- Email: [email protected]
Mg–Zn–Si composites
Journal of Alloys and Compounds 487 (2009) 293–297
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The effect of Ba addition on microstructure of in situ synthesized Mg2 Si/Mg–Zn–Si composites K. Chen, Z.Q. Li ∗ , J.S. Liu, J.N. Yang, Y.D. Sun, S.G. Bian College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, No. 29 Yudao Street, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 10 February 2009 Received in revised form 17 July 2009 Accepted 20 July 2009 Available online 28 July 2009 Keywords: Mg2 Si Ba addition Microstructure Modification Heterogenous nucleation
a b s t r a c t In order to modify in situ synthesized Mg2 Si phase in Mg–6Zn–4Si alloy, Ba addition of 0.1–3.0 wt.% had been explored. The effect of Ba addition on the microstructure of Mg2 Si/Mg–Zn–Si composites was investigated by means of optical microscope, scanning electron microscope, X-ray diffraction and energy dispersive spectrometer. The results indicate that the morphology of primary Mg2 Si in the composites changes from large dendrite to fine polygon with the increasing Ba content. The average size of primary and eutectic Mg2 Si sharply decreases with increasing Ba content up to 1.0 wt.% and then slowly increases. Theoretical calculation and experimental analysis show that tiny BaMg2 Si2 particles, formed by adding Ba, act as the heterogenous nucleation substrates for primary Mg2 Si. Therefore, Si consumption due to BaMg2 Si2 formation and primary Mg2 Si nucleation results in the inhibition of Mg2 Si growth. It is also found that the BaMg2 Si2 phase in primary Mg2 Si is obviously coarsened as Ba content exceeds 1.5 wt.% and some needle-like Ba2 Mg3 Si4 is found in alloy with 3.0 wt.% Ba. These are responsible for the over modification effect. Therefore, it can be concluded that proper Ba addition can effectively modify and refine primary Mg2 Si and decrease the amount of eutectic Mg2 Si. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction In recent years, in situ synthesized Mg matrix composites are considered to have great potential in modern industries due to their excellent heat-resistance and creep-resistance, low density and high specific strength as well as better cohesion between the reinforcement and matrix [1–3]. One research hotspot is in situ synthesized intermetallic compound Mg2 Si reinforced Mg matrix composite [3,4]. Mg2 Si is characterized by high melting temperature of 1085 ◦ C, low density of 1.99 × 103 kg m−3 , high hardness of 4.5 × 109 N m−2 , low thermal expansion coefficient of 7.5 × 10−6 K−1 and high elastic modulus of 120 GPa [5], which greatly improve the heat-resistance and wear-resistance of the alloys. However, a mass of coarse-dendritical primary Mg2 Si (with size even exceeding 100 m) and bulk-Chinese-character eutectic Mg2 Si appearing in high-Si magnesium alloys, deteriorate the mechanical and deformation properties of materials and impede their applications [6,7]. How to modify Mg2 Si phase in high-Si magnesium alloy has attracted considerable attention. Various processing techniques have been employed, such as mechanical alloying [1], hot extrusion [2,8], heat treatment [9], rapid solidi-
fication [10,11], and modification treatment [12–15]. Among those methods, modification during solidification process of alloys is a relatively convenient and economical method. For example, the primary Mg2 Si dendrite was changed to polyhedral particles with size about 20 m when Y, KBF4 , B2 O3 and Bi were added to Mg–Si alloy, and the modification mechanism can be regarded as poisoning [12–14]. It is also reported that the combination of Sr–Sb modification and heat treatment can alter the primary Mg2 Si dendrite to short rod-shape or granular [9]. But most of those researches focused on the variation of primary Mg2 Si. It is necessary to develop more effective modifier in order to modify both the primary Mg2 Si dendrite and Chinese-characters-like eutectic Mg2 Si. It is well known that Al–Ba alloy has been used to modify the primary Si in Al–Si alloy. Considering the similarity between Si modification in Al–Si alloy and Mg2 Si modification in Mg–Si alloy, Ba may be a good modifier for Mg2 Si in Mg–Si alloys. The objectives of this work are to find out the effect of Ba addition on the microstructure of Mg–6Zn–4Si alloy, to explore the modification mechanism of Ba modifier on in situ synthesized Mg2 Si during solidification processing, and to prepare Mg matrix composites with fine microstructure. 2. Experimental 2.1. Raw materials and smelting
∗ Corresponding author. Tel.: +86 25 8489 2797; fax: +86 25 8489 0521. E-mail address: [email protected] (Z.Q. Li).
Commercial Mg ingot (>99.9 wt.%), Zn ingot (>99.8 wt.%) and Si (>99.8 wt.%) were used to prepare master alloy Mg–6Zn–4Si. Mg and Zn ingots were melted up to 700 ◦ C
0925-8388/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.111
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K. Chen et al. / Journal of Alloys and Compounds 487 (2009) 293–297
Table 1 Experimental alloy number and nominal Ba addition. Alloy No.
0
1
2
3
4
5
6
Ba content (wt.%)
0
0.1
0.5
1.0
1.5
2.0
3.0
in a steel crucible, and then Si particles were added. The melt was continuously heated to above 750 ◦ C and kept for 20 min. The melt was well-proportioned by mechanical agitation for 5 min and deslaged, then cast into a steel mould which was preheated at 250 ◦ C while the melt temperature is below 700 ◦ C. The master alloy ingots were sliced for subsequent experiments. Rationed amount of pure Ba (>99 wt.%) was added to remelted master alloys. Then the melts were kept at 750 ◦ C for 15 min. After being stirred and deslaged, the melts were cast into the preheated steel mould (with the size of 100 mm × 80 mm × 20 mm). The whole smelting was carried on in an electric resistance furnace and protected by the gas mixture of 1 vol.% SF6 + 99 vol.% CO2 . The Ba contents of ingots are shown in Table 1. 2.2. Microstructure investigation After kibbled and polished, samples cut from the middle of castings were etched by 0.4 vol.% nitric alcohol solution. XJP-300 Optical Microscope (OM) was used to observe the microstructure of the samples. The average areas of primary Mg2 Si grains in every optical image were measured by Image-pro Plus 5.0 software. The crystal structures and chemical composition of phases were detected and analyzed by Bruker D8-advance X-ray diffraction (XRD) and energy dispersive spectrometer (EDS) affiliated to the LEO 1550 scanning electron microscope (SEM), respectively.
Fig. 1. Microstructure of alloy 0.
3. Results The microstructure of Mg–6Zn–4Si master alloys is consisted of primary Mg2 Si, eutectic Mg2 Si, intermetallic compound MgZn
Fig. 2. OM of contained Ba alloys: (a) alloy 1, (b) alloy 2, (c) alloy 3, (d) alloy 4, (e) alloy 5 and (f) alloy 6.
K. Chen et al. / Journal of Alloys and Compounds 487 (2009) 293–297
Fig. 3. The relationship between Ba content and average area of primary Mg2 Si grains.
and ␣-Mg, as shown in Fig. 1. The primary Mg2 Si exists as coarse dendrites or polygons. Eutectic Mg2 Si exhibits as Chinese character. Semicontinuous net of MgZn is distributed among ␣-Mg and eutectic Mg2 Si. OM images of the alloys containing different Ba contents are presented in Fig. 2. The morphology of primary Mg2 Si in the composites significantly changes from large dendrites to fine polygonal even spherical particles with the increasing of Ba content. The average size of primary Mg2 Si sharply decreases first with increasing Ba content up to 1.0 wt.% and then slowly increases. Quantitative results of primary Mg2 Si grain size in every alloy, characterized by normalized average area of primary Mg2 Si (the average areas of
295
primary Mg2 Si in alloy 0 is fixed to 1), are shown in Fig. 3. The smallest average grain area, appeared in Mg–6Zn–4Si–1Ba alloy, is about 200.5 × 10−12 m2 (normalized to 0.172 in Fig. 3), where corresponding primary Mg2 Si size is less than 20 m. When Ba content is over 1.0 wt.%, primary Mg2 Si grain becomes slightly larger with the increasing Ba content. Normalized average areas of primary Mg2 Si grain in alloy 4, alloy 5 and alloy 6 are 0.195, 0.257 and 0.684, respectively. Compared with primary phase, the amount and average size of eutectic Mg2 Si also show similar change trend: decreasing at first and then increasing as Ba increases. Particularly, little eutectic Mg2 Si appears in the alloy with 1.0 wt.% Ba and its size is very small. SEM images of alloy 3 and alloy 5 are shown in Fig. 4. A tiny particle (as white arrow pointed) is found inside the primary Mg2 Si. EDS test of this phase (Fig. 5) indicates that it contains Mg, Ba and Si. But it is hard to find this phase in XRD patterns (Fig. 6) as its amount is too small. This phase is obviously coarsened from about 0.5 m to 3 m (Figs. 4 and 5) as the content of Ba exceeds 1.5 wt.%. It is also found that some needle-like phase with a length of 10–100 m appears when the Ba content is 3.0 wt.% (Fig. 2(f)). It is proved to be Ba2 Mg3 Si4 by EDS and XRD patterns (Figs. 7 and 6, respectively).
4. Discussion Two major mechanisms of modification and refinement of Mg2 Si grains were discussed in previous work. One mechanism is the increase of nucleation. The formation of large amounts of nuclei in the melt leads to the decrease of Mg2 Si grain size [16,17]. The other is inhibition of grain growth through changing the solidification condition and the modification is due to poisoning. For example, adsorption of B, Y atoms at Mg2 Si growing surface front changes the solidification condition and the primary Mg2 Si is modified [12–14].
Fig. 4. SEM micrographs of alloys contained Ba: (a) alloy 3 and (b) alloy 5.
Fig. 5. EDS results of a tiny granular phase inside primary Mg2 Si grain in alloy 3.
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K. Chen et al. / Journal of Alloys and Compounds 487 (2009) 293–297
may acts as the substrate of Mg2 Si nucleus. According to the Bramfitt theory of heterogenous nucleation, lattice misfitting between Ba–Mg–Si compound and Mg2 Si were calculated. The lattice misfitting mathematical mode [18] by Bramfitt is:
k l)s ı(h = (h k l)
i 3 d i cos − d [u v w]s [u v w]n
n
i=1
Fig. 6. XRD patterns of alloy 3 and alloy 6.
Here Ba was added into the Mg–Zn–Si melt. Since Ba scarcely dissolves in Mg crystal lattice, it will precipitate or react with the melt and form binary or ternary intermetallic compounds during solidification. The above results of microstructure observation and EDS test indicate that, in alloys with Ba addition, tiny compound particles contained Ba exist inside primary Mg2 Si. This compound
i 3d[u v w]
× 100%
n
where the (h k l)s is low index crystal plane of the heteronucleus, the [u v w]s is low index crystal orientation in the (h k l)s ; the (h k l)n is low index crystal plane of new crystal nucleus, the [u v w]n is low index crystal orientation in the (k h l)n ; d[u v w]s and d[u v w]n are atomic spatial distance along the [u v w]s and the [u v w]n ; is the angle between [u v w]s and [u v w]n ( < 90◦ ). The fcc Mg2 Si belongs to the space group of Fm-3m(225) [19], and its Pearson symbol is cF12. The tetragonal BaMg2 Si2 belongs to the space group of I4/mmm(139) and it is with Pearson symbol of tI10 [20]. The crystal structures and atomic arrangements of Mg2 Si and BaMg2 Si2 are shown schematically in Figs. 8 and 9. One can see that atoms distribute on (0 0 1)Mg2 Si and (0 0 1)BaMg2 Si2 in a similar way. The misfitting of these two planes is calculated according to Brafitt theory and formula. The results are shown in Table 2. The misfitting between (0 0 1)Mg2 Si and (0 0 1)BaMg2 Si2 is 3.01%, much smaller than misfitting between other planes. The interface coherent theory and Bramfitt theory point out: if the mismatch of two planes is below 15%, one phase can be heterogenous nucleation
Fig. 7. EDS test of needle-like phase in alloy 6.
Fig. 8. Crystal structures of Mg2 Si and BaMg2 Si2 .
K. Chen et al. / Journal of Alloys and Compounds 487 (2009) 293–297
297
Fig. 9. Comparison of atomic arrangements in (0 0 1)Mg2 Si and (0 0 1)BaMg2 Si2 Table 2 Calculated values of planar disregistry between Mg2 Si and BaMg2 Si2 phase. (0 0 1)BaMg2 Si2 //(1 1 0)Mg2 Si
(0 0 1)BaMg2 Si2 //(0 0 1)Mg2 Si
(0 0 1)BaMg2 Si2 //(1 1 1)Mg2 Si
[h k l]BaMg2 Si2
[0 1 0]
[1¯ 1 0]
[1¯ 0 0]
[0 1 0]
[1¯ 1 0]
[1¯ 0 0]
[0 1 0]
[1¯ 1 0]
[1¯ 0 0]
[h k l]Mg2 Si
[1¯ 1 0]
[1¯ 1 1]
[0 0 1]
[1 1 0]
[0 1 0]
[1¯ 1 0]
[1¯ 1 0]
[1¯ 1 1]
[1¯ 0 1]
d[h k l]BaMg2 Si2
4.65
6.57
4.65
4.65
6.57
4.65
4.65
6.57
4.65
d[h k l]Mg2 Si
4.51
11.068
6.39
4.51
6.39
4.51
4.51
7.82
4.51
0 15.83%
9.7
0
0 3.01%
0
0
0 10.88%
15
30
◦
( ) ı
substrate for the other; furthermore, if the mismatch of two planes is below 6%, one phase can be an extremely effective heteronuclei for the other. Therefore, BaMg2 Si2 is a highly effective heterogenous nucleation substrate for primary Mg2 Si grains. In other words, the formation of large amounts of tiny BaMg2 Si2 particles leads to the increase of Mg2 Si nucleation rate. As a result, a mass of primary Mg2 Si nuclei cannot grow and develop to dendrite before solidification ends, i.e. the primary Mg2 Si grain is refined. At the beginning of solidification of alloy with 1.0 wt.% Ba, the rapid growth of many primary Mg2 Si nuclei and the formation of BaMg2 Si2 consume most Si atoms. The Si content in residual melt is much less than the eutectic content, so there is little eutectic Mg2 Si formed in solidification. This process is nonequilibrium solidification at a high cooling rate. The reduction of Chinese-character-like eutectic Mg2 Si will definitely improve the mechanical properties of the materials. In this case, Mg2 Si grains are almost polyhedral particles. The alloy can be regarded as Mg2 Si particles reinforced Mg–Zn–Si matrix composite with great application potentials. When Ba content exceeds 1.0 wt.%, the grain size of primary Mg2 Si particles slightly rebounds and eutectic Mg2 Si also increases, which means that the refinement and modification effect of Ba have been weakened. The reason is that BaMg2 Si2 granules gradually grow or conglomerate when Ba is continuously added. For example, the size of BaMg2 Si2 in alloy 3 is about 0.5 m (Fig. 5), while it is about 3 m in alloy 5 (Fig. 4). As a result, the amount of effective heteronulei reduces and over modification occurs. Furthermore, primary Mg2 Si aggrandizes and tends to be dendrite when Ba content is 3.0 wt.%. This is because the formation of needle-like Ba2 Mg3 Si4 in alloy 6 consumes some Si and most Ba, so there is little BaMg2 Si2 in the alloy to act as heteronulei. Meanwhile, Ba2 Mg3 Si4 cannot be heteronulei of Mg2 Si for the rather large crystal lattice misfitting between them. It can be concluded that excessive Ba addition has little effect of modification and refinement on Mg2 Si. 5. Conclusions (1) Ba can effectively modify and refine Mg2 Si in Mg2 Si/Mg–Zn–Si composite: primary Mg2 Si is modified to fine polygonal particles at first and then coarsen as Ba content increases; the
amount and average size of eutectic Mg2 Si also decrease at first and then increase as Ba increases. The best modification effect is obtained when Ba content is 1.0 wt.%. (2) Tiny BaMg2 Si2 particles are found inside the primary Mg2 Si grains of composite containing Ba; the crystal lattice misfitting between BaMg2 Si2 and Mg2 Si is so small that BaMg2 Si2 can act as the heterogenous nucleation substrate for primary Mg2 Si; the numerous nucleation of primary Mg2 Si leads to its refinement. (3) With further increase of Ba content, the agglomeration and growth of BaMg2 Si2 leads to the weakening of modification. When Ba content is 3.0%, needle-like Ba2 Mg3 Si4 exists in alloy which results in over modification. References [1] L. Lu, K.K. Thong, M. Gupta, Compos. Sci. Technol. 63 (2003) 627–632. [2] R. Tsuzuki, K. Kondoh, Pricm 5: The Fifth Pacific Rim International Conference on Advanced Materials and Processing, Pts 1–5, vols. 475–479, 2005, pp. 497– 500. [3] K. Asano, H. Yoneda, Mater. Trans. 48 (2007) 1469–1475. [4] S.K. Thakur, H. Dieringa, B.K. Dhindaw, N. Hort, K.U. Kainer, Trans. Indian Inst. Met. 58 (2005) 653–659. [5] L. Lu, M.O. Lai, M.L. Hoe, Nanostruct. Mater. 10 (1998) 551–563. [6] B.L. Mordike, J. Mater. Process. Technol. 117 (2001) 391–394. [7] Y.C. Pan, X.F. Liu, H. Yang, Mater. Charact. 55 (2005) 241–247. [8] R. Tsuzuki, K. Kondoh, W.B. Du, T. Aizawa, E. Yuasa, Mater. Sci. Forum. 419–422 (2003) 789–794. [9] H.Y. Wang, M. Zha, B. Liu, D.M. Wang, Q.C. Jiang, J. Alloys Compd. 480 (2009) L25–L28. [10] M. Mabuchi, K. Higashi, Acta Metall. 44 (1996) 4611–4618. [11] S.D. Sheng, D. Chen, Z.H. Chen, J. Alloys Compd. 470 (2009) L17–L20. [12] Q.C. Jiang, H.Y. Wang, Y. Wang, B.X. Ma, J.G. Wang, Mater. Sci. Eng. A 392 (2005) 130–135. [13] H.Y. Wang, Q.C. Jiang, B.X. Ma, Y. Wang, J.G. Wang, J.B. Li, J. Alloys Compd. 387 (2005) 105–108. [14] N. Zheng, H.Y. Wang, Z.H. Gu, W. Wang, Q.C. Jiang, J. Alloys Compd. 463 (2008) L1–L4. [15] E.J. Guo, B.X. Ma, L.P. Wang, J. Mater. Process. Technol. 206 (2008) 161–166. [16] L.H. Liao, X.Q. Zhang, H.W. Wang, X.F. Li, N.H. Ma, J. Alloys Compd. 430 (2007) 292–296. [17] J. Jeon, S. Lee, B. Kim, B. Park, Y. Park, I. Park, J. Korean Inst. Met. Mater. 46 (2008) 304–309. [18] B.L. Bramfitt, Metall. Trans. 7 (1970) 197–201. [19] G. Frommeyer, S. Beer, K.V. Oldenburg, Z. Metallkd. 85 (1994) 372–377. [20] B. Eisenmann, H. Schäfer, Z. Anorg. Allg. Chem. 403 (1974) 163–172.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jallcom
The effect of Ba addition on microstructure of in situ synthesized Mg2 Si/Mg–Zn–Si composites K. Chen, Z.Q. Li ∗ , J.S. Liu, J.N. Yang, Y.D. Sun, S.G. Bian College of Materials Science and Technology, Nanjing University of Aeronautics and Astronautics, No. 29 Yudao Street, Nanjing 210016, China
a r t i c l e
i n f o
Article history: Received 10 February 2009 Received in revised form 17 July 2009 Accepted 20 July 2009 Available online 28 July 2009 Keywords: Mg2 Si Ba addition Microstructure Modification Heterogenous nucleation
a b s t r a c t In order to modify in situ synthesized Mg2 Si phase in Mg–6Zn–4Si alloy, Ba addition of 0.1–3.0 wt.% had been explored. The effect of Ba addition on the microstructure of Mg2 Si/Mg–Zn–Si composites was investigated by means of optical microscope, scanning electron microscope, X-ray diffraction and energy dispersive spectrometer. The results indicate that the morphology of primary Mg2 Si in the composites changes from large dendrite to fine polygon with the increasing Ba content. The average size of primary and eutectic Mg2 Si sharply decreases with increasing Ba content up to 1.0 wt.% and then slowly increases. Theoretical calculation and experimental analysis show that tiny BaMg2 Si2 particles, formed by adding Ba, act as the heterogenous nucleation substrates for primary Mg2 Si. Therefore, Si consumption due to BaMg2 Si2 formation and primary Mg2 Si nucleation results in the inhibition of Mg2 Si growth. It is also found that the BaMg2 Si2 phase in primary Mg2 Si is obviously coarsened as Ba content exceeds 1.5 wt.% and some needle-like Ba2 Mg3 Si4 is found in alloy with 3.0 wt.% Ba. These are responsible for the over modification effect. Therefore, it can be concluded that proper Ba addition can effectively modify and refine primary Mg2 Si and decrease the amount of eutectic Mg2 Si. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved.
1. Introduction In recent years, in situ synthesized Mg matrix composites are considered to have great potential in modern industries due to their excellent heat-resistance and creep-resistance, low density and high specific strength as well as better cohesion between the reinforcement and matrix [1–3]. One research hotspot is in situ synthesized intermetallic compound Mg2 Si reinforced Mg matrix composite [3,4]. Mg2 Si is characterized by high melting temperature of 1085 ◦ C, low density of 1.99 × 103 kg m−3 , high hardness of 4.5 × 109 N m−2 , low thermal expansion coefficient of 7.5 × 10−6 K−1 and high elastic modulus of 120 GPa [5], which greatly improve the heat-resistance and wear-resistance of the alloys. However, a mass of coarse-dendritical primary Mg2 Si (with size even exceeding 100 m) and bulk-Chinese-character eutectic Mg2 Si appearing in high-Si magnesium alloys, deteriorate the mechanical and deformation properties of materials and impede their applications [6,7]. How to modify Mg2 Si phase in high-Si magnesium alloy has attracted considerable attention. Various processing techniques have been employed, such as mechanical alloying [1], hot extrusion [2,8], heat treatment [9], rapid solidi-
fication [10,11], and modification treatment [12–15]. Among those methods, modification during solidification process of alloys is a relatively convenient and economical method. For example, the primary Mg2 Si dendrite was changed to polyhedral particles with size about 20 m when Y, KBF4 , B2 O3 and Bi were added to Mg–Si alloy, and the modification mechanism can be regarded as poisoning [12–14]. It is also reported that the combination of Sr–Sb modification and heat treatment can alter the primary Mg2 Si dendrite to short rod-shape or granular [9]. But most of those researches focused on the variation of primary Mg2 Si. It is necessary to develop more effective modifier in order to modify both the primary Mg2 Si dendrite and Chinese-characters-like eutectic Mg2 Si. It is well known that Al–Ba alloy has been used to modify the primary Si in Al–Si alloy. Considering the similarity between Si modification in Al–Si alloy and Mg2 Si modification in Mg–Si alloy, Ba may be a good modifier for Mg2 Si in Mg–Si alloys. The objectives of this work are to find out the effect of Ba addition on the microstructure of Mg–6Zn–4Si alloy, to explore the modification mechanism of Ba modifier on in situ synthesized Mg2 Si during solidification processing, and to prepare Mg matrix composites with fine microstructure. 2. Experimental 2.1. Raw materials and smelting
∗ Corresponding author. Tel.: +86 25 8489 2797; fax: +86 25 8489 0521. E-mail address: [email protected] (Z.Q. Li).
Commercial Mg ingot (>99.9 wt.%), Zn ingot (>99.8 wt.%) and Si (>99.8 wt.%) were used to prepare master alloy Mg–6Zn–4Si. Mg and Zn ingots were melted up to 700 ◦ C
0925-8388/$ – see front matter. Crown Copyright © 2009 Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2009.07.111
294
K. Chen et al. / Journal of Alloys and Compounds 487 (2009) 293–297
Table 1 Experimental alloy number and nominal Ba addition. Alloy No.
0
1
2
3
4
5
6
Ba content (wt.%)
0
0.1
0.5
1.0
1.5
2.0
3.0
in a steel crucible, and then Si particles were added. The melt was continuously heated to above 750 ◦ C and kept for 20 min. The melt was well-proportioned by mechanical agitation for 5 min and deslaged, then cast into a steel mould which was preheated at 250 ◦ C while the melt temperature is below 700 ◦ C. The master alloy ingots were sliced for subsequent experiments. Rationed amount of pure Ba (>99 wt.%) was added to remelted master alloys. Then the melts were kept at 750 ◦ C for 15 min. After being stirred and deslaged, the melts were cast into the preheated steel mould (with the size of 100 mm × 80 mm × 20 mm). The whole smelting was carried on in an electric resistance furnace and protected by the gas mixture of 1 vol.% SF6 + 99 vol.% CO2 . The Ba contents of ingots are shown in Table 1. 2.2. Microstructure investigation After kibbled and polished, samples cut from the middle of castings were etched by 0.4 vol.% nitric alcohol solution. XJP-300 Optical Microscope (OM) was used to observe the microstructure of the samples. The average areas of primary Mg2 Si grains in every optical image were measured by Image-pro Plus 5.0 software. The crystal structures and chemical composition of phases were detected and analyzed by Bruker D8-advance X-ray diffraction (XRD) and energy dispersive spectrometer (EDS) affiliated to the LEO 1550 scanning electron microscope (SEM), respectively.
Fig. 1. Microstructure of alloy 0.
3. Results The microstructure of Mg–6Zn–4Si master alloys is consisted of primary Mg2 Si, eutectic Mg2 Si, intermetallic compound MgZn
Fig. 2. OM of contained Ba alloys: (a) alloy 1, (b) alloy 2, (c) alloy 3, (d) alloy 4, (e) alloy 5 and (f) alloy 6.
K. Chen et al. / Journal of Alloys and Compounds 487 (2009) 293–297
Fig. 3. The relationship between Ba content and average area of primary Mg2 Si grains.
and ␣-Mg, as shown in Fig. 1. The primary Mg2 Si exists as coarse dendrites or polygons. Eutectic Mg2 Si exhibits as Chinese character. Semicontinuous net of MgZn is distributed among ␣-Mg and eutectic Mg2 Si. OM images of the alloys containing different Ba contents are presented in Fig. 2. The morphology of primary Mg2 Si in the composites significantly changes from large dendrites to fine polygonal even spherical particles with the increasing of Ba content. The average size of primary Mg2 Si sharply decreases first with increasing Ba content up to 1.0 wt.% and then slowly increases. Quantitative results of primary Mg2 Si grain size in every alloy, characterized by normalized average area of primary Mg2 Si (the average areas of
295
primary Mg2 Si in alloy 0 is fixed to 1), are shown in Fig. 3. The smallest average grain area, appeared in Mg–6Zn–4Si–1Ba alloy, is about 200.5 × 10−12 m2 (normalized to 0.172 in Fig. 3), where corresponding primary Mg2 Si size is less than 20 m. When Ba content is over 1.0 wt.%, primary Mg2 Si grain becomes slightly larger with the increasing Ba content. Normalized average areas of primary Mg2 Si grain in alloy 4, alloy 5 and alloy 6 are 0.195, 0.257 and 0.684, respectively. Compared with primary phase, the amount and average size of eutectic Mg2 Si also show similar change trend: decreasing at first and then increasing as Ba increases. Particularly, little eutectic Mg2 Si appears in the alloy with 1.0 wt.% Ba and its size is very small. SEM images of alloy 3 and alloy 5 are shown in Fig. 4. A tiny particle (as white arrow pointed) is found inside the primary Mg2 Si. EDS test of this phase (Fig. 5) indicates that it contains Mg, Ba and Si. But it is hard to find this phase in XRD patterns (Fig. 6) as its amount is too small. This phase is obviously coarsened from about 0.5 m to 3 m (Figs. 4 and 5) as the content of Ba exceeds 1.5 wt.%. It is also found that some needle-like phase with a length of 10–100 m appears when the Ba content is 3.0 wt.% (Fig. 2(f)). It is proved to be Ba2 Mg3 Si4 by EDS and XRD patterns (Figs. 7 and 6, respectively).
4. Discussion Two major mechanisms of modification and refinement of Mg2 Si grains were discussed in previous work. One mechanism is the increase of nucleation. The formation of large amounts of nuclei in the melt leads to the decrease of Mg2 Si grain size [16,17]. The other is inhibition of grain growth through changing the solidification condition and the modification is due to poisoning. For example, adsorption of B, Y atoms at Mg2 Si growing surface front changes the solidification condition and the primary Mg2 Si is modified [12–14].
Fig. 4. SEM micrographs of alloys contained Ba: (a) alloy 3 and (b) alloy 5.
Fig. 5. EDS results of a tiny granular phase inside primary Mg2 Si grain in alloy 3.
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K. Chen et al. / Journal of Alloys and Compounds 487 (2009) 293–297
may acts as the substrate of Mg2 Si nucleus. According to the Bramfitt theory of heterogenous nucleation, lattice misfitting between Ba–Mg–Si compound and Mg2 Si were calculated. The lattice misfitting mathematical mode [18] by Bramfitt is:
k l)s ı(h = (h k l)
i 3 d i cos − d [u v w]s [u v w]n
n
i=1
Fig. 6. XRD patterns of alloy 3 and alloy 6.
Here Ba was added into the Mg–Zn–Si melt. Since Ba scarcely dissolves in Mg crystal lattice, it will precipitate or react with the melt and form binary or ternary intermetallic compounds during solidification. The above results of microstructure observation and EDS test indicate that, in alloys with Ba addition, tiny compound particles contained Ba exist inside primary Mg2 Si. This compound
i 3d[u v w]
× 100%
n
where the (h k l)s is low index crystal plane of the heteronucleus, the [u v w]s is low index crystal orientation in the (h k l)s ; the (h k l)n is low index crystal plane of new crystal nucleus, the [u v w]n is low index crystal orientation in the (k h l)n ; d[u v w]s and d[u v w]n are atomic spatial distance along the [u v w]s and the [u v w]n ; is the angle between [u v w]s and [u v w]n ( < 90◦ ). The fcc Mg2 Si belongs to the space group of Fm-3m(225) [19], and its Pearson symbol is cF12. The tetragonal BaMg2 Si2 belongs to the space group of I4/mmm(139) and it is with Pearson symbol of tI10 [20]. The crystal structures and atomic arrangements of Mg2 Si and BaMg2 Si2 are shown schematically in Figs. 8 and 9. One can see that atoms distribute on (0 0 1)Mg2 Si and (0 0 1)BaMg2 Si2 in a similar way. The misfitting of these two planes is calculated according to Brafitt theory and formula. The results are shown in Table 2. The misfitting between (0 0 1)Mg2 Si and (0 0 1)BaMg2 Si2 is 3.01%, much smaller than misfitting between other planes. The interface coherent theory and Bramfitt theory point out: if the mismatch of two planes is below 15%, one phase can be heterogenous nucleation
Fig. 7. EDS test of needle-like phase in alloy 6.
Fig. 8. Crystal structures of Mg2 Si and BaMg2 Si2 .
K. Chen et al. / Journal of Alloys and Compounds 487 (2009) 293–297
297
Fig. 9. Comparison of atomic arrangements in (0 0 1)Mg2 Si and (0 0 1)BaMg2 Si2 Table 2 Calculated values of planar disregistry between Mg2 Si and BaMg2 Si2 phase. (0 0 1)BaMg2 Si2 //(1 1 0)Mg2 Si
(0 0 1)BaMg2 Si2 //(0 0 1)Mg2 Si
(0 0 1)BaMg2 Si2 //(1 1 1)Mg2 Si
[h k l]BaMg2 Si2
[0 1 0]
[1¯ 1 0]
[1¯ 0 0]
[0 1 0]
[1¯ 1 0]
[1¯ 0 0]
[0 1 0]
[1¯ 1 0]
[1¯ 0 0]
[h k l]Mg2 Si
[1¯ 1 0]
[1¯ 1 1]
[0 0 1]
[1 1 0]
[0 1 0]
[1¯ 1 0]
[1¯ 1 0]
[1¯ 1 1]
[1¯ 0 1]
d[h k l]BaMg2 Si2
4.65
6.57
4.65
4.65
6.57
4.65
4.65
6.57
4.65
d[h k l]Mg2 Si
4.51
11.068
6.39
4.51
6.39
4.51
4.51
7.82
4.51
0 15.83%
9.7
0
0 3.01%
0
0
0 10.88%
15
30
◦
( ) ı
substrate for the other; furthermore, if the mismatch of two planes is below 6%, one phase can be an extremely effective heteronuclei for the other. Therefore, BaMg2 Si2 is a highly effective heterogenous nucleation substrate for primary Mg2 Si grains. In other words, the formation of large amounts of tiny BaMg2 Si2 particles leads to the increase of Mg2 Si nucleation rate. As a result, a mass of primary Mg2 Si nuclei cannot grow and develop to dendrite before solidification ends, i.e. the primary Mg2 Si grain is refined. At the beginning of solidification of alloy with 1.0 wt.% Ba, the rapid growth of many primary Mg2 Si nuclei and the formation of BaMg2 Si2 consume most Si atoms. The Si content in residual melt is much less than the eutectic content, so there is little eutectic Mg2 Si formed in solidification. This process is nonequilibrium solidification at a high cooling rate. The reduction of Chinese-character-like eutectic Mg2 Si will definitely improve the mechanical properties of the materials. In this case, Mg2 Si grains are almost polyhedral particles. The alloy can be regarded as Mg2 Si particles reinforced Mg–Zn–Si matrix composite with great application potentials. When Ba content exceeds 1.0 wt.%, the grain size of primary Mg2 Si particles slightly rebounds and eutectic Mg2 Si also increases, which means that the refinement and modification effect of Ba have been weakened. The reason is that BaMg2 Si2 granules gradually grow or conglomerate when Ba is continuously added. For example, the size of BaMg2 Si2 in alloy 3 is about 0.5 m (Fig. 5), while it is about 3 m in alloy 5 (Fig. 4). As a result, the amount of effective heteronulei reduces and over modification occurs. Furthermore, primary Mg2 Si aggrandizes and tends to be dendrite when Ba content is 3.0 wt.%. This is because the formation of needle-like Ba2 Mg3 Si4 in alloy 6 consumes some Si and most Ba, so there is little BaMg2 Si2 in the alloy to act as heteronulei. Meanwhile, Ba2 Mg3 Si4 cannot be heteronulei of Mg2 Si for the rather large crystal lattice misfitting between them. It can be concluded that excessive Ba addition has little effect of modification and refinement on Mg2 Si. 5. Conclusions (1) Ba can effectively modify and refine Mg2 Si in Mg2 Si/Mg–Zn–Si composite: primary Mg2 Si is modified to fine polygonal particles at first and then coarsen as Ba content increases; the
amount and average size of eutectic Mg2 Si also decrease at first and then increase as Ba increases. The best modification effect is obtained when Ba content is 1.0 wt.%. (2) Tiny BaMg2 Si2 particles are found inside the primary Mg2 Si grains of composite containing Ba; the crystal lattice misfitting between BaMg2 Si2 and Mg2 Si is so small that BaMg2 Si2 can act as the heterogenous nucleation substrate for primary Mg2 Si; the numerous nucleation of primary Mg2 Si leads to its refinement. (3) With further increase of Ba content, the agglomeration and growth of BaMg2 Si2 leads to the weakening of modification. When Ba content is 3.0%, needle-like Ba2 Mg3 Si4 exists in alloy which results in over modification. References [1] L. Lu, K.K. Thong, M. Gupta, Compos. Sci. Technol. 63 (2003) 627–632. [2] R. Tsuzuki, K. Kondoh, Pricm 5: The Fifth Pacific Rim International Conference on Advanced Materials and Processing, Pts 1–5, vols. 475–479, 2005, pp. 497– 500. [3] K. Asano, H. Yoneda, Mater. Trans. 48 (2007) 1469–1475. [4] S.K. Thakur, H. Dieringa, B.K. Dhindaw, N. Hort, K.U. Kainer, Trans. Indian Inst. Met. 58 (2005) 653–659. [5] L. Lu, M.O. Lai, M.L. Hoe, Nanostruct. Mater. 10 (1998) 551–563. [6] B.L. Mordike, J. Mater. Process. Technol. 117 (2001) 391–394. [7] Y.C. Pan, X.F. Liu, H. Yang, Mater. Charact. 55 (2005) 241–247. [8] R. Tsuzuki, K. Kondoh, W.B. Du, T. Aizawa, E. Yuasa, Mater. Sci. Forum. 419–422 (2003) 789–794. [9] H.Y. Wang, M. Zha, B. Liu, D.M. Wang, Q.C. Jiang, J. Alloys Compd. 480 (2009) L25–L28. [10] M. Mabuchi, K. Higashi, Acta Metall. 44 (1996) 4611–4618. [11] S.D. Sheng, D. Chen, Z.H. Chen, J. Alloys Compd. 470 (2009) L17–L20. [12] Q.C. Jiang, H.Y. Wang, Y. Wang, B.X. Ma, J.G. Wang, Mater. Sci. Eng. A 392 (2005) 130–135. [13] H.Y. Wang, Q.C. Jiang, B.X. Ma, Y. Wang, J.G. Wang, J.B. Li, J. Alloys Compd. 387 (2005) 105–108. [14] N. Zheng, H.Y. Wang, Z.H. Gu, W. Wang, Q.C. Jiang, J. Alloys Compd. 463 (2008) L1–L4. [15] E.J. Guo, B.X. Ma, L.P. Wang, J. Mater. Process. Technol. 206 (2008) 161–166. [16] L.H. Liao, X.Q. Zhang, H.W. Wang, X.F. Li, N.H. Ma, J. Alloys Compd. 430 (2007) 292–296. [17] J. Jeon, S. Lee, B. Kim, B. Park, Y. Park, I. Park, J. Korean Inst. Met. Mater. 46 (2008) 304–309. [18] B.L. Bramfitt, Metall. Trans. 7 (1970) 197–201. [19] G. Frommeyer, S. Beer, K.V. Oldenburg, Z. Metallkd. 85 (1994) 372–377. [20] B. Eisenmann, H. Schäfer, Z. Anorg. Allg. Chem. 403 (1974) 163–172.