Modification of Mg2Si in Mg–Si alloys with K2TiF6, KBF4 and KBF4 + K2TiF6

Modification of Mg2Si in Mg–Si alloys with K2TiF6, KBF4 and KBF4 + K2TiF6

Journal of Alloys and Compounds 387 (2005) 105–108 Modification of Mg2 Si in Mg–Si alloys with K2 TiF6, KBF4 and KBF4 + K2 TiF6 H.Y. Wang, Q.C. Jiang...

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Journal of Alloys and Compounds 387 (2005) 105–108

Modification of Mg2 Si in Mg–Si alloys with K2 TiF6, KBF4 and KBF4 + K2 TiF6 H.Y. Wang, Q.C. Jiang∗ , B.X. Ma, Y. Wang, J.G. Wang, J.B. Li Key Laboratory of Automobile Materials of Ministry of Education and Department of Materials Science and Engineering, Jilin University at Nanling Campus, No. 142 Renmin Street, Changchun 130025, PR China Received 26 April 2004; received in revised form 1 June 2004; accepted 1 June 2004

Abstract The modification effect of KBF4 on the primary and eutectic Mg2 Si in Mg–Si alloy is better than that of K2 TiF6 . This may be the reason of the presence of B in melts. However, the modification effect of KBF4 is greatly weakened by the simultaneous addition of K2 TiF6 due to the decreasing of B content in the melts by the formation of TiB and TiB2 . © 2004 Elsevier B.V. All rights reserved. Keywords: Modification; Mg–Si alloy; Mg2 Si; Composite; K2 TiF6 ; KBF4

1. Introduction In recent years, there has been an increasing growth of magnesium alloy and its composites which can be used as structural materials in the engineering applications where weight saving is critical for improved performance [1–3]. Mg-high Si alloys are in situ Mg metal matrix composites (MMCs) containing hard particles of Mg2 Si [4–6], since a maximum solid solubility of Si into Mg is only 0.003 at.% and Si atoms react with Mg atoms and are precipitated as an intermetallic compound of Mg2 Si [7,8]. Recently, it has being shown that Mg–Si alloys have high potential as heat resistant light metals because Mg2 Si exhibits a high melting temperature, low density, low thermal expansion coefficient, and a reasonably high elastic modulus [6,9–11]. However, ingot metallurgy (I/M) Mg–Si alloys showed very low ductility and strength because of the large Mg2 Si particle size and brittle eutectic structures [5,7,11]. More recently, it has been reported that the mechanical properties of Mg–Si alloys could be improved by the applica∗ Corresponding author. Tel.: +86-431-509-5592; fax: +86-431-509-5592. E-mail address: [email protected] (Q.C. Jiang).

0925-8388/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2004.06.027

tion of advanced processing techniques such as hot extrusion (HE) [4,12,13], rapid solidification (RS) [14], directional solidification (DS) [15], and mechanical alloying (MA) [9,16]. Refinement of microstructure is mainly responsible for the improvement in the mechanical properties [4–8]. Unfortunately, such techniques are too expensive and complex to be accepted by the engineering community for general applications. Therefore, to prepare the Mg–Si alloys by simple cast modification process seems to be the most hopeful route when facing further commercial demand. Traditionally, many of the studies have been focused on the modification effect of rare earth elements, strontium, sodium salt, and halide salts on the eutectic and primary Si crystals in Al–Si alloys [17–24] and on Mg2 Si in Al–Si–Mg alloys [11,25–28], while less work has been carried on the modification effect of Mg2 Si in Mg–Si alloys. In the present study, the modification effects of K2 TiF6 , KBF4 and KBF4 + K2 TiF6 additions on the Mg2 Si phase (both eutectic and primary Mg2 Si crystals) in hypereutectic Mg–5 wt.% Si alloys were investigated. The purpose of this study is to develop a simplified casting process route to produce a relatively fine Mg2 Si particle reinforced magnesium alloy composite. It is expected that the preliminary results can

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be significant in promoting the development of fabrication of high quality Mg–Si system alloys.

2. Experimental Industrially pure Mg ingot (99.85 wt.% purity) and Si (99.95 wt.% purity) were used as starting materials to prepare Mg–5 wt.% Si alloy, which corresponds to a compositions of Mg–14 wt.% Mg2 Si composite. Pure Mg was selected as the matrix material in order to avoid the influence of impurity elements on the modification result, since they have a certain effect on the morphology and chemical composition of Mg2 Si [6]. About 1000 g of pure magnesium melt was prepared at 780 ◦ C in a graphite crucible in an electric resistance furnace of 5 kW under an argon protective atmosphere. Fiftythree grams of Si preheated at 200 ◦ C in a vacuum oven was then added to the molten magnesium. After about 15 min, 53 g (5 wt.% of the melts) of K2 TiF6 , KBF4 or a KBF4 + K2 TiF6 mixture (with a weight ratio of 4:1) were added to the Mg–Si melts, respectively. Then, the melts were manually stirred for about 5 min using a graphite impeller to facilitate the reactions between the salts and melts. After that, the melts were poured into a steel mold preheated at 200 ◦ C to produce tabulate samples of 12 mm × 120 mm × 180 mm. Metal-

lographic samples were prepared in accordance with standard procedures used for metallographic preparation of metal samples, and etched with 3 vol.% HF solution for 5–10 s at 25 ◦ C. Microstructure and phase analyses were investigated by using scanning electron microscopy (SEM) (Model JSM5310, Japan) equipped with energy-dispersive spectrometer (EDS) (Model Link-Isis, Britain) and X-ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan).

3. Results and discussion According to the Mg–Si binary phase diagram, the Mg2 Si particle was formed in situ in melts during the solidification process. Figs. 1 and 2 show the SEM microstructures and XRD patterns of Mg–5 wt.% Si alloys unmodified and modified by K2 TiF6 , KBF4 or KBF4 + K2 TiF6 , respectively. XRD results reveal that the components of the obtained alloys are only Mg2 Si and Mg phases as expected. The reaction products between the salts and molten magnesium can not be detected by the XRD due to its low intensity, as shown in Fig. 2. It can be seen from Fig. 1(a) that the unmodified microstructure consists of coarse primary Mg2 Si crystal, Chinese script type eutectic Mg2 Si particle, and grey ␣-Mg phase.

Fig. 1. SEM microstructures of Mg–5 wt.% Si alloys (a) unmodified and modified by (b) 5 wt.% K2 TiF6 ; (c) 5 wt.% KBF4 ; and (d) 5 wt.% KBF4 + K2 TiF6 mixtures with a weight ratio of 4:1, respectively.

H.Y. Wang et al. / Journal of Alloys and Compounds 387 (2005) 105–108

Fig. 2. XRD patterns of Mg–5 wt.% Si alloys (a) unmodified and modified by (b) 5 wt.% K2 TiF6 ; (c) 5 wt.% KBF4 ; and (d) 5 wt.% KBF4 + K2 TiF6 mixtures with a weight ratio of 4:1, respectively.

When 5 wt.% K2 TiF6 was added to the melt, the sizes of the primary and eutectic Mg2 Si became only slightly reduced in the alloy, as shown in Fig. 1(b). When 5 wt.% KBF4 was added to the melt, the average size of primary Mg2 Si significantly decreased from more than 100 ␮m to about 20 ␮m or less, and its morphology changed from a dendritic shape to a polyhedral shape; furthermore, the Chinese script type Mg2 Si particle also exhibited modified morphology as fine fibers, as shown in Fig. 1(c). When 5 wt.% KBF4 + K2 TiF6 mixtures with a weight ratio of 4:1 were added to the melt, compared with that in Fig. 1(c), the primary Mg2 Si became coarser, and the morphology also changed to an irregular shape; however, the eutectic Mg2 Si still exhibited modified morphology with a fine size, as shown in Fig. 1(d). When the salts were added to the melts, they decomposed and reacted with molten magnesium to form low melting point compounds. During the solidification, in addition to the segregation of K, Ti, and B at the liquid–solid interface, some of them might become adsorbed on the Mg2 Si crystal plane boundary and change the surface energy of the Mg2 Si crystals by lattice distortion, since the atomic sizes of them are different from those of Mg and Si. This may effectively

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poison the growth steps and suppress the anisotropic growth of the Mg2 Si crystals. Comparing Fig. 1(b) to (c), we can conclude that the modification effect of KBF4 is more effective than that of K2 TiF6 . This, in turn, implies that the poisoning effect of B is higher than that of Ti. Zhao et al. [27] studied the modification of Mg2 Si and primary Si in Al–Si–Mg alloy by the addition of K2 TiF6 , and concluded that the particle sizes of Mg2 Si and primary Si decreased from 80 to 30 ␮m and from 100 to 50 ␮m, respectively. However, the results obtained in the present study on Mg–Si alloys are still unexpected. Matin et al. [29] have discussed the reactions between KBF4 + K2 TiF6 mixtures and molten magnesium based on the experimental result and thermodynamic calculations. They concluded that the direct reaction between KBF4 and K2 TiF6 in molten magnesium is thermodynamically unfavorable and hence theoretically impossible. However, the reaction between KBF4 and molten magnesium may proceed as follows [29]: 8KBF4(l) + 10Mg(l) = Mg(BF4 )2(l) + MgB6 + 8KF(l) + 8MgF2(l)

(1)

When the KBF4 + K2 TiF6 mixtures were simultaneously added to the melt, in addition to reaction (1), another reaction might occur as follows [29]: Mg(BF4 )2(l) + 2K2 TiF6(l) + 7Mg = 2TiB(in alloy) + 4KF(l) + 8MgF2(l)

(2)

As the TiB and TiB2 ceramics exhibit high melting points and are thermodynamically stable, some of them will precipitate out of the melts [29], which results in a significant decrease of the B content in the magnesium melts. As a result, the modification effect on the primary Mg2 Si is greatly weakened, as shown in Fig. 1(d). This result is similar to some of the experimental findings in near-eutectic Al–Si alloys [30]. A mutual poisoning effect is found when the contents of Sr and B go beyond a certain limit due to the formation of a

Fig. 3. Typical SEM micrograph of primary Mg2 Si in (a) unmodified and (b) modified with 5 wt.% KBF4 in the deeply etched samples.

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(Sr,B) compound [30]. However, the refining effect on the eutectic Mg2 Si still exists, since an excess of B is present in the melts. In order to study the detailed morphology of primary Mg2 Si, the samples were then deeply etched by using a mixture of 15 vol.% HNO3 + 5 vol.% HCl distilled water solution for 30 h. Fig. 3(a) and (b) show the typical SEM micrographs of the deeply etched primary Mg2 Si in the unmodified and modified with 5 wt.% KBF4 samples, respectively. It is interesting to observe that there are many chaps distributed in the magnesium matrix, and the eutectic Mg2 Si particles occur within the chaps. The exact reason of the chap formation is not clear. It might be caused by the etched process, since it cannot be found in the samples etched with 3 vol.% HF solution, as shown in Fig. 1(a)–(d). It can be clearly seen that the coarse primary Mg2 Si is formed by a preferred (anisotropic) growth that occurs at the tips of branches in the unmodified alloy, which results in complex dendritic morphologies (Fig. 3(a)). However, when the alloy is modified with 5 wt.% KBF4 , the preferred growth manner of the primary Mg2 Si was depressed. Another type of isotropic growth mode is present in the modified alloy, which results in the polyhedral shapes (Fig. 3(b)).

4. Conclusions 1) When 5 wt.% K2 TiF6 was added to the Mg–5 wt.% Si melts, the sizes of primary and eutectic Mg2 Si particles became only slightly reduced in the alloy. When 5 wt.% KBF4 was added to the melt, the average size of the primary Mg2 Si became significantly decreased from more than 100 ␮m to about 20 ␮m or less, and its morphology changed from a dendritic shape to a polyhedral shape. Furthermore, also the Chinese script type eutectic Mg2 Si particles exhibited a modified morphology as fine fibers. When 5 wt.% KBF4 + K2 TiF6 mixtures with a weight ratio of 4:1 were added to the melt, the primary Mg2 Si became coarser again, and the morphology also changed to irregular shapes. However, the eutectic Mg2 Si still exhibited a modified morphology with a fine size. 2) The modification effect of KBF4 on the primary and eutectic Mg2 Si in Mg–Si alloy is more effective than that of K2 TiF4 . The reason may be the presence of B in the melts. However, the modification effect of KBF4 is strongly weakened by the simultaneous addition of K2 TiF6 due to a decreasing B content in the melts by the formation of TiB and TiB2 . 3) The preferred growth mode of the primary Mg2 Si was depressed by the KBF4 addition, and therefore, an isotropic growth mode became enhanced in the modified alloy, resulting in the formation of polyhedral primary Mg2 Si.

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