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Influence of the amount of KBF4 on the morphology of Mg2Si in Mg–5Si alloys
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Materials Chemistry and Physics 108 (2008) 353–358
Influence of the amount of KBF4 on the morphology of Mg2Si in Mg–5Si alloys Hui-Yuan Wang, Wei Wang, Min Zha, Na Zheng, Zhen-Hua Gu, Dong Li, Qi-Chuan Jiang ∗ Key Laboratory of Automobile Materials of Ministry of Education and Department of Materials Science and Engineering, Jilin University, Nanling Campus, No. 5988 Renmin Street, Changchun 130025, PR China Received 3 July 2007; received in revised form 4 October 2007; accepted 8 October 2007
Abstract The modification of Mg2 Si in Mg–5Si alloys with KBF4 was investigated in detail. With the increase of KBF4 content, the alloys exhibited sub-modified and fully modified microstructures, but no over-modified microstructure was found. In the fully modified alloys, most of the primary Mg2 Si particles exhibited the morphology of octahedrons. It is believed that B which came from the decomposition of KBF4 effectively confined the growth of primary Mg2 Si to the octahedron morphology. © 2007 Elsevier B.V. All rights reserved. Keywords: Modification; Magnesium; Mg2 Si; Growth
1. Introduction Demands from the automobile and aerospace industry for increasingly lightweight components thereby reducing energy consumption and air pollution have resulted in an increased interest in magnesium alloys and its composites [1,2]. It is well known that Si is a kind of very important alloying element in magnesium alloys. However, it can be concluded from the binary phase diagram of Mg–Si system that the maximum solid solubility of Si into Mg is only 0.003 at.% and Si atoms react with Mg atoms to precipitate as an intermetallic compound of Mg2 Si during solidification [3–5]. Because Mg2 Si exhibits a high-melting temperature, low density, low-thermal expansion coefficient, and a reasonably high-elastic modulus [6–9], it is believed that Mg–Si alloys have high potential as heat-resistant light metals. However, because of the large primary Mg2 Si particles and the brittle eutectic phase obtained by traditional casting route [4,9,10], Mg–Si alloys show low ductility and strength, which severely confine the development and applications of Mg–Si alloys. It is widely known that refinement of microstructure is mainly responsible for the improvement in the mechanical properties [3,6], while changing growth manners, namely mod-
∗
Corresponding author. Tel.: +86 431 8509 4699; fax: +86 431 8509 4699. E-mail address: [email protected] (Q.-C. Jiang).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.10.006
ification, is an effective path to refine microstructure. Therefore, it seems to be the most promising route to prepare the Mg–Si alloys by simple casting modification process. Recently, it has been reported that many studies have been focused on the modification effects of rare earth (RE) elements, sodium and halide salts on the primary Mg2 Si in Al–Si–Mg alloys [9,11–12], also Sr, Sb, Ca and P on the eutectic Mg2 Si in Mg–Al–Zn–Si alloys [13–15], but less work on Mg–Si alloys, especially on Mg–high-Si alloys. More recently, Wang et al. have investigated the modification effects of K2 TiF6 , KBF4 and K2 TiF6 + KBF4 additions on Mg2 Si phase in hypereutectic Mg–5Si alloys [16]. It was found that KBF4 has a much better modification effect than both K2 TiF6 and K2 TiF6 + KBF4 additions. However, only single modifier content (5 wt.% of the melts) was given out in the research and there was no further discussion about KBF4 on the modified morphology of Mg2 Si in Mg–high-Si alloys [16]. The aim of this paper is to study the influence of the amount of KBF4 on the size and morphology of Mg2 Si in Mg–5 wt.% Si alloys designed, and the modification mechanism of KBF4 is also discussed. 2. Experimental Industrially pure Mg ingot (99.85 wt.% purity) and Si (99.02 wt.% purity) were used as starting materials to prepare the designed Mg–5Si (wt.%) alloys. Details of the fabrication process were described elsewhere [16]. Different KBF4
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contents of 0, 0.1, 0.5, 1.0, 2.0, 4.0, 6.0, 8.0 and 10.0 wt.% were added to the Mg–Si melts, respectively. After stirred manually and held on for about 5 min, the melts were poured into a steel mold preheated at 200 ◦ C to produce tabulate samples of 12 mm × 120 mm × 180 mm. To study the reactions among KBF4 , Mg and Si, the Mg powder (99.6 wt.% purity, ∼150 m), Si powder (99.5 wt.% purity, ∼15 m) and KBF4 powder (98.0 wt.% purity) were mixed for 6h with the designed compositions (wt.%) of Mg–40KBF4 and Mg–5Si–40KBF4 , and then pressed into cylindrical compacts (20 mm diameter); subsequently, the cylindrical compacts were heated to 780 ◦ C for about 20 min protected by high-purity argon gas. Metallographic samples were prepared and etched with a mixture of 1 vol.% HNO3 + 2 vol.% HCl alcohol solution for about 6 h. Microstructure and phase analyses were investigated by scanning electron microscopy (SEM) (Model QUANTA200, America, and JSM-5310, Japan) and X-ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan). Average grain size was estimated by using the line intercept method from the maximum apparent size of Mg2 Si under low magnification of the microstructures.
3. Results and discussion Fig. 1 shows the XRD patterns of Mg–5Si alloys unmodified and modified by 1.0, 2.0, 8.0 wt.% KBF4 , respectively. The results reveal that the phase constituents of the obtained alloys are only Mg and Mg2 Si phases, which indicates that there is no obvious change of the phase composition in Mg–5Si alloys after KBF4 modification. Fig. 2(a)–(d) show SEM microstructures unmodified and modified by 1.0, 2.0 and 6.0 wt.% KBF4 , respectively. Because the cooling rate is higher than that of the equilibrium solidification, the as-cast unmodified microstructures consist of the coarse dendritic primary Mg2 Si, ␣-Mg halos and Chinese script type eutectic Mg + Mg2 Si phases (Fig. 2(a)). The formation of ␣-Mg halos has been discussed in the previous work [10]. When 1.0 wt.% KBF4 was added, the sizes of the primary and eutectic Mg2 Si became slightly reduced (Fig. 2(b)),
which exhibited the sub-modified microstructures. However, when KBF4 contents increased from 2.0 to 6.0 wt.%, the morphology of the primary Mg2 Si exhibited fine polygonal shapes (Fig. 2(c) and (d)), which was called fully modified microstructures. At the same time, the Chinese script type eutectic Mg2 Si also exhibited a modified morphology as fine fibers. Fig. 3 shows the influence of the amount of KBF4 on the size of primary Mg2 Si. Obviously, when KBF4 content increased from 0 to 1.0 wt.%, the average sizes of primary Mg2 Si decreased from more than 90 m to about 40 m, which corresponded to the sub-modified structures. However, when KBF4 contents further increased from 2.0 to 10.0 wt.%, the average sizes almost remained unchanged (about 20 m), which corresponded to the fully modified structures. According to Figs. 2 and 3, it is worth noting that no over-modified microstructure was found even with 10.0 wt.% KBF4 added. This phenomenon is very different from many other modifiers, such as Y in Mg–Si alloys and Na in Al–Si alloys [10,17]. It should be mentioned that, however, there was too much slag when 8.0 or 10.0 wt.% KBF4 was added, which resulted in the poor fluidity in making a casting. As a result, the 2.0–6.0 wt.% KBF4 additions are the preferred contents. Fig. 4 shows the high-magnification SEM of the primary Mg2 Si. The morphologies of the primary Mg2 Si exhibit quadrilateral, square, triangular and hexagonal outlines, respectively. It is interesting to note that these polygonal outlines nicely correspond with the sections of the octahedrons shown in Fig. 5. So it can be concluded that most morphologies of the primary Mg2 Si shown in Fig. 2(c) and (d) or other fully modified microstructures are octahedrons, including both perfect and defective ones. It is imaginable that the primary Mg2 Si crystals will be cut at random angles during polishing process, and therefore a variety of polygonal outlines of the crystals can be observed in the polished section. To reveal the modification mechanism of KBF4 on the primary Mg2 Si in hypereutectic Mg–Si alloys, it is necessary to study the reaction of KBF4 in Mg–5Si alloys. Therefore, an experiment was designed using the unsintered Mg–5Si–40KBF4 compact, and the sintered Mg–40KBF4 and Mg–5Si–40KBF4 compacts. The reason for the high-KBF4 content in the compacts is that the products of reaction can be detected by XRD easily, as shown in Fig. 6. As expected, the phase compositions of the unsintered compact were Mg, Si and KBF4 (Fig. 6(a)). However, the KBF4 cannot be observed in the sintered Mg–40 wt.% KBF4 compact, while three new compounds correlated to KBF4 were detected, namely KMgF3 , MgF2 and MgB2 (Fig. 6(b)). This indicates that a reaction might take place between Mg and KBF4 , as follows: 2 KBF4 + 4Mg = 2KMgF3 + MgF2 + MgB2
Fig. 1. XRD patterns of Mg–5 wt.% Si alloys: (a) unmodified and modified by (b) 1.0 wt.%, (c) 2.0 wt.% and (d) 8.0 wt.% KBF4 , respectively.
(1)
When 5 wt.% Si was added to the Mg–40 wt.% KBF4 compact, the products were the same as that shown in Fig. 6(b) except Mg2 Si, as shown in Fig. 6(c), which indicates that no reaction occurred between Si and KBF4 . Based on the results of the sintering experiment, when KBF4 was added to the melts, it reacted with molten magnesium to
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Fig. 2. SEM microstructures of Mg–5 wt.% Si alloys: (a) unmodified and modified by (b) 1.0 wt.%, (c) 2.0 wt.% and (d) 6.0 wt.% KBF4 , respectively.
Fig. 3. Influence of the amount of KBF4 on the size of primary Mg2 Si.
form new compounds, such as KMgF3 , MgF2 and MgB2 . The KMgF3 and MgF2 compounds can be used in making slags and removed by raking off the slags, while B in MgB2 may exist in the dissolved state in the molten Mg melts (or B-containing liquid phase). Since K can modify the eutectic silicon in Al–Si alloys, it is necessary to discuss the influence of K on Mg2 Si in Mg–5Si alloys. Thus, 0.5, 1.0, 2.0 and 4.0 wt.% KCl were used to modify Mg–5Si alloys, respectively. When 0.5 wt.% KCl was added, the alloys exhibited sub-modified microstructures. With KCl content increasing from 1.0 to 2.0 wt.%, only slight modification effect on primary Mg2 Si occurred and some large Mg2 Si dendrites still appeared. When the amount of KCl further increased to 4.0 wt.%, however, the alloys exhibited over-modified microstructures, as shown in Fig. 7(a) and (b). As a result, the mainly effective modification element should be B in KBF4 . During solidification, besides the enrichment of boron content at the liquid–solid interface, some B atoms might be adsorbed on the Mg2 Si crystal plane and change the surface
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Fig. 4. SEM micrographs of the sections of primary Mg2 Si in the fully modified Mg–5Si alloys after deeply etched: (a) a quadrilateral outline, (b) a square outline, (c) a triangular outline and (d) a hexagonal outline.
Fig. 5. A schematic illustration of the polygonal outlines of octahedral primary Mg2 Si: (a) a quadrilateral outline, (b) a square outline, (c) a triangular outline and (d) a hexagonal outline.
H.-Y. Wang et al. / Materials Chemistry and Physics 108 (2008) 353–358
Fig. 6. XRD pattern of phase composition of (a) the unsintered Mg–5 wt.% Si–40 wt.% KBF4 compact and sintered compacts of (b) Mg–40 wt.% KBF4 and (c) Mg–5 wt.% Si–40 wt.% KBF4 (held at 780 ◦ C for 20 min protected by high-purity argon).
energy of the Mg2 Si crystals. This adsorption may effectively poison the growth steps and suppress the anisotropic growth of the Mg2 Si crystals [16]. To prove the modification effect of B, the Mg–30 wt.% B master alloy, a compact fabricated by the mixtures of Mg and B powders which was sintered at 250 ◦ C for about 20 min protected by high-purity argon, was used to modify Mg–5Si alloys. Fig. 8 shows the SEM micrographs modified by 1.0 wt.% B. It is clear that the primary Mg2 Si modified by elementary substance B become fine particles with the size of about 10 m, which corresponds to fully modified structures. Furthermore, the octa-
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hedral primary Mg2 Si particles can also be found, as shown in Fig. 8(b). Comparing Fig. 2 with Fig. 8, we also observed that the eutectic Mg2 Si exhibits a modified morphology as fine fibers, indicating that boron is the element mainly responsible for the change in morphology of eutectic regions. This experiment strongly confirms that B can modify Mg–Si alloys effectively. Therefore, B that came from the decomposition of KBF4 must play the most important role in modifying Mg–Si alloys. Fig. 9 is the schematic illustration which shows that the Mg2 Si crystal grows from nucleation to octahedron, and then to dendritic crystal; and this process has been marked with stage (i)–(v), respectively. As we known, the lattice of Mg2 Si belongs to face center cubic (fcc) system [13]. The preferred growth direction of fcc crystals is in [1 0 0] direction, and the (1 1 1) plane, which is the dense plane of Mg2 Si crystal, grows slower than (1 0 0) plane. So it can grow to octahedron, and then to the dendritic crystal along [1 0 0]. But when KBF4 with the proper content is added to the melts, the growth of (1 0 0) is depressed by the adsorption and poison of B. Therefore, the growth process of Mg2 Si is confined to stage (ii) or (iii). The above analyses can well interpret the phenomenon that the fully modified structures are almost full of octahedrons. Besides the adsorption and poisoning mechanisms, however, it is reasonable to assume that boron effect is also associated with accelerated nucleation of the Mg2 Si phase, since the amount of Mg2 Si modified by KBF4 increases evidently (Fig. 2(a)–(d)). In fact, the faceted growth is very sensitive to the solidification conditions, and even very small amounts of inoculant could show a critical influence on the nucleation of the faceted phases [9]. The B-containing clusters in the melts may act as an inoculant for the nucleation process, and therefore, they may promote the nucleation of the primary Mg2 Si phase, so large amount of Mg2 Si nuclei can be formed during the solidification. This result is similar to the solidification process of RE-containing liquid alloy [9]. However, the exact mechanism for enhanced nucleation of the primary Mg2 Si phase in modified Mg–Si alloys requires further investigation.
Fig. 7. Influence of the amount of KCl on the morphology of primary Mg2 Si: (a) 2 wt.% and (b) 4.0 wt.% KCl additions.
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Fig. 8. SEM micrographs of primary Mg2 Si: (a) modified by 1.0 wt.% B and (b) local magnification of (a).
Fig. 9. (i–v) Schematic illustration of primary Mg2 Si growing from a core to dendritic crystal.
4. Conclusions
References
(1) The KBF4 additions have a significant modification effect on the primary and eutectic Mg2 Si in the Mg–5Si alloys. When KBF4 content increased from 0 to 1.0 wt.%, the sizes of primary Mg2 Si decreased from more than 90 m to about 40 m, which exhibited sub-modification structures. When the content further increased from 2.0 to 10.0 wt.%, the sizes of primary Mg2 Si remained unchanged (about 20 m), which exhibited fully modified structures. However, no over-modification structure was found. (2) In the fully modified structures, the primary Mg2 Si exhibited the octahedrons, including both perfect and defective ones. Furthermore, the Chinese script type eutectic Mg2 Si exhibited a modified morphology as fine fibers. (3) The KBF4 decomposed due to reacting with molten Mg, and the products formed were KMgF3 , MgF2 and MgB2 . It is believed that B which came from the decomposition of KBF4 played the most important role in modifying Mg–Si alloys.
[1] X. Zhang, L. Liao, N. Ma, H. Wang, Mater. Chem. Phys. 96 (2006) 9–15. [2] J.M. Kim, K. Shin, K.T. Kim, W.J. Jung, Scripta Mater. 49 (2003) 687– 691. [3] Y.Z. L¨u, Q.D. Wang, X.Q. Zeng, Y.P. Zhu, W.J. Ding, Mater. Sci. Eng. A301 (2001) 255–258. [4] M. Mabuchi, K. Higashi, Acta Mater. 44 (1996) 4611–4618. [5] M. Mabuchi, K. Kubota, K. Higashi, J. Mater. Sci. 31 (1996) 1529– 1535. [6] G.H. Li, H.S. Gill, R.A. Varin, Metall. Trans. A 24 (1993) 2383–2391. [7] L. Lu, M.O. Lai, M.L. Hoe, Nanostruct. Mater. 10 (1998) 551–563. [8] J. Zhang, Y.Q. Wang, B. Yang, B.L. Zhou, J. Mater. Res. 14 (1999) 68–74. [9] J. Zhang, Z. Fan, Y.Q. Wang, B.L. Zhou, Mater. Sci. Eng. A281 (2000) 104–112. [10] Q.C. Jiang, H.Y. Wang, Y. Wang, B.X. Ma, J.G. Wang, Mater. Sci. Eng. A392 (2005) 130–135. [11] J. Zhang, Z. Fan, Y.Q. Wang, B.L. Zhou, J. Mater. Sci. Lett. 18 (1999) 783–784. [12] Y.G. Zhao, Q.D. Qin, W. Zhou, Y.H. Liang, J. Alloys Compd. 389 (2005) L1–L4. [13] H.C. Liao, Y. Sun, G.X. Sun, Mater. Sci. Eng. A358 (2003) 164–170. [14] J.J. Kim, D.H. Kim, K.S. Shin, N.J. Kim, Scripta Mater. 41 (1999) 333– 340. [15] G.Y. Yuan, Z.Y. Liu, Q.D. Wang, W.J. Ding, Mater. Lett. 56 (2002) 53–58. [16] H.Y. Wang, Q.C. Jiang, B.X. Ma, Y. Wang, J.G. Wang, J.B. Li, J. Alloys Compd. 387 (2005) 105–108. [17] K.F. Kobayashi, L.M. Hogan, J. Mater. Sci. 20 (1985) 1961–1975.
Acknowledgements This work is supported by The National Natural Science Foundation of China (no. 50501010) and The Project 985Automotive Engineering of Jilin University.
Materials Chemistry and Physics 108 (2008) 353–358
Influence of the amount of KBF4 on the morphology of Mg2Si in Mg–5Si alloys Hui-Yuan Wang, Wei Wang, Min Zha, Na Zheng, Zhen-Hua Gu, Dong Li, Qi-Chuan Jiang ∗ Key Laboratory of Automobile Materials of Ministry of Education and Department of Materials Science and Engineering, Jilin University, Nanling Campus, No. 5988 Renmin Street, Changchun 130025, PR China Received 3 July 2007; received in revised form 4 October 2007; accepted 8 October 2007
Abstract The modification of Mg2 Si in Mg–5Si alloys with KBF4 was investigated in detail. With the increase of KBF4 content, the alloys exhibited sub-modified and fully modified microstructures, but no over-modified microstructure was found. In the fully modified alloys, most of the primary Mg2 Si particles exhibited the morphology of octahedrons. It is believed that B which came from the decomposition of KBF4 effectively confined the growth of primary Mg2 Si to the octahedron morphology. © 2007 Elsevier B.V. All rights reserved. Keywords: Modification; Magnesium; Mg2 Si; Growth
1. Introduction Demands from the automobile and aerospace industry for increasingly lightweight components thereby reducing energy consumption and air pollution have resulted in an increased interest in magnesium alloys and its composites [1,2]. It is well known that Si is a kind of very important alloying element in magnesium alloys. However, it can be concluded from the binary phase diagram of Mg–Si system that the maximum solid solubility of Si into Mg is only 0.003 at.% and Si atoms react with Mg atoms to precipitate as an intermetallic compound of Mg2 Si during solidification [3–5]. Because Mg2 Si exhibits a high-melting temperature, low density, low-thermal expansion coefficient, and a reasonably high-elastic modulus [6–9], it is believed that Mg–Si alloys have high potential as heat-resistant light metals. However, because of the large primary Mg2 Si particles and the brittle eutectic phase obtained by traditional casting route [4,9,10], Mg–Si alloys show low ductility and strength, which severely confine the development and applications of Mg–Si alloys. It is widely known that refinement of microstructure is mainly responsible for the improvement in the mechanical properties [3,6], while changing growth manners, namely mod-
∗
Corresponding author. Tel.: +86 431 8509 4699; fax: +86 431 8509 4699. E-mail address: [email protected] (Q.-C. Jiang).
0254-0584/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2007.10.006
ification, is an effective path to refine microstructure. Therefore, it seems to be the most promising route to prepare the Mg–Si alloys by simple casting modification process. Recently, it has been reported that many studies have been focused on the modification effects of rare earth (RE) elements, sodium and halide salts on the primary Mg2 Si in Al–Si–Mg alloys [9,11–12], also Sr, Sb, Ca and P on the eutectic Mg2 Si in Mg–Al–Zn–Si alloys [13–15], but less work on Mg–Si alloys, especially on Mg–high-Si alloys. More recently, Wang et al. have investigated the modification effects of K2 TiF6 , KBF4 and K2 TiF6 + KBF4 additions on Mg2 Si phase in hypereutectic Mg–5Si alloys [16]. It was found that KBF4 has a much better modification effect than both K2 TiF6 and K2 TiF6 + KBF4 additions. However, only single modifier content (5 wt.% of the melts) was given out in the research and there was no further discussion about KBF4 on the modified morphology of Mg2 Si in Mg–high-Si alloys [16]. The aim of this paper is to study the influence of the amount of KBF4 on the size and morphology of Mg2 Si in Mg–5 wt.% Si alloys designed, and the modification mechanism of KBF4 is also discussed. 2. Experimental Industrially pure Mg ingot (99.85 wt.% purity) and Si (99.02 wt.% purity) were used as starting materials to prepare the designed Mg–5Si (wt.%) alloys. Details of the fabrication process were described elsewhere [16]. Different KBF4
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contents of 0, 0.1, 0.5, 1.0, 2.0, 4.0, 6.0, 8.0 and 10.0 wt.% were added to the Mg–Si melts, respectively. After stirred manually and held on for about 5 min, the melts were poured into a steel mold preheated at 200 ◦ C to produce tabulate samples of 12 mm × 120 mm × 180 mm. To study the reactions among KBF4 , Mg and Si, the Mg powder (99.6 wt.% purity, ∼150 m), Si powder (99.5 wt.% purity, ∼15 m) and KBF4 powder (98.0 wt.% purity) were mixed for 6h with the designed compositions (wt.%) of Mg–40KBF4 and Mg–5Si–40KBF4 , and then pressed into cylindrical compacts (20 mm diameter); subsequently, the cylindrical compacts were heated to 780 ◦ C for about 20 min protected by high-purity argon gas. Metallographic samples were prepared and etched with a mixture of 1 vol.% HNO3 + 2 vol.% HCl alcohol solution for about 6 h. Microstructure and phase analyses were investigated by scanning electron microscopy (SEM) (Model QUANTA200, America, and JSM-5310, Japan) and X-ray diffraction (XRD) (Model D/Max 2500PC Rigaku, Japan). Average grain size was estimated by using the line intercept method from the maximum apparent size of Mg2 Si under low magnification of the microstructures.
3. Results and discussion Fig. 1 shows the XRD patterns of Mg–5Si alloys unmodified and modified by 1.0, 2.0, 8.0 wt.% KBF4 , respectively. The results reveal that the phase constituents of the obtained alloys are only Mg and Mg2 Si phases, which indicates that there is no obvious change of the phase composition in Mg–5Si alloys after KBF4 modification. Fig. 2(a)–(d) show SEM microstructures unmodified and modified by 1.0, 2.0 and 6.0 wt.% KBF4 , respectively. Because the cooling rate is higher than that of the equilibrium solidification, the as-cast unmodified microstructures consist of the coarse dendritic primary Mg2 Si, ␣-Mg halos and Chinese script type eutectic Mg + Mg2 Si phases (Fig. 2(a)). The formation of ␣-Mg halos has been discussed in the previous work [10]. When 1.0 wt.% KBF4 was added, the sizes of the primary and eutectic Mg2 Si became slightly reduced (Fig. 2(b)),
which exhibited the sub-modified microstructures. However, when KBF4 contents increased from 2.0 to 6.0 wt.%, the morphology of the primary Mg2 Si exhibited fine polygonal shapes (Fig. 2(c) and (d)), which was called fully modified microstructures. At the same time, the Chinese script type eutectic Mg2 Si also exhibited a modified morphology as fine fibers. Fig. 3 shows the influence of the amount of KBF4 on the size of primary Mg2 Si. Obviously, when KBF4 content increased from 0 to 1.0 wt.%, the average sizes of primary Mg2 Si decreased from more than 90 m to about 40 m, which corresponded to the sub-modified structures. However, when KBF4 contents further increased from 2.0 to 10.0 wt.%, the average sizes almost remained unchanged (about 20 m), which corresponded to the fully modified structures. According to Figs. 2 and 3, it is worth noting that no over-modified microstructure was found even with 10.0 wt.% KBF4 added. This phenomenon is very different from many other modifiers, such as Y in Mg–Si alloys and Na in Al–Si alloys [10,17]. It should be mentioned that, however, there was too much slag when 8.0 or 10.0 wt.% KBF4 was added, which resulted in the poor fluidity in making a casting. As a result, the 2.0–6.0 wt.% KBF4 additions are the preferred contents. Fig. 4 shows the high-magnification SEM of the primary Mg2 Si. The morphologies of the primary Mg2 Si exhibit quadrilateral, square, triangular and hexagonal outlines, respectively. It is interesting to note that these polygonal outlines nicely correspond with the sections of the octahedrons shown in Fig. 5. So it can be concluded that most morphologies of the primary Mg2 Si shown in Fig. 2(c) and (d) or other fully modified microstructures are octahedrons, including both perfect and defective ones. It is imaginable that the primary Mg2 Si crystals will be cut at random angles during polishing process, and therefore a variety of polygonal outlines of the crystals can be observed in the polished section. To reveal the modification mechanism of KBF4 on the primary Mg2 Si in hypereutectic Mg–Si alloys, it is necessary to study the reaction of KBF4 in Mg–5Si alloys. Therefore, an experiment was designed using the unsintered Mg–5Si–40KBF4 compact, and the sintered Mg–40KBF4 and Mg–5Si–40KBF4 compacts. The reason for the high-KBF4 content in the compacts is that the products of reaction can be detected by XRD easily, as shown in Fig. 6. As expected, the phase compositions of the unsintered compact were Mg, Si and KBF4 (Fig. 6(a)). However, the KBF4 cannot be observed in the sintered Mg–40 wt.% KBF4 compact, while three new compounds correlated to KBF4 were detected, namely KMgF3 , MgF2 and MgB2 (Fig. 6(b)). This indicates that a reaction might take place between Mg and KBF4 , as follows: 2 KBF4 + 4Mg = 2KMgF3 + MgF2 + MgB2
Fig. 1. XRD patterns of Mg–5 wt.% Si alloys: (a) unmodified and modified by (b) 1.0 wt.%, (c) 2.0 wt.% and (d) 8.0 wt.% KBF4 , respectively.
(1)
When 5 wt.% Si was added to the Mg–40 wt.% KBF4 compact, the products were the same as that shown in Fig. 6(b) except Mg2 Si, as shown in Fig. 6(c), which indicates that no reaction occurred between Si and KBF4 . Based on the results of the sintering experiment, when KBF4 was added to the melts, it reacted with molten magnesium to
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Fig. 2. SEM microstructures of Mg–5 wt.% Si alloys: (a) unmodified and modified by (b) 1.0 wt.%, (c) 2.0 wt.% and (d) 6.0 wt.% KBF4 , respectively.
Fig. 3. Influence of the amount of KBF4 on the size of primary Mg2 Si.
form new compounds, such as KMgF3 , MgF2 and MgB2 . The KMgF3 and MgF2 compounds can be used in making slags and removed by raking off the slags, while B in MgB2 may exist in the dissolved state in the molten Mg melts (or B-containing liquid phase). Since K can modify the eutectic silicon in Al–Si alloys, it is necessary to discuss the influence of K on Mg2 Si in Mg–5Si alloys. Thus, 0.5, 1.0, 2.0 and 4.0 wt.% KCl were used to modify Mg–5Si alloys, respectively. When 0.5 wt.% KCl was added, the alloys exhibited sub-modified microstructures. With KCl content increasing from 1.0 to 2.0 wt.%, only slight modification effect on primary Mg2 Si occurred and some large Mg2 Si dendrites still appeared. When the amount of KCl further increased to 4.0 wt.%, however, the alloys exhibited over-modified microstructures, as shown in Fig. 7(a) and (b). As a result, the mainly effective modification element should be B in KBF4 . During solidification, besides the enrichment of boron content at the liquid–solid interface, some B atoms might be adsorbed on the Mg2 Si crystal plane and change the surface
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Fig. 4. SEM micrographs of the sections of primary Mg2 Si in the fully modified Mg–5Si alloys after deeply etched: (a) a quadrilateral outline, (b) a square outline, (c) a triangular outline and (d) a hexagonal outline.
Fig. 5. A schematic illustration of the polygonal outlines of octahedral primary Mg2 Si: (a) a quadrilateral outline, (b) a square outline, (c) a triangular outline and (d) a hexagonal outline.
H.-Y. Wang et al. / Materials Chemistry and Physics 108 (2008) 353–358
Fig. 6. XRD pattern of phase composition of (a) the unsintered Mg–5 wt.% Si–40 wt.% KBF4 compact and sintered compacts of (b) Mg–40 wt.% KBF4 and (c) Mg–5 wt.% Si–40 wt.% KBF4 (held at 780 ◦ C for 20 min protected by high-purity argon).
energy of the Mg2 Si crystals. This adsorption may effectively poison the growth steps and suppress the anisotropic growth of the Mg2 Si crystals [16]. To prove the modification effect of B, the Mg–30 wt.% B master alloy, a compact fabricated by the mixtures of Mg and B powders which was sintered at 250 ◦ C for about 20 min protected by high-purity argon, was used to modify Mg–5Si alloys. Fig. 8 shows the SEM micrographs modified by 1.0 wt.% B. It is clear that the primary Mg2 Si modified by elementary substance B become fine particles with the size of about 10 m, which corresponds to fully modified structures. Furthermore, the octa-
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hedral primary Mg2 Si particles can also be found, as shown in Fig. 8(b). Comparing Fig. 2 with Fig. 8, we also observed that the eutectic Mg2 Si exhibits a modified morphology as fine fibers, indicating that boron is the element mainly responsible for the change in morphology of eutectic regions. This experiment strongly confirms that B can modify Mg–Si alloys effectively. Therefore, B that came from the decomposition of KBF4 must play the most important role in modifying Mg–Si alloys. Fig. 9 is the schematic illustration which shows that the Mg2 Si crystal grows from nucleation to octahedron, and then to dendritic crystal; and this process has been marked with stage (i)–(v), respectively. As we known, the lattice of Mg2 Si belongs to face center cubic (fcc) system [13]. The preferred growth direction of fcc crystals is in [1 0 0] direction, and the (1 1 1) plane, which is the dense plane of Mg2 Si crystal, grows slower than (1 0 0) plane. So it can grow to octahedron, and then to the dendritic crystal along [1 0 0]. But when KBF4 with the proper content is added to the melts, the growth of (1 0 0) is depressed by the adsorption and poison of B. Therefore, the growth process of Mg2 Si is confined to stage (ii) or (iii). The above analyses can well interpret the phenomenon that the fully modified structures are almost full of octahedrons. Besides the adsorption and poisoning mechanisms, however, it is reasonable to assume that boron effect is also associated with accelerated nucleation of the Mg2 Si phase, since the amount of Mg2 Si modified by KBF4 increases evidently (Fig. 2(a)–(d)). In fact, the faceted growth is very sensitive to the solidification conditions, and even very small amounts of inoculant could show a critical influence on the nucleation of the faceted phases [9]. The B-containing clusters in the melts may act as an inoculant for the nucleation process, and therefore, they may promote the nucleation of the primary Mg2 Si phase, so large amount of Mg2 Si nuclei can be formed during the solidification. This result is similar to the solidification process of RE-containing liquid alloy [9]. However, the exact mechanism for enhanced nucleation of the primary Mg2 Si phase in modified Mg–Si alloys requires further investigation.
Fig. 7. Influence of the amount of KCl on the morphology of primary Mg2 Si: (a) 2 wt.% and (b) 4.0 wt.% KCl additions.
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Fig. 8. SEM micrographs of primary Mg2 Si: (a) modified by 1.0 wt.% B and (b) local magnification of (a).
Fig. 9. (i–v) Schematic illustration of primary Mg2 Si growing from a core to dendritic crystal.
4. Conclusions
References
(1) The KBF4 additions have a significant modification effect on the primary and eutectic Mg2 Si in the Mg–5Si alloys. When KBF4 content increased from 0 to 1.0 wt.%, the sizes of primary Mg2 Si decreased from more than 90 m to about 40 m, which exhibited sub-modification structures. When the content further increased from 2.0 to 10.0 wt.%, the sizes of primary Mg2 Si remained unchanged (about 20 m), which exhibited fully modified structures. However, no over-modification structure was found. (2) In the fully modified structures, the primary Mg2 Si exhibited the octahedrons, including both perfect and defective ones. Furthermore, the Chinese script type eutectic Mg2 Si exhibited a modified morphology as fine fibers. (3) The KBF4 decomposed due to reacting with molten Mg, and the products formed were KMgF3 , MgF2 and MgB2 . It is believed that B which came from the decomposition of KBF4 played the most important role in modifying Mg–Si alloys.
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Acknowledgements This work is supported by The National Natural Science Foundation of China (no. 50501010) and The Project 985Automotive Engineering of Jilin University.