**2.2 Heat treatment**

Properties in particulate-reinforced aluminium matrix composites are primarily dictated by the uniformity of the second-phase dispersion in the matrix. The distribution is controlled by solidification and can be later modified during secondary processing. In particular, due to the addition of magnesium in the A359 alloy, the mechanical properties of this material can be greatly improved by heat treatment process. There are many different heat treatment sequences and each one can modify the microstructural behaviour as desired [20]. Precipitation heat treatments generally are low temperature, long-term processes. Temperatures range from 110°C to 195°C for 5 to 48 hours. The selection of the time temperature cycles for precipitation heat treatment should receive careful consideration. Larger precipitate particulates result from longer times and higher temperatures. On the other hand, the desired number of larger particles formed in the material in relation to their interparticle spacing is a crucial factor for optimising the strengthening behaviour of the composite. The objective is to select the heat treatment cycle that produces the most

Aluminium – silicon – magnesium alloys (A359) are important materials in many industrial applications, including aerospace and automotive applications. The alloys from the Al-Si-Mg system are the most widely used in the foundry industry thanks to their good castability and high strength to weight ratio. Materials based on A359 matrix reinforced with varying

Four types of material are used: 1) Ingot as received A359/40%SiC, with an average particle size of 19±1 micron, 2) Ingot as received A359/25%SiC, with an average particle size of 17±1 micron, 3) Hot rolled as received A359/31%SiC with an average particle size of 17±1 micron and 4) Cast alloy as received A359/30%SiC with particles of F400grit, with an average particle sizes of 17±1 micron. Table 1, contains the details of the chemical composition of the matrix alloy as well as the amount of silicon carbide particles in the metal matrix

**TYPES Si Mg Mn Cu Fe Zn SiC** 

A359 9.5 0.5 0.1 0.2 0.2 0.1 40

A359 9.5 0.5 0.1 0.2 0.2 0.1 25

A359 9.5 0.5 0.1 0.2 0.2 0.1 30

D A359 9.5 0.5 0.1 0.2 0.2 0.1 <sup>31</sup>

The microstructure of such materials consists of a major phase, aluminium or silicon and the eutectic mixture of these two elements. In this system, each element plays a role in the material's overall behaviour. In particular, Si improves the fluidity of Al and also Si particles are hard and improve the wear resistance of Al. By adding Mg, Al – Si alloy become age

Properties in particulate-reinforced aluminium matrix composites are primarily dictated by the uniformity of the second-phase dispersion in the matrix. The distribution is controlled by solidification and can be later modified during secondary processing. In particular, due to the addition of magnesium in the A359 alloy, the mechanical properties of this material can be greatly improved by heat treatment process. There are many different heat treatment sequences and each one can modify the microstructural behaviour as desired [20]. Precipitation heat treatments generally are low temperature, long-term processes. Temperatures range from 110°C to 195°C for 5 to 48 hours. The selection of the time temperature cycles for precipitation heat treatment should receive careful consideration. Larger precipitate particulates result from longer times and higher temperatures. On the other hand, the desired number of larger particles formed in the material in relation to their interparticle spacing is a crucial factor for optimising the strengthening behaviour of the composite. The objective is to select the heat treatment cycle that produces the most

**2. SiC-particulate reinforced aluminium matrix composites** 

amounts of silicon carbide particles are discussed in this chapter.

INGOT

INGOT

CAST

ROLLE

Table 1. Types and composition of the material

hardenable through the precipitation of Mg2Si particulates.

**2.1 Materials** 

composites.

**2.2 Heat treatment** 

favourable precipitate size and distribution pattern. However, the cycle used for optimising one property, e.g. tensile strength, is usually different from the one required to optimise a different property, e.g. yield strength, corrosion resistance.

Heat treatment of composites though has an additional aspect to consider, the particles introduced in the matrix. These particles may alter the alloy's surface characteristics and increase the surface energies [21].

The heat treatments were performed in Carbolite RHF 1200 furnaces with thermocouples attached, ensuring constant temperature inside the furnace. There were two different heat treatments used in the experiments, T6 and modified-T6 (HT-1) [21, 22].

The T6 heat treatment consists of the following steps: solution heat treatment, quench and age hardening (Fig. 1).

Fig. 1. T6 Heat treatment diagram showing the stages of the solution treatment for 2 hours and artificial ageing for 5h

In the solution heat treatment, the alloys have been heated to a temperature just below the initial melting point of the alloy for 2 hours at 530±5ºC where all the solute atoms are allowed to dissolve to form a single phase solid solution. Magnesium is highly reactive with Silicon at this temperature and precipitation of Mg2Si is expected to be formed. The alloys were then quenched to room temperature. In age hardening, the alloys were heated to an intermediate temperature of 155ºC for 5 hours where nucleation and growth of the β' phase. The desired β phase Mg2Si precipitated at that temperature and then cooled at room temperature conditions. The precipitate phase nucleates within the grains at grain boundaries and in areas close to the matrix-reinforcement interface, as uniformly dispersed particles. The holding time plays a key role in promoting precipitation and growth which results in higher mechanical deformation response of the composite.

The second heat treatment process was the modified-T6 (HT-1) heat treatment, where in the solution treatment the alloys have been heated to a temperature lower than the T6 heat treatment, at 450±5ºC for 1 hour, and then quenched in water. Subsequently the alloys were heated to an intermediate temperature of 170±5ºC for 24 hours in the age hardened stage and then cooled in air (Fig. 2).

Deformation Characteristics of Aluminium Composites for Structural Applications 353

stage which are not fully grown. This evidence shows that β' phase has been formed with magnesium and silicon reacting together but β phases forming platelets of precipitates have not been formed in this HT-1 heat treatment, and this is probably due to the solution treatment temperature that did not allow enough reactivity time between the main alloying

In the rolled 20% SiC the microstructure of HT-1 heat treatment shows an increase of the Silicon phase as shown in the image (Fig. 6). Silicon has been expanded during solidification and subsequent ageing. This formed round areas around the whole area of the composite. Comparing with the cast 30% SiC sample, in the rolled material the silicon phase is

Fig. 3. Microstructure of cast 30% SiC in the as received condition showing four distinct phases: Aluminium matrix, SiC particles, eutectic region of aluminium and silicon and Mg

Fig. 4. Microstructure of rolled 31% SiC in the as received condition showing matrix-

elements.

phase

reinforcement interfaces

Fig. 2. Modified T6 (HT-1) showing stages of solution treatment for 1 hour and artificial ageing for 24h

In both heat treatments undesired formation of phases, like the Al4C3, is a possibility and selection of the solution treatment as well as the age hardening processes should be carefully considered. Temperature and time control, therefore, is extremely important during heat treatment. If the melt temperature of SiC/Al composite materials rises above a critical value, Al4C3 is formed increasing the viscosity of the molten material, which can result in severe loss of corrosion resistance and degradation of mechanical properties in the cast composite; excessive formation of Al4C3 makes the melt unsuitable for casting. In the A359/SiC composite high silicon percentage added in excess aids to the formation of some oxides (SiO2) around the SiC reinforcement, something that retards the formation of Al4C3, since such oxides prevent the dissolution of SiC particles [22].

#### **2.3 Metallographic examination**

In order to analyse the microstructure, a series of sample preparation exercises were carried out, consisted of the cutting, mounting, grinding and polishing of the samples. The microstructures were investigated by SEM, EDAX, and XRD to determine the Al/SiC area percentage, size and count of particulates.

The microstructures of the examined MMCs in the as received condition have four distinct micro phases as clearly marked on the image micrograph, which are as follows: the aluminium matrix, the SiC particles, the eutectic region of aluminium and silicon and the Mg phase (Fig. 3). The distribution of SiC particles was found to be more or less uniform, however, instances of particle free zones and particle clustered zones were observed.

Matrix-reinforcement interfaces were identified by using high magnification Nano-SEM. In the as received hot rolled images the Al Matrix/SiC reinforcement interface is clearly identified (Fig. 4). These interfaces attain properties coming from both individual phases of constituents and facilitate the strengthening behaviour of the composite material.

In the modified T6 (HT-1) condition the microsturucture of the cast 30% SiC has the same phases as in the as received state, plus one rod-shape phase (Fig.5) along the matrix and at the matrix-reinforcement interface has been identified to be Mg2Si precipitates in an early

*Quench* 

since such oxides prevent the dissolution of SiC particles [22].

**<sup>1</sup> <sup>26</sup> Solution Treatment Age Hardening**

Fig. 2. Modified T6 (HT-1) showing stages of solution treatment for 1 hour and artificial

In both heat treatments undesired formation of phases, like the Al4C3, is a possibility and selection of the solution treatment as well as the age hardening processes should be carefully considered. Temperature and time control, therefore, is extremely important during heat treatment. If the melt temperature of SiC/Al composite materials rises above a critical value, Al4C3 is formed increasing the viscosity of the molten material, which can result in severe loss of corrosion resistance and degradation of mechanical properties in the cast composite; excessive formation of Al4C3 makes the melt unsuitable for casting. In the A359/SiC composite high silicon percentage added in excess aids to the formation of some oxides (SiO2) around the SiC reinforcement, something that retards the formation of Al4C3,

In order to analyse the microstructure, a series of sample preparation exercises were carried out, consisted of the cutting, mounting, grinding and polishing of the samples. The microstructures were investigated by SEM, EDAX, and XRD to determine the Al/SiC area

The microstructures of the examined MMCs in the as received condition have four distinct micro phases as clearly marked on the image micrograph, which are as follows: the aluminium matrix, the SiC particles, the eutectic region of aluminium and silicon and the Mg phase (Fig. 3). The distribution of SiC particles was found to be more or less uniform,

Matrix-reinforcement interfaces were identified by using high magnification Nano-SEM. In the as received hot rolled images the Al Matrix/SiC reinforcement interface is clearly identified (Fig. 4). These interfaces attain properties coming from both individual phases of

In the modified T6 (HT-1) condition the microsturucture of the cast 30% SiC has the same phases as in the as received state, plus one rod-shape phase (Fig.5) along the matrix and at the matrix-reinforcement interface has been identified to be Mg2Si precipitates in an early

however, instances of particle free zones and particle clustered zones were observed.

constituents and facilitate the strengthening behaviour of the composite material.

**Hours** 

**Temperature °C** 

ageing for 24h

20

**2.3 Metallographic examination** 

percentage, size and count of particulates.

170

0

450

stage which are not fully grown. This evidence shows that β' phase has been formed with magnesium and silicon reacting together but β phases forming platelets of precipitates have not been formed in this HT-1 heat treatment, and this is probably due to the solution treatment temperature that did not allow enough reactivity time between the main alloying elements.

In the rolled 20% SiC the microstructure of HT-1 heat treatment shows an increase of the Silicon phase as shown in the image (Fig. 6). Silicon has been expanded during solidification and subsequent ageing. This formed round areas around the whole area of the composite. Comparing with the cast 30% SiC sample, in the rolled material the silicon phase is

Fig. 3. Microstructure of cast 30% SiC in the as received condition showing four distinct phases: Aluminium matrix, SiC particles, eutectic region of aluminium and silicon and Mg phase

Fig. 4. Microstructure of rolled 31% SiC in the as received condition showing matrixreinforcement interfaces

Deformation Characteristics of Aluminium Composites for Structural Applications 355

the interface region compared to the matrix. In the case of presence of a crack in the matrix, the precipitates act as strengthening aids promoting crack deflection at the interface resulting in an increase of the composite's fracture toughness [20, 23]. Furthermore, the precipitates formed in the matrix act as support to strengthening mechanisms of the

(a) (b)

the interphacial region between the matrix and the reinforcement.

when SiO2 is reacting with liquid aluminium.

oxygen formed this oxide.

Fig. 7. (a) Hot rolled 31% SiC –T6 showing precipitate formed around the reinforcement. (b) Hot rolled 31% SiC – T6 showing Mg2Si precipitates formed between the SiC reinforcement interface in a platelet shape of around 1-3 μm. A porous close to the interface has been

The X-ray diffraction was carried out on the MMCs in the as received, as well as, in the heat treatment conditions, in samples with 20%, 30% and 31% of SiC particulates. Even though some peaks were superimposed, the results clearly showed the phases present in the microstructures. In particular, in the as received condition and in the heat treatment conditions the results showed existence of aluminium matrix material, eutectic silicon, SiC, Mg2Si, SiO2 phases as the distinct ones, and also MgAl2O4 and Al2O3 phases. MgAl2O4 and Al2O3 oxides give good cohesion between matrix and reinforcement when forming a continuous film at the interface. The presence of MgAl2O4 (spinel) shows that low percentage of magnesium reacted with SiO2 at the surface of SiC and formed this layer in

The layers of MgAl2O4 protect the SiC particles from the liquid aluminium during production or remelting of the composites. This layer provides more than twice bonding strength compared to Al4C3. Furthermore, the layer of Al2O3 oxide is formed as a coating

The same phases have been identified in the HT-1 modified condition. In the T6 condition XRD results showed one more phase present which is the spinel-type mixed oxide MgFeAl04 showing that Fe trace reacted with Mg and in the presence of aluminium and

2 2 <sup>4</sup> 2 2 *SiO Al Mg MgAl O Si* ++→ + 2 (1)

2 2 <sup>3</sup> 3 42 3 *SiO Al Al O Si* +→ + (2)

reinforcement-matrix interface.

identified in a similar size

increased by 6%. This increase under the same heat treatment conditions is explained by the difference in the percentage of reinforcement in the material. Therefore, it becomes evident that the introduction of SiC reinforcement promotes zone kinetics and phase formation reactions during heat treatment process. The reinforcement, depending on its percentage in the matrix material, accelerates or restrains events such as precipitation and segregation. This is further supported by the fact that precipitation has not been observed in the HT-1 heat treated 20% SiC rolled material, where lower percentage of SiC reinforcement sloweddown the precipitation kinetics and β' phases could not be created in a similar manner as the 30% SiC cast sample.

In the T6 condition the microstructural results showed that in the rolled 31% SiC sample precipitates of Mg2Si have been formed in the matrix in a platelet shape with a size of around 1-3 μm, as well as in areas close to the interface (Fig. 7). The higher solution temperature and lower age hardening holding time that exist in the T6 heat treatment process, promoted the forming of this type of precipitates which more densely populated

Fig. 5. Microstructure of cast 30% SiC in the HT-1 condition showing rod shape β' phases of Mg2Si around the matrix and the interface of the reinforcement

Fig. 6. Hot rolled HT-1 sample showing phases of Aluminium, SiC, Silicon, Mg

increased by 6%. This increase under the same heat treatment conditions is explained by the difference in the percentage of reinforcement in the material. Therefore, it becomes evident that the introduction of SiC reinforcement promotes zone kinetics and phase formation reactions during heat treatment process. The reinforcement, depending on its percentage in the matrix material, accelerates or restrains events such as precipitation and segregation. This is further supported by the fact that precipitation has not been observed in the HT-1 heat treated 20% SiC rolled material, where lower percentage of SiC reinforcement sloweddown the precipitation kinetics and β' phases could not be created in a similar manner as the

In the T6 condition the microstructural results showed that in the rolled 31% SiC sample precipitates of Mg2Si have been formed in the matrix in a platelet shape with a size of around 1-3 μm, as well as in areas close to the interface (Fig. 7). The higher solution temperature and lower age hardening holding time that exist in the T6 heat treatment process, promoted the forming of this type of precipitates which more densely populated

Fig. 5. Microstructure of cast 30% SiC in the HT-1 condition showing rod shape β' phases of

Fig. 6. Hot rolled HT-1 sample showing phases of Aluminium, SiC, Silicon, Mg

Mg2Si around the matrix and the interface of the reinforcement

30% SiC cast sample.

the interface region compared to the matrix. In the case of presence of a crack in the matrix, the precipitates act as strengthening aids promoting crack deflection at the interface resulting in an increase of the composite's fracture toughness [20, 23]. Furthermore, the precipitates formed in the matrix act as support to strengthening mechanisms of the reinforcement-matrix interface.

Fig. 7. (a) Hot rolled 31% SiC –T6 showing precipitate formed around the reinforcement. (b) Hot rolled 31% SiC – T6 showing Mg2Si precipitates formed between the SiC reinforcement interface in a platelet shape of around 1-3 μm. A porous close to the interface has been identified in a similar size

The X-ray diffraction was carried out on the MMCs in the as received, as well as, in the heat treatment conditions, in samples with 20%, 30% and 31% of SiC particulates. Even though some peaks were superimposed, the results clearly showed the phases present in the microstructures. In particular, in the as received condition and in the heat treatment conditions the results showed existence of aluminium matrix material, eutectic silicon, SiC, Mg2Si, SiO2 phases as the distinct ones, and also MgAl2O4 and Al2O3 phases. MgAl2O4 and Al2O3 oxides give good cohesion between matrix and reinforcement when forming a continuous film at the interface. The presence of MgAl2O4 (spinel) shows that low percentage of magnesium reacted with SiO2 at the surface of SiC and formed this layer in the interphacial region between the matrix and the reinforcement.

$$2\text{SiO}\_2 + 2\text{Al} + \text{Mg} \rightarrow \text{MgAl}\_2\text{O}\_4 + 2\text{Si} \tag{1}$$

The layers of MgAl2O4 protect the SiC particles from the liquid aluminium during production or remelting of the composites. This layer provides more than twice bonding strength compared to Al4C3. Furthermore, the layer of Al2O3 oxide is formed as a coating when SiO2 is reacting with liquid aluminium.

$$\text{\ $}\text{\$ SiO}\_2 + \text{\text{4Al}} \rightarrow \text{\text{2Al}}\_2\text{O}\_3 + \text{\text{3Si}} \tag{2}$$

The same phases have been identified in the HT-1 modified condition. In the T6 condition XRD results showed one more phase present which is the spinel-type mixed oxide MgFeAl04 showing that Fe trace reacted with Mg and in the presence of aluminium and oxygen formed this oxide.

Deformation Characteristics of Aluminium Composites for Structural Applications 357

manufacturing process. In particular, in the rolled 20% SiC material the increase in

Furthermore, variability in microhardness values was observed when comparing cast and rolled materials with different percentage of SiC. However, this variability varied when samples processed at different heat treatment conditions were compared. Highest variability showed samples in the as received condition, whereas lowest variability showed samples in the T6 condition, with samples in the HT-1 condition in between. This can be explained by the fact that precipitates act as strengthening mechanisms and affect the micromechanical

In the absence of precipitates (in the as received condition), the volume percentage of SiC and the manufacturing processing play a significant role in micromechanical behaviour of the composite. As precipitates are formed due to heat treatment process they assume the main role in the micromechanical behaviour of the material. In the HT-1 heat treatment condition there is presence of β' precipitates which affect the micromechanical behaviour in a lesser degree than in the case of T6 heat treatment condition where fully grown β precipitates are formed. It becomes clear that after a critical stage, which if related to the formation of β precipitates in the composite the dominant strengthening mechanism is

While Figure 8 shows results in areas that include the interface region (where precipitates are concentrated) Figure 9, shows results on microhardness values in the aluminium matrix (where precipitates are dispersed). In Figure 9 there is similar variability for all three materials processing states, as received, HT-1, and T6. This implies that in the matrix material the percentage of the reinforcements, the manufacturing process, as well as the

**Microhardness Vs Heat Treatment-AL**

AL- AS-RECEIVED AL- HT-1 AL- T6

Figure 10 shows microhardness measurements obtained from areas around the matrixreinforcement interface in a composite heat treated in the T6 condition. The microhardness

**Heat Treatment Cycles**

precipitation hardening, are strengthening mechanisms of equal importance.

 ROLLED20SIC ROLLED31SIC CAST30SIC

Fig. 9. Microhardness Vs. Heat treatment cycles for Aluminium areas

microhardness values is in the order of 90%.

behaviour of the composite material.

precipitation hardening.

Hv
