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Grain refinement of Mg–Al based alloys by a new Al–C master alloy
Journal of Alloys and Compounds 467 (2009) 202–207
Grain refinement of Mg–Al based alloys by a new Al–C master alloy Han Guang, Liu Xiangfa ∗ , Ding Haimin Key Laboratory of Materials Liquid Structure and Heredity, Ministry of Education, Shandong University, Jinan 250061, China Received 21 October 2007; accepted 6 December 2007 Available online 15 December 2007
Abstract A new Al–C master alloy for grain refinement on Mg–Al based alloys has been fabricated by powder metallurgy. Al4 C3 is formed by reaction between part of f.c.c Al(C) solid solution and aluminum during fabrication, and the net-like mixture of Al4 C3 and Al(C) solid solution is located at the boundaries of aluminum particles. Milling and pressing of graphite and aluminum powders change the nature of graphite and ensure the kinetics conditions of the reaction between the two phases. Grain refinement experiment shows Al–1C master alloy can efficiently refine AZ31 (Mg–3%Al–1%Zn). At 760 ◦ C, when adding 2 wt.% Al–1C master alloy, the grain size is reduced sharply from 400 m to 140 m comparing to the sample added 2 wt.% commercial pure aluminum, and the grain morphology of ␣-Mg transits from a characteristic sixfold symmetrical shape to a petal-like shape. © 2007 Elsevier B.V. All rights reserved. Keywords: Metals and alloys; Powder metallurgy; Microstructure; Metallography; X-ray diffraction
1. Introduction Magnesium alloys, as lightweight structural materials, are receiving increased attention on replacing other metals in automobile industry. Grain refinement is an important method to improve the mechanical properties and workability of both cast and wrought magnesium alloys [1–6]. Mg–Al based alloys are the most common commercial magnesium alloys to which many grain refining methods have been applied, such as super-heating, Elfinal, carbon inoculation and addition of ceramic particles [1–13]. Among these methods, carbon inoculation is known to be the most effective for operating at a low temperature and less fading with long-time holding [1,2]. Amounts of carbon-containing agents such as C2 Cl6 [7], SiC [8], Al4 C3 [9] and carbon powder [14] have been reported to successfully refine Mg–Al based alloys. Among them, C2 Cl6 is the most useful grain refiner for commercial application; however, it cannot satisfy the industry demand for releasing toxic gas during refining. So a reliable, efficient and convenient commer-
∗
Corresponding author. Fax: +86 531 88395414. E-mail address: [email protected] (L. Xiangfa).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.013
cial grain refiner should be developed. Among the added agents of grain refining, adding master alloy is an important research direction for its convenient operation, less pollution and fine grain refinement efficiency. The mechanism of carbon inoculation refining method to Mg–Al based alloys is very important to the research and design of master alloy. Among the hypotheses proposing to explain the mechanism, Al4 C3 nuclei hypothesis [9–15] has been the most commonly accepted theory, as grain refinement by an addition of carbon is only confined to aluminum-containing magnesium alloys. Though much research work has been conducted to achieve fine grains in magnesium alloys, a mature master alloy still remains to be developed. In previous work of our research group, Pan [5,6] fabricated Al–1C master alloy by melt in situ reaction, which can efficiently refine AZ63B. However, bad wettability between graphite and aluminum melt and uncontrollable graphite absorption by the melt makes the Al–1C master alloy difficult to fabricate by melt in situ reaction. Therefore, in this paper, a new Al–C master alloy for the grain refinement of AZ31 has been developed by powder metallurgy, also, its microstructure and refining performance are studied and the possible refining mechanism is investigated.
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2. Experimental procedure Two kinds of Al–C master alloys with different carbon contents were fabricated by powder metallurgy. For preparation of the Al–C master alloys, the mixture of aluminum powders and graphite powders was milled in a planetary ball mill for 6–8 h in order to fabricate Al–10C ground powders. The ground powders were mixed with appropriate proportion of aluminum powders, and then pressed into Al–1C and Al–2.5C columned pallets with a diameter of 57 mm at the pressure of 30–40 MPa. All handling of the powders was carried out under a purified argon atmosphere to prevent additional oxidation of the ground powders. Subsequently, the pressed pallets were sintered at 710–730 ◦ C for 1–2 h in vacuum condition and cooled down to room temperature in the furnace. XRD (Xray diffraction) and EPMA (electron probe microanalysis) were employed for analysis of phase identification and microstructures of the Al–1C and Al–2.5C master alloys. For grain refinement test, commercial AZ31 (Mg–3%Al–1%Zn) alloy was melted and heated up to 760 ◦ C in a steel-made crucible using a resistance furnace under the protection of flux, and then 1 wt.% and 2 wt.% of the Al–1C master alloys were added to the molten AZ31 respectively. After being held for 30 min, the melt was stirred for 30 s and cast in cylindrical steel mold with a size of 25 mm × 60 mm. As aluminum was brought into the melt when adding Al–1C master alloy, samples added 1 wt.% and 2 wt.% commercial pure aluminum under the same condition were performed for comparison of grain size. In order to reveal grain boundaries, the ingots were held at 420 ◦ C for 12 h in an air furnace and then water-cooled (T4 treatment). Owning to the different cooling rates between the center and edge of the ingots, all samples were taken from the center of the ingots for grain size measurement. For optical microscopy, all specimens were sectioned, mounted, polished and then etched in a solution of picric and acetic acid. Microstructures were investigated using a high scope video microscope (HSVM). The grain sizes of the ingots were measured applying the line-intersect method.
3. Results and discussions 3.1. Microstructures and their forming mechanism of Al–C master alloys Fig. 1 shows the XRD patterns of a series of Al–2.5C pressed pellets. Peaks of graphite disappeared in Al–2.5C pressed pellet made of Al–10C ground powders comparing to that made of Al–10C mechanical mixture. This indicates that graphite particles were obviously refined during fabricating Al–10C ground powders, and most of them dissolved in aluminum and formed f.c.c Al(C) solid solution after long-time milling [16–18], leading to the sharp decrease of graphite peaks. Besides, evident
Fig. 1. XRD patterns of Al–2.5C pressed pellets: (a) made of Al and Al–10C mechanical mixture, (b) made of Al and Al–10C ground powders and (c) Al–2.5C master alloy.
peaks of Al4 C3 were detected in XRD pattern of Al–2.5C master alloy indicating formation of Al4 C3 by reaction between Al(C) solid solution and aluminum. It is also observed that XRD pattern of Al–2.5C master alloy showed no graphite peaks, indicating the original graphite either dissolved in aluminum or reacted with aluminum. The microstructures of Al–1C and Al–2.5C master alloys are presented in Fig. 2. A kind of black phase is located at the boundaries of aluminum particles, showing net-like distribution, and the net-like phase is more bulky and dense in the Al–2.5C master alloy. Moreover, some brown particles are located at the interface between the black net-like phase and aluminum particles. Electron probe microanalysis (EPMA) of Al–1C and Al–2.5C master alloys is shown in Fig. 3. Fig. 3a shows that a brown phase layer is located at the boundaries of black netlike phase, and some brown particles with a size of 2–3 m are next to the net-like phase. According to EMPA line analysis, the brown particles and the brown phase layer contain C and Al in addition to O, and the black net-like phase enriches in C
Fig. 2. Micrographs of: (a) Al–1C master alloy and (b) Al–2.5C master alloy.
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Fig. 3. EMPA of Al–C master alloys: (a) back-scattered electron image of Al–1C master alloy, (b) EPMA line analysis along line A-B in (a), (c) back-scattered electron image of Al–2.5C master alloy and (d) EPMA line analysis along line C–D in (c).
Fig. 4. Schematic diagram for fabrication process of Al–C alloys.
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especially which is not like Al4 C3 phase. The existence of O might have been introduced by preparing the specimen through the following chemical reaction: Al4 C3 + 12H2 O → 4Al(OH)3 + 3CH4 ↑
(1)
Therefore, it is deduced that the brown particles are Al4 C3 phase, the brown boundary is Al4 C3 thin layer, and the black net-like phase is f.c.c Al(C) solid solution. So when fabricating Al–1C master alloy, small amount of f.c.c Al(C) solid solution reacts with aluminum to form Al4 C3 and the reaction is favorable on
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the interface between aluminum and f.c.c Al(C) solid solution. In the same manner, in Al–2.5C master alloy, the brown boundary is Al4 C3 layer with a width of 3–10 m, which is more bulky than that of Al–1C master alloy. Also, the black phase is f.c.c Al(C) solid solution. All in all, Al4 C3 is formed by reaction between part of f.c.c Al(C) solid solution and aluminum during fabrication of Al–C master alloys, and the mixture of Al4 C3 and f.c.c Al(C) solid solution is located at the boundaries of aluminum particles, showing net-like distribution.
Fig. 5. Optical micrographs of as-cast AZ31 alloy ingots with addition of: (a) no master alloy, (b) 1 wt.% commercial pure Al, (c) 1 wt.% Al–1C master alloy, (d) 2 wt.% commercial pure Al and (e) 2 wt.% Al–1C master alloy.
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According to the research of Kennedy et al. [19] and Wang et al. [20], the reaction: Al(l) + C(s) → Al4 C3 (s)
(2)
can occur at about 705 ◦ C. So sintering at the temperature of 710–730 ◦ C ensure the thermodynamics conditions of the above reaction. During the fabrication of Al–C master alloys, f.c.c Al(C) solid solution reacts with aluminum to form Al4 C3 . It is believed that milling and pressing of graphite and aluminum powders ensure the kinetics conditions of the reaction between the two phases. After milling, graphite particles decrease sharply in size and dissolve in aluminum particles to form Al(C) solid solution leading to a higher reaction velocity and shorter reaction time. Besides, pressing ensure the contiguity of the two phases. So milling and pressing change the nature of graphite and make it easily reacts with aluminum at the interface between the two phases to form Al4 C3 at a relatively low temperature. Fabrication of Al–C master alloys by powder metallurgy and reaction process between graphite and aluminum can be displayed in Fig. 4. 3.2. Grain refinement of AZ31 by Al–1C master alloy and its refining mechanism The microstructures of as-cast AZ31 alloy ingots with and without addition of grain refiners are presented in Fig. 5. Fig. 5a shows a typical equiaxed dendritic structure with inter-dendritic angle of 60◦ and many nonequilibrium eutectic zones formed in the inter-dendritic region. According to Lee et al. [1], the grain size of Mg–Al based alloys decreases sharply as the increasing of aluminum when the content of aluminum is lower than 5 wt.%, so it is necessary to take the samples added equal amount of commercial pure aluminum as compared samples. As the Fig. 5c and e show, the decrease of grain size as well as the transition of primary phases from a fully developed sixfold symmetrical dendrite structure to a less developed dendrite structure is readily noticeable. At 760 ◦ C, when adding 1 wt.% Al–1C master alloy, the grain size is reduced to 1/2 of the sample added 1 wt.% commercial pure aluminum, and the first dentrites become undeveloped; when adding 2 wt.% Al–1C master alloy, the grain size is reduced to 1/3 of the sample added 2 wt.% commercial pure aluminum, and the grain morphology of ␣-Mg transits from a characteristic sixfold symmetrical shape to a petal–like shape. Average grain sizes of as-cast AZ31 alloy ingots after T4 solution heat treatment measured using the line-intersect method are shown in Fig. 6. The grain size of AZ31 without refinement is nearly 580 m; when adding 1 wt.% Al–1C master alloy, grain size decreases from 460 m to 230 m in contrast to sample added 1 wt.% commercial pure aluminum, and grain size of sample added 2 wt.% Al–1C master alloy decreases from 400 m to 140 m comparing to sample added equal amount of commercial pure aluminum. Eliminating the influence of aluminum brought by adding Al–1C master alloy, the Al–1C master alloy presents good grain refining efficiency on AZ31. ˚ c = 24.967 A) ˚ has a crystallographic simAl4 C3 (a = 3.335 A, ˚ c = 5.211 A), ˚ and is a kind of ilarity to ␣-Mg (a = 3.209 A,
Fig. 6. Grain sizes of T4 treated AZ31 alloy samples after addition of commercial pure Al and Al–1C master alloy.
compound with a high melting point, which can exist at normal melting temperature, so it can be a nucleant for ␣-Mg. Among the hypotheses proposing to explain the mechanism of carbon inoculation refining method to Mg–Al based alloys, Al4 C3 nuclei hypothesis [9–15] has been the most commonly accepted theory. Besides, the size of heterogeneous nuclei is a vital factor deciding nucleation potency. It is predicted that 3 m or less is the optimum mean particle size in the literature [10,21,22] for high performance heterogeneous nuclei using a model based on free growth control of grain initiation. So in this article, it is believed that Al4 C3 particles with a size 3 m or less are proposed to be the nucleation sites during solidification of ␣-Mg. During the fabrication of Al–1C master alloy, Al(C) solid solution can easily react with aluminum to form Al4 C3 even in a relatively low temperature 705 ◦ C. In the Al–1C master alloy, Al4 C3 particles with a size of 2–3 m are formed by the reaction between Al(C) solid solution and aluminum, besides, Al4 C3 thin layer is formed at the interface of net-like solid solution. When adding Al–1C master alloy into AZ31 melt, Al4 C3 particles distribute evenly and Al4 C3 thin layer are cracked and disperse in the melt. Moreover, Al(C) solid solution without reaction can react with aluminum in the melt at a higher temperature to form Al4 C3 particles with a fine size. Large numbers of Al4 C3 heterogeneous nuclei lead to excellent grain refinement. 4. Conclusions 1. A new Al–C master alloy has been fabricated by powder metallurgy. Al4 C3 is formed by reaction between part of f.c.c Al(C) solid solution and aluminum during fabrication, and the mixture of Al4 C3 and Al(C) solid solution is located at the boundaries of aluminum particles, showing net-like distribution. Ball milling and briquetting ensure the formation of Al4 C3 phase. 2. Al–1C master alloy in which Al4 C3 particle has a size of 2–3 m shows preferable grain refining efficiency for AZ31. With suitable addition of Al–1C master alloy, the sharp decrease of grain size as well as the transition of primary ␣-Mg from a fully developed sixfold symmetrical dendrite
H. Guang et al. / Journal of Alloys and Compounds 467 (2009) 202–207
structure to a less developed dendrite structure is readily obtained. Acknowledgements This work was supported by a grant from National Science Fund for Distinguished Young Scholars (No. 50625101) and Key Project of Science and Technology Research of Ministry of Education of China (No. 106103).
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
References [1] Y.C. Lee, A.K. Dahle, D.H. StJohn, Metal. Mater. Trans. 31A (2000) 2895–2906. [2] E.F. Emley, Principles of Magnesium Technology, Pergamon Press, Oxford, 1966, pp. 200–211. [3] D.H. StJohn, M. Qian, M.A. Easton, P. Cao, Z. Hidebrand, Metall. Mater. Trans. 36A (2005) 1669–1679. [4] Y.M. Kim, C.D. Yim, B.S. You, Scripta Mater. 57 (2007) 691–694. [5] Y.C. Pan, X.F. Liu, H. Yang, J. Mater. Sci. Technol. 21 (2005) 822–826. [6] Y.C. Pan, X.F. Liu, H. Yang, Z. Metallkd. 96 (2005) 1398–1403. [7] Q. Jin, J.P. Eom, S.G. Lim, W.W. Park, B.S. You, Scripta Mater. 49 (2003) 1129–1132.
[18] [19] [20] [21] [22]
207
R. G¨unther, Ch. Hartig, R. Bormann, Acta Mater. 54 (2006) 5591–5597. L. Lu, A.K. Dahle, D.H. StJohn, Scripta Mater. 53 (2005) 517–522. L. Lu, A.K. Dahle, D.H. StJohn, Scripta Mater. 54 (2006) 2197–2201. M. Qian, P. Cao, Scripta Mater. 52 (2005) 415–419. P. Cao, M. Qian, D.H. StJohn, Scripta Mater. 53 (2005) 841–844. P. Cao, M. Qian, D.H. StJohn, Scripta Mater. 56 (2007) 633–636. T. Motegi, Mater. Sci. Eng. A 413–414 (2005) 408–411. M.X. Zhang, P.M. Kelly, M. Qian, J.A. Taylor, Acta Mater. 53 (2005) 3261–3270. Z.Q. Wang, X.F. Liu, J.Y. Zhang, X.F. Bian, J. Mater. Sci. 39 (2004) 2179–2181. N.Q. Wu, J.M. Wu, G.X. Wang, Z.Z. Li, J. Alloy Compd. 260 (1997) 121–126. Y. Zhou, Z.Q. Li, J. Alloy Compd. 414 (2006) 107–112. A.R. Kennedy, D.P. Weston, M.I. Jones, C. Enel, Scripta Mater. 42 (2000) 1187–1192. Z.Q. Wang, X.F. Liu, J.Y. Zhang, X.F. Bian, J. Mater. Sci. Lett. 22 (2003) 1427–1429. A.L. Greer, A.M. Bunn, A. Tronche, P.V. Evans, D.J. Bristow, Acta Mater. 48 (2000) 2823–2835. A. Tronche, A.L. Greer, Design of Grain Refiners for Aluminum Alloys Paper Presented at Light Metals 2000, TMS, Pennsylvania, USA, 2000, pp. 827–832.
Grain refinement of Mg–Al based alloys by a new Al–C master alloy Han Guang, Liu Xiangfa ∗ , Ding Haimin Key Laboratory of Materials Liquid Structure and Heredity, Ministry of Education, Shandong University, Jinan 250061, China Received 21 October 2007; accepted 6 December 2007 Available online 15 December 2007
Abstract A new Al–C master alloy for grain refinement on Mg–Al based alloys has been fabricated by powder metallurgy. Al4 C3 is formed by reaction between part of f.c.c Al(C) solid solution and aluminum during fabrication, and the net-like mixture of Al4 C3 and Al(C) solid solution is located at the boundaries of aluminum particles. Milling and pressing of graphite and aluminum powders change the nature of graphite and ensure the kinetics conditions of the reaction between the two phases. Grain refinement experiment shows Al–1C master alloy can efficiently refine AZ31 (Mg–3%Al–1%Zn). At 760 ◦ C, when adding 2 wt.% Al–1C master alloy, the grain size is reduced sharply from 400 m to 140 m comparing to the sample added 2 wt.% commercial pure aluminum, and the grain morphology of ␣-Mg transits from a characteristic sixfold symmetrical shape to a petal-like shape. © 2007 Elsevier B.V. All rights reserved. Keywords: Metals and alloys; Powder metallurgy; Microstructure; Metallography; X-ray diffraction
1. Introduction Magnesium alloys, as lightweight structural materials, are receiving increased attention on replacing other metals in automobile industry. Grain refinement is an important method to improve the mechanical properties and workability of both cast and wrought magnesium alloys [1–6]. Mg–Al based alloys are the most common commercial magnesium alloys to which many grain refining methods have been applied, such as super-heating, Elfinal, carbon inoculation and addition of ceramic particles [1–13]. Among these methods, carbon inoculation is known to be the most effective for operating at a low temperature and less fading with long-time holding [1,2]. Amounts of carbon-containing agents such as C2 Cl6 [7], SiC [8], Al4 C3 [9] and carbon powder [14] have been reported to successfully refine Mg–Al based alloys. Among them, C2 Cl6 is the most useful grain refiner for commercial application; however, it cannot satisfy the industry demand for releasing toxic gas during refining. So a reliable, efficient and convenient commer-
∗
Corresponding author. Fax: +86 531 88395414. E-mail address: [email protected] (L. Xiangfa).
0925-8388/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.jallcom.2007.12.013
cial grain refiner should be developed. Among the added agents of grain refining, adding master alloy is an important research direction for its convenient operation, less pollution and fine grain refinement efficiency. The mechanism of carbon inoculation refining method to Mg–Al based alloys is very important to the research and design of master alloy. Among the hypotheses proposing to explain the mechanism, Al4 C3 nuclei hypothesis [9–15] has been the most commonly accepted theory, as grain refinement by an addition of carbon is only confined to aluminum-containing magnesium alloys. Though much research work has been conducted to achieve fine grains in magnesium alloys, a mature master alloy still remains to be developed. In previous work of our research group, Pan [5,6] fabricated Al–1C master alloy by melt in situ reaction, which can efficiently refine AZ63B. However, bad wettability between graphite and aluminum melt and uncontrollable graphite absorption by the melt makes the Al–1C master alloy difficult to fabricate by melt in situ reaction. Therefore, in this paper, a new Al–C master alloy for the grain refinement of AZ31 has been developed by powder metallurgy, also, its microstructure and refining performance are studied and the possible refining mechanism is investigated.
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2. Experimental procedure Two kinds of Al–C master alloys with different carbon contents were fabricated by powder metallurgy. For preparation of the Al–C master alloys, the mixture of aluminum powders and graphite powders was milled in a planetary ball mill for 6–8 h in order to fabricate Al–10C ground powders. The ground powders were mixed with appropriate proportion of aluminum powders, and then pressed into Al–1C and Al–2.5C columned pallets with a diameter of 57 mm at the pressure of 30–40 MPa. All handling of the powders was carried out under a purified argon atmosphere to prevent additional oxidation of the ground powders. Subsequently, the pressed pallets were sintered at 710–730 ◦ C for 1–2 h in vacuum condition and cooled down to room temperature in the furnace. XRD (Xray diffraction) and EPMA (electron probe microanalysis) were employed for analysis of phase identification and microstructures of the Al–1C and Al–2.5C master alloys. For grain refinement test, commercial AZ31 (Mg–3%Al–1%Zn) alloy was melted and heated up to 760 ◦ C in a steel-made crucible using a resistance furnace under the protection of flux, and then 1 wt.% and 2 wt.% of the Al–1C master alloys were added to the molten AZ31 respectively. After being held for 30 min, the melt was stirred for 30 s and cast in cylindrical steel mold with a size of 25 mm × 60 mm. As aluminum was brought into the melt when adding Al–1C master alloy, samples added 1 wt.% and 2 wt.% commercial pure aluminum under the same condition were performed for comparison of grain size. In order to reveal grain boundaries, the ingots were held at 420 ◦ C for 12 h in an air furnace and then water-cooled (T4 treatment). Owning to the different cooling rates between the center and edge of the ingots, all samples were taken from the center of the ingots for grain size measurement. For optical microscopy, all specimens were sectioned, mounted, polished and then etched in a solution of picric and acetic acid. Microstructures were investigated using a high scope video microscope (HSVM). The grain sizes of the ingots were measured applying the line-intersect method.
3. Results and discussions 3.1. Microstructures and their forming mechanism of Al–C master alloys Fig. 1 shows the XRD patterns of a series of Al–2.5C pressed pellets. Peaks of graphite disappeared in Al–2.5C pressed pellet made of Al–10C ground powders comparing to that made of Al–10C mechanical mixture. This indicates that graphite particles were obviously refined during fabricating Al–10C ground powders, and most of them dissolved in aluminum and formed f.c.c Al(C) solid solution after long-time milling [16–18], leading to the sharp decrease of graphite peaks. Besides, evident
Fig. 1. XRD patterns of Al–2.5C pressed pellets: (a) made of Al and Al–10C mechanical mixture, (b) made of Al and Al–10C ground powders and (c) Al–2.5C master alloy.
peaks of Al4 C3 were detected in XRD pattern of Al–2.5C master alloy indicating formation of Al4 C3 by reaction between Al(C) solid solution and aluminum. It is also observed that XRD pattern of Al–2.5C master alloy showed no graphite peaks, indicating the original graphite either dissolved in aluminum or reacted with aluminum. The microstructures of Al–1C and Al–2.5C master alloys are presented in Fig. 2. A kind of black phase is located at the boundaries of aluminum particles, showing net-like distribution, and the net-like phase is more bulky and dense in the Al–2.5C master alloy. Moreover, some brown particles are located at the interface between the black net-like phase and aluminum particles. Electron probe microanalysis (EPMA) of Al–1C and Al–2.5C master alloys is shown in Fig. 3. Fig. 3a shows that a brown phase layer is located at the boundaries of black netlike phase, and some brown particles with a size of 2–3 m are next to the net-like phase. According to EMPA line analysis, the brown particles and the brown phase layer contain C and Al in addition to O, and the black net-like phase enriches in C
Fig. 2. Micrographs of: (a) Al–1C master alloy and (b) Al–2.5C master alloy.
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Fig. 3. EMPA of Al–C master alloys: (a) back-scattered electron image of Al–1C master alloy, (b) EPMA line analysis along line A-B in (a), (c) back-scattered electron image of Al–2.5C master alloy and (d) EPMA line analysis along line C–D in (c).
Fig. 4. Schematic diagram for fabrication process of Al–C alloys.
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especially which is not like Al4 C3 phase. The existence of O might have been introduced by preparing the specimen through the following chemical reaction: Al4 C3 + 12H2 O → 4Al(OH)3 + 3CH4 ↑
(1)
Therefore, it is deduced that the brown particles are Al4 C3 phase, the brown boundary is Al4 C3 thin layer, and the black net-like phase is f.c.c Al(C) solid solution. So when fabricating Al–1C master alloy, small amount of f.c.c Al(C) solid solution reacts with aluminum to form Al4 C3 and the reaction is favorable on
205
the interface between aluminum and f.c.c Al(C) solid solution. In the same manner, in Al–2.5C master alloy, the brown boundary is Al4 C3 layer with a width of 3–10 m, which is more bulky than that of Al–1C master alloy. Also, the black phase is f.c.c Al(C) solid solution. All in all, Al4 C3 is formed by reaction between part of f.c.c Al(C) solid solution and aluminum during fabrication of Al–C master alloys, and the mixture of Al4 C3 and f.c.c Al(C) solid solution is located at the boundaries of aluminum particles, showing net-like distribution.
Fig. 5. Optical micrographs of as-cast AZ31 alloy ingots with addition of: (a) no master alloy, (b) 1 wt.% commercial pure Al, (c) 1 wt.% Al–1C master alloy, (d) 2 wt.% commercial pure Al and (e) 2 wt.% Al–1C master alloy.
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According to the research of Kennedy et al. [19] and Wang et al. [20], the reaction: Al(l) + C(s) → Al4 C3 (s)
(2)
can occur at about 705 ◦ C. So sintering at the temperature of 710–730 ◦ C ensure the thermodynamics conditions of the above reaction. During the fabrication of Al–C master alloys, f.c.c Al(C) solid solution reacts with aluminum to form Al4 C3 . It is believed that milling and pressing of graphite and aluminum powders ensure the kinetics conditions of the reaction between the two phases. After milling, graphite particles decrease sharply in size and dissolve in aluminum particles to form Al(C) solid solution leading to a higher reaction velocity and shorter reaction time. Besides, pressing ensure the contiguity of the two phases. So milling and pressing change the nature of graphite and make it easily reacts with aluminum at the interface between the two phases to form Al4 C3 at a relatively low temperature. Fabrication of Al–C master alloys by powder metallurgy and reaction process between graphite and aluminum can be displayed in Fig. 4. 3.2. Grain refinement of AZ31 by Al–1C master alloy and its refining mechanism The microstructures of as-cast AZ31 alloy ingots with and without addition of grain refiners are presented in Fig. 5. Fig. 5a shows a typical equiaxed dendritic structure with inter-dendritic angle of 60◦ and many nonequilibrium eutectic zones formed in the inter-dendritic region. According to Lee et al. [1], the grain size of Mg–Al based alloys decreases sharply as the increasing of aluminum when the content of aluminum is lower than 5 wt.%, so it is necessary to take the samples added equal amount of commercial pure aluminum as compared samples. As the Fig. 5c and e show, the decrease of grain size as well as the transition of primary phases from a fully developed sixfold symmetrical dendrite structure to a less developed dendrite structure is readily noticeable. At 760 ◦ C, when adding 1 wt.% Al–1C master alloy, the grain size is reduced to 1/2 of the sample added 1 wt.% commercial pure aluminum, and the first dentrites become undeveloped; when adding 2 wt.% Al–1C master alloy, the grain size is reduced to 1/3 of the sample added 2 wt.% commercial pure aluminum, and the grain morphology of ␣-Mg transits from a characteristic sixfold symmetrical shape to a petal–like shape. Average grain sizes of as-cast AZ31 alloy ingots after T4 solution heat treatment measured using the line-intersect method are shown in Fig. 6. The grain size of AZ31 without refinement is nearly 580 m; when adding 1 wt.% Al–1C master alloy, grain size decreases from 460 m to 230 m in contrast to sample added 1 wt.% commercial pure aluminum, and grain size of sample added 2 wt.% Al–1C master alloy decreases from 400 m to 140 m comparing to sample added equal amount of commercial pure aluminum. Eliminating the influence of aluminum brought by adding Al–1C master alloy, the Al–1C master alloy presents good grain refining efficiency on AZ31. ˚ c = 24.967 A) ˚ has a crystallographic simAl4 C3 (a = 3.335 A, ˚ c = 5.211 A), ˚ and is a kind of ilarity to ␣-Mg (a = 3.209 A,
Fig. 6. Grain sizes of T4 treated AZ31 alloy samples after addition of commercial pure Al and Al–1C master alloy.
compound with a high melting point, which can exist at normal melting temperature, so it can be a nucleant for ␣-Mg. Among the hypotheses proposing to explain the mechanism of carbon inoculation refining method to Mg–Al based alloys, Al4 C3 nuclei hypothesis [9–15] has been the most commonly accepted theory. Besides, the size of heterogeneous nuclei is a vital factor deciding nucleation potency. It is predicted that 3 m or less is the optimum mean particle size in the literature [10,21,22] for high performance heterogeneous nuclei using a model based on free growth control of grain initiation. So in this article, it is believed that Al4 C3 particles with a size 3 m or less are proposed to be the nucleation sites during solidification of ␣-Mg. During the fabrication of Al–1C master alloy, Al(C) solid solution can easily react with aluminum to form Al4 C3 even in a relatively low temperature 705 ◦ C. In the Al–1C master alloy, Al4 C3 particles with a size of 2–3 m are formed by the reaction between Al(C) solid solution and aluminum, besides, Al4 C3 thin layer is formed at the interface of net-like solid solution. When adding Al–1C master alloy into AZ31 melt, Al4 C3 particles distribute evenly and Al4 C3 thin layer are cracked and disperse in the melt. Moreover, Al(C) solid solution without reaction can react with aluminum in the melt at a higher temperature to form Al4 C3 particles with a fine size. Large numbers of Al4 C3 heterogeneous nuclei lead to excellent grain refinement. 4. Conclusions 1. A new Al–C master alloy has been fabricated by powder metallurgy. Al4 C3 is formed by reaction between part of f.c.c Al(C) solid solution and aluminum during fabrication, and the mixture of Al4 C3 and Al(C) solid solution is located at the boundaries of aluminum particles, showing net-like distribution. Ball milling and briquetting ensure the formation of Al4 C3 phase. 2. Al–1C master alloy in which Al4 C3 particle has a size of 2–3 m shows preferable grain refining efficiency for AZ31. With suitable addition of Al–1C master alloy, the sharp decrease of grain size as well as the transition of primary ␣-Mg from a fully developed sixfold symmetrical dendrite
H. Guang et al. / Journal of Alloys and Compounds 467 (2009) 202–207
structure to a less developed dendrite structure is readily obtained. Acknowledgements This work was supported by a grant from National Science Fund for Distinguished Young Scholars (No. 50625101) and Key Project of Science and Technology Research of Ministry of Education of China (No. 106103).
[8] [9] [10] [11] [12] [13] [14] [15] [16] [17]
References [1] Y.C. Lee, A.K. Dahle, D.H. StJohn, Metal. Mater. Trans. 31A (2000) 2895–2906. [2] E.F. Emley, Principles of Magnesium Technology, Pergamon Press, Oxford, 1966, pp. 200–211. [3] D.H. StJohn, M. Qian, M.A. Easton, P. Cao, Z. Hidebrand, Metall. Mater. Trans. 36A (2005) 1669–1679. [4] Y.M. Kim, C.D. Yim, B.S. You, Scripta Mater. 57 (2007) 691–694. [5] Y.C. Pan, X.F. Liu, H. Yang, J. Mater. Sci. Technol. 21 (2005) 822–826. [6] Y.C. Pan, X.F. Liu, H. Yang, Z. Metallkd. 96 (2005) 1398–1403. [7] Q. Jin, J.P. Eom, S.G. Lim, W.W. Park, B.S. You, Scripta Mater. 49 (2003) 1129–1132.
[18] [19] [20] [21] [22]
207
R. G¨unther, Ch. Hartig, R. Bormann, Acta Mater. 54 (2006) 5591–5597. L. Lu, A.K. Dahle, D.H. StJohn, Scripta Mater. 53 (2005) 517–522. L. Lu, A.K. Dahle, D.H. StJohn, Scripta Mater. 54 (2006) 2197–2201. M. Qian, P. Cao, Scripta Mater. 52 (2005) 415–419. P. Cao, M. Qian, D.H. StJohn, Scripta Mater. 53 (2005) 841–844. P. Cao, M. Qian, D.H. StJohn, Scripta Mater. 56 (2007) 633–636. T. Motegi, Mater. Sci. Eng. A 413–414 (2005) 408–411. M.X. Zhang, P.M. Kelly, M. Qian, J.A. Taylor, Acta Mater. 53 (2005) 3261–3270. Z.Q. Wang, X.F. Liu, J.Y. Zhang, X.F. Bian, J. Mater. Sci. 39 (2004) 2179–2181. N.Q. Wu, J.M. Wu, G.X. Wang, Z.Z. Li, J. Alloy Compd. 260 (1997) 121–126. Y. Zhou, Z.Q. Li, J. Alloy Compd. 414 (2006) 107–112. A.R. Kennedy, D.P. Weston, M.I. Jones, C. Enel, Scripta Mater. 42 (2000) 1187–1192. Z.Q. Wang, X.F. Liu, J.Y. Zhang, X.F. Bian, J. Mater. Sci. Lett. 22 (2003) 1427–1429. A.L. Greer, A.M. Bunn, A. Tronche, P.V. Evans, D.J. Bristow, Acta Mater. 48 (2000) 2823–2835. A. Tronche, A.L. Greer, Design of Grain Refiners for Aluminum Alloys Paper Presented at Light Metals 2000, TMS, Pennsylvania, USA, 2000, pp. 827–832.