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Refinement and modification performance of Al–P master alloy on primary Mg2Si in Al–Mg–Si alloys
Journal of Alloys and Compounds 465 (2008) 145–150
Refinement and modification performance of Al–P master alloy on primary Mg2Si in Al–Mg–Si alloys Chong Li, Xiangfa Liu ∗ , Yuying Wu Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, 73 Jingshi Road, Jinan 250061, PR China Received 30 August 2007; received in revised form 22 October 2007; accepted 23 October 2007 Available online 4 November 2007
Abstract Refinement and modification performance of Al–3P master alloy on primary Mg2 Si in Al–12.67Mg–10.33Si alloys were investigated in this paper. The experimental results show that the perfect effect can be obtained after the addition of 3% Al–3P master alloy into the Al–Mg–Si alloys. The morphologies of primary Mg2 Si particulates change from dendritic to polygonal shape, and their average sizes decrease from ∼100 to ∼20 m. Further, the ultimate tensile strength increases from 252 to 275 MPa. Also, the melt treating technological parameters were established in this paper, treating temperature is 850 ◦ C, holding time is 30 min. EPMA results show that AlP particles act as the nuclei of primary Mg2 Si. © 2007 Elsevier B.V. All rights reserved. Keywords: Al–Mg–Si alloys; Primary Mg2 Si; Refinement; Modification; Al–P master alloy
1. Introduction The characteristic properties of Al–Mg–Si alloys, like high specific strength, high specific modulus, heat-resistant, fatigueresistant and excellent physical properties make them possess considerable potential market and application foreground [1,2]. Al–Mg–Si alloys can be used to product cylinder heads, cylinder frames, pistons, brake disks and so on. However, in normal cast Al–Mg–Si alloys, primary Mg2 Si is usually very coarse. It seriously separates aluminum matrix and leads to stress concentration at sharp edges and corners of Mg2 Si. Therefore, to refine and modify primary Mg2 Si seems to be important when facing further application demand. Recently, it was reported that the effective measures for Mg2 Si are additions of refiners or modifiers, such as rare earth [3,4], potassium fluotitanate [5], Al–Sr master alloys [6,7], sodium salt [8] and phosphorus [9]. However, some of them exist internal defects, for example, potassium fluotitanate [5] addition only decreases the size of primary Mg2 Si and cannot change its morphology; besides, sodium salt [8] addition can change the morphology and size, but the amount of addition
∗
Corresponding author. Tel.: +86 531 88395414; fax: +86 531 88395414. E-mail address: [email protected] (X. Liu).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.10.111
is too high (10 wt.%). Al–P master alloy applied in this article is an efficient grain refiner for Al–Mg–Si alloys. It can show stabilizing effect with the addition of small amount. 2. Experimental procedures The Al–12.67Mg–10.33Si alloys used in the experiments were produced by a 25 kW medium frequency induction furnace using 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%). Different refinement and modification experiments were conducted in clay-bonded graphite crucibles in a 5 kW electric resistant-heating furnace. The first group of experiments is to treat Al–12.67Mg–10.33Si alloys with different adding levels of Al–3P master alloy at the same temperature. The alloys were remelted at 850 ◦ C and held at this temperature. Then, Al–3P master alloy was added to the melts with 0, 1, 2, 3 and 4% Al–3P master alloy, respectively. After holding 30 min, the melts were poured into a cast iron mold and tensile test bars were obtained. The second group of experiments is to refine the alloy with 3% Al–3P master alloy at different temperatures 750, 800 and 850 ◦ C, respectively. Three samples were taken after holding 30 min. Then, in order to confirm holding time, the alloys were remelted at 850 ◦ C and held at this temperature until they melted completely. Samples were obtained after 10, 30 and 60 min after the addition of 3% Al–3P master alloy, respectively. All the samples were poured into the same type of cast iron mold preheated at about 150 ◦ C before pouring. Metallographic specimens were cut at the same position of the tabulate samples and polished using standard routines. The microstructure analysis was
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C. Li et al. / Journal of Alloys and Compounds 465 (2008) 145–150
conducted by using a high scope video microscope (HSVM) (model KH-2200, Japan). The tensile test bars were heated at 520 ◦ C for 3 h and water-quenched to room temperature. Then, they were artificially aged at 160 ◦ C for 6 h. After that, the tensile strength at room temperature was determined by a universal testing machine (model CMT700, China). The nucleation mechanism of primary Mg2 Si was studied by electron probe micro-analyzer (EPMA) (model JXA-8840, Japan).
3. Results and discussion 3.1. The melt treating technological parameters of Al–3P master alloy 3.1.1. The addition amount of Al–3P mater alloy Fig. 1 shows the microstructures of Al–12.67Mg–10.33Si alloys before and after the addition of Al–3P master alloy. There are a few primary Mg2 Si grains in Al–12.67Mg–10.33Si alloys, and they are coarse dendritic and their distributions are uneven,
as shown in Fig. 1a. They seriously separate aluminum matrix and lead to alloys become brittle. After the addition of 1% Al–3P master alloy, although the size of primary Mg2 Si decreases, its morphology is still dendritic (Fig. 1b). It is interesting to note that with the increasing of Al–3P addition beyond a certain limit, not only the size of primary Mg2 Si changes, but also the morphology changes, namely, the morphology changes from dendritic to polygonal shape. The favorable effect is obtained with addition of 3% Al–3P master alloy. The grains of primary Mg2 Si are very fine and uniformly distributed. Its average size is about 20 m or less. But when the Al–3P content adds up to 4%, in contrast with Fig. 1d, the primary Mg2 Si has no apparent change, as shown in Fig. 1e. 3.1.2. Melt treating temperature Melt treating temperature is an important factor for the refinement and modification effect of Al–3P master alloy. If the temperature is too low, the Al–3P cannot bring refining and
Fig. 1. Microstructures of Al–12.67Mg–10.33Si alloys before and after the addition of Al–3P master alloy: (a) without Al–3P; (b) adding 1% Al–3P; (c) adding 2% Al–3P; (d) adding 3% Al–3P; (e) adding 4% Al–3P.
C. Li et al. / Journal of Alloys and Compounds 465 (2008) 145–150
Fig. 2. Variation of average size of primary Mg2 Si in Al–12.67Mg–10.33Si alloys with temperature.
modifying effect into full play. On the contrary, over-high temperature increases suction gas and oxidation of the treated alloys. Fig. 2 presents the size change law of primary Mg2 Si at different treating temperature conditions with the addition of 3% Al–3P master alloy and holding 30 min. It can be seen that the effect of Al–3P is not satisfying at 750 ◦ C. When the temperature adds up to 800 ◦ C, the size of primary Mg2 Si decreases obviously and is about 35 m. With further increase of temperature, the effect of Al–3P increases. The average size of primary Mg2 Si decreases to 20 m, when treatment temperature is 850 ◦ C. 3.1.3. Holding time Fig. 3a shows the microstructure of Al–12.67Mg–10.33Si alloys holding 10 min after the addition of 3% Al–3P master alloy. It is clear that primary Mg2 Si is coarse, compared with Fig. 1d. Based on the above experimental results, it gets favorable effect after holding 30 min under the condition of all other state being the same. The refining and modifying effect is still perfect, when holding time increases to 60 min, as shown in Fig. 3b. 3.2. The tensile strength change of Al–12.67Mg–10.33Si alloys at room temperature It is evident from Fig. 4 that the tensile strength of Al–12.67Mg–10.33Si alloys is low, about 252 MPa without
147
Fig. 4. Relation between ultimate tensile strength of Al–12.67Mg–10.33Si alloys and adding level of Al–3P.
Al–3P master alloy. With the increase of the addition of Al–3P, the tensile strength of the alloys increases. When the Al–3P content is 3%, the tensile strength is improved to 275 MPa. The main reason is that the primary Mg2 Si changes from coarse dendritic to fine polygonal shape. Consequently it dose not separate aluminum matrix seriously and its tensile strength is improved. 3.3. The refinement and modification mechanism of Al–3P master alloy Al–3P master alloy can refine and modify primary Mg2 Si of Al–12.67Mg–10.33Si alloys effectively. It indicates that Al–3P plays an important influence on the process of nucleation and growth of primary Mg2 Si. It was reported that P could refine primary Mg2 Si, due to P acting with Mg in the melts to form Mg3 (PO4 )2 , Mg3 (PO4 )2 particles act as the nuclei of the primary Mg2 Si, increasing the quantity of Mg2 Si and decreasing the size [10]. This article argues that AlP not Mg3 (PO4 )2 acts as heterogeneous nucleating site for Mg2 Si. It is well known that AlP is a zinc-blende structure with lat˚ as shown in Fig. 5. The Al atoms, tice parameter: a = 5.42 A, shown in black, form a normal face-centered cubic, while the P atoms, shown in white, are distributed in the tetrahedral interstices of the crystal cell, with each P atom surrounded by four proximate Al atoms. Mg2 Si is a fluorite-type structure with lat˚ (Fig. 6). There are 12 atoms in Mg2 Si tice parameter: a = 6.39 A
Fig. 3. Microstructures of Al–12.67Mg–10.33Si after different holding times with 3%Al–3P: (a) holding 10 min; (b) holding 60 min.
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Fig. 5. Crystal structure of AlP.
Fig. 7. The crystallographic relationship at the interface between the (2 2 0) of AlP and the (3 1 1) of Mg2 Si.
Fig. 6. Crystal structure of Mg2 Si.
cell. Si atoms are located on the corners and surface centers of the face-centered cubic and Mg atoms are in the eight tetrahedral interstices of the crystal cell. When there is a good coherent relationship existing on the interface of two types of phase, one phase can act as fine heterogeneous nucleating site for the other phase [11]. The interatomic distance of the crystal face of the two phases should be close to each other, and the atomic arrangement 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, it is clear that several possible coherent interfaces between AlP and Mg2 Si can be obtained, and the disregistry is less than 5%. Generally the disregistry δ between the substrate phase and the crystallization phase is calculated by the following Turnbull–Vonnegut equation: δ=
|as − ac | × 100% ac
(1)
Table 1 Possible coherent interface of AlP and Mg2 Si crystals Number
1 2 3 4 5 6
AlP
Mg2 Si
where as and ac are the interatomic/interplanar distance of substrate plane and crystallization plane without deformation, respectively. Due to the inherent limitation of the Turnbull–Vonnegut equation, it is not applicable to crystallographic combinations of two phases with planes of different atomic arrangements. So it is necessary to adjust the equation in terms of the angular difference between the crystallographic directions within the planes [12]. The adjustment equation is expressed in the follwing form: (h k l)s δ(h k l)n =
3 |(d [u v w]is cos θ) − d[u v w]in |/d[u v w]in
3
i=1
× 100% (2)
where (h k l)s is a low-index plane of the substrate, [u v w]s is a low-index direction in (h k l)s , (h k l)n is a low-index plane of the nucleated solid, [u v w]n is a low-index direction in (h k l)n , d[u v w]s is the interatomic distance along [u v w]s , d[u v w]n is the interatomic distance along [u v w]n and θ is the angle between the [u v w]s and [u v w]n . Eq. (2) is taken as an example to research the atomic arrangement of the crystal plane, as shown in Table 1. Fig. 7 shows the typical planar atomic arrangement in the (3 1 1) crystal face of Mg2 Si and in the (2 2 0) crystal face of AlP. The shaded circles represent the P atoms in the (2 2 0) crystal face of AlP, while broken-line circles represent Si atoms in the (3 1 1) crys-
δ (%)
˚ d (A)
(h k l)
˚ d (A)
(h k l)
3.1292 1.9162 1.5646 1.3550 1.2434 1.1063
111 220 222 400 331 422
3.1955 1.9269 1.5977 1.3045 1.2299 1.1297
200 311 400 422 330 440
2.07 0.56 2.07 3.87 1.1 2.07
Table 2 Parameters for Eq. (2) Case
d[u v w]s
d[u v w]n
θ (◦ )
d[u v w]s cos θ
(2 2 0)AlP||(3 1 1) Mg2 Si
8.13 23.00 10.84
9.04 21.77 10.10
0 24.8 28.32
8.13 20.87 9.54
C. Li et al. / Journal of Alloys and Compounds 465 (2008) 145–150
149
Fig. 8. EPMA analysis of a primary Mg2 Si: (a) SEI of the primary Mg2 Si; (b–f) the X-ray images for respective elements, Al, Mg, Si, P and O.
Fig. 9. Line distribution of chemical composition along the line across the primary Mg2 Si: (a) line M and N across the primary Mg2 Si; (b) chemical composition distribution along line M and N.
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C. Li et al. / Journal of Alloys and Compounds 465 (2008) 145–150
tal face of Mg2 Si. The required parameters for Eq. (2) are listed in Table 2.
(2 2 0)AlP
δ(3 1 1)Mg
2 Si
(|8.13 − 9.04|)/9.04 + (|20.87 − 21.77|)/ 21.77 + (|9.54 − 10.10|)/10.10 = 3 ×100% ≈ 6.58%
effect will be achieved by treating the melt at 850 ◦ C and holding 30 min after addition of Al–3P master alloy. (3) Crystal lattice correspondence indicates AlP has a good lattice matching coherence relationship with Mg2 Si, and the disregistry is only 6.58%, so Mg2 Si may nucleate on AlP surface. Besides, EPMA results show that AlP particles can act as the nuclei of the primary Mg2 Si. Acknowledgements
This indicates that AlP compound is a good nucleating site for Mg2 Si compound and Mg2 Si can easily nucleate. Fig. 8 presents the EPMA of a primary Mg2 Si nucleus in Al–12.67Mg–10.33Si alloys with the addition of 3% Al–3P master alloy and holding 30 min. The X-ray images show that the core of Mg2 Si contains Al and P elements, indicating that it maybe AlP compound. In order to further confirm whether it is AlP or not, the composition along the line M and N across the primary Mg2 Si in Fig. 9a is illustrated in Fig. 9b. The P exhibits a peak corresponds to the Al peak. It can be concluded that there is AlP compound in the nucleus of the primary Mg2 Si. AlP can act as nucleating sites of the primary Mg2 Si during solidification. 4. Conclusions (1) With 3% Al–3P master alloy to Al–12.67Mg–10.33Si alloy, it can be obtained favorable refining and modifying effect, the average size of primary Mg2 Si decreasing from ∼100 to ∼20 m, the morphology changing from dendritic to polygonal shape. Due to the structure improvement, the ultimate tensile strength of the alloy increases from 252 to 275 MPa. (2) Both treating temperature and holding time are main factors influencing refinement and modification. A satisfactory
This work was supported by National Science Fund for Distinguished Young Scholars (No. 50625101), Key Project of Science and Technology Research of Ministry of Education of China (No. 106103). References [1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12]
J. Liu, Aluminum Fabrication 164 (2005) 1. M. Ma, j. Bo, Adv. Mater. Ind. 6 (2006) 37. X. Ai, Y. Li, Foundry 54 (3) (2005) 238. J. Zhang, Z. Fan, Y.Q. Wang, B.L. Zhou, Mater. Sci. Eng. A 281 (2000) 104. Y.G. Zhao, Q.D. Qin, Y.Q. Zhao, Y.H. Liang, Q.C. Jiang, Mater. Lett. 58 (2004) 2192. H. Liao, Y. Ding, G. Sun, Foundry 51 (2) (2002) 80. Y.G. Zhao, Q.D. Qin, Y.H. Liang, W. Zhou, Q.C. Jiang, J. Mater. Sci. 40 (2005) 1831. J. Zhang, Y.Q. Wang, B. Yang, B.L. Zhou, J. Mater. Res. 14 (1999) 68. Q.D. Qin, Y.G. Zhao, W. Zhou, P.J. Cong, Mater. Sci. Eng. A 447 (2007) 186. E.E. Schmid, K. Olderburg, G. Frommeyer, Z. Metallkde 81 (1990) 809. J.M. Rigsbee, H.I. Aaronson, Acta Metall. Mater. 27 (1979) 351. L.B. Bruce, Metall. Trans. 1 (1970) 1987.
Refinement and modification performance of Al–P master alloy on primary Mg2Si in Al–Mg–Si alloys Chong Li, Xiangfa Liu ∗ , Yuying Wu Key Laboratory of Liquid Structure and Heredity of Materials, Ministry of Education, Shandong University, 73 Jingshi Road, Jinan 250061, PR China Received 30 August 2007; received in revised form 22 October 2007; accepted 23 October 2007 Available online 4 November 2007
Abstract Refinement and modification performance of Al–3P master alloy on primary Mg2 Si in Al–12.67Mg–10.33Si alloys were investigated in this paper. The experimental results show that the perfect effect can be obtained after the addition of 3% Al–3P master alloy into the Al–Mg–Si alloys. The morphologies of primary Mg2 Si particulates change from dendritic to polygonal shape, and their average sizes decrease from ∼100 to ∼20 m. Further, the ultimate tensile strength increases from 252 to 275 MPa. Also, the melt treating technological parameters were established in this paper, treating temperature is 850 ◦ C, holding time is 30 min. EPMA results show that AlP particles act as the nuclei of primary Mg2 Si. © 2007 Elsevier B.V. All rights reserved. Keywords: Al–Mg–Si alloys; Primary Mg2 Si; Refinement; Modification; Al–P master alloy
1. Introduction The characteristic properties of Al–Mg–Si alloys, like high specific strength, high specific modulus, heat-resistant, fatigueresistant and excellent physical properties make them possess considerable potential market and application foreground [1,2]. Al–Mg–Si alloys can be used to product cylinder heads, cylinder frames, pistons, brake disks and so on. However, in normal cast Al–Mg–Si alloys, primary Mg2 Si is usually very coarse. It seriously separates aluminum matrix and leads to stress concentration at sharp edges and corners of Mg2 Si. Therefore, to refine and modify primary Mg2 Si seems to be important when facing further application demand. Recently, it was reported that the effective measures for Mg2 Si are additions of refiners or modifiers, such as rare earth [3,4], potassium fluotitanate [5], Al–Sr master alloys [6,7], sodium salt [8] and phosphorus [9]. However, some of them exist internal defects, for example, potassium fluotitanate [5] addition only decreases the size of primary Mg2 Si and cannot change its morphology; besides, sodium salt [8] addition can change the morphology and size, but the amount of addition
∗
Corresponding author. Tel.: +86 531 88395414; fax: +86 531 88395414. E-mail address: [email protected] (X. Liu).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.10.111
is too high (10 wt.%). Al–P master alloy applied in this article is an efficient grain refiner for Al–Mg–Si alloys. It can show stabilizing effect with the addition of small amount. 2. Experimental procedures The Al–12.67Mg–10.33Si alloys used in the experiments were produced by a 25 kW medium frequency induction furnace using 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%). Different refinement and modification experiments were conducted in clay-bonded graphite crucibles in a 5 kW electric resistant-heating furnace. The first group of experiments is to treat Al–12.67Mg–10.33Si alloys with different adding levels of Al–3P master alloy at the same temperature. The alloys were remelted at 850 ◦ C and held at this temperature. Then, Al–3P master alloy was added to the melts with 0, 1, 2, 3 and 4% Al–3P master alloy, respectively. After holding 30 min, the melts were poured into a cast iron mold and tensile test bars were obtained. The second group of experiments is to refine the alloy with 3% Al–3P master alloy at different temperatures 750, 800 and 850 ◦ C, respectively. Three samples were taken after holding 30 min. Then, in order to confirm holding time, the alloys were remelted at 850 ◦ C and held at this temperature until they melted completely. Samples were obtained after 10, 30 and 60 min after the addition of 3% Al–3P master alloy, respectively. All the samples were poured into the same type of cast iron mold preheated at about 150 ◦ C before pouring. Metallographic specimens were cut at the same position of the tabulate samples and polished using standard routines. The microstructure analysis was
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C. Li et al. / Journal of Alloys and Compounds 465 (2008) 145–150
conducted by using a high scope video microscope (HSVM) (model KH-2200, Japan). The tensile test bars were heated at 520 ◦ C for 3 h and water-quenched to room temperature. Then, they were artificially aged at 160 ◦ C for 6 h. After that, the tensile strength at room temperature was determined by a universal testing machine (model CMT700, China). The nucleation mechanism of primary Mg2 Si was studied by electron probe micro-analyzer (EPMA) (model JXA-8840, Japan).
3. Results and discussion 3.1. The melt treating technological parameters of Al–3P master alloy 3.1.1. The addition amount of Al–3P mater alloy Fig. 1 shows the microstructures of Al–12.67Mg–10.33Si alloys before and after the addition of Al–3P master alloy. There are a few primary Mg2 Si grains in Al–12.67Mg–10.33Si alloys, and they are coarse dendritic and their distributions are uneven,
as shown in Fig. 1a. They seriously separate aluminum matrix and lead to alloys become brittle. After the addition of 1% Al–3P master alloy, although the size of primary Mg2 Si decreases, its morphology is still dendritic (Fig. 1b). It is interesting to note that with the increasing of Al–3P addition beyond a certain limit, not only the size of primary Mg2 Si changes, but also the morphology changes, namely, the morphology changes from dendritic to polygonal shape. The favorable effect is obtained with addition of 3% Al–3P master alloy. The grains of primary Mg2 Si are very fine and uniformly distributed. Its average size is about 20 m or less. But when the Al–3P content adds up to 4%, in contrast with Fig. 1d, the primary Mg2 Si has no apparent change, as shown in Fig. 1e. 3.1.2. Melt treating temperature Melt treating temperature is an important factor for the refinement and modification effect of Al–3P master alloy. If the temperature is too low, the Al–3P cannot bring refining and
Fig. 1. Microstructures of Al–12.67Mg–10.33Si alloys before and after the addition of Al–3P master alloy: (a) without Al–3P; (b) adding 1% Al–3P; (c) adding 2% Al–3P; (d) adding 3% Al–3P; (e) adding 4% Al–3P.
C. Li et al. / Journal of Alloys and Compounds 465 (2008) 145–150
Fig. 2. Variation of average size of primary Mg2 Si in Al–12.67Mg–10.33Si alloys with temperature.
modifying effect into full play. On the contrary, over-high temperature increases suction gas and oxidation of the treated alloys. Fig. 2 presents the size change law of primary Mg2 Si at different treating temperature conditions with the addition of 3% Al–3P master alloy and holding 30 min. It can be seen that the effect of Al–3P is not satisfying at 750 ◦ C. When the temperature adds up to 800 ◦ C, the size of primary Mg2 Si decreases obviously and is about 35 m. With further increase of temperature, the effect of Al–3P increases. The average size of primary Mg2 Si decreases to 20 m, when treatment temperature is 850 ◦ C. 3.1.3. Holding time Fig. 3a shows the microstructure of Al–12.67Mg–10.33Si alloys holding 10 min after the addition of 3% Al–3P master alloy. It is clear that primary Mg2 Si is coarse, compared with Fig. 1d. Based on the above experimental results, it gets favorable effect after holding 30 min under the condition of all other state being the same. The refining and modifying effect is still perfect, when holding time increases to 60 min, as shown in Fig. 3b. 3.2. The tensile strength change of Al–12.67Mg–10.33Si alloys at room temperature It is evident from Fig. 4 that the tensile strength of Al–12.67Mg–10.33Si alloys is low, about 252 MPa without
147
Fig. 4. Relation between ultimate tensile strength of Al–12.67Mg–10.33Si alloys and adding level of Al–3P.
Al–3P master alloy. With the increase of the addition of Al–3P, the tensile strength of the alloys increases. When the Al–3P content is 3%, the tensile strength is improved to 275 MPa. The main reason is that the primary Mg2 Si changes from coarse dendritic to fine polygonal shape. Consequently it dose not separate aluminum matrix seriously and its tensile strength is improved. 3.3. The refinement and modification mechanism of Al–3P master alloy Al–3P master alloy can refine and modify primary Mg2 Si of Al–12.67Mg–10.33Si alloys effectively. It indicates that Al–3P plays an important influence on the process of nucleation and growth of primary Mg2 Si. It was reported that P could refine primary Mg2 Si, due to P acting with Mg in the melts to form Mg3 (PO4 )2 , Mg3 (PO4 )2 particles act as the nuclei of the primary Mg2 Si, increasing the quantity of Mg2 Si and decreasing the size [10]. This article argues that AlP not Mg3 (PO4 )2 acts as heterogeneous nucleating site for Mg2 Si. It is well known that AlP is a zinc-blende structure with lat˚ as shown in Fig. 5. The Al atoms, tice parameter: a = 5.42 A, shown in black, form a normal face-centered cubic, while the P atoms, shown in white, are distributed in the tetrahedral interstices of the crystal cell, with each P atom surrounded by four proximate Al atoms. Mg2 Si is a fluorite-type structure with lat˚ (Fig. 6). There are 12 atoms in Mg2 Si tice parameter: a = 6.39 A
Fig. 3. Microstructures of Al–12.67Mg–10.33Si after different holding times with 3%Al–3P: (a) holding 10 min; (b) holding 60 min.
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Fig. 5. Crystal structure of AlP.
Fig. 7. The crystallographic relationship at the interface between the (2 2 0) of AlP and the (3 1 1) of Mg2 Si.
Fig. 6. Crystal structure of Mg2 Si.
cell. Si atoms are located on the corners and surface centers of the face-centered cubic and Mg atoms are in the eight tetrahedral interstices of the crystal cell. When there is a good coherent relationship existing on the interface of two types of phase, one phase can act as fine heterogeneous nucleating site for the other phase [11]. The interatomic distance of the crystal face of the two phases should be close to each other, and the atomic arrangement 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, it is clear that several possible coherent interfaces between AlP and Mg2 Si can be obtained, and the disregistry is less than 5%. Generally the disregistry δ between the substrate phase and the crystallization phase is calculated by the following Turnbull–Vonnegut equation: δ=
|as − ac | × 100% ac
(1)
Table 1 Possible coherent interface of AlP and Mg2 Si crystals Number
1 2 3 4 5 6
AlP
Mg2 Si
where as and ac are the interatomic/interplanar distance of substrate plane and crystallization plane without deformation, respectively. Due to the inherent limitation of the Turnbull–Vonnegut equation, it is not applicable to crystallographic combinations of two phases with planes of different atomic arrangements. So it is necessary to adjust the equation in terms of the angular difference between the crystallographic directions within the planes [12]. The adjustment equation is expressed in the follwing form: (h k l)s δ(h k l)n =
3 |(d [u v w]is cos θ) − d[u v w]in |/d[u v w]in
3
i=1
× 100% (2)
where (h k l)s is a low-index plane of the substrate, [u v w]s is a low-index direction in (h k l)s , (h k l)n is a low-index plane of the nucleated solid, [u v w]n is a low-index direction in (h k l)n , d[u v w]s is the interatomic distance along [u v w]s , d[u v w]n is the interatomic distance along [u v w]n and θ is the angle between the [u v w]s and [u v w]n . Eq. (2) is taken as an example to research the atomic arrangement of the crystal plane, as shown in Table 1. Fig. 7 shows the typical planar atomic arrangement in the (3 1 1) crystal face of Mg2 Si and in the (2 2 0) crystal face of AlP. The shaded circles represent the P atoms in the (2 2 0) crystal face of AlP, while broken-line circles represent Si atoms in the (3 1 1) crys-
δ (%)
˚ d (A)
(h k l)
˚ d (A)
(h k l)
3.1292 1.9162 1.5646 1.3550 1.2434 1.1063
111 220 222 400 331 422
3.1955 1.9269 1.5977 1.3045 1.2299 1.1297
200 311 400 422 330 440
2.07 0.56 2.07 3.87 1.1 2.07
Table 2 Parameters for Eq. (2) Case
d[u v w]s
d[u v w]n
θ (◦ )
d[u v w]s cos θ
(2 2 0)AlP||(3 1 1) Mg2 Si
8.13 23.00 10.84
9.04 21.77 10.10
0 24.8 28.32
8.13 20.87 9.54
C. Li et al. / Journal of Alloys and Compounds 465 (2008) 145–150
149
Fig. 8. EPMA analysis of a primary Mg2 Si: (a) SEI of the primary Mg2 Si; (b–f) the X-ray images for respective elements, Al, Mg, Si, P and O.
Fig. 9. Line distribution of chemical composition along the line across the primary Mg2 Si: (a) line M and N across the primary Mg2 Si; (b) chemical composition distribution along line M and N.
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tal face of Mg2 Si. The required parameters for Eq. (2) are listed in Table 2.
(2 2 0)AlP
δ(3 1 1)Mg
2 Si
(|8.13 − 9.04|)/9.04 + (|20.87 − 21.77|)/ 21.77 + (|9.54 − 10.10|)/10.10 = 3 ×100% ≈ 6.58%
effect will be achieved by treating the melt at 850 ◦ C and holding 30 min after addition of Al–3P master alloy. (3) Crystal lattice correspondence indicates AlP has a good lattice matching coherence relationship with Mg2 Si, and the disregistry is only 6.58%, so Mg2 Si may nucleate on AlP surface. Besides, EPMA results show that AlP particles can act as the nuclei of the primary Mg2 Si. Acknowledgements
This indicates that AlP compound is a good nucleating site for Mg2 Si compound and Mg2 Si can easily nucleate. Fig. 8 presents the EPMA of a primary Mg2 Si nucleus in Al–12.67Mg–10.33Si alloys with the addition of 3% Al–3P master alloy and holding 30 min. The X-ray images show that the core of Mg2 Si contains Al and P elements, indicating that it maybe AlP compound. In order to further confirm whether it is AlP or not, the composition along the line M and N across the primary Mg2 Si in Fig. 9a is illustrated in Fig. 9b. The P exhibits a peak corresponds to the Al peak. It can be concluded that there is AlP compound in the nucleus of the primary Mg2 Si. AlP can act as nucleating sites of the primary Mg2 Si during solidification. 4. Conclusions (1) With 3% Al–3P master alloy to Al–12.67Mg–10.33Si alloy, it can be obtained favorable refining and modifying effect, the average size of primary Mg2 Si decreasing from ∼100 to ∼20 m, the morphology changing from dendritic to polygonal shape. Due to the structure improvement, the ultimate tensile strength of the alloy increases from 252 to 275 MPa. (2) Both treating temperature and holding time are main factors influencing refinement and modification. A satisfactory
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