A. Structural characterization

Figure 3 depicts typical microstructures of the Sn-free Mg–Mg₂Si composite in as-cast condition.

FIG. 3.

Micrograph of the as-cast Al–15% Mg₂Si composite microstructure.

From Fig. 3 and previous investigations, it can be seen that the microstructure of the unmodified Al–Mg₂Si composite consists of coarse dark particles of primary Mg₂Si and eutectic Mg₂Si + α-Al phases. ^{Reference Khorshidi, Raouf, Emamy and Campbell3} Figure 3 clearly reveals that the morphology of primary Mg₂Si particles is often polygonal with hollow inside and also Mg₂Si phase in eutectic matrix demonstrates flake-like morphology. ^{Reference Li, Liu and Wu16}

The optical micrographs of thick section casting specimens (with 30 mm diameter) of Al–Mg₂Si composite with different Sn contents are shown in Fig. 4.

FIG. 4.

Optical microscope microstructures of in situ Al–15 wt% Mg₂Si composite with different Sn additions: (a) 0 wt%, (b) 0.3 wt%, (c) 0.5 wt%, (d) 1.0 wt%, (e) 3.0 wt%, (f) 5.0 wt%, (g) 7.0 wt% and (h) 10 wt%.

Figure 5 also depicts the variation in particle size of the primary particles with different Sn contents.

FIG. 5.

The size of primary particles as a function of Sn additions.

From Fig. 4, it can be seen that the morphology of primary particles does not change seriously with the addition 0.5 wt% Sn to the composite. But, the microstructural study of cast specimens depicts that the use of higher amounts of Sn (>0.5 wt%) does not alter the morphology of primary particles significantly, but its size notably deceases from 48 µm to 7 µm, shown in Figs. 4 and 5. Further reduction in the size of primary phase (7 µm) is observed by the addition of Sn content up to 5 wt%. Figs. 4 and 5 clearly indicate that not only the particle size of primary phases reduces significantly, but also the number of Mg₂Si-rich particles increases by adding Sn content. This implies an increase in the number of nuclei for the primary Mg₂Si-rich particles during solidification. It has been reported that Sn addition increases the kinetics of forming Mg₂Si precipitates. ^{Reference Azarbarmas, Emamy, Rassizadehghani and Alipour17}

From Figs. 6 and 7, based on the X-ray diffraction (XRD) patterns and EDS analysis results obtained from Al–15% Mg₂Si–5% Sn composite, the presence of Mg and Sn-containing products was confirmed.

FIG. 6.

XRD patterns of the Al–15% Mg₂Si–5% Sn composite.

FIG. 7.

(a) SEM image of Al–15% Mg₂Si composite and its related map for: (b) Al, (c) Si, (d) Mg, (e) Sn, and (f) Al–Mg–Si–Sn.

It is important to note that Sn has no solid solubility in both aluminum and silicon and also it does not form any intermetallic phases with Al and Si. But, when the amount of Sn increases in this system, it tends to introduce new intermetallic phase. ^{Reference Mohamed, Samuel, Samuel, Doty and Valtierra18} In this work, the presence of Mg₂Sn intermetallic has been confirmed by XRD pattern of Fig. 6, when 5 wt% Sn is added to the Al–15% Mg₂Si composite. From Fig. 7(e), Sn element is clearly seen in the Mg₂Si intermetallic. From previous investigation, the solubility of Sn in Mg₂Si was found to be 4.4 at.% and the value of Si solubility in Mg₂Sn was 14 at.%. ^{Reference Jung, Kang, Park, Kim and Ahn19} Some researchers have reported the significant segregation of tin during solidification demonstrates itself in very small precipitates of Mg₂Sn and Sn. ^{Reference Hirsch, Skrotzki and Gottstein20} So, it can be concluded when the amount of tin is low, Mg₂Si-rich phase [Mg₂(Si,Sn)] is formed as primary phase, but when the amount of tin increases up to higher levels (i.e., 10 wt%), the Mg₂Sn-rich phase [Mg₂(Sn,Si)] may play the role of primary phase, which is confirmed by the result of Kozlov et al. ^{Reference Kozlov, Gröbner and Schmid-Fetzer21} The line scan results obtained from EDS analysis, shown in Fig. 8, indicates the variations of Si and Sn atoms and the presence of Sn atom in the Mg₂Si phase.

FIG. 8.

SEM image of Al–15% Mg₂Si–5% Sn composite and its related line scan.

The mechanisms of refinement in Al–Mg₂Si composite by adding some elements have been also discussed by several researchers. ^{Reference Ghorbani, Emamy, Khorshidi, Rasizadehghani and Emami6,Reference Wu, Zhang and Wu10}

It truly is well known that heterogeneous nucleation and poisoning effect are two major types of modification mechanisms. ^{Reference Ghorbani, Emamy, Khorshidi, Rasizadehghani and Emami6} From the results of the current research, the positive role of Sn (especially at lower content) in refining of Mg₂Si can be attributed to the heterogeneous nucleation mechanism. Because, there is a crystallochemical correspondence between Mg₂Si and Mg₂Sn phases, which have the same cubic CaF₂ type crystal structure. ^{Reference Mohamed, Samuel, Samuel, Doty and Valtierra18} And also, solidification path of Al–15% Mg₂Si composite confirms that Sn can enhance the nucleation sites for primary Mg₂Si phase in Al melt, on the other hand, equilibrium phase diagrams of Al–Mg₂Si ^{Reference Azarbarmas, Emamy, Rassizadehghani and Alipour17} and Mg–Sn ^{Reference Nayeb-Hashemi and Clark22} depict that Mg₂Si particles form as the primary phase (Mg₂Si_P) during initial solidification stage (at 600 °C) in Al–15% Mg₂Si composite, whereas the formation of Mg₂Sn intermetallic occurs at 770 °C. Also, Bramfitt has shown that the disregistry of nucleant planes must be as low as possible (i.e., <12%) for nucleation of a solid from liquid phase. ^{Reference Bramfitt23} In literature, it was reported that Mg₂Sn intermetallic has a cubic crystal with lattice parameters; a = 6.739 Å and also the important planes for Mg₂Sn phase are (111), (110), and (100).¹⁸ Due to the proposed mathematical equation, Eq. (1) can be written as follows ^{Reference Bramfitt23} :

that:

${\left( {hkl} \right)_{\rm{s}}}$ = low index plane of the substrate, ${\left[ {uvw} \right]_{\rm{s}}}$ = the low index direction in this plane, ${\left( {hkl} \right)_{\rm{n}}}$ = low index plane in the nucleated solid, ${\left[ {uvw} \right]_{\rm{n}}}$ = the low index direction in this plane, ${\rm{d}}{\left[ {uvw} \right]_{\rm{s}}}$ = the atomic spacing's along the ${\left[ {uvw} \right]_{\rm{s}}}$ , ${\rm{d}}{\left[ {uvw} \right]_{\rm{n}}}$ = the atomic spacing's along the ${\left[ {uvw} \right]_{\rm{n}}}$ , and Θ = is the angle between the ${\rm{d}}{\left[ {uvw} \right]_{\rm{n}}}$ and ${\rm{d}}{\left[ {uvw} \right]_{\rm{s}}}$ .

Table I also shows the results of calculation for some possible crystallographic orientations for Mg₂Si nucleation on the Mg₂Sn particles.

TABLE I.

Calculated values of planer disregistry between Mg₂Si and Mg₂Sn phases.

From Table I, it can be seen that the lowest disregistry (5.4%) has been found for $(001)_{{\rm{Mg}}_2 {\rm{Sn}}} {\parallel} (001)_{{\rm{Mg}}_2 {\rm{Si}}}$ , showing that Mg₂Sn acts as a potent heterogeneous nucleation site for Mg₂Si phase. The primary Mg₂Si particles maintain proper morphology in Al–15% Mg₂Si–5% Sn composite.

Figure 9 shows the cooling curves recorded during the solidification process of Al–15% Mg₂Si alloys without Sn and with 5 wt% Sn addition. There are some temperature arrests in the cooling curve that are hardly recognized by cooling curve. So, the first derivative curves have been used to determine these temperatures. As depicted in Fig. 9, when the melts begin to solidify, the absolute value of first derivate begins to increase, which means that the cooling rate reduces initially and then it start to diminish. In solidification process, the formation of each phase results in an increase in the value of first derivative because of the exothermic nature of phase formation that heats the system and slows down heat release from the system. On the contrary, the minimum value of first derivative is called solidus temperature (T _S), after T _S point, first derivative value enhances.

FIG. 9.

Cooling curve, first derivative of in situ Al–15 wt% Mg₂Si composite with different Sn additions (a) 0 wt% and (b) 5 wt%.

The Mg₂Si nucleation temperature (liquidus temperature), T _N, is shown in Fig. 9. Slight reduction in nucleation temperature (T _N) is seen in the cooling curves, varying from 603 °C for Sn-free specimen to 601 °C for 5% Sn added MMC. As shown in Fig. 9(b), a reaction takes place at 540 °C in Al–15% Mg₂Si–5% Sn system, which can be related to the formation of Mg₂(Sn,Si)-rich intermetallic (Fig. 7). Also, it can be concluded that the solidification time increases from 379 s for the base alloy to 409 s for the alloy containing 5% Sn.

A magnified liquidus region of the cooling curve and its first derivative curve are shown in Fig. 10. It is seen that minimum temperature (T _min) and growth temperature (T _G) decrease with the addition of 5% Sn. After T _N, the cooling rate decreases due to the growing stable nuclei lessens the energy of the system which results in heat release into the melt. Then, in T _min, the formed heat of nucleation overcomes the heat loss of the system. Afterward, the melt becomes hotter up to T _G point. After T _G point, the growth of the present nucleus occurs without forming new crystal nucleates. The difference between T _G and T _min is named the recalescence undercooling. It can be concluded from Fig. 10 after adding 5 wt% Sn to the molten alloy, T _min and T _G are reduced and also the recalescence undercooling temperature is increased from 0.44 °C to 1.13 °C.

FIG. 10.

Magnified liquidus region of the cooling curve and its first derivative curve for (a) unrefined and (b) 5 wt% Sn refined Al–15 wt% Mg₂Si composite.

In similar alloy systems, from several factors affecting the quality of the castings, the role of oxide films has been well understood. ^{Reference Khorshidi, Raouf, Emamy and Campbell3} Previous investigations have shown that Al alloys are generally full of bifilms which not only act as substrates for the precipitation of second phases (like Mg₂Si-rich phase) but also introduce real cracks in the solidified castings. ^{Reference Campbell24,Reference Dispinar and Campbell25} Several defects may result from the presence of bifilms such as porosity and hot tearing. Improper morphologies of second phases can be considered as other deleterious effects of bifilms which impair the reliability of mechanical properties. Cracks may form through the centers of particles due to forming particles on both sides of bifilm. ^{Reference Campbell26}

Similar investigation on Sn-free composite clearly shows that the favorable formation of Mg₂Si on bifilm leads to the evolution of latent heat at a high temperature and delaying the general freezing of the alloy. Because of delayed solidification at higher temperatures, all particles will be coarsened. ^{Reference Khorshidi, Raouf, Emamy and Campbell3} With the addition Sn to this composite, the lower minimum temperature (T _min) and growth temperature (T _G) of modified composite may be another reason of decreasing the bifilm due to the presence of Sn. So the bifilms appear to become deactivated as substrates for the precipitation of Mg₂Si particles.

It is interesting to note that after 5% Sn addition, the cracks on the Mg₂Si particle are hardly seen. The addition of Sn seems to render the oxide substrates unfavorable as substrates for precipitation of Mg₂Si, therefore the melt now continues to new lower temperatures. Finally, oxide bifilms reach a temperature at which they will act as substrates, or some other nucleation sites become active, this necessarily is accompanied by a refinement of the Mg₂Si. But, when the amount of Sn increases up to 10%, some cracks on the particle are observed [Fig. 4(h)]. So, it seems that the effect of Sn on the reduction of bifilm at higher addition is destroyed.

In the current study, although efforts were made to prepare sound castings, the production of a bifilm-free casting is so hard and the use of filtering does not usually eliminate the bifilms completely. If a bifilm-free casting was available, less cracks were expected and with higher Sn content, much Sn or Sn-containing phase would be possible, without the formation of coarse and evidently brittle phases.

B. Mechanical properties

The ultimate tensile strength (UTS) and elongation values of the composite specimens are plotted in Fig. 11 as a function of Sn content in the Al–Mg₂Si composite.

FIG. 11.

UTS and elongation values of the MMC specimens as a function of added Sn contents.

From Fig. 11, it is seen that UTS increases from ∼152 MPa to ∼184 MPa and elongation percentage improves from ∼2.5% to ∼3.4%. The observed improvement in tensile properties of the composite with Sn addition could be a result of the effective refining action of Sn on primary Mg₂Si particles.

On the other hand, the discrete Mg₂Sn intermetallic precipitates mainly at grain/cell boundaries and may restrain the movement of dislocations, resulting in a strengthening effect. The dispersion strengthening of the Mg₂Sn phases also plays an important role in the rising of the tensile strength. ^{Reference Liu, Chen, Tang, Wei and Niu27}

Materials with fine microstructures have a lower tendency for low energy brittle cracking. To obtain acceptable morphology and degree of dispersion of the components, one can interfere at the crystallization stage by modifying the structural characteristics of the material. The modifying of the Mg₂Si precipitates network and their spheroidization are also very important. This allows the reduction in the stress–concentration factor value [K _n in Eq. (2)] in these regions, depending on brittle phase morphology ^{Reference Hertzberg28,Reference Dieter29} :

where, a is one-half the length of a particle and b is one-half the width of a particle.

The elongation percentage enhancement in the composite with Sn contents up to 0.5 wt% is probably due to the refined microstructure and also the formation of more α-Al phase in the microstructure, as seen in Figs. 4 and 11. The presence of α-Al phase in the microstructure causes more fine dimples and ductile fracture in the composite. But, Fig. 11 depicts a reducing trend in tensile properties of the Al–Mg₂Si composite when Sn content is more than 5 wt%. This reduction can be attributed to the fact that the formation of brittle, coarse, and irregular Mg₂Sn intermetallic phase encourages more crack nucleation and crack propagation sites.

C. Fracture characteristics

Figure 12 demonstrates the difference between the fracture surfaces of Al–15% Mg₂Si MMC castings containing 0 and 5 wt% Sn.

FIG. 12.

Fracture surfaces of Al–15% Mg₂Si composites with different Sn contents: (a) 0 wt% and (b) 5 wt%.

It can be understood from Fig. 12a that fracture planes of coarse Mg₂Si particle show clear cleavage characteristic, creating a rapid fracture deriving from their intrinsic and pre-crack structure. ^{Reference Warmuzek30} As can be depicted in Fig. 12(b), by adding 5 wt% Sn to the Al–Mg₂Si composite, some dimples and fewer brittle cleavage surfaces and decohered particles are seen on the fracture surfaces of the samples, consequently the Al–15% Mg₂Si–5% Sn composite clearly leads to a higher elongation, as compared with the lower elongation of unmodified composite.

According to the modified morphology of Mg₂Si particles, it also can be described that with the addition of 5 wt% Sn, the primary Mg₂Si phase becomes finer and more compacted that may lead to the reduced stress-concentration sites. The fracture surfaces also show both decohered and cracked particles in the cast MMC, Fig. 12(b). In Sn added composite, decohesion at the interface between aluminum and Mg₂Si was rarely found, while the secondary cracks in the brittle phases were often observed. It seems that Sn addition alters the fracture mode from brittle to ductile. As expected, soft matrix surrounds the brittle particles, because of rather strong cohesion at the interface between α-Al and Mg₂Si, the matrix is deformed under local active stress. Babout et al. proposed a model to simulate the fracture behavior in MMCs suggesting that regardless of the volume fraction of the particles, interface fracture was strength controlled, and that decohesion occurred when the average tensile stress in the particles surpassed a critical value known as the interface strength. In in situ composites, interface is strong enough and when load is transferred to Mg₂Si primary particle, fracture occurs as soon as the threshold stress is reached. However, if the matrix is soft and the interface is weak, decohesion prevails. ^{Reference Babout, Brechet, Maire and Fougeres31}

According to Griffith's theory, ^{Reference Dieter29} a particle breaks when its fracture stress exceeds the Griffith criterion given by

where, k _c is the fracture toughness of the particles and d is the diameter of the particle. Thus, higher strength values are expected in samples containing Sn. As shown in Fig. 5, Sn addition has been resulted in fine and compacted Mg₂Si particles. Hence, nucleation sites for cracking were reduced [Fig. 12(b)] which led to the improved tensile properties.