Effect of Sb on microstructure and mechanical properties of

Mg₂Si/Al-Si composites

REN Bo(任 波)¹, LIU Zhong-xia(刘忠侠)¹, ZHAO Rui-feng(赵瑞锋)², ZHANG Tian-qing(张天清)³,

LIU Zhi-yong(刘志勇)¹, WANG Ming-xing(王明星)¹, WENG Yong-gang(翁永刚)¹

1. School of Physics Engineering, Key Laboratory of Material Physics, Ministry of Education,

Zhengzhou University, Zhengzhou 450052, China;

2. No. 19 Middle School of Zhengzhou, Zhengzhou 450007, China;

3. DDJT Aluminum alloys Co. Ltd., Dengfeng 452477, China

Received 8 September 2009; accepted 23 March 2010

Abstract: The effect of Sb on the microstructure and mechanical properties of Mg₂Si/Al-Si composites was investigated. The results show that Sb can improve the microstructure and mechanical properties of Mg₂Si/Al-Si composites. When the content of Sb is 0.4%, the morphology of primary Mg₂Si changes from dendrites to fine particles, the average size of Mg₂Si particles is refined from 52 to 25 μm, and the ultimate tensile strength and elongation of the composites increase from 102.1 MPa and 0.26% to 138.6 MPa and 0.36%, respectively. The strengthening mechanism can be attributed to the fine-grain strengthening. However, excessive Sb is disadvantageous to the modification of the composites.

Key words: Mg₂Si/Al-Si composite; Sb; mechanical property; fine-grain strengthening

1 Introduction

Particulate-reinforced aluminum and magnesium metal matrix composites (MMCs) have attractive advantages of high specific tensile strength, modulus, high wear resistance and improved mechanical properties[1-2], which makes them potential materials in the field of automotive industry, such as cylinder blocks, cylinder heads, pistons and valve lifters[3]. In situ Mg₂Si/Al composites have a high wear resistance since the intermetallic compound Mg₂Si has a high melting temperature, low density, high hardness, low thermal expansion coefficient and reasonably high elastic modulus[4-8]. However, these expected excellent properties of Mg₂Si/Al composites can be seriously affected by the coarse Mg₂Si particles and brittle eutectic matrix[5, 8]. Therefore, controlling the primary and eutectic Mg₂Si phase is a key problem for acquiring excellent mechanical properties.

It was reported that the microstructure and mechanical properties of the Mg₂Si/Al composites can be improved by advanced processing techniques, such as hot extrusion, rapid solidification processing, directional solidification, mechanical alloying, electromagnetic stirring and electromagnetic separation[9-10]. However, these processing methods lead to the increase of production cost. Recently, the primary and eutectic Mg₂Si phase in Mg₂Si/Al composites have been modified by means of adding modification elements, and desirable mechanical properties are obtained.

The physical properties of Mg₂Si and Si are very similar, and the solidification characteristics of Al-Mg₂Si and Al-Si systems are also considerable similar (see Tables 1 and 2)[11]. Modification elements, such as Sr, Ba, P, RE, Sb and Ti, can be used to modify primary Si and eutectic structure in Al-Si alloy systems (including hypoeutectic, eutectic and hypereutectic alloys) effectively[12-18], and they can be used as refiners to modify primary Mg₂Si in Al-Mg₂Si composites. It has been also reported that Na salts, K₂TiF₆, mischmetal, P, extra Si，La, Li and Sr can refine the primary Mg₂Si in Al-Mg₂Si or Mg-Mg₂Si alloy systems [1-3, 5, 19-27]. Recently, Sb has been used to modify the primary Mg₂Si

Table 1 Physical properties of Mg₂Si and Si[11]

Table 2 Solidification characteristics of Al-Mg₂Si and Al-Si systems[11]

in Mg-Al-Si-Zn alloy and Mg-Si alloy, which changes the  morphology of Mg₂Si particles from coarse dendrites to fine particles [28-29]. However, few investigations were carried out on the modification of Mg₂Si by the addition of Sb in Mg₂Si/Al composites. In this study, the in situ 20Mg₂Si/Al-5Si composites were produced by means of common gravity casting process. The effect of Sb on the microstructure and mechanical properties of the composites were investigated. In addition, the modification mechanism of Sb on the primary Mg₂Si is analyzed.

2 Experimental

Industrial pure Al, Mg, Si and Al-3Sb master alloy were used to produce 20Mg₂Si/Al-5Si (mass fraction, %) composites. An excess of 20% (mass fraction) Mg was added in order to balance the oxidation loss. All the alloys were melted in a 7.5 kW resistance furnace. The melt of Al-Si alloy was degassed in Ar at 750 °C. The Mg and Al-3Sb master alloy were added into the Al-Si melt at 765 °C and 780 °C, respectively. The Addition amount of Sb was 0, 0.2%, 0.4% and 0.6% (mass fraction), respectively. After cleaning the slag, the melts were poured into a steel mold preheated at 200 °C. Two kinds of composites billets were cast, which were named tensile test billets and analysis billets, respectively. The tensile test billets were machined to cylindrical samples with a diameter of 8 mm and a gauge length of 80 mm. The tensile tests were conducted on an MTS810 tester at a strain rate of 5×10^-4/s. The microstructure of the tested composites was observed and quantitatively analyzed using an Olympus BX51 metallographic microscope with an image collection and analysis system, and JSM6700F scanning electron microscope (SEM). The chemical compositions of these alloys were analyzed by energy dispersive spectrometer (EDS) assembled in SEM. X-ray diffractometer (XRD, X’Pert PROX) was used to identify the crystalline structures with scanning area ranging from 20? to 100? at a speed of 6 (?)/min.

3 Results

3.1 Microstructure of Mg₂Si/Al-Si composites

The XRD patterns of Mg₂Si/Al-Si composites are presented in Fig.1. The microstructures of these composites consist of α(Al), primary Mg₂Si and (Mg₂Si+Si +Al) eutectic.

Fig. 1 XRD patterns of Mg₂Si/Al-Si composites with different Sb content: (a) 0; (b) 0.2%; (c) 0.4%; (d) 0.6%

The microstructures of Mg₂Si/Al-Si composites are shown in Fig.2. Many coarse dendritic or Chinese script type of primary Mg₂Si can be found in the unmodified composites (Fig.2(a)). The average size of primary Mg₂Si decreases with increasing Sb content, and the edges and angles are passivated (Fig.2(b-d)). Moreover, α(Al) phase becomes finer. The primary Mg₂Si changes into fine particles when the content of Sb increases to 0.4%, while excessive Sb results in coarsening of primary Mg₂Si.

Fig.3 presents the effects of Sb on the volume fraction, average size and roundness of primary Mg₂Si. We can see that the volume fraction of primary Mg₂Si and the particles roundness C²/A (C and A are the circumference and area of primary Mg₂Si particles,

Fig.2 Microstructures of Mg₂Si/Al-Si composites with different Sb additions: (a) 0; (b) 0.2%; (c) 0.4%; (d) 0.6%

Fig.3 Correlation parameters of primary Mg₂Si particles as function of Sb content: (a) Volume fraction; (b) Roundness and average size

respectively) have a slightly increase with the increase of Sb; on the contrary, the average size of primary Mg₂Si is refined to 25 μm after adding 0.4% Sb. However, when the content of Sb exceeds 0.4%, the variation tendency of volume fraction, roundness and average size moves to the opposite direction.

3.2 Tensile properties

The ultimate tensile strength (UTS) and elongation of Mg₂Si/Al-Si composites with different Sb content are shown in Fig.4. It can be seen that the UTS and elongation increase from 102.1 MPa and 0.26% to 138.6 MPa and 0.36%, respectively, with increasing Sb content. When the Sb content exceeds 0.4%, however, the UTS and elongation begin to decrease and the tendency is similar to that of the modifications of primary Mg₂Si.

Fig.5 shows the fractograph difference between primary and modified composites. In Fig.5(a), there are many coarse dark inhomogeneous distributed cleavage facets in the fracture. For modified composites, however, the size of cleavage facets is smaller and the distribution is very homogeneous, as seen in Fig.5(b). These fractured particles are identified as Mg₂Si particles according to the EDS analysis.

Fig.4 Mechanical properties of composites as function of Sb content

Fig.5 Fractographs of Mg₂Si/Al-Si composites with different Sb addition: (a) 0% Sb; (b) 0.4% Sb

4 Discussion

4.1 Evolution of phase structure

According to Ref.[5], there is a narrow ternary phase region of (liquid + α(Al) + Mg₂Si) between 583.5 and 594 ℃ at the pseudo-eutectic point in equilibrium diagram of Al-Mg₂Si system. The composition of the pseudo-eutectic is 13.9% Mg₂Si and the solubility of Mg₂Si in the α(Al) at 583.5 ℃ is 1.91%. However, the pseudo binary eutectic is converted to ternary eutectic system when excessive Si is added into the alloys. Fig.6 shows the vertical section of the Al-Mg₂Si-Si equilibrium ternary phase diagram[30]. It can be found that the equilibrium solidification paths of the alloys depend on the alloys’ compositions:

Mg₂Si/Al:

   (1)

Mg₂Si/Al-5Si:

             (2)

where P and E represent the primary and eutectic phase, respectively. Accordingly, primary Mg₂Si solidifies first from the liquid in all cases followed by binary eutectic reactions to form either Al or Si eutectic with Mg₂Si. Afterwards, the ternary eutectic forms if extra Si is added into the alloys in the range from A to C in Fig. 6. It is clear that the components of the obtained alloys are primary Mg₂Si, α(Al) and ternary eutectic (Al+Mg₂Si+Si), which is consistent with the XRD analyses.

Fig.6 Phase diagram of Al-Mg₂Si-Si equilibrium ternary alloy[30]

It is noticed that some α(Al) phase around Mg₂Si particles can be found in the microstructure. According to the solidification paths of (1) and (2), no α(Al) phase is produced under equilibrium solidification[30]. But the high cooling rate in experiments restrains the diffusion of Mg and Si in liquid aluminum. There is an Al-rich region around each Mg₂Si particle. Once the under-cooling at the solid-liquid interface is large enough, α(Al) may nucleate on the facets of the Mg₂Si crystals. The growth of Mg₂Si may be restrained, which is similar to the α(Al) haloes around primary Si in hypereutectic Al-Si alloys and halos around primary Mg₂Si in Mg-Si alloys [6].

4.2 Effects of Sb on primary Mg₂Si

Usually, the modification mechanism of Sb in Al-Si alloys is related to the AlSb compounds which act as heterogeneous crystallization nucleus for the eutectic silicon, hence resulting in fine lamellar morphology of eutectic silicon[14]. However, the reason of primary Mg₂Si refined in Mg-Si alloys and Al-Mg₂Si systems is still not clear. As for Mg-Si alloys, it is believed that Sb on the surface not only prompts the formation of homogeneous nucleation but also restrains the growth of primary Mg₂Si, because Sb aggregates on the interface between Mg₂Si and melt [28]. As for the Al-Mg₂Si composites, the above mentioned modification mechanism may be one reason for the formation of refined Mg₂Si particles.

Fig.7(a) shows the microstructure of Mg₂Si in the composites. We can see that some Mg₂Si particles contain small white particles which may act as nucleation sites for Mg₂Si particles.

The results of area scanning and EDS (as shown in Fig.8) indicate that the nucleus are enriched in Si, Mg and Sb. It is believed that the nucleus should be Mg₃Sb₂ compounds. Similar results were observed in Mg-Al- Zn-Si alloys, and the heterogeneous crystallization nucleus Mg₃Sb₂ was also observed inside the Mg₂Si particles [25]. It is found that the misfit between lattice parameters of Mg₃Sb₂ and Mg₂Si is the lowest (5.1%) when the orientations relationship between Mg₂Si and

Mg₃Sb₂ phase isTherefore,

Mg₃Sb₂ can act as the heterogeneous crystallization nucleus for the Mg₂Si phase, which results in the modification of primary Mg₂Si.

Fig.7 SEM image (a) of modified Mg₂Si particles with 0.4 % Sb and area scanning: (b) Si element; (c) Mg element; and (d) Sb element

Fig.8 EDS analysis of Mg₂Si particles: (a) Spect 1 and (b) Spect 2

4.3 Effects of Sb on tensile properties

The investigations of hypereutectic Al-Si alloys showed that their mechanical properties are mainly determined by some metallurgical factors such as [14]:  1) the morphology, size and distribution of the primary silicon crystal particles; 2) the cohesion between the silicon crystal particles and the matrix; and 3) the ease with which the particles crack. The strengthening mechanism of Al-Mg₂Si composites is very similar to the hypereutectic Al-Si alloys. The mechanical properties of Mg₂Si/Al-Si composites are determined by the size, morphology and distribution of primary Mg₂Si particles.

As for the unmodified or inadequate modified Mg₂Si/Al-Si composites, the high stress concentration on the coarse dendritic Mg₂Si particles is produced by the tensile load. The microcrack is easily initiated from the primary Mg₂Si particles by debonding Mg₂Si particles from matrix or self-cracking of Mg₂Si particles, which results in the decrease of mechanical properties of the composites. As for the modified composites, the fine particles and perfect shape make the particles subject to less stress concentration and have higher resistance of crack comparatively. The initiation probability of microcrack also decreases effectively. Moreover, the increasing volume fraction of Mg₂Si particles and the excellent bonding between fine Mg₂Si particles and Al matrix may further increase the strength of the composites. Therefore, the strengthening mechanism of Mg₂Si/Al-Si composites is mainly fine-grain strengthening.

5 Conclusions

1) When the mass fraction of Sb is 0.4%, the average size of primary Mg₂Si is refined to 25 μm, and the morphology of primary Mg₂Si changed from coarse Chinese script type or dendritic shape to fine particles. Excessive Sb content results in coarsening of primary Mg₂Si particles.

2) The refinement mechanism is related to the formation of Mg₃Sb₂, which is easy to act as the excellent heterogeneous crystallization nucleus of Mg₂Si particles and results in the modification of primary Mg₂Si particles.

3) As the Sb content increases to 0.4%, the ultimate tensile strength and elongation of Mg₂Si/Al-Si composites increase from 102.1 MPa and 0.26% to 138.6 MPa and 0.36%, respectively. The refined primary Mg₂Si particles subject to less stress concentration and have higher resistance of crack, which leads to the enhancement of mechanical properties. Therefore, the strengthening mechanism of Mg₂Si/Al-Si composites is mainly fine-grain strengthening.

Acknowledgments

The authors are grateful to the DDJT Aluminum alloys Co. Ltd. for providing testing materials and facilities.

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(Edited by FANG Jing-hua)

Corresponding author: LIU Zhong-xia, Tel: +86-371-67767776; Fax: +86-371-67767776; E-mail: liuzhongxia@zzu.edu.cn

DOI: 10.1016/S1003-6326(09)60306-X