Mechanical properties of Al–15Mg₂Si composites prepared under different solidification cooling rates

1.   Introduction

The distinguished properties of in-situ Al−15Mg₂Si composites, owing to the existence of primary (Mg₂Si_P) and eutectic Mg₂Si (Mg₂Si_E) reinforcements in their microstructure, render them suitable candidates for automotive, aerospace, and other industries, where weight saving, compactness, good tribological properties, and high temperature resistance are needed. Examples of such applications are engine blocks, piston heads, cylinder frame, cylinder heads, gear box, brake disk, and steering rod [1−3].

Mg₂Si is an intermetallic compound with an antifluorite structure and a face-centered cubic lattice (a = b = c = 0.6351 nm) [4]. This compound possesses a low density (1.99 × 10³ kg/m³), high melting point (1085°C), great elastic modulus (120 GPa), excellent hardness (4500 MN/m²), low thermal expansion coefficient (7.5×10⁻⁶ K⁻¹) [2,4], and brittle property [5]. Moreover, Mg₂Si exhibits a weak facet interface, with the matrix stemming from its high fusion entropy [6]. However, under equilibrium growth conditions, Mg₂Si particles are intrinsically crystallized as coarse irregular-shaped particles that are unevenly distributed within the matrix and can deteriorate mechanical properties. Therefore, thus far, many efforts have been exerted to overcome the negative effects of unmodified Mg₂Si particles on the mechanical, tribological, or corrosion behavior of Al−Mg₂Si composites, the most important of which are chemical modification through the addition of appropriate modifiers elements [1−2,7−8], molten metal processing [9], mechanical/thermo-mechanical processing via friction stir processing, equal channel angular pressing, multi-directional forging, hot extrusion, accumulative back extrusion, repetitive continuous extrusion forming [10−16], and heat treatment [17]. Regardless of their effectiveness, several of these techniques are cost intensive, require especial equipment, suffer from sample size and shape limitations, and can locally modify microstructures. Chemical modification is a simple and cost-effective technique that can thoroughly modify the microstructure of Al−Mg₂Si composites. However, the technical/metallurgical issues associated with chemical modification, such as oxidation loss of the modifier element, formation of unwanted detrimental compounds rich in modifier elements, and/or increased porosity content in the microstructure, may restrict its application [8,18−19]. Increasing the solidification cooling rate is also used to refine/modify as-cast microstructures. In addition, an increased cooling rate decreases the size and promotes the uniform distribution of microconstituents and gas/shrinkage porosities in a microstructure [20−22]. Moreover, an increased cooling rate refines the grains and reduces the elemental micro-segregations [22].

Studies have dealt with the influence of solidification cooling rate on the mechanical properties and microstructural evolution of Al−Mg₂Si composites. Wang et al. [23] investigated the influence of different cooling rates on morphological changes and the manner by which Mg₂Si_P particles are distributed in hypereutectic Al−Mg₂Si composites. They observed that under slow cooling rates (~1°C/s), Mg₂Si particles were crystallized as octahedral particles enveloped by {111} faces. However, at medium (~12.5°C/s) and high (~32°C/s) cooling rates, Mg₂Si_P particles precipitated as truncated octahedrons. Thus, the higher the cooling rate, the larger the area of {100} faces. In another study, Wang et al. [24] investigated the combined influence of cooling rate and Ca modification (0.1wt%) on the microstructure of Al−20Mg₂Si composites. Their results indicated that the increase in the solidification cooling rate changed the 3D morphology of Mg₂Si_P particles from a mixture of octahedrons and equiaxed dendrites to truncated octahedrons, truncated cubes, and perfect cubes. They claimed that this modification enhanced the toughness and strength of the composite. Hadian et al. [25] reported the influence of cooling rate on the mechanical and microstructural properties of Al−15Mg₂Si composites in non-modified and Li-modified conditions. They showed that the increased cooling rate noticeably reduced the size of Mg₂Si_P particles and altered the flake-like morphology of Mg₂Si_E particles to a divorced eutectic structure comprising irregular-shaped Mg₂Si particles embedded in an α-Al phase. They also revealed that the higher the cooling rate, the greater the elongation and the higher the tensile strength of the composite. Li et al. [26] and Shimosaka et al. [27] also showed that augmenting the cooling rate changed Mg₂Si_E from lamellae to a rod-shaped structure.

Most of these studies have focused on the effect of cooling rate on the microstructural evolution of Al−Mg₂Si composites, whereas a limited number of them have dealt with the influence of cooling rate on the mechanical properties. The present work aimed to study the effect of solidification cooling rate on the mechanical properties and microstructural evolution of Al−15Mg₂Si composite using computer-aided thermal analysis and simultaneous monitoring of solidification behavior.

2.   Experimental

Pure Al (99.9wt%), pure Mg (99.9wt%), and pure Si (99.8wt%) were used to prepare the primary Al−15Mg₂Si ingots. Melting was performed in a SiC crucible using an electrical resistance furnace (AZAR-VM10L 1200) at 750°C. Extra Si and Mg were added to compensate for the losses of these elements during the melting process. The melt was continually heated at 750°C for 15 min, stirred gently for 20 s to homogenize its composition, and cast into a steel mold. Table 1 provides the average chemical composition of the ingots. The obtained ingots were re-melted in a clay-graphite crucible using another electrical resistance furnace (AZAR-VM2L 1200). The melt was degassed by dry C₂Cl₆ tablets, skimmed, and gently stirred until homogenization. Then, the melts were cast into the molds with identical geometries (Fig. 1(a)) but made of different materials to simulate different cooling rates to study the effect of solidification cooling rate on the mechanical properties and microstructural evolution of the Al−15Mg₂Si composite. Table 2 lists the mold coding system and their average solidification cooling rates.

Table  1.  Chemical composition of Al−15Mg₂Si composite wt%

Mg	Si	Fe	Mn	Cr	Ti	Cu	Al
9.70	5.75	0.14	0.01	0.01	0.01	0.01	bulk

 | Show Table

DownLoad: CSV

Fig. 1.  Schematic showing the dimensions and geometry of (a) cast-iron mold/final casting and (b) tensile test sample.

DownLoad: Full-Size Img PowerPoint

Table  2.  Coding system and average cooling rates of the molds used in the present study

Code	Mold		Average cooling rate / (°C·s−1)
SM	Ceramic	SiO2-based	2.7
AM		Al2O3-based	5.5
StM	Metallic	Mild steel	17.1
CM		Copper (water-cooled)	57.5

 | Show Table

DownLoad: CSV

A K-type thermocouple was inserted into the mold center, where its tip was about 25 mm above the mold bottom, to determine the solidification behavior of the composites. The thermocouple was connected to a high-speed data acquisition system to record the melt temperature during the solidification process. Prior to each experiment, the thermocouple was calibrated with a high-purity Al liquid (99.99wt%). An analog-to-digital converter with a sensitive 16-bit sensor (resolution of 0.0015%), high-accuracy detection, and response time of 0.02 s was employed to visualize the temperature–time data on the computer screen. The data were recorded by the frequency of 10 readings per second. The thermal analysis modulus in the software can concurrently alert any abrupt thermocouple displacements (or detachments) and display the cooling curve on the screen showing the in-situ monitoring of the recoding process. Origin pro. 9.2 software was employed to reveal the curve under a stylish format and perform noise smoothing.

A Gnehm-Harteprufer FM100 microhardness tester was employed to measure the Vickers microhardness of the samples, and the vertical force of 30 N and dwelling time of 20 s were used in accordance with the ASTM E92 [28]. The reported final value was the average of eight measurements. A Zwick/Roell Z100 tensile testing machine with the constant crosshead speed of 0.5 mm/min was used to perform the tensile tests. Fig. 1(b) shows the dimensions and geometry of the tensile test samples. The reported final value was the average of four results.

For metallographic purposes, standard metallographic procedures were used to prepare the samples, and a solution consisting of 2 mL HF and 98 mL distilled H₂O was used to etch each cross-section for 8–10 s and reveal their microstructure. Tucker’s reagent (45 mL HCl, 15 mL HNO₃, 15 mL HF, 25 mL H₂O) was also used to reveal the grain structure of composites. A Tescan-Vega scanning electron microscope (SEM) coupled with an energy dispersive spectroscope (EDS) was employed to study the microstructure of composites and perform phase microanalysis, and an Olympus (BX51M) optical microscope (OM) coupled with a portrait image analysis system (Clemex Vision, PE) was used to evaluate the composite microstructure. A Carl Zeiss Axioskop-2-MAT microscope using the linear intercept method was used to measure the grain size in accordance with ASTM E112-12 [29]. The sample density was calculated using Archimedes’ immersion method (BP210S Balance, Sartorius AG, Gottingen, Germany) to determine the porosity volume fraction [30].

3.   Results and discussion

3.1   Microstructural characterization

Fig. 2 depicts that the microstructure of Al–15Mg₂Si composites solidified under different cooling rates. Considering the liquidus projection of Al–Mg–Si ternary phase diagram at the Al-rich corner [31], the solidification of the composite started with the precipitation of Mg₂Si_P particles, followed by the eutectic reaction L → α-Al + Mg₂Si_E. Therefore, regardless of the cooling rate applied during solidification, the microstructure of the composite was composed of polyhedral-shaped Mg₂Si_P and plate-like Mg₂Si_E particles, which were embedded within an α-Al matrix. Under low cooling rates (Fig. 2(a) and (b)), hopper-like Mg₂Si_P particles were formed. This finding may be due to the formation of an Al-rich layer on the {111} face centers of Mg₂Si_P particles, which restricted their growth [32]. Several Fe-rich compounds were also observed in the microstructure, and their formation can be explained by the existence of small amounts of Fe impurity in the composite analysis (Table 1). Fig. 3 shows the EDS analyses of Mg₂Si_P and Mg₂Si_E particles and Fe-rich compounds.

Fig. 2.  SEM microstructure of composites at different cooling rates: (a) 2.7°C/s (SM composite); (b) 5.5°C/s (AM composite); (c) 17.1°C/s (StM composite); (d) 57.5°C/s (CM composite).

DownLoad: Full-Size Img PowerPoint

Fig. 3.  EDS analyses of the phases shown in Fig. 2: (a) point A, (b) point B, (c) point C, and (d) point D.

DownLoad: Full-Size Img PowerPoint

Changes in external conditions can substantially modify the size and morphology of alloy microconstituents, as determined by their intrinsic crystal structure. According to Fig. 2, augmenting the cooling rate modified the Mg₂Si_P and Mg₂Si_E particles and improved their distribution within the matrix. Fig. 4 demonstrates the results of image analysis showing the effect of solidification cooling rate on the geometrical parameters of Mg₂Si_P particles. The average radius of Mg₂Si_P particles decreased from about 20 µm under the cooling rate of 2.7°C/s to about 17.5, 12, and 10 µm under the cooling rates of about 5.5, 17.1, and 57.5°C/s, respectively.

Fig. 4.  Effect of solidification cooling rate on the geometrical parameters of Mg₂Si_P particles: (a) average radius, (b) perimeter, (c) area, and (d) area fraction (R²—The square of the correlation coefficient).

DownLoad: Full-Size Img PowerPoint

The morphology of Mg₂Si_P particles also changed from hopper-like shape in the SM composite to a truncated octahedron in the CM composite. This finding can be explained by the restricted growth of high-energy {100} faces of Mg₂Si_P particles in the <100> directions due to the low coarsening time at high cooling rates [23]. Fig. 5 displays the 3D morphology of Mg₂Si_E particles. An increase in the solidification cooling rate altered the morphology of the coarse Mg₂Si_E particles from flake-like in the SM composite (Fig. 5(a)) to a combination of fine flakes and fibers in the StM composite (Fig. 5(b)) and fine fibers (Fig. 5(c)) in the CM composite. No exact mechanism has been proposed thus far for this transformation.

Fig. 5.  3D morphology of Mg₂Si particles at different solidification cooling rates: (a) 2.7°C/s (SM composite), (b) 17.1°C/s (StM composite), and (c) 57.5°C/s (CM composite).

DownLoad: Full-Size Img PowerPoint

The increased cooling rate also refined and modified the morphology of Fe-rich compounds. As shown in Fig. 2(a)–(c), as the cooling rate increased from 2.7 to about 17.1°C/s, the large plate-like β-Fe particles with the chemical analysis shown in Fig. 3(c) were refined. The further increase in the cooling rate from 17.1 to about 57.5°C/s not only refined the Fe-rich compounds but might have also altered their morphology to blocky shapes/Chinese scripts, with the chemical analysis findings close to that of π-phase (Al₈FeMg₃Si₆) [33]. These results are consistent with those of previous investigations. According to Ref. [34], the existence of Mg can convert a large proportion of β-Al₅FeSi particles to the Chinese script Al₈Mg₃FeSi₆ phase. Sjölander and Seifeddine [35] reported that the increased cooling rate increased the fraction of π-Al₈FeMg₃Si₆ in the microstructure of Al–Si–Mg alloys at the expense of β particles. Wen et al. [36] reported that high cooling rates greatly hindered the formation of β-Al₅FeSi. Narayanan et al. [37] also indicated that the increase in solidification cooling rate modified the morphology and size of Fe-intermetallics in Al alloys through affecting their nucleation and growth.

Fig. 6 depicts the influence of cooling rate on the porosity content and grain structure of the Al–15Mg₂Si composite. As presented in Fig. 6(a), the increased cooling rate substantially refined the composite grains, with the average grain size decreasing from 1.7 mm in the SM composite to 1.1, 0.6, and 0.1 mm in the AM, StM, and CM composites, respectively. Increasing the solidification cooling rate enhances the thermal undercooling component of the total undercooling (as will be discussed later) and restricts diffusion-controlled transformations. Therefore, the refinement of grain, Mg₂Si particles, and other microconstituents with the cooling rate can be ascribed to the synergy between the increased frequency of nucleation, which is mediated by a large driving force at high cooling rates [38], and low diffusion rate of solute atoms, which is required for the growth stage [39]. Moreover, an increase in the solidification undercooling is likely to decrease the time available for coalescence/settling of the effective heterogeneous nuclei within the melt.

Fig. 6.  Effect of cooling rate on (a) the morphology and size of grains and (b) porosity content in the experimental composites.

DownLoad: Full-Size Img PowerPoint

Fig. 6(b) indicates the influence of cooling rate on the porosity content of the Al–15Mg₂Si composite. The higher the cooling rate, the lower the fraction of micropores. According to the density measurement results, the volume percent of micropores decreased from 7.3% in the SM composite to about 5%, 3.8%, and 2% in the AM, StM, and CM composites, respectively. Different factors are responsible for the low content of porosities within the microstructure of the composites solidified at high cooling rates, and the most important of them are as follows: (i) less time available for the diffusion/accumulation of hydrogen atoms into/at the existing nucleation sites (such as bifilms) and the growth of already-nucleated micropores; (ii) a supersaturated solid solution in the solidified solid that prevents the nucleated pores from absorbing the hydrogen required for their growth; (iii) less time available for unfurling/inflation of entrained oxide bifilms (as potential sites for hydrogen pore nucleation); (iv) high nucleation frequency that reduces the size of the remaining liquid pockets at the end of solidification process and volumetric porosity content/space constriction [40–41]. As will be discussed later, the presence of micropores reduces the effective load-bearing cross-section of the composite and facilitates the nucleation of microcracks, thereby impairing its tensile properties.

3.2   Thermal analysis

Fig. 7 shows the cooling and first derivative curves of the Al–15Mg₂Si composite solidified under different cooling rates. The two red dots on each cooling curve show the nucleation and solidus temperatures (T_N and T_S), respectively. Time/temperature intervals between these points are designated as solidification time/temperature ranges (∆t_S/∆T_S) that are divided into zones I, II, and III on their corresponding first derivative curves.

Fig. 7.  Cooling and first derivative curves of the Al–15Mg₂Si composite solidified at different solidification cooling rates: (a) 2.7, (b) 5.5, (c) 17.1, and (d) 57.5°C/s.

DownLoad: Full-Size Img PowerPoint

Zone I shows the interval within which Mg₂Si_P started to nucleate and grow. The nucleation released the latent heat into the solidifying system, which generally enhanced the values of solidification cooling rate (dT/dt or $\dot{T}$) [42]. In terms of temperature variation over the time within zone I, the mold with high thermal resistivity showed resistance in conducting out the released latent heat. Therefore, high dT/dt values were expected to be obtained [43]. In extreme conditions (Fig. 7(a)), the first derivative curve can reach the extremum value at which dT/dt = 0, and a small increase emerged on the cooling curve at 575–580°C. This stage is known as recalescence undercooling (ΔT_R), and it increases the temperature at the vicinity of solid/liquid interface. However, the solidification continues by further formation of Mg₂Si as the temperature decreases. A low probability of ΔT_R was observed in the molds with high cooling rate/thermal conductivity (Fig. 7(b)–(d)).

Zone II corresponds to the eutectic transformation during which Mg₂Si_E forms almost isothermally over a broad time interval. The isothermal Mg₂Si_E possibly forms owing to the balance between the latent heat released from fusion of Mg₂Si_E particles on the one hand and the extraction of heat from the molds on the other hand. The increase in the solidification cooling rate or equivalent reduction in the time interval of zone II refined the size and altered the morphology of Mg₂Si_E phase from flaky to fibrous (Figs. 2 and 5). Similar results have been reported elsewhere [44–45].

The decreased diffusion rate of solute atoms, which is mediated by inadequate time at high dT/dt, also impairs the long-range diffusion of the solutes. Therefore, the atoms can be clustered within short diffusion paths to compensate for the lack of sufficient diffusion time. Consequently, a repetitive array of fine Mg₂Si_E particles spread out over the entire microstructure, as observed in the microstructure of the CM composite (Fig. 2(d)).

In the last stage of solidification, β-Al₅FeSi phase forms during the post-eutectic reaction, as illustrated in zone III. The formation of Si- and Fe-bearing phases usually generates high amount of latent heat that significantly enhances dT/dt values [46]. This finding is shown by a sharp peak that emerges within zone III.

Fig. 8 plots the quantitative measurements of thermal analysis data, including T_N, the eutectic temperature (T_E), T_S, ΔT_S, and Δt_S, at different dT/dt or $\dot{T}$. Given Fig. 8(a)–(c), the increase in $\dot{T}$ reduced all the critical temperatures with different rates. For instance, with the increase in $\dot{T}$ from 2.7 to 57.5°C/s, T_N, T_E, and T_S experienced 15°C, 68°C, and 54°C drop in their values, respectively. The gentler drop in T_N may be due to the higher diffusion rate of the liquid phase, which can better compensate the lack of sufficient time at high $\dot{T}$, compared with the solid phase at high fraction near the solidus temperature. Moreover, given the high fraction of the solid phase or system thermal conductivity, a high reduction in the temperature of solid formation at high values of $\dot{T}$ was expected.

Fig. 8.  Variations in (a) T_N, (b) T_S, (c) T_E, and (d) ΔT_S and Δt_S against $\dot{\mathit{T}}$ and the corresponding mathematical equations.

DownLoad: Full-Size Img PowerPoint

Comparison of Fig. 8(b) and (c) showed that at medium $\dot{T}$ (i.e., 17.1°C/s), the temperature difference between T_E and T_S was the maximum (almost 100°C). Given that a eutectic reaction occurs at approximately constant temperature, this huge difference shows the broad temperature interval of zone III at 17.1°C/s. A high volume fraction of post-eutectic β-Al₅FeSi was expected in this condition, which can be confirmed by SEM analyses (Fig. 2(c)).

As the thermal conductivity of the mold increases (higher $\dot{T}$), the thermal gradient becomes steeper, raising the growth velocity during solidification. In addition, ΔT_S became larger. As mentioned previously, such phenomenon occurs because under non-equilibrium solidification, the deviation of the solidus line is greater than that of the liquidus line. As a result, the alloy solidifying at high $\dot{T}$ assumes a broad mushy state.

3.3   Mechanical properties

Fig. 9 depicts the effect of solidification cooling rate on the tensile properties and hardness of Al–15Mg₂Si composite. Increasing the cooling rate from 2.7°C/s in the SM composite to about 57.5°C/s in the CM composite increased the tensile strength, fracture strain, and hardness by 135%, 260%, and 22%, respectively. The effect of solidification cooling rate on the tensile properties can be further interpreted using the quality index that combines the tensile strength and fracture strain as follows:

Fig. 9.  Effect of solidification cooling rate on the tensile properties of Al–15Mg₂Si composite: (a) tensile strength, (b) fracture strain, (c) quality index, and (d) hardness.

DownLoad: Full-Size Img PowerPoint

where Q_i is the quality index in MPa, UTS is the tensile strength in MPa, the constant k is equal to 150 MPa, and El% is the percentage of fracture strain [47].

Based on our calculations (Fig. 9(c)), the Q_i value increased from about 90 MPa in the SM composite to about 325 MPa in the CM composite, with the latter falling within the range of Q_i values found by other researchers after chemical modification of Al–15Mg₂Si composites. Ghandvar et al. [48] observed that adding 0.5wt% Gd increased the quality index of Al–15Mg₂Si composite from 265 to about 300 MPa. This value was further improved to about 370 MPa by hot extrusion. Emamy et al. [49] reported the positive impact of Fe (~1.0wt%) on the quality index of Al–15Mg₂Si composites, where the Q_i increased from 284 to about 300 MPa (i.e., 5% improvement). Soltani et al. [50] reported the beneficial influence of 0.5wt% Ti modification followed by hot extrusion on the quality index of Al–15Mg₂Si composites. They reported the Q_i value of about 378 MPa. Khorshidi et al. [51] investigated the effect of heat treatment on the Q_i in Na-modified Al–15Mg₂Si composites. According to their findings, the Q_i of the heat-treated composite (~355 MPa) was higher than that of the non-heat-treated composite by about 6%.

The beneficial influence of solidification cooling rate on the tensile properties of Al–15Mg₂Si composites can be explained preliminary by the effective refinement/modification of Mg₂Si_P and Mg₂Si_E particles and post-eutectic intermetallics. The Mg₂Si phase has a cubic antifluorite (CaF₂-type) structure and is brittle due to the strong covalent bond between its Si atoms [52]. Moreover, the coarse and irregular morphologies of Mg₂Si particles along with their sharp edges and weak faceted interfaces result in their susceptibility to microcracking. As a result, when the composite experiences a tensile loading, Mg₂Si compounds, especially the hopper-like primary particles, are likely to be fractured or de-bonded from the matrix.

Fig. 10 shows the fracture surface morphology of SM and AM composites. In agreement with the results above, the fracture surface of SM composite consists of cracked Mg₂Si_P particles and cleavage zones corresponding to the brittle fracture of Mg₂Si_E platelets, which prove their crucial role in the brittle fracture of composites. Table 3 shows the EDS point analyses of Mg₂Si_P (point A) and Mg₂Si_E (point B) particles (Fig. 10(a)). A large porosity is also evident in the micrograph Fig. 10(b). According to the EDS analysis of point C (Table 3), the micropores are nucleated on the entrained oxide, as reported elsewhere [53]. The presence of micropores reduces the effective load-bearing cross-section of the composite and facilitates the nucleation of microcracks [54], thereby impairing its tensile properties. Despite the 100% growth in the cooling rate, the coarse and fractured Mg₂Si_P particles and cleavage fracture patterns are visible on the fracture surface of the AM composite (Fig. 10(c) and (d)).

Fig. 10.  Fracture surface morphology of (a, b) SM composite (2.7°C/s) and (c, d) AM composite (5.5°C/s). The presence of coarse-fractured Mg₂Si particles is evident on the surfaces.

DownLoad: Full-Size Img PowerPoint

Table  3.  EDS analyses of the phases shown in Figs. 10 and 11 wt%

Point	O	Mg	Si	Fe
A	—	63.01	36.99	—
B	—	67.31	32.69	—
C	27.16	14.68	9.34	1.05
D	—	64.90	35.10	—
E	—	66.37	33.63	—

 | Show Table

DownLoad: CSV

The increase in the solidification cooling rate from 5.5°C/s in the AM composite to about 17.1°C/s and 57.5°C/s in the StM and CM composites, respectively, greatly changes the fracture mode (Fig. 11). According to these microfractographs, which are in agreement with the microstructural characterization results (Fig. 2), the coarse, hopper-like Mg₂Si_P particles present on the fracture surface of SM and AM composites (Fig. 10) are replaced by finer Mg₂Si_P particles with less clustering behavior in the StM (Fig. 11(a) and (b)) and CM (Fig. 11(c) and (d)) composites.

Fig. 11.  Fracture surface morphology of (a, b) StM composite (17.1°C/s) and (c, d) CM composite (57°C/s). The arrows show the debonded particles.

DownLoad: Full-Size Img PowerPoint

The decrease in size and increase in the density of Mg₂Si_P particles decreased the amplitude of the stress applied on individual particles, improving their resistance against cracking and/or interfacial debonding. The improved distribution and altered morphology of Mg₂Si_P particles from hopper-like to solid truncated octahedrons reduced their susceptibility to microcracking. The modification of Mg₂Si_E particles also significantly increased the energy required for the initiation and propagation of microcracks, which in turn promoted a ductile fracture mode [55]. Such a result was observed because the already-initiated microcracks preferentially circumvented the soft zones (i.e., α-Al) and easily propagated through the brittle eutectic region [56].

The positive effect of solid solution strengthening and refinement of composite microconstituents is another factor responsible for improving the mechanical properties of the composites fabricated at high cooling rates. Augmenting the cooling rate negatively affects the capability of solutes to diffuse and creates groups with critical radius for nucleation of thermodynamically stable precipitates [57]. This condition increases the concentration of solutes substitutionally dissolved in the host lattice. Therefore, given the different atomic radii of Si and Mg solutes compared with that of Al host, high elastic strains possibly develop around Al atoms. These strains hinder the movement of dislocations, and as a result, the hardness and strength of the matrix improve [58]. According to the Fleischer equation [59], the higher the difference in size/lattice parameter of host and solute atoms and the higher the concentration of solute atoms, the higher the degree of solid solution strengthening. The hardness results also reveal that the microhardness of α-Al matrix increased from HV_0.005 (60.0 ± 0.5) in the SM composite to about HV_0.005 (62.2 ± 0.9), (72.8 ± 0.6), and (89.3 ± 3.1) in the AM, StM, and CM composites, respectively, and these increases are due to the high levels of matrix distortion at high cooling rates [57,59].

4.   Conclusions

The influence of solidification cooling rate on the mechanical properties and microstructural evolution of Al–15Mg₂Si composite was investigated. The conclusions are as follows:

(1) An increase in the $\dot{T}$ narrowed the time interval of pre-eutectic, eutectic, and post-eutectic reactions, whereas it broadened the $\Delta {T}_{\mathrm{S}}$. Thus, solidification occurred rapidly but over a wide range of temperature. The increase in $\dot{T}$ also caused a sharp drop in the probability of $\Delta {T}_{\mathrm{R}}$ and critical solidification temperatures. The latter became more remarkable with the evolution of the solid structure. Therefore, T_S was exposed to a higher drop compared with T_N.

(2) The increased solidification cooling rate decreased the size and altered the morphology of the Mg₂Si_P and Mg₂Si_E particles to more compacted forms with less microcracking tendency. A high cooling rate also improved the distribution of microconstituents and decreased the grain size and volume fraction of micropores.

(3) An increase in the cooling rate from 2.7 to about 57.5°C/s increased the hardness and quality index of the composite by 25% and 245%, respectively.

(4) The fractography results reveal that the fracture surface morphology of the composite changed from a brittle pattern comprising severely cracked Mg₂Si particles to a more ductile fracture pattern consisting of well-refined Mg₂Si particles and extensive dimpled zones.

Conflict of Interest

The authors have no funding, financial relationships, or conflict of interest to report.