplified in Refs. [45, 50]. Alizadeh [50] employed Al2O3/B4C nanoparticles as the reinforcement materials, and no local maximum σ y was monitored in the produced composite up to 4 vol% of Al2O3/B4C. In his work, microstructural characterization revealed a fairly uniform distribution of the nano-sized Al2O3/B4C particles. The author claimed that increased number of Al and Al2O3/B4C layers, the metal extrusion through particle clusters, and the sheet elongation due to rolling, are the main reasons to observe uniform distribution of the Al2O3/B4C reinforcements. No σ y peak position was observed in Ref. [45] as well, as the SiC clusters and the SiC-free regions were found to decrease with increasing the SiC contents. However, it is not currently certain that how much reinforcement vol% could be added to these samples without σ y peak positions beyond 4 vol%.

Figure 2
figure 2

Effects of particle volume fractions on a σ y, b σ us, and c ɛ f for pure Al MMNC systems

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In Fig. 2b, the σ us variations in Al MMNCs with reinforcement vol% are presented. As explained before, minimal degrees of particle clustering are reported in the samples of Refs. [45] and [50]; therefore, σ us in these examples continuously increases as the reinforcement vol% increases. When the results for the effects of the reinforcement vol% on σ y and σ us are compared, the optimum vol% values of the reinforcement contents to show the best performance are nearly identical for σ y and σ us. This is generally true for the cases of both MMC and MMNC. These optimal reinforcement vol% values are determined by many factors including material types and processing histories. It is once more generally expected that, with increasing reinforcement contents, ɛ f of MMNC decreases because the distance between the hard phases decreases with a higher amount of reinforcements, which hinders the dislocation movements. Note that ɛ f measurement values are not reported for Al MMNC in many literature most likely because Al MMNC composites containing nano-sized reinforcement particles often show very poor ductility. Figure 2c shows two selected examples of the ɛ f variations with the reinforcement vol% in Al MMNC. Different starting points and decreasing rates of ɛ f were observed in the ɛ f vs. reinforcement vol% plots that may be resulted from different material types, microstructures, and processing conditions.

Particle size effects

It is identified that the reinforcement sizes can impose an indirect effect on the mechanical properties of the Al MMC, an increasing trend in σ y and σ us with decreasing particle sizes. This trend is clearly reported in Ref. [42], as shown in Fig. 1a, b. Here, when the Al2O3 reinforcement particulate size increases from 3 to 48 μm, measured σ y and σ us were substantially decreased in the entire reinforcement vol% ranges. This can be explained by the dispersion strengthening mechanism in Al matrix [52]; decreasing the particulate size in a certain volume fraction leads to a decrease in the distance between the particulates, which increases the required tension for dislocation movements between the reinforcement particulates, thereby increasing the strength of composites. Also, smaller particles will indirectly increase the strength by slowing down the grain growth during processing (i.e., grain refinement mechanism). An increasing trend in ɛ f with decreasing particle size is expected for Al MMCs as presented in Fig. 1c [41, 42]; however, there would be an optimal minimum size of reinforcements because smaller particulates show much higher tendency for agglomeration upon synthesis, which will substantially deteriorate the strength and the ductility of composites. Therefore, the optimal particulate size must be carefully determined considering the material types and synthesis conditions. As for the impacts of particulate size on the mechanical performance of pure Al MMNCs, not sufficient data are currently available partly because the variation of nano-sized particulate sizes is rather limited and a uniform size of particulates is routinely added into the Al matrix.

Matrix grain size effects

The matrix grain size of Al MMC/MMNC is also a vital factor to determine various mechanical properties of composites. The grain sizes of synthesized composites are highly influenced by the amounts of added reinforcement particles [39, 45, 50]. It is generally understood that as greater reinforcement contents are added with smaller particles, more refined grain structures are obtained [53]. This is because the particulates can pin the grain boundaries and prevent the grain growth of Al matrix during synthesis. Such grain boundary pinning effects are observed nearly all Al MMC/MMNC including Al2O3, B4C, and SiC particulates. In Ref. [41], it was observed that any increase in reinforcement particulate contents leads to finer grain sizes in pure Al MMC samples containing 5–20 wt% Al2O3. Kang and Chan [39] showed that the grain sizes of Al/Al2O3 composites decreased when the embedded reinforcement contents increased from 0 to 4 vol% and remained unchanged for further addition of reinforcements. It also has been found that the B4C nanoparticles can lock the Al grain boundaries and prevent a significant grain growth during hot pressing [49] and that SiC particles can retard the grain growth during sintering producing ultrafine grain structures [45]. Although it is known that the reinforcement contents largely affect the matrix grain size, there could be a wide range of differences in the quantitative impacts of reinforcement particulate on the final grain size, and the observed difference is generally associated with the reinforcement particulate sizes/amounts, starting monolithic matrix grain sizes, and fabrication conditions.

Strengthening mechanisms

In Fig. 3a, we present selected examples of the comparison between experimental (measured) and theoretical (calculated) σ y values for pure Al MMNC samples listed in Table 4. Theoretical σ y of particulate-reinforced MMNC materials can be estimated by the following considerations [54, 55].

$$ {\text{Arithmetic}}\;{\text{summation:}}\,{}\sigma_{y} = \sigma_{0} + \Delta \sigma_{\text{HP}} + \Delta \sigma_{\text{OR}} + \Delta \sigma_{\text{CTE}} + \Delta \sigma_{l} $$
(1)
$$ {\text{Quadratic}}\;{\text{summation:}}\sigma_{y} = \sigma_{0} + \sqrt {\Delta \sigma_{\text{HP}}^{ 2} + \Delta \sigma_{\text{OR}}^{ 2} + \Delta \sigma_{\text{CTE}}^{ 2} + \Delta \sigma_{l}^{2} } $$
(2)
$$ \Delta \sigma_{\text{HP}} = k_{\text{y}} \left( {\frac{1}{{\sqrt {d_{\text{MMNC}} } }} - \frac{1}{{\sqrt {d_{0} } }}} \right) $$
(3)
$$ \Delta \sigma_{\text{OR}} = \frac{{0.13G_{\text{m}} b}}{\lambda }\ln \left( {\frac{{d_{\text{p}} }}{2b}} \right) $$
(4)
$$ \Delta \sigma_{\text{CTE}} = \sqrt 3 \beta G_{\text{m}} b\sqrt {\rho_{\text{CTE}} } $$
(5)
$$ \Delta \sigma_{\text{l}} = \frac{1}{2}V_{\text{p}} \sigma_{0} $$
(6)

where σ 0 is the original yield strength of the monolithic metal matrix, and Δσ HP, Δσ OR, Δσ CTE, and Δσ l are the yield strength increases from the CTE mismatch between the reinforcements and metal matrix, Orowan strengthening, Hall–Petch strengthening, and load-bearing strengthening contributions, respectively. To theoretically evaluate the individual contributions from different strengthening mechanisms, material and processing variables including k y (Hall–Petch constant), G m (shear modulus), b (Burgers’ vector), β (dislocation strengthening coefficient), and α (CTE values) need to be quantified. In Eqs. (3)–(6), d MMNC, d 0, d p, V p, \( \lambda = d_{\text{p}} \left( {\frac{1}{{\left( {2V_{\text{p}} } \right)^{1/3} }} - 1} \right) \), and \( \rho_{\text{CTE}} = \frac{{12V_{\text{p}} \Delta \alpha \Delta T}}{{\left( {1 - V_{\text{p}} } \right)bd_{\text{p}} }} \), respectively, stand for the average grain size of MMNC composites, average grain size of monolithic Al, particulate diameter, particulate volume, interparticle mean free path, and dislocation density created by the CTE strengthening. As introduced in Eqs. (1) and (2), the arithmetic and quadratic summation methods can be independently applied in the theoretical estimation of σ y, but it was claimed that the quadratic summation method produces more reasonable results compared with the arithmetic summation method [55]. In the current study, for the theoretical increase of σ y shown in Fig. 3a, the material parameters listed in Table 5 are used [38, 50]. The CTE was assumed as a constant for Al, Al2O3, B4C, and SiC; although the CTE values of these materials will vary with temperature changes, it was expected that the error originated from this constant CTE assumption would be as large as ~2% [54]. In Fig. 3a, theoretically calculated σ y results for pure Al MMNC using the arithmetic and the quadratic summation approaches are given by the circular and cross-marked symbols, respectively. Results from Refs. [39, 45, 50] are reported here because these are the only resources to report all necessary microstructural parameters including matrix grain sizes. The diagonal line in Fig. 3a represents the one-to-one correspondence between calculated and measured σ y. If individual contributions from different strengthening mechanisms are examined, the contributions from Δσ HP, Δσ OR, and Δσ CTE are largely dominant and that of Δσ l is relatively minor. When theoretically estimated σ y is compared with experimentally measured σ y, one can readily notice that the σ y values based on the quadratic summation are generally much closer to the experimental measurements. In addition, in many cases, the theoretical calculation overestimates σ y, which indicates that the individual strengthening mechanisms addressed in Eqs. (1) and (2) are not fully activated in these Al MMNC systems.

Figure 3
figure 3

a Comparison between experimental (measured) and theoretical (calculated) σ y values and b variations of experimental σ y with the inverse of square root of matrix grain sizes (D −1/2) for pure Al MMNC systems

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Table 5 Parameters used for the σ y estimation of pure Al MMNC systems [38, 50]
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To further examine the strengthening mechanisms in pure Al MMNC systems, in Fig. 3b, we plotted the variations of measured σ y for the Al composites in Fig. 3a with changes of inverse of square root of average matrix grain size (D −1/2). The dotted line in Fig. 3b represents the expected σ y from the Hall–Petch relation using the σ 0 and k y parameters in Table 5. In the figure, measured σ y values of monolithic Al materials are indicated using circles, which should be theoretically positioned on the Hall–Petch line. The measured σ y of monolithic Al (i.e., blue circle in Fig. 3b) for Ref. [45] is much higher than the expected value probably due to the intensive mechanical alloying during synthesis. When the variations of measured σ y of the MMNC materials are examined for Al2O3 composites studied in Refs. [39, 50], the initial slope (indicated as a dashed line) is much greater than that of Hall–Petch line, which obviously elucidates that other strengthening mechanisms in addition to the grain refinement have been activated. However, if more reinforcement particles are added beyond a certain limit, the slope becomes nearly parallel to that of the Hall–Petch line shown as the solid trend lines in Fig. 3b. This implies that the strength increase is merely dominated by the grain refinement and other strengthening routes are essentially deactivated. For the SiC composites in Ref. [45], the initial σ y is much higher, but Δσ y is mostly governed by the grain refinement mechanism without considerable contribution from other strengthening mechanisms. From these results, it is thought that adding nano-sized particles beyond a few vol% (e.g., 2–3 vol% for Refs. [39, 50]) would not effectively devote to Δσ y from anticipated Orowan strengthening mechanism.

Alloyed Al systems

Now, in Tables 6 and 7, we summarize the currently available data for the mechanical properties of Al alloy MMC (Table 6) and MMNC (Table 7) materials, respectively, along with the information of reinforcement material types, reinforcement contents, reinforcement particle sizes, and matrix grain sizes [17, 25, 70, 71, 86,87,88,89,90,91,92,93]. Because Al alloys generally possess much higher strength compared with pure Al, there is a greater volume of reported data based on Al alloy metal matrices. Various Al alloys including 2024 AA, 356 AA, 6061 AA, 7075 AA, Al-Cu alloy, etc., have been utilized as the base metal matrices. Because of the general aspects for the impacts of reinforcement particle volume fractions, particle sizes, and matrix grain sizes on σ y, σ us, and ɛ f of Al alloy MMCs/MMNCs, the Al alloy system sections were simply divided into MMC and MMNC, and results based on the collective data sets are discussed in this section.

Table 6 Reinforcement types, processing methods, microstructural features, and mechanical properties of Al alloy MMC systems
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Table 7 Reinforcement types, processing methods, microstructural features, and mechanical properties of Al alloy MMNC systems
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MMCs

In Al alloy MMC systems, it is claimed that as the volume fraction of reinforcement particles increases, the interfacial area between the particulates and the matrix increases and more load can be transferred from the matrix to the reinforcement particles, thereby improving σ y and σ us of the composites. Similar to pure Al systems, further increase in the reinforcement contents may lead to a reduction in the strength of alloyed Al composites, which could be the results of greater agglomeration of particles and a higher degree of defects/micro-porosity formation [93]. For example, Slipenyuk et al.’s [63] research on the SiC/Al composites showed a maximum σ y at 10 vol% of 3-µm SiC particulates, and reduction in the σ y values was observed when more reinforcements were added. The critical concentration of reinforcements was found to be greater for the composites containing larger reinforcements, i.e., 14-µm SiC, which also confirms the general trend of the particle size effects discussed for the pure Al systems. As another example, σ y and σ us started decreasing at higher SiC contents (i.e., beyond 7 vol%) in SiC/Al composites studied in Ref. [65] mostly due to the increased degree of particulate clustering. The composites produced in this work exhibited heterogeneous microstructures with elongated clusters of particles along the extrusion direction, and the local volumes of these clusters varied between 16% for 3 vol% SiC, 12% for 5 vol% of SiC, and 32% for 10 vol% SiC samples, respectively [65]. In Table 6, the SiC/Al composites examined in Ref. [63] present relatively high σ y and σ us, which can be explained by the effects of post-processing (i.e., aging, age hardening, precipitation hardening) steps in synthesizing the samples. The formation of finely dispersed precipitates in the alloys was the goal of the precipitation hardening process; however, measured ɛ f for these composites showed a substantial reduction.

As discussed earlier for pure Al composite materials, for a given particulate volume fraction in a composite system, a clear relationship between the particulate size and σ y and/or σ us can be identified in alloyed Al systems as well, a decreasing trend with increasing particulate size. For example, the composites reinforced with 16 μm Al2O3 in Ref. [73] showed the greatest tensile strength compared with the results from the composites reinforced with 32- and/or 66-μm Al2O3 particles. Doel et al. [64] found only a minor difference in σ y and σ us of the 5- and 13-μm SiC-reinforced composites, but the authors observed that the 60-μm particulate-reinforced materials exhibited much lower σ y and σ us. A larger ceramic reinforcement particle is more susceptible to fracture at a given applied stress because it is likely to contain a higher quantity of defects larger than critical size. Moreover, it is expected that damaged 60-μm particle would introduce larger-sized flaws or defects within the microstructures and the increase in the stress over the rest of composites for the system with 60-μm particles is likely to be much higher [64]. Therefore, synthesizing composites incorporating smaller particle sizes could provide higher strength and better fabricability as long as the reinforcement concentration does not exceed a certain critical value. We note that the effects of alloy matrix grain sizes on the mechanical properties are not readily available for Al alloy MMC systems because many of the prior studies do not contain the details for the measured grain sizes of metal matrix as shown (i.e., blank data) in Table 6.

Figure 4 epitomizes typical examples of mechanical property changes with (a) reinforcement fractions and (b) yield strength increase (Δσ y) for collectively selected Al alloy MMC materials from Table 6. Here, we particularly selected the data sets to include ɛ f measurements, because the strength and elongation data are both important for practical application of MMC materials. In Fig. 4a, collective data variations for Δσ y, Δσ us, and Δɛ f as a function of added reinforcement fractions are presented as indicated by the orange, green, and purple symbols, respectively. Vol% or wt% were mixedly used in the plot depending on the units reported in the corresponding literatures. The reinforcement contents were set as the major variable to affect mechanical properties in the results of Fig. 4a, because it is recognized as the prime consideration in synthesizing MMC. Although there are some scatters due to the nature of collective data sets irrespective of processing methods and microstructural features, the Δσ y, Δσ us, and Δɛ f variations shown in the figure clearly elucidate the contemporary status of Al alloy MMC materials. From a collective point of view, reinforcement particles may be added up to ~20 and ~7–8 vol% (or wt%) to increase σ y and σ us, respectively, but adding larger amounts of particulates can substantially decrease ɛ f. In fact, all results containing more than 20% of reinforcements suffered from more than 50% reduction in ɛ f. The collective property change data with Δσ y variations given in Fig. 4b clearly confirm this; when Δσ y is greater than 60%, ɛ f of the sample essentially approaches to 0 by losing at least 70% of original elongation ability. For instance, in Ref. [63], Δσ y = 83.44% in the Al-Cu sample with addition of 20 vol% SiC is compensated by Δɛ f = −92.46%. σ us may increase as σ y increases by ~50%; however, a decreasing trend in relative Δσ us is apparent when σ y increases beyond ~50%. It is considered that minimization of such sacrifice in Δσ us and Δɛ f with high Δσ y is still a formidable challenge in developing advanced high-strength high-ductility Al MMC systems.

Figure 4
figure 4

Mechanical property changes with a reinforcement fractions and b Δσ y for Al alloy MMC systems

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The ultimate intent of developing MMC materials is to enhance the mechanical performance of composites starting from the monolithic metal matrix. Therefore, it is of pivotal importance to assess how much improvements can be made from the monolithic metal matrix by adding reinforcements. In Fig. 5, we present the variations of (a) Δσ y, (b) Δσ us, and (c) Δɛ f as a function of respective monolithic properties for Al alloy MMCs using the same data sets included in Fig. 4. Symbols with identical types and colors have been used to denote the same specimen. For Δσ y case, the majority of testing samples exhibited Δσ y values under ~30–40% and maximum Δσ y was observed when the σ y of monolithic metal matrix is near ~150 MPa. However, such maximum Δσ y reported in Al-Cu alloys in Ref. [63] is thought to be the results of heat treatment and aging rather than those of particulate incorporation. The 356 AA composites reinforced with SiC-RHA (rice husk ash) studied in Ref. [69] showed a decent combination of Δσ y (=53.57%) and Δɛ f (=−33.33%). Other than these two examples, generally, it is summarized that Δσ y could be achieved up to a maximum of ~30–40% over the entire range of starting monolithic σ y. From the figure, one can apparently notice that the Δσ us variations with starting σ us are much smaller than Δσ y variations and that some of σ us exhibited even lower values than the original monolithic σ us. In the data set shown in Fig. 5b, maximum Δσ us was found in the samples to contain SiC-RHA reinforcements started from σ us = 263 MPa [69]. Δσ us reported in Bharath et al.’s [79] work also showed a relatively high value (Δσ us = 29.59%) when 12 vol% of Al2O3 was added to 6061 AA. From the figure, Δσ us is typically measured smaller than ~20%. Although the Δɛ f variations with initial monolithic ɛ f are widely spread out, the scatter plots displayed in Fig. 5c generally confirm that ɛ f of composites is greatly reduced when the monolithic ɛ f is large; if the starting ɛ f is greater than 20%, Δɛ f of alloyed Al MMC systems unanimously experienced higher degrees of reduction more than −30%. In Fig. 5c, the relatively smaller Δɛ f (less than −30%) is mostly correlated with the smaller amount of reinforcement addition and therefore smaller Δσ y. We must again note that although the collective data sets contained in Figs. 4 and 5 are based on the selected experimental measurements listed in Table 6 that contain ɛ f information, they clearly elucidate the current status of Al alloy MMC systems regarding the improvements of three important mechanical properties, i.e., Δσ y, Δσ us, and Δɛ f.

Figure 5
figure 5

Variations of a Δσ y, b Δσ us, and c Δɛ f as a function of respective monolithic properties for Al alloy MMC systems

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MMNCs

In general, a similar trend for the effects of particle volume fractions on the mechanical properties is observed for Al alloy MMNC; there is a certain critical value where nano-composites exhibit their maximum σ y and σ us. It was claimed that such critical volume fraction is the point where the matrix is particle-saturated and that further addition of the particulate volume fraction would not impose substantial impacts on the grain size of the matrix; therefore, the grain refinement effects remain independent of the particulate volume fraction beyond this critical point [93]. For instance, it was found that additional increase of Al2O3 reinforcement contents beyond 3 wt% caused a reduction in σ y in Sajjadi et al.’s [72] study. The authors showed that higher degrees of particle agglomeration, defects, and micro-porosity were observed with higher contents of Al2O3 reinforcements. As an exception for this trend, the SiC/Al nano-composites in Ref. [89] exhibited monotonous increase in σ y and σ us with reinforcement additions up to 15 wt%. It is considered that the low-temperature aging treatment chosen (125 °C for 8 h) in this work improved the mechanical property values. The authors claimed that because of homogeneous particle distribution without a significant clustering, no peaks were found in σ y and σ us of these composites [89]. However, the SiC particle size embedded in the composites was < 500 nm, which is relatively larger than the dimension of typical nano-sized ceramic particulates (i.e., < ~100 nm). We also find that ɛ f in general gradually decreases with increasing the contents of nano-sized particles. Besides greater agglomeration, the increased mass fraction of nanoparticles will decrease the effective slip distance of dislocations during deformation, which will in turn decrease ɛ f [94, 95].

In Fig. 6, the collective mechanical property changes with (a) reinforcement fractions and (b) Δσ y for Al alloy MMNCs are summarized. Again, the currently available data sets to contain ɛ f measurements have been included in the figure. As shown in Fig. 6, the numbers of Al alloy MMNC data sets to report ɛ f values are limited because many of the prior literatures primarily focused only on the strength improvement ignoring ɛ f. Here, it is first noticed that much smaller reinforcement fractions are added in MMNCs, i.e., up to ~5 vol% (or wt%), because of higher propensity for particle agglomeration due to the greater instability of nano-sized particulates embedded in MMNC. From Fig. 6a, most of the MMNC specimen exhibit Δσ y and Δσ us values under ~40%. Δσ us and Δɛ f are generally expected to increase and decrease with increasing Δσ y for Al alloy MMNC systems as well, but the scatter data shown in Fig. 6b do not clearly show such trend partly due to the lack of available data sets and partly due to the variety of processing techniques in synthesizing MMNC materials.

Figure 6
figure 6

Mechanical property changes with a reinforcement fractions and b Δσ y for Al alloy MMNC systems

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In addition to the property changes with reinforcement fractions and Δσ y, in Fig. 7, we present the collective data of (a) Δσ y, (b) Δσ us, and (c) Δɛ f as a function of respective monolithic properties for Al alloy MMNC. Again, identical types and colors of symbols have been used to indicate the same alloyed Al MMNC samples. The 2024 AA composites reinforced with Al2O3 prepared by Su et al. [17] showed much improvements (nearly ~100% or higher) for Δσ y, but these samples have been subjected to a substantial decrease in ɛ f (nearly ~100%) as evidenced in Fig. 7c. Such ɛ f loss can stem from the excessive generation of dislocations during the ball-milling process applied to synthesize the samples. On the other hand, when Δσ y is relatively small, corresponding Δɛ f was also small as shown in the example of Ref. [72]. Out of the alloyed Al MMNC materials in Table 7, the 6061 AA systems with 500-nm particles studied by Knowles et al. [89] exhibited a decent combination of Δσ y = 74%, Δσ us = 60%, and Δɛ f = −15%. Except the Δɛ f values reported in Ref. [89], similar to the Al alloy MMC systems shown in Fig. 5c, we again find that generally ɛ f loss of composites is larger when the monolithic ɛ f is larger, which implies that we may lose nearly all ductility of alloyed Al MMNC materials even starting from relatively ductile metal matrix.

Figure 7
figure 7

Variations of a Δσ y, b Δσ us, and c Δɛ f as a function of respective monolithic properties for Al alloy MMNC systems

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As mentioned earlier, there is an obvious impact of reinforcement material types and contents on the matrix grain sizes of composites, and the grain sizes of matrix can play an important role in determining mechanical properties of final polycrystalline products. Figure 8 illustrates the selected examples of (a) σ y and (b) ɛ f variations as a function of metal-matrix grain sizes for alloyed Al MMNCs listed in Table 7. In Ref. [91], reduced matrix grain sizes were observed by adding more Al2O3 particles, and consequently the flow stress of the composite was found to increase with greater contents of Al2O3 particles in the composites. With higher Al2O3 contents, the Al metal matrix experiences considerable constraints to the plastic deformation due to smaller inter-particle distance and finer matrix grain sizes. SiC particles have shown similar effects on the matrix grain size, where grain refinement strengthening effects were identified in Mazahery et al.’s [87] work. In their study, the grain refinement strengthening mechanism was greatly evidenced with increasing particulate volume fraction because the reinforcement particles can serve as the heterogeneous nucleation catalyst [87]. When the slopes of σ y vs. D −1/2 are quantified, the values are regressively approximated as ~180, ~170, and ~1800 MPa μm1/2 for the samples of Refs. [87], [92], and [91], respectively. Here, the data set for the samples synthesized with adding Cu particles [i.e., open symbols in Fig. 8a] has not been analyzed. Recognizing that the Hall–Petch constant (k y) is ~160–170 MPa μm1/2 for 356 AA alloys [96], Δσ y occurred in the composites of Refs. [87] and [92] is mostly governed by the grain refinement effects, not by the Orowan strengthening mechanism. On the other hand, it seems that Δσ y in Ref. [91] was attained by activating other strengthening mechanisms such as Orowan or solid solution hardening mechanisms. As for ɛ f, it is noticed that ɛ f is generally decreased with decreasing grain sizes as shown in Fig. 8b. Such reduction in ɛ f with adding nano-sized particles and resultant smaller grain sizes is clearly reported in the samples of Refs. [87] and [92]. The 356 AA materials reinforced with Al2O3 nanoparticles and Al (or Cu) exhibited relatively high σ y; however, the ɛ f values of these samples were minimal irrespective of the matrix grain sizes.

Figure 8
figure 8

Variations of experimental a σ y and b ɛ f with the inverse of square root of matrix grain sizes (D −1/2) for Al alloy MMNCs (356 AA)

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From Tables 6 and 7, we find that maximum σ y of Al alloy MMC is 385 MPa in an Al-20 vol% SiC MMC [63], while maximum σ y of Al alloy MMNCs is 307 MPa in an Al-15 wt% SiC MMNC [89]. When the strength increase is considered for these systems, 88 and 81 MPa of σ y were improved, respectively. As for σ us, Al/10 vol% Al2O3 MMC with an ultimate tensile strength of 482.5 MPa [74] exhibited the highest value among the Al alloy MMC systems, and Al/15 wt% SiC MMNC with a maximum ultimate tensile strength of 449 MPa [89] was the best out of the alloyed Al MMNC samples. Generally speaking, Δσ y and Δσ us are expected in the range of less than ~40 and ~20% both for Al alloy MMC and MMNC materials. Some samples showed much enhanced strength, but they typically exhibited a substantial reduction in their ductility, implying that the strengthening mechanism is originated from the generation of extensive dislocations upon processing, not from the activation of desired load transfer or Orowan strengthening mechanisms. The situations for pure Al MMC and MMNC materials are similar to those for alloyed Al MMC/MMNC. Because the intrinsic material properties of pure Al are more ductile with a lower strength value, σ y and σ us of pure Al could be increased much more than the alloyed Al composite cases; however, the impacts of adding nano-sized reinforcement particulates are not prominent. Therefore, it is thought that, based on the collected data from existing literature, not a substantial difference between the mechanical performances of the Al MMCs and Al MMNCs samples was found by adding micron-sized or nano-sized particulates both from the absolute and the relative strength increase point of views. Although there has been a discernable progress in the field of particulate-reinforced Al MMCs/MMNCs, the synthesized products still show relatively low ductility and ɛ f. More in-depth understanding for the relationship among the processing, microstructure, and mechanical performance will be necessary to minimize such inferior ductility of Al MMC/MMNC composites. It is anticipated that the benefit of adding nano-sized reinforcements could be maximized by developing more advanced synthesis techniques such as near-net shape, in situ fabrications [97,98,99,100] and by applying more adequate heat treatments that can prevent the agglomeration and aid the homogeneous distribution of reinforcement particulates.

Summary

In the present study, we have thoroughly analyzed the mechanical performance of pure Al MMC/MMNC and alloyed Al MMC/MMNC systems using a comprehensive collection of currently available data sets. The analysis includes the effects of the particle volume fractions, particle sizes, and matrix grain sizes on the σ y, σ us, and ɛ f variations of composites. Results showed that the reinforcement particulate contents directly affect σ y, σ us, and ɛ f of pure and alloyed Al MMC/MMNC. Depending on the fabrication process, particle size ratio (PSR), and post-processing treatments and heat treatments, an optimum particle volume fraction exists where maximum σ y and σ us are observed. Although the metal-matrix grain size is one of the most basic parameters to determine the mechanical properties of Al MMC/MMNC, adequate measurements for the grain sizes have not been conducted in many previous studies. Addition of reinforcements clearly reduced the ductility of monolithic metal matrix, and the degree of losing ductility was generally found to be greater as the initial ductility of monolithic matrix is higher. While one may expect to attain significant advantages of adding nano-sized reinforcements over the micron-sized reinforcements, not a clear benefit to incorporate nano-sized particulate is identified in the mechanical performance improvements for particulate-reinforced Al-based composite systems. Although there are certain limitations in the mechanical performance of contemporary Al-based MMNC materials, in the future, developing more advanced Al-based MMNC materials is highly expected by applying innovative synthesis techniques along with relevant heat treatments.