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TiB2 coupled compounds on primary Mg2Si in Al–Mg–Si alloys
Materials Science and Engineering A 497 (2008) 432–437
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Heterogeneous nucleating role of TiB2 or AlP/TiB2 coupled compounds on primary Mg2 Si in Al–Mg–Si alloys Chong Li a , Xiangfa Liu a,b,∗ , Guohua Zhang b a b
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China Shandong Binzhou Bohai Piston Co. Ltd., Binzhou 256602, Shandong, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2008 Received in revised form 20 July 2008 Accepted 21 July 2008 Keywords: Al–Mg–Si alloys Primary Mg2 Si Crystal structure Nucleation
a b s t r a c t Refinement and modification performance of Al–Ti–B master alloy alone or with Al–P master alloy on primary Mg2 Si in Al–12.67% Mg–10.33% Si alloy was investigated in this paper. The experimental results show that perfect effect can be obtained after addition of 1% Al–5Ti–1B master alloy into the Al–Mg–Si alloy. The morphologies of primary Mg2 Si particulates change from dendritic to polygonal shape, and their average sizes decrease from ∼100 to ∼20 m. The ultimate tensile strength of the alloy increases. EPMA and TEM results show that TiB2 particles act as the nuclei of primary Mg2 Si. Crystal lattice correspondence indicates TiB2 has a good lattice matching coherence relationship with Mg2 Si, and the disregistry is only 4.64%. Furthermore, the addition of Al–5Ti–1B master alloy can significantly improve the refinement and modification effect of AlP on primary Mg2 Si. It is also found that coupling particles of AlP and TiB2 exist in the center of primary Mg2 Si. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In Al–Mg–Si alloys, the intermetallic compound Mg2 Si exhibits a high melting temperature (1085 ◦ C), low density (1.99 × 103 kg m−3 ), high hardness (4.5 × 109 N m−2 ), a low coefficient of thermal expansion (7.5 × 10−6 K−1 ) and a high elastic modulus (120 GPa) [1], which makes the alloys with Mg2 Si particles an attractive candidate material for aerospace, automotive, and other applications [2,3]. Al–Mg–Si alloys can be used to product cylinder heads, pistons, brake disks and so on. However, in normal cast Al–Mg–Si alloys, primary Mg2 Si is usually very coarse and its distribution is uneven. As a result, it seriously separates aluminum matrix and leads to poor properties [4]. Therefore, coarse primary Mg2 Si particles need to be refined and modified to obtain adequate mechanical strength and ductility. Refinement of microstructure is mainly responsible for the improvement in the mechanical properties. It has been reported that the microstructures and mechanical properties of Mg2 Si reinforced Mg- and Al-based alloys can be improved by the application of advanced processing techniques such as hot extrusion [5], rapid solidification processing [6,7] and mechanical alloying [8,9]. How-
∗ Corresponding author at: Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China. Tel.: +86 531 88395414; fax: +86 531 88395414. E-mail address: xfl[email protected] (X. Liu). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.07.034
ever, these processing methods may lead to the increase of the production cost. Recently, more practical methods to refine and modify primary Mg2 Si are additions of rare earth [10–12], Al–Sr master alloys [13,14], sodium salt [15] and phosphorus [16]. The effects of K2 TiF6 and KBF4 on primary Mg2 Si have been also investigated [17,18]. However, some of them exist internal defects, for example, sodium salt [15] addition can change the morphology and size, but the amount of addition is too high (10 wt.%); besides, potassium fluotitanate [17] addition only decreases the size of primary Mg2 Si and can not change its morphology. The refinement and modification mechanism of Ti or B elements did not carry out [17,18]. In the article, the nucleation mechanism of Ti and B on primary Mg2 Si in Al–Mg–Si alloys was investigated. In addition, the combination effect of Al–Ti–B and Al–P master alloys on Mg2 Si was discussed.
2. Experimental procedures Commercial pure Al (99.7%, all compositions quoted in this work are in wt.% unless otherwise stated), commercial pure crystalline Si (99.9%) and commercial pure Mg (99.8%) were used as starting materials to prepare Al–12.67% Mg–10.33% Si alloy (named Al–12.67Mg–10.33Si alloy below) in a 25 kW medium frequency induction furnace. Different refinement and modification experiments were conducted in clay-bonded graphite crucibles in a 5 kW electric resistant-heating furnace.
C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
Three groups of samples were carried out in present work, and were henceforth designated as group 1, group 2 and group 3, respectively. Samples in group 1 were fabricated under following condition: the Al–12.67Mg–10.33Si alloy was remelted at 850 ◦ C and held at this temperature, then Al–5Ti–1B master alloy was added to the melt with 0, 0.2 and 1% respectively. After holding 10 min, the melt was poured into a cast iron mold and tensile test bars were obtained. In order to investigate the combination effect of Al–P and Al–Ti–B master alloys on primary Mg2 Si, Al–12.67Mg–10.33Si melt was added with 1% Al–3P for 30 min, part of the melt was poured and Sample-2 was obtained. Then, 0.2% Al–5Ti–1B master alloy was added into the Al–3P refined and modified Al–12.67Mg–10.33Si melt and holding 10 min. The melt was poured and Sample-3 was obtained. Metallographic specimens were cut at the same position of the tabulate samples and polished through standard procedure. Structure and qualitative analysis were conducted by using high scope video microscope (HSVM) (model KH-2200, Japan), electron probe micro-analyzer (EPMA) (model JXA-8840, Japan) and transmission electron microscope (TEM) (model H-800, Japan). The tensile test bars were machined to ‘dog-bone’ type specimens (shown in Fig. 1) and then tensile-tested at room temperature by universal testing machine (model CMT700, China). 3. Results and discussion 3.1. Effect of Al–5Ti–1B on the primary Mg2 Si Fig. 2 shows the change of primary Mg2 Si phase with different amounts of Al–5Ti–1B master alloy in the Al–12.67Mg–10.33Si alloy. The addition of 0.2% Al–5Ti–1B master alloy leads to the size of Mg2 Si decreasing, but its morphology being still dendritic,
433
Fig. 1. The specimen used for tensile strength testing at room temperature.
compared with the microstructure of the alloy without Al–5Ti–1B addition. The size of primary Mg2 Si decreases from ∼100 to ∼20 m with the addition of 1% Al–5Ti–1B, and the morphology changes from dendritic to polygonal shape. So it is suggested that Al–5Ti–1B master alloy is an efficient grain refiner for Al–Mg–Si alloys. Fig. 3 presents the EPMA of a primary Mg2 Si nucleus in Al–12.67Mg–10.33Si alloy with the addition of 1% Al–5Ti–1B master alloy and holding 10 min. The X-ray images show that the center of Mg2 Si contains Ti and B elements. In order to further analyze its component, the composition along the line M–N across the primary Mg2 Si in Fig. 3(a) is illustrated in Fig. 4. The peak of B overlaps the Ti peak. It is indicated that it may be TiB2 compound. In order to further confirm whether it is TiB2 or not, the TEM analysis of a primary Mg2 Si particle is illustrated in Fig. 5. It is concluded that there are TiB2 particles in the center of primary Mg2 Si. Also it is interesting to note that sometimes several TiB2 particles rather one act as the nuclei of the primary Mg2 Si, as shown in Fig. 5. TiB2 is hexagonal with the lattice parameters: a = 3.03 Å, c = 3.23 Å [19,20], and its crystal structure can be illustrated in Fig. 6. The Ti atoms, shown in black, locate on the corner angles of hexagon and in the center of the back surface, while the B atoms, shown in
Fig. 2. Microstructures of Al–12.67Mg–10.33Si alloy before and after the addition of Al–5Ti–1B master alloy: (a) without Al–5Ti–1B; (b) adding 0.2% Al–5Ti–1B; (c) adding 1% Al–5Ti–1B.
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C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
Fig. 3. EPMA analysis of a primary Mg2 Si: (a) BEI of the primary Mg2 Si; (b–f) the X-ray images for respective elements, Al, Mg, Si, Ti and B.
white, distribute in the center of the triangular prism composed of the Ti atoms. Therefore, the Ti atomic plane alternates with the B atomic plane to form a two-dimensional planar netted structure. Mg2 Si is a fluorite-type structure with the lattice parameter: a = 6.39 Å (Fig. 7). There are 12 atoms in the Mg2 Si cell. The Si atoms locate on the corners and surface centers of the face-centered cubic, and the Mg atoms are in the eight tetrahedral interstices of the crystal cell. One phase can act as a fine heterogeneous nucleating site for the other phase, when there is a good coherent relationship existing on the interface of two types of phases [21]. The interatomic distances of the crystal faces of the two phases should be close to each other, and the atomic arrangements of the crystal faces should be similar. It is necessary to point out that there should be some similar interplanar distances when the interface of two phases has a good coherent relationship. From Table 1, several possible coherent interfaces between TiB2 and Mg2 Si can be obtained, and the disregistry is less than 5%. Generally the disregistry ı between the substrate phase and the crystallization phase is calculated by the following
Turnbull–Vonnegut equation: ı=
|as − ac | × 100% ac
(1)
where as and ac are the interatomic/interplanar distances of substrate plane and crystallization plane without deformation, respectively. It is not applicable to crystallographic combinations of two phases with planes of different atomic arrangements, because of the inherent limitation of the Turnbull–Vonnegut equation. So it is necessary to adjust the equation in terms of the angular difference between the crystallographic directions within the planes [22]. The adjustment equation is expressed in the following form: k l)s ı(h = (h k l)
3 |(d[u v w]i cos ) − d[u v w]i )|/d[u v w]i s
n
3
n
n
× 100%
(2)
i=1
where (h k l)s is a low-index plane of the substrate, [u v w]s a lowindex direction in (h k l)s , (h k l)n a low-index plane of the nucleated
C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
435
Fig. 4. The distribution of chemical composition along the line from M to N in Fig. 3(a).
Fig. 7. Crystal structure of Mg2 Si.
of TiB2 and in the (2 0 0) crystal face of Mg2 Si. The shaded circles represent the Ti atoms in the (0 0 1) crystal face of TiB2 , while broken-line circles represent Si atoms in the (2 0 0) crystal face of Mg2 Si. The required parameters for Eq. (2) are listed in Table 2: Fig. 5. TEM image of center of a primary Mg2 Si.
solid, [u v w]n a low-index direction in (h k l)n , d[u v w]s the interatomic distance along [u v w]s , d[u v w]n the interatomic distance along [u v w]n , and is the angle between the [u v w]s and [u v w]n . The number ‘1’ is taken as an example to research the atomic arrangement of the crystal plane, as shown in Table 1. Fig. 8 shows the typical planar atomic arrangements in the (0 0 1) crystal face
0 1)TiB ı(0 2 (2 0 0)
Mg2 Si
=
(|6.06 − 6.39|/6.39) + (|21 − 19.71|/19.71) + (|8.84 − 9.04|/9.04) 3 × 100% ≈ 4.64%
This indicates that TiB2 compound is a good nucleating site for Mg2 Si compound, and Mg2 Si can nucleate on the TiB2 nucleation substrates.
Table 1 Possible coherent interfaces of TiB2 and Mg2 Si crystals Number
Fig. 6. Crystal structure of TiB2 .
1 2 3 4 5 6
TiB2
Mg2 Si
ı (%)
d (Å)
(h k l)
d (Å)
(h k l)
3.2295 1.6145 1.5153 1.3751 1.2156 1.1049
001 002 110 102 201 112
3.1955 1.5977 1.4662 1.4290 1.2299 1.1297
200 400 331 420 333 440
1.06 1.05 3.35 3.77 1.16 2.20
436
C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
Table 2 Parameters for Eq. (2) Case
d[u v w]s
d[u v w]n
(◦ )
d[u v w]s cos
(0 0 1)TiB2 ||(2 0 0)Mg2 Si
6.06 21 9.09
6.39 19.71 9.04
0 0 15
6.06 21 8.84
Fig. 9. Relation between ultimate tensile strength of Al–12.67Mg–10.33Si alloy and adding level of Al–5Ti–1B.
3.3. Effects of combined additions of Al–5Ti–1B and Al–3P
Fig. 8. The crystallographic relationship at the interface between the (0 0 1) of TiB2 and the (2 0 0) of Mg2 Si.
3.2. The tensile strength change of Al–12.67Mg–10.33Si alloy at room temperature It is evident from Fig. 9 that TiB2 refinement and modification raises the ultimate tensile strength of the alloy and there is an increasing trend in the strength of the alloy with the increase of the addition of Al–5Ti–1B. After addition of Al–5Ti–1B master alloy into the Al–Mg–Si alloy, the primary Mg2 Si changes from coarse dendritic to fine polygonal shape (shown in Fig. 2). The improvement of strength is attributed to the fine and uniform distribution of primary Mg2 Si particles. The fine primary Mg2 Si dose not separate aluminum matrix seriously and tensile strength of the alloy is improved.
Fig. 10 shows the microstructures of Al–12.67Mg–10.33Si alloy before and after the addition of Al–3P or (and) Al–5Ti–1B master alloys. The effect of Al–P master alloy on the primary Mg2 Si in Al–Mg–Si alloys has been investigated in previous work [16]. It has been found that Al–P master alloy can refine and modify primary Mg2 Si. But it cannot obtain favorable effect with the addition of 1% Al–3P master alloy. The size of primary Mg2 Si does not decrease obviously, and the morphology of primary Mg2 Si is still dendritic, as shown in Fig. 10(a). But after the addition of 0.2% Al–5Ti–1B, the size of primary Mg2 Si decreases significantly, as shown in Fig. 10(b). This indicates that the addition of Al–5Ti–1B master alloy can significantly improve the refinement and modification effect of AlP on primary Mg2 Si in Al–12.67Mg–10.33Si alloy. Yu et al. [19] reported that the addition of Al–5Ti–1B master alloy could significantly improve the phosphorous modification effect on primary Si, in the Al–P modified Al–Si alloy. And it is reported that TiB2 has a good lattice matching coherence relationship with AlP, for example, the interfacial relationship (1 1 2)TiB2 ||(4 4 2)Alp possesses a low lattice misfit (about 7.27%). In this study, it was also found the phenomenon of coupling particles of AlP and TiB2 in Al–Mg–Si alloys. EPMA analysis of a primary Mg2 Si nucleus is illustrated in Fig. 11, the black phase is AlP, and the light phase is TiB2 . TiB2 has strong absorbability to AlP, so AlP crystals can easily nucleate on TiB2 surfaces and form quantities of coupling com-
Fig. 10. Microstructures of Al–12.67Mg–10.33Si alloy before and after the addition of Al–3P or (and) Al–5Ti–1B master alloys: (a) Al–12.67Mg–10.33Si + 1% Al–3P; (b) Al–12.67Mg–10.33Si + 1% Al–3P + 0.2% Al–5Ti–1B.
C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
437
Fig. 11. Line distribution of chemical composition along the line across the primary Mg2 Si: (a) line E–F across the primary Mg2 Si; (b) chemical composition distribution along line E–F.
pounds. These compounds can also act as heterogeneous nuclei for the primary Mg2 Si. The quantity of primary Mg2 Si increases, and accordingly the size decreases.
Project of Science and Technology Research of Ministry of Education of China (no. 106103) and “Taishan Scholar” Construction Project for financial support of Shandong Province in China.
4. Conclusions
References
(1) With the addition of 1% Al–5Ti–1B master alloy into the Al–Mg–Si alloy, the morphologies of primary Mg2 Si particulates change from dendritic to polygonal shape, and their average sizes decrease from ∼100 to ∼20 m. Due to the structure improvement, the ultimate tensile strength of the alloy increases. (2) EPMA and TEM results show that TiB2 particles can act as the nuclei of the primary Mg2 Si. Besides, crystal lattice correspondence indicates TiB2 has a good lattice matching coherence relationship with Mg2 Si, and the disregistry is only 4.64%. (3) The addition of Al–5Ti–1B master alloy can significantly improve the refinement and modification effect of AlP on primary Mg2 Si in Al–12.67Mg–10.33Si alloy. In addition, it is found that coupling compounds of AlP and TiB2 exist in the center of primary Mg2 Si.
[1] L. Lu, M.O. Lai, M.L. Hoe, Nanostruct. Mater. 10 (1998) 551–563. [2] Y.L. Liu, S.B. Kang, H.W. Kim, Mater. Lett. 41 (1999) 267–272. [3] S.P. Li, S.X. Zhao, M.X. Pan, D.Q. Zhao, X.C. Chen, O.M. Barabash, J. Mater. Sci. 36 (2001) 1569–1575. [4] M. Mabuchi, K. Kubota, K. Higashi, J. Mater. Sci. 31 (1996) 1529–1535. [5] M. Mabuchi, K. Kubota, K. Higashi, Mater. Lett. 19 (1994) 247–250. [6] M. Mabuchi, K. Kubota, K. Higashi, Scr. Metall. Mater. 33 (1995) 331–335. [7] M. Mabuchi, K. Higashi, Acta Mater. 44 (1996) 4611–4618. [8] G. Frommeyer, S. Beer, K. Von Oldenburg, Z. Metallkde. 85 (1994) 372–377. [9] L. Lu, K.K. Thong, M. Gupta, Compos. Sci. Technol. 63 (2003) 627–632. [10] X. Ai, Y. Li, Foundry 54 (3) (2005) 238–240. [11] Y.G. Zhao, Q.D. Qin, W. Zhou, Y.H. Liang, J. Alloys Compd. 389 (2005) L1–L4. [12] J. Zhang, Z. Fan, Y.Q. Wang, B.L. Zhou, Mater. Sci. Eng. A 281 (2000) 104–112. [13] H. Liao, Y. Ding, G. Sun, Foundry 51 (2) (2002) 80–84. [14] Y.G. Zhao, Q.D. Qin, Y.H. Liang, W. Zhou, Q.C. Jiang, J. Mater. Sci. 40 (2005) 1831–1833. [15] J. Zhang, Z. Fan, Y. Wang, B. Zhou, J. Mater. Sci. Lett. 18 (1999) 783–784. [16] C. Li, X.F. Liu, Y.Y. Wu, J. Alloys Compd. 465 (2008) 145–150. [17] Y.G. Zhao, Q.D. Qin, Y.Q. Zhao, Y.H. Liang, Q.C. Jiang, Mater. Lett. 58 (2004) 2192–2194. [18] H.Y. Wang, W. Wang, M. Zha, N. Zheng, Z.H. Gu, D. Li, Q.C. Jiang, Mater. Chem. Phys. 108 (2008) 353–358. [19] L.N. Yu, X.F. Liu, H.M. Ding, X.F. Bian, J. Alloys Compd. 432 (2007) 156–162. [20] P.S. Mohanty, J.E. Gruzleski, Acta Metall. Mater. 43 (1995) 2001–2012. [21] J.M. Rigsbee, H.I. Aaronson, Acta Metall. Mater. 27 (1979) 351–363. [22] L.B. Bruce, Metall. Trans. 1 (1970) 1987–1995.
Acknowledgments This work was supported by a grant from National Science Fund for Distinguished Young Scholars of China (no. 50625101), Key
Contents lists available at ScienceDirect
Materials Science and Engineering A journal homepage: www.elsevier.com/locate/msea
Heterogeneous nucleating role of TiB2 or AlP/TiB2 coupled compounds on primary Mg2 Si in Al–Mg–Si alloys Chong Li a , Xiangfa Liu a,b,∗ , Guohua Zhang b a b
Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China Shandong Binzhou Bohai Piston Co. Ltd., Binzhou 256602, Shandong, PR China
a r t i c l e
i n f o
Article history: Received 27 March 2008 Received in revised form 20 July 2008 Accepted 21 July 2008 Keywords: Al–Mg–Si alloys Primary Mg2 Si Crystal structure Nucleation
a b s t r a c t Refinement and modification performance of Al–Ti–B master alloy alone or with Al–P master alloy on primary Mg2 Si in Al–12.67% Mg–10.33% Si alloy was investigated in this paper. The experimental results show that perfect effect can be obtained after addition of 1% Al–5Ti–1B master alloy into the Al–Mg–Si alloy. The morphologies of primary Mg2 Si particulates change from dendritic to polygonal shape, and their average sizes decrease from ∼100 to ∼20 m. The ultimate tensile strength of the alloy increases. EPMA and TEM results show that TiB2 particles act as the nuclei of primary Mg2 Si. Crystal lattice correspondence indicates TiB2 has a good lattice matching coherence relationship with Mg2 Si, and the disregistry is only 4.64%. Furthermore, the addition of Al–5Ti–1B master alloy can significantly improve the refinement and modification effect of AlP on primary Mg2 Si. It is also found that coupling particles of AlP and TiB2 exist in the center of primary Mg2 Si. © 2008 Elsevier B.V. All rights reserved.
1. Introduction In Al–Mg–Si alloys, the intermetallic compound Mg2 Si exhibits a high melting temperature (1085 ◦ C), low density (1.99 × 103 kg m−3 ), high hardness (4.5 × 109 N m−2 ), a low coefficient of thermal expansion (7.5 × 10−6 K−1 ) and a high elastic modulus (120 GPa) [1], which makes the alloys with Mg2 Si particles an attractive candidate material for aerospace, automotive, and other applications [2,3]. Al–Mg–Si alloys can be used to product cylinder heads, pistons, brake disks and so on. However, in normal cast Al–Mg–Si alloys, primary Mg2 Si is usually very coarse and its distribution is uneven. As a result, it seriously separates aluminum matrix and leads to poor properties [4]. Therefore, coarse primary Mg2 Si particles need to be refined and modified to obtain adequate mechanical strength and ductility. Refinement of microstructure is mainly responsible for the improvement in the mechanical properties. It has been reported that the microstructures and mechanical properties of Mg2 Si reinforced Mg- and Al-based alloys can be improved by the application of advanced processing techniques such as hot extrusion [5], rapid solidification processing [6,7] and mechanical alloying [8,9]. How-
∗ Corresponding author at: Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China. Tel.: +86 531 88395414; fax: +86 531 88395414. E-mail address: xfl[email protected] (X. Liu). 0921-5093/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2008.07.034
ever, these processing methods may lead to the increase of the production cost. Recently, more practical methods to refine and modify primary Mg2 Si are additions of rare earth [10–12], Al–Sr master alloys [13,14], sodium salt [15] and phosphorus [16]. The effects of K2 TiF6 and KBF4 on primary Mg2 Si have been also investigated [17,18]. However, some of them exist internal defects, for example, sodium salt [15] addition can change the morphology and size, but the amount of addition is too high (10 wt.%); besides, potassium fluotitanate [17] addition only decreases the size of primary Mg2 Si and can not change its morphology. The refinement and modification mechanism of Ti or B elements did not carry out [17,18]. In the article, the nucleation mechanism of Ti and B on primary Mg2 Si in Al–Mg–Si alloys was investigated. In addition, the combination effect of Al–Ti–B and Al–P master alloys on Mg2 Si was discussed.
2. Experimental procedures Commercial pure Al (99.7%, all compositions quoted in this work are in wt.% unless otherwise stated), commercial pure crystalline Si (99.9%) and commercial pure Mg (99.8%) were used as starting materials to prepare Al–12.67% Mg–10.33% Si alloy (named Al–12.67Mg–10.33Si alloy below) in a 25 kW medium frequency induction furnace. Different refinement and modification experiments were conducted in clay-bonded graphite crucibles in a 5 kW electric resistant-heating furnace.
C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
Three groups of samples were carried out in present work, and were henceforth designated as group 1, group 2 and group 3, respectively. Samples in group 1 were fabricated under following condition: the Al–12.67Mg–10.33Si alloy was remelted at 850 ◦ C and held at this temperature, then Al–5Ti–1B master alloy was added to the melt with 0, 0.2 and 1% respectively. After holding 10 min, the melt was poured into a cast iron mold and tensile test bars were obtained. In order to investigate the combination effect of Al–P and Al–Ti–B master alloys on primary Mg2 Si, Al–12.67Mg–10.33Si melt was added with 1% Al–3P for 30 min, part of the melt was poured and Sample-2 was obtained. Then, 0.2% Al–5Ti–1B master alloy was added into the Al–3P refined and modified Al–12.67Mg–10.33Si melt and holding 10 min. The melt was poured and Sample-3 was obtained. Metallographic specimens were cut at the same position of the tabulate samples and polished through standard procedure. Structure and qualitative analysis were conducted by using high scope video microscope (HSVM) (model KH-2200, Japan), electron probe micro-analyzer (EPMA) (model JXA-8840, Japan) and transmission electron microscope (TEM) (model H-800, Japan). The tensile test bars were machined to ‘dog-bone’ type specimens (shown in Fig. 1) and then tensile-tested at room temperature by universal testing machine (model CMT700, China). 3. Results and discussion 3.1. Effect of Al–5Ti–1B on the primary Mg2 Si Fig. 2 shows the change of primary Mg2 Si phase with different amounts of Al–5Ti–1B master alloy in the Al–12.67Mg–10.33Si alloy. The addition of 0.2% Al–5Ti–1B master alloy leads to the size of Mg2 Si decreasing, but its morphology being still dendritic,
433
Fig. 1. The specimen used for tensile strength testing at room temperature.
compared with the microstructure of the alloy without Al–5Ti–1B addition. The size of primary Mg2 Si decreases from ∼100 to ∼20 m with the addition of 1% Al–5Ti–1B, and the morphology changes from dendritic to polygonal shape. So it is suggested that Al–5Ti–1B master alloy is an efficient grain refiner for Al–Mg–Si alloys. Fig. 3 presents the EPMA of a primary Mg2 Si nucleus in Al–12.67Mg–10.33Si alloy with the addition of 1% Al–5Ti–1B master alloy and holding 10 min. The X-ray images show that the center of Mg2 Si contains Ti and B elements. In order to further analyze its component, the composition along the line M–N across the primary Mg2 Si in Fig. 3(a) is illustrated in Fig. 4. The peak of B overlaps the Ti peak. It is indicated that it may be TiB2 compound. In order to further confirm whether it is TiB2 or not, the TEM analysis of a primary Mg2 Si particle is illustrated in Fig. 5. It is concluded that there are TiB2 particles in the center of primary Mg2 Si. Also it is interesting to note that sometimes several TiB2 particles rather one act as the nuclei of the primary Mg2 Si, as shown in Fig. 5. TiB2 is hexagonal with the lattice parameters: a = 3.03 Å, c = 3.23 Å [19,20], and its crystal structure can be illustrated in Fig. 6. The Ti atoms, shown in black, locate on the corner angles of hexagon and in the center of the back surface, while the B atoms, shown in
Fig. 2. Microstructures of Al–12.67Mg–10.33Si alloy before and after the addition of Al–5Ti–1B master alloy: (a) without Al–5Ti–1B; (b) adding 0.2% Al–5Ti–1B; (c) adding 1% Al–5Ti–1B.
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C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
Fig. 3. EPMA analysis of a primary Mg2 Si: (a) BEI of the primary Mg2 Si; (b–f) the X-ray images for respective elements, Al, Mg, Si, Ti and B.
white, distribute in the center of the triangular prism composed of the Ti atoms. Therefore, the Ti atomic plane alternates with the B atomic plane to form a two-dimensional planar netted structure. Mg2 Si is a fluorite-type structure with the lattice parameter: a = 6.39 Å (Fig. 7). There are 12 atoms in the Mg2 Si cell. The Si atoms locate on the corners and surface centers of the face-centered cubic, and the Mg atoms are in the eight tetrahedral interstices of the crystal cell. One phase can act as a fine heterogeneous nucleating site for the other phase, when there is a good coherent relationship existing on the interface of two types of phases [21]. The interatomic distances of the crystal faces of the two phases should be close to each other, and the atomic arrangements of the crystal faces should be similar. It is necessary to point out that there should be some similar interplanar distances when the interface of two phases has a good coherent relationship. From Table 1, several possible coherent interfaces between TiB2 and Mg2 Si can be obtained, and the disregistry is less than 5%. Generally the disregistry ı between the substrate phase and the crystallization phase is calculated by the following
Turnbull–Vonnegut equation: ı=
|as − ac | × 100% ac
(1)
where as and ac are the interatomic/interplanar distances of substrate plane and crystallization plane without deformation, respectively. It is not applicable to crystallographic combinations of two phases with planes of different atomic arrangements, because of the inherent limitation of the Turnbull–Vonnegut equation. So it is necessary to adjust the equation in terms of the angular difference between the crystallographic directions within the planes [22]. The adjustment equation is expressed in the following form: k l)s ı(h = (h k l)
3 |(d[u v w]i cos ) − d[u v w]i )|/d[u v w]i s
n
3
n
n
× 100%
(2)
i=1
where (h k l)s is a low-index plane of the substrate, [u v w]s a lowindex direction in (h k l)s , (h k l)n a low-index plane of the nucleated
C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
435
Fig. 4. The distribution of chemical composition along the line from M to N in Fig. 3(a).
Fig. 7. Crystal structure of Mg2 Si.
of TiB2 and in the (2 0 0) crystal face of Mg2 Si. The shaded circles represent the Ti atoms in the (0 0 1) crystal face of TiB2 , while broken-line circles represent Si atoms in the (2 0 0) crystal face of Mg2 Si. The required parameters for Eq. (2) are listed in Table 2: Fig. 5. TEM image of center of a primary Mg2 Si.
solid, [u v w]n a low-index direction in (h k l)n , d[u v w]s the interatomic distance along [u v w]s , d[u v w]n the interatomic distance along [u v w]n , and is the angle between the [u v w]s and [u v w]n . The number ‘1’ is taken as an example to research the atomic arrangement of the crystal plane, as shown in Table 1. Fig. 8 shows the typical planar atomic arrangements in the (0 0 1) crystal face
0 1)TiB ı(0 2 (2 0 0)
Mg2 Si
=
(|6.06 − 6.39|/6.39) + (|21 − 19.71|/19.71) + (|8.84 − 9.04|/9.04) 3 × 100% ≈ 4.64%
This indicates that TiB2 compound is a good nucleating site for Mg2 Si compound, and Mg2 Si can nucleate on the TiB2 nucleation substrates.
Table 1 Possible coherent interfaces of TiB2 and Mg2 Si crystals Number
Fig. 6. Crystal structure of TiB2 .
1 2 3 4 5 6
TiB2
Mg2 Si
ı (%)
d (Å)
(h k l)
d (Å)
(h k l)
3.2295 1.6145 1.5153 1.3751 1.2156 1.1049
001 002 110 102 201 112
3.1955 1.5977 1.4662 1.4290 1.2299 1.1297
200 400 331 420 333 440
1.06 1.05 3.35 3.77 1.16 2.20
436
C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
Table 2 Parameters for Eq. (2) Case
d[u v w]s
d[u v w]n
(◦ )
d[u v w]s cos
(0 0 1)TiB2 ||(2 0 0)Mg2 Si
6.06 21 9.09
6.39 19.71 9.04
0 0 15
6.06 21 8.84
Fig. 9. Relation between ultimate tensile strength of Al–12.67Mg–10.33Si alloy and adding level of Al–5Ti–1B.
3.3. Effects of combined additions of Al–5Ti–1B and Al–3P
Fig. 8. The crystallographic relationship at the interface between the (0 0 1) of TiB2 and the (2 0 0) of Mg2 Si.
3.2. The tensile strength change of Al–12.67Mg–10.33Si alloy at room temperature It is evident from Fig. 9 that TiB2 refinement and modification raises the ultimate tensile strength of the alloy and there is an increasing trend in the strength of the alloy with the increase of the addition of Al–5Ti–1B. After addition of Al–5Ti–1B master alloy into the Al–Mg–Si alloy, the primary Mg2 Si changes from coarse dendritic to fine polygonal shape (shown in Fig. 2). The improvement of strength is attributed to the fine and uniform distribution of primary Mg2 Si particles. The fine primary Mg2 Si dose not separate aluminum matrix seriously and tensile strength of the alloy is improved.
Fig. 10 shows the microstructures of Al–12.67Mg–10.33Si alloy before and after the addition of Al–3P or (and) Al–5Ti–1B master alloys. The effect of Al–P master alloy on the primary Mg2 Si in Al–Mg–Si alloys has been investigated in previous work [16]. It has been found that Al–P master alloy can refine and modify primary Mg2 Si. But it cannot obtain favorable effect with the addition of 1% Al–3P master alloy. The size of primary Mg2 Si does not decrease obviously, and the morphology of primary Mg2 Si is still dendritic, as shown in Fig. 10(a). But after the addition of 0.2% Al–5Ti–1B, the size of primary Mg2 Si decreases significantly, as shown in Fig. 10(b). This indicates that the addition of Al–5Ti–1B master alloy can significantly improve the refinement and modification effect of AlP on primary Mg2 Si in Al–12.67Mg–10.33Si alloy. Yu et al. [19] reported that the addition of Al–5Ti–1B master alloy could significantly improve the phosphorous modification effect on primary Si, in the Al–P modified Al–Si alloy. And it is reported that TiB2 has a good lattice matching coherence relationship with AlP, for example, the interfacial relationship (1 1 2)TiB2 ||(4 4 2)Alp possesses a low lattice misfit (about 7.27%). In this study, it was also found the phenomenon of coupling particles of AlP and TiB2 in Al–Mg–Si alloys. EPMA analysis of a primary Mg2 Si nucleus is illustrated in Fig. 11, the black phase is AlP, and the light phase is TiB2 . TiB2 has strong absorbability to AlP, so AlP crystals can easily nucleate on TiB2 surfaces and form quantities of coupling com-
Fig. 10. Microstructures of Al–12.67Mg–10.33Si alloy before and after the addition of Al–3P or (and) Al–5Ti–1B master alloys: (a) Al–12.67Mg–10.33Si + 1% Al–3P; (b) Al–12.67Mg–10.33Si + 1% Al–3P + 0.2% Al–5Ti–1B.
C. Li et al. / Materials Science and Engineering A 497 (2008) 432–437
437
Fig. 11. Line distribution of chemical composition along the line across the primary Mg2 Si: (a) line E–F across the primary Mg2 Si; (b) chemical composition distribution along line E–F.
pounds. These compounds can also act as heterogeneous nuclei for the primary Mg2 Si. The quantity of primary Mg2 Si increases, and accordingly the size decreases.
Project of Science and Technology Research of Ministry of Education of China (no. 106103) and “Taishan Scholar” Construction Project for financial support of Shandong Province in China.
4. Conclusions
References
(1) With the addition of 1% Al–5Ti–1B master alloy into the Al–Mg–Si alloy, the morphologies of primary Mg2 Si particulates change from dendritic to polygonal shape, and their average sizes decrease from ∼100 to ∼20 m. Due to the structure improvement, the ultimate tensile strength of the alloy increases. (2) EPMA and TEM results show that TiB2 particles can act as the nuclei of the primary Mg2 Si. Besides, crystal lattice correspondence indicates TiB2 has a good lattice matching coherence relationship with Mg2 Si, and the disregistry is only 4.64%. (3) The addition of Al–5Ti–1B master alloy can significantly improve the refinement and modification effect of AlP on primary Mg2 Si in Al–12.67Mg–10.33Si alloy. In addition, it is found that coupling compounds of AlP and TiB2 exist in the center of primary Mg2 Si.
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Acknowledgments This work was supported by a grant from National Science Fund for Distinguished Young Scholars of China (no. 50625101), Key