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Efficient use of iron impurity in Al–Si alloys
Journal of Alloys and Compounds 615 (2014) 594–597
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Efficient use of iron impurity in Al–Si alloys Yong Zhang, Hongliang Zheng, Yue Liu, Lei Shi, Qingming Zhao, Xuelei Tian ⇑ Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China
a r t i c l e
i n f o
Article history: Received 22 May 2014 Received in revised form 16 June 2014 Accepted 18 June 2014 Available online 26 June 2014 Keywords: Aluminum alloys Iron impurity Silicon Microstructure Solidification
a b s t r a c t An efficient way of using iron to optimize the as-cast microstructure of Al–Si alloys was investigated in this study. Before pouring, Al–12.1%Si–0.1%Fe melts were cooled from 830 °C and held for 10 min at 587 °C, 600 °C and 620 °C, respectively, and then these melts were homogenized at 830 °C for another 20 min. Both the cooling curves of melts and the as-cast microstructure indicate that a lowertemperature holding can result in a considerable amount of (AlFeSi) clusters which subsequently enhance the silicon phase nucleation. Due to the late occurrence of these (AlFeSi) clusters in naturally cooled melts and without a low-temperature holding, the enhancement effect of iron on silicon nucleation is too weak and often neglected. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Al-Si alloys are the dominant member of casting aluminum alloys due to their excellent casting formability and acceptable mechanical properties. Therefore, it is easy to find their applications in industry, e.g., piston [1]. Usually, the silicon phase in piston alloys plays a key role in determining the casting’s quality, and several methods have been proposed to control the solidified microstructure [2,3]. Inoculating melts by introducing potent nucleating substrates [2,4] is the most efficient way to optimize the solidified microstructures. But this still needs extra additions, i.e., nucleating substrates, which is also an additional expense. If the morphology of the silicon phase in Al–Si alloys can be controlled by the alloying elements, it can reduce the corresponding manufacturing cost. Iron in aluminum alloys is always viewed as an inevitable impurity, whose amount increases continuously with recycling. Combined with other elements, iron sometimes can play a beneficial role in strengthening some special alloys [5], but this is not a universal phenomenon. In most cases, iron levels have to be carefully controlled, in order to avoid potential reduction in mechanical properties accompanied by large iron-rich particles [6]. Even though extensive studies have focused on the iron-rich phases in Al–Si alloys [7–10], only few reports have concentrated on the beneficial effects of iron in Al–Si alloys. Recently, an iron-rich particle, b-Al5FeSi, was used as an inoculant and found to evolve into a kind
⇑ Corresponding author. Tel.: +86 531 88392727; fax: +86 531 88395011. E-mail address: [email protected] (X. Tian). http://dx.doi.org/10.1016/j.jallcom.2014.06.114 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
of Mackay icosahedral (AlFeSi) clusters, and enhance the silicon phase nucleation [11,12]. Though the inoculating effect is pronounced, it still relies on exogenous addition, i.e., an Al–10Si–2Fe master alloy containing b-Al5FeSi. Since b-Al5FeSi is a natural outcome of solidification in commercial purity Al–Si alloys [13,14], one may ask why the endogenous iron-rich particles do not significantly enhance the nucleation of silicon phase? Theoretically, the Mackay icosahedral (AlFeSi) clusters should occur before the b-Al5FeSi phase [12,13], implying that the endogenous iron may show remarkable effect in enhancing the nucleation of silicon. Therefore, it is necessary to investigate the method that can trigger the intrinsic potential of endogenous iron in Al–Si alloys, and this is also the topic of this study. 2. Experimental procedures Nominal Al–12%Si alloy was prepared from commercial purity aluminum ingot and silicon and its actual composition was Al–12.1 wt.%Si–0.10 wt.%Fe measured by an optical emission spectrometer (other elements were in ppm levels). The alloy was remelted in a medium frequency induction furnace (at 900 °C) and separated into three parts, and then transferred into an electric resistance furnace. The first samples of every part were held at 830 °C for 20 min before pouring into a graphite mold (inner diameter: 27 mm, wall thickness: 3 mm); the second samples of every part were further cooled to 587 ± 5 °C, 600 ± 5 °C and 620 ± 5 °C and held for 10 min, respectively, and then were heated to 830 °C and held for 20 min, followed by pouring into the mold. To determine the effects of this treatment on solidification behavior of these melts, the cooling curves during solidification of some melts were measured using a K type thermocouple, and detailed information can be found in early work [12]. Metallographic samples were prepared through standard routine and further characterized by optical microscopy and scanning electron microscopy (JSM6610-LV, a secondary electron detector and energy dispersive spectroscopy (EDS) detector were used for imaging and composition analysis, respectively). The sample foil for the high-resolution transmission electron microscopy (HRTEM)
Y. Zhang et al. / Journal of Alloys and Compounds 615 (2014) 594–597 study was cut from the cast and mechanically thinned to 60 lm and finally prepared using a Gatan precision ion polishing system (Gatan 691). HRTEM was then carried out with a high resolution transmission electron microscope (Tecnai G2 F30) operating at an acceleration voltage of 300 kV.
3. Results and discussion The typical as-cast microstructures of samples with different preparation methods are displayed in Fig. 1. All the samples show common features, namely the presence of polyhedral silicon particles (marked by arrows). This is unexpected because the compositions of all the samples are in the vicinity of eutectic point (12.6% Si). Theoretically, if the melts solidify according to the phase diagram, no polyhedral silicon is to be expected and all the silicon particles should be restricted to the eutectic silicon phase. Nevertheless, the existence of impurities, e.g., AlP, can promote the nucleation of the silicon phase, which can grow without coupling with the eutectic aluminum phase, finally forming these polyhedral silicon particles [4,15]. After a low-temperature holding, the separation and morphology of eutectic silicon change significantly and aluminum dendrites disappear (Fig. 1a and c compared with b and d). This phenomenon is similar as that found in Al–Si alloys showing enhanced nucleation of silicon phase [3]. It is necessary to note that, in the studied temperature range, the effect of enhanced nucleation of silicon phase by a low-temperature holding evolves monotonically with decreasing temperature. If holding temperature is not low enough, i.e., above 620 °C, the enhancement effect can be negligible (Fig. 1e and f). Fig. 2 shows the cooling curves of Al–12.1Si–0.1Fe melts with or without a low-temperature holding. It is evident that a lowtemperature holding makes the corresponding melt nucleate and grow at a shallower undercooling. Usually, a shallower undercooling means easier nucleation, and it is traditionally achieved by enhancing nucleation through specific substrates [16]. Therefore, the lowtemperature holding enhances the silicon phase nucleation, and this is consistent with the result shown in Fig. 1. However, this phenomenon is difficult to comprehend given that no inoculants were added. A possible explanation is that a low-temperature holding triggers some inherent mechanism that can enhance nucleation, such as the formation of some nucleating substrates. Fig. 3 shows the HRTEM image of a silicon particle in the sample with a low-temperature holding at 587 °C. In the silicon matrix,
595
there are two isolated nanoparticles with special crystal structures. The corresponding fast Fourier transforms (FFTs) of these nanoparticles reveal that they cannot be identified as any well-established crystals known in Al–Si–Fe alloy systems, and this agrees well with the nanoparticles earlier reported in Al–Si alloys inoculated with the (AlFeSi) clusters [11,12]. Therefore, it is reasonable to conclude that (AlFeSi) clusters developing in the melts play the dominant role in enhancing the nucleation of silicon phase. It is clear that the lowtemperature holding directly is responsible for this effect. But, in fact, the (AlFeSi) clusters always form in the studied melts during cooling and finally evolve into a-Al8Fe2Si or b-Al5FeSi [13], and they should enhance the nucleation of the silicon phase. But, when directly cooled from the pouring temperature without a lowtemperature holding (i.e., naturally cooled melts), the enhancement effect is too weak and is often neglected. Why does a further lowtemperature holding enhance the function of the (AlFeSi) clusters? It is commonly recognised that atom clusters with a similar or related structure with solid phase will develop continuously as temperature approaches the liquidus line [17]. Since the (AlFeSi) clusters are the building blocks of a-Al8Fe2Si [18,19] and a-Al8Fe2Si is the metastable counterpart of b-Al5FeSi, a-Al8Fe2Si is easily stabilised by high cooling rates during solidification in commercial purity Al–Si alloys [13]. Therefore, with temperature decreasing, the amount of (AlFeSi) clusters increases continuously, and this is consistent with our early in-situ XRD characterization of Al–10Si–2Fe melt [12]. Consequently, a low-temperature holding provides an appropriate condition where abundant (AlFeSi) clusters can develop and a lower holding temperature induces a larger amount of these clusters. This is why a lower holding temperature shows a better efficiency in optimizing the microstructures (Fig. 1). Due to the inherent covalent bonds in these clusters [20], these developed (AlFeSi) clusters can remain relatively stable against increased temperatures in the studied temperature and time range. Furthermore these (AlFeSi) clusters can trigger the segregation of aluminum and silicon atoms around them, which evolve into iron-poor precursors through the migration of iron atoms out [11]. These nanoparticles shown in Fig. 3 are reminiscent of these iron-poor precursors that participate in silicon nucleation and growth by aggregation and subsequent structural reconfiguration. As mentioned above, the (AlFeSi) clusters developing during a low-temperature holding are the same as those developing in
Fig. 1. Typical microstructures of Al–12.1Si–0.1Fe samples with or without a low-temperature holding obtained using an optical microscope, (a, c and e) without a lowtemperature holding, (b) held at 587 °C, (d) held at 600 °C, (f) held at 620 °C.
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Y. Zhang et al. / Journal of Alloys and Compounds 615 (2014) 594–597
Fig. 2. Cooling curves of Al–12.1Si–0.1Fe melts, (a) without a low-temperature holding and (b) holding at 600 °C for 10 min.
naturally cooled melts. The different efficiency in enhancing nucleation should be attributed to the temperature range where the (AlFeSi) clusters promote the precursors. It is logical to conjecture that the action temperature range where the (AlFeSi) clusters trigger and produce the iron-poor precursors is higher than the temperature where abundant (AlFeSi) clusters can occur in naturally cooled melts. In this scenario, although the (AlFeSi) clusters always appear naturally from the melts, in traditional conditions (i.e., naturally cooled melts), these (AlFeSi) clusters appear late and lose their intrinsic potential to promote the iron-poor precursors. However, in the case of (AlFeSi) clusters developing during a low-temperature holding, they will be stable over the temperature range used for the heat treatments. As a result, these (AlFeSi) clusters can enhance silicon nucleation as depicted above and deplete themselves, with iron atoms migrating into the melts.
Fig. 3. HRTEM image of a silicon particle in Al–12.1Si–01.Fe alloy with a lowtemperature holding (at 587 °C). The insets are corresponding FFTs of the nanoparticles (indicated by arrows).
With the temperature of the melts further decreasing and approaching the formation temperature of the iron-rich phases, there will not be any remaining (AlFeSi) clusters that can assist, and iron-rich phases will form exclusively through newly formed clusters. This is the reason why a further low-temperature holding seldom affects the iron-rich particles compared with the naturally solidified samples (see Fig. 4). All of these particles are a-Al8Fe2Si, according to their morphology. If the action temperature of the
Fig. 4. Iron-rich particles (marked by arrows) of Al–12.1Si–0.1Fe alloys, (a and c) without a low-temperature holding, (b) held at 587 °C, (d) held at 600 °C. The insets are corresponding EDS results of particles indicated by white arrows. Al, Si and Fe elements were detected in these particles and oxygen was an artifact from samples and SEM equipment.
Y. Zhang et al. / Journal of Alloys and Compounds 615 (2014) 594–597
(AlFeSi) clusters were not higher than the appearance temperature of the (AlFeSi) clusters in the naturally cooled melts, the remnant (AlFeSi) clusters should affect the formation of iron-rich particles. These remnant (AlFeSi) clusters are perfect nucleating embryos of iron-rich phases, and thus can significantly affect the dimension and crystal structure of iron-rich particles in Al–10Si–xFe (x = 0.1– 1.0) alloys inoculated with (AlFeSi) clusters [21]. This is in stark contrast to the results in Fig. 4. 4. Conclusions In summary, a low-temperature holding (587 °C, 600 °C) can effectively optimize the as-cast microstructures of Al–12.1Si– 0.1Fe alloys. By cooling curve analysis and HRTEM characterization, the (AlFeSi) clusters developing during holding were determined to be responsible for the enhanced nucleation of silicon phase in the Al–12.1Si–0.1Fe alloys. However, (AlFeSi) clusters appear late to enhance nucleation in naturally cooled melts. This is the reason why, without a low-temperature holding, the enhancement effect of iron on silicon nucleation is too weak and often neglected. Acknowledgments We gratefully acknowledge the financial support of National Natural Science Foundation of China (NSFC) Project (No. 51371109). Y.Z. thanks A/Prof. Laure Bourgeois of Monash University for reading the manuscript.
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References [1] S. Manasijevic, R. Radisa, S. Markovic, Z. Acimovic-Pavlovic, K. Raic, Intermetallics 19 (2011) 486–492. [2] Q. Zhang, X. Liu, H. Dai, J. Alloys Comp. 480 (2009) 376–381. [3] X. Liu, Y. Wu, X. Bian, J. Alloys Comp. 391 (2005) 90–94. [4] S.-M. Liang, R. Schmid-Fetzer, Acta Mater. 72 (2014) 41–56. [5] Q. Zhao, M. Slagsvold, B. Holmedal, Scr. Mater. 67 (2012) 217–220. [6] M. Sha, S. Wu, L. Wan, Mater. Sci. Eng. A 554 (2012) 142–148. [7] M. Timpel, N. Wanderka, B.S. Murty, J. Banhart, Acta Mater. 58 (2010) 6600– 6608. [8] S. Terzi, J. Taylor, Y. Cho, L. Salvo, M. Suéry, E. Boller, A. Dahle, Acta Mater. 58 (2010) 5370–5380. [9] C. Puncreobutr, A. Phillion, J. Fife, P. Lee, Acta Mater. 64 (2014) 316–325. [10] C. Puncreobutr, P.D. Lee, K.M. Kareh, T. Connolley, J.L. Fife, A.B. Phillion, Acta Mater. 68 (2014) 42–51. [11] Y. Zhang, H. Zheng, Y. Liu, L. Shi, R. Xu, X. Tian, Acta Mater. 70 (2014) 162–173. [12] Y. Zhang, H. Zheng, Y. Liu, R. Xu, L. Shi, X. Tian, Mater. Lett. 123 (2014) 224– 228. [13] A. Gorny, J. Manickaraj, Z. Cai, S. Shankar, J. Alloys Comp. 577 (2013) 103–124. [14] M. Timpel, N. Wanderka, R. Grothausmann, J. Banhart, J. Alloys Comp. 558 (2013) 18–25. [15] A.K. Dahle, K. Nogita, S.D. McDonald, C. Dinnis, L. Lu, Mater. Sci. Eng. A 413– 414 (2005) 243–248. [16] J. Wannasin, R. Canyook, S. Wisutmethangoon, M. Flemings, Acta Mater. 61 (2013) 3897–3903. [17] L. Xiong, H. Lou, X. Wang, T. Debela, Q. Cao, D. Zhang, S. Wang, C. Wang, J. Jiang, Acta Mater. 68 (2014) 1–8. [18] V. Elser, C.L. Henley, Phys. Rev. Lett. 55 (1985) 2883–2886. [19] G.G. Long, K.W. Chapman, P.J. Chupas, L.A. Bendersky, L.E. Levine, F. Mompiou, J.K. Stalick, J.W. Cahn, Phys. Rev. Lett. 111 (2013) 015502. [20] K. Kirihara, T. Nakata, M. Takata, Y. Kubota, E. Nishibori, K. Kimura, M. Sakata, Phys. Rev. Lett. 85 (2000) 3468. [21] Y. Liu, Y. Zhang, X. Tian, To be published, 2014.
Contents lists available at ScienceDirect
Journal of Alloys and Compounds journal homepage: www.elsevier.com/locate/jalcom
Letter
Efficient use of iron impurity in Al–Si alloys Yong Zhang, Hongliang Zheng, Yue Liu, Lei Shi, Qingming Zhao, Xuelei Tian ⇑ Key Laboratory for Liquid–Solid Structural Evolution and Processing of Materials, Ministry of Education, Shandong University, Jinan 250061, PR China
a r t i c l e
i n f o
Article history: Received 22 May 2014 Received in revised form 16 June 2014 Accepted 18 June 2014 Available online 26 June 2014 Keywords: Aluminum alloys Iron impurity Silicon Microstructure Solidification
a b s t r a c t An efficient way of using iron to optimize the as-cast microstructure of Al–Si alloys was investigated in this study. Before pouring, Al–12.1%Si–0.1%Fe melts were cooled from 830 °C and held for 10 min at 587 °C, 600 °C and 620 °C, respectively, and then these melts were homogenized at 830 °C for another 20 min. Both the cooling curves of melts and the as-cast microstructure indicate that a lowertemperature holding can result in a considerable amount of (AlFeSi) clusters which subsequently enhance the silicon phase nucleation. Due to the late occurrence of these (AlFeSi) clusters in naturally cooled melts and without a low-temperature holding, the enhancement effect of iron on silicon nucleation is too weak and often neglected. Ó 2014 Elsevier B.V. All rights reserved.
1. Introduction Al-Si alloys are the dominant member of casting aluminum alloys due to their excellent casting formability and acceptable mechanical properties. Therefore, it is easy to find their applications in industry, e.g., piston [1]. Usually, the silicon phase in piston alloys plays a key role in determining the casting’s quality, and several methods have been proposed to control the solidified microstructure [2,3]. Inoculating melts by introducing potent nucleating substrates [2,4] is the most efficient way to optimize the solidified microstructures. But this still needs extra additions, i.e., nucleating substrates, which is also an additional expense. If the morphology of the silicon phase in Al–Si alloys can be controlled by the alloying elements, it can reduce the corresponding manufacturing cost. Iron in aluminum alloys is always viewed as an inevitable impurity, whose amount increases continuously with recycling. Combined with other elements, iron sometimes can play a beneficial role in strengthening some special alloys [5], but this is not a universal phenomenon. In most cases, iron levels have to be carefully controlled, in order to avoid potential reduction in mechanical properties accompanied by large iron-rich particles [6]. Even though extensive studies have focused on the iron-rich phases in Al–Si alloys [7–10], only few reports have concentrated on the beneficial effects of iron in Al–Si alloys. Recently, an iron-rich particle, b-Al5FeSi, was used as an inoculant and found to evolve into a kind
⇑ Corresponding author. Tel.: +86 531 88392727; fax: +86 531 88395011. E-mail address: [email protected] (X. Tian). http://dx.doi.org/10.1016/j.jallcom.2014.06.114 0925-8388/Ó 2014 Elsevier B.V. All rights reserved.
of Mackay icosahedral (AlFeSi) clusters, and enhance the silicon phase nucleation [11,12]. Though the inoculating effect is pronounced, it still relies on exogenous addition, i.e., an Al–10Si–2Fe master alloy containing b-Al5FeSi. Since b-Al5FeSi is a natural outcome of solidification in commercial purity Al–Si alloys [13,14], one may ask why the endogenous iron-rich particles do not significantly enhance the nucleation of silicon phase? Theoretically, the Mackay icosahedral (AlFeSi) clusters should occur before the b-Al5FeSi phase [12,13], implying that the endogenous iron may show remarkable effect in enhancing the nucleation of silicon. Therefore, it is necessary to investigate the method that can trigger the intrinsic potential of endogenous iron in Al–Si alloys, and this is also the topic of this study. 2. Experimental procedures Nominal Al–12%Si alloy was prepared from commercial purity aluminum ingot and silicon and its actual composition was Al–12.1 wt.%Si–0.10 wt.%Fe measured by an optical emission spectrometer (other elements were in ppm levels). The alloy was remelted in a medium frequency induction furnace (at 900 °C) and separated into three parts, and then transferred into an electric resistance furnace. The first samples of every part were held at 830 °C for 20 min before pouring into a graphite mold (inner diameter: 27 mm, wall thickness: 3 mm); the second samples of every part were further cooled to 587 ± 5 °C, 600 ± 5 °C and 620 ± 5 °C and held for 10 min, respectively, and then were heated to 830 °C and held for 20 min, followed by pouring into the mold. To determine the effects of this treatment on solidification behavior of these melts, the cooling curves during solidification of some melts were measured using a K type thermocouple, and detailed information can be found in early work [12]. Metallographic samples were prepared through standard routine and further characterized by optical microscopy and scanning electron microscopy (JSM6610-LV, a secondary electron detector and energy dispersive spectroscopy (EDS) detector were used for imaging and composition analysis, respectively). The sample foil for the high-resolution transmission electron microscopy (HRTEM)
Y. Zhang et al. / Journal of Alloys and Compounds 615 (2014) 594–597 study was cut from the cast and mechanically thinned to 60 lm and finally prepared using a Gatan precision ion polishing system (Gatan 691). HRTEM was then carried out with a high resolution transmission electron microscope (Tecnai G2 F30) operating at an acceleration voltage of 300 kV.
3. Results and discussion The typical as-cast microstructures of samples with different preparation methods are displayed in Fig. 1. All the samples show common features, namely the presence of polyhedral silicon particles (marked by arrows). This is unexpected because the compositions of all the samples are in the vicinity of eutectic point (12.6% Si). Theoretically, if the melts solidify according to the phase diagram, no polyhedral silicon is to be expected and all the silicon particles should be restricted to the eutectic silicon phase. Nevertheless, the existence of impurities, e.g., AlP, can promote the nucleation of the silicon phase, which can grow without coupling with the eutectic aluminum phase, finally forming these polyhedral silicon particles [4,15]. After a low-temperature holding, the separation and morphology of eutectic silicon change significantly and aluminum dendrites disappear (Fig. 1a and c compared with b and d). This phenomenon is similar as that found in Al–Si alloys showing enhanced nucleation of silicon phase [3]. It is necessary to note that, in the studied temperature range, the effect of enhanced nucleation of silicon phase by a low-temperature holding evolves monotonically with decreasing temperature. If holding temperature is not low enough, i.e., above 620 °C, the enhancement effect can be negligible (Fig. 1e and f). Fig. 2 shows the cooling curves of Al–12.1Si–0.1Fe melts with or without a low-temperature holding. It is evident that a lowtemperature holding makes the corresponding melt nucleate and grow at a shallower undercooling. Usually, a shallower undercooling means easier nucleation, and it is traditionally achieved by enhancing nucleation through specific substrates [16]. Therefore, the lowtemperature holding enhances the silicon phase nucleation, and this is consistent with the result shown in Fig. 1. However, this phenomenon is difficult to comprehend given that no inoculants were added. A possible explanation is that a low-temperature holding triggers some inherent mechanism that can enhance nucleation, such as the formation of some nucleating substrates. Fig. 3 shows the HRTEM image of a silicon particle in the sample with a low-temperature holding at 587 °C. In the silicon matrix,
595
there are two isolated nanoparticles with special crystal structures. The corresponding fast Fourier transforms (FFTs) of these nanoparticles reveal that they cannot be identified as any well-established crystals known in Al–Si–Fe alloy systems, and this agrees well with the nanoparticles earlier reported in Al–Si alloys inoculated with the (AlFeSi) clusters [11,12]. Therefore, it is reasonable to conclude that (AlFeSi) clusters developing in the melts play the dominant role in enhancing the nucleation of silicon phase. It is clear that the lowtemperature holding directly is responsible for this effect. But, in fact, the (AlFeSi) clusters always form in the studied melts during cooling and finally evolve into a-Al8Fe2Si or b-Al5FeSi [13], and they should enhance the nucleation of the silicon phase. But, when directly cooled from the pouring temperature without a lowtemperature holding (i.e., naturally cooled melts), the enhancement effect is too weak and is often neglected. Why does a further lowtemperature holding enhance the function of the (AlFeSi) clusters? It is commonly recognised that atom clusters with a similar or related structure with solid phase will develop continuously as temperature approaches the liquidus line [17]. Since the (AlFeSi) clusters are the building blocks of a-Al8Fe2Si [18,19] and a-Al8Fe2Si is the metastable counterpart of b-Al5FeSi, a-Al8Fe2Si is easily stabilised by high cooling rates during solidification in commercial purity Al–Si alloys [13]. Therefore, with temperature decreasing, the amount of (AlFeSi) clusters increases continuously, and this is consistent with our early in-situ XRD characterization of Al–10Si–2Fe melt [12]. Consequently, a low-temperature holding provides an appropriate condition where abundant (AlFeSi) clusters can develop and a lower holding temperature induces a larger amount of these clusters. This is why a lower holding temperature shows a better efficiency in optimizing the microstructures (Fig. 1). Due to the inherent covalent bonds in these clusters [20], these developed (AlFeSi) clusters can remain relatively stable against increased temperatures in the studied temperature and time range. Furthermore these (AlFeSi) clusters can trigger the segregation of aluminum and silicon atoms around them, which evolve into iron-poor precursors through the migration of iron atoms out [11]. These nanoparticles shown in Fig. 3 are reminiscent of these iron-poor precursors that participate in silicon nucleation and growth by aggregation and subsequent structural reconfiguration. As mentioned above, the (AlFeSi) clusters developing during a low-temperature holding are the same as those developing in
Fig. 1. Typical microstructures of Al–12.1Si–0.1Fe samples with or without a low-temperature holding obtained using an optical microscope, (a, c and e) without a lowtemperature holding, (b) held at 587 °C, (d) held at 600 °C, (f) held at 620 °C.
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Y. Zhang et al. / Journal of Alloys and Compounds 615 (2014) 594–597
Fig. 2. Cooling curves of Al–12.1Si–0.1Fe melts, (a) without a low-temperature holding and (b) holding at 600 °C for 10 min.
naturally cooled melts. The different efficiency in enhancing nucleation should be attributed to the temperature range where the (AlFeSi) clusters promote the precursors. It is logical to conjecture that the action temperature range where the (AlFeSi) clusters trigger and produce the iron-poor precursors is higher than the temperature where abundant (AlFeSi) clusters can occur in naturally cooled melts. In this scenario, although the (AlFeSi) clusters always appear naturally from the melts, in traditional conditions (i.e., naturally cooled melts), these (AlFeSi) clusters appear late and lose their intrinsic potential to promote the iron-poor precursors. However, in the case of (AlFeSi) clusters developing during a low-temperature holding, they will be stable over the temperature range used for the heat treatments. As a result, these (AlFeSi) clusters can enhance silicon nucleation as depicted above and deplete themselves, with iron atoms migrating into the melts.
Fig. 3. HRTEM image of a silicon particle in Al–12.1Si–01.Fe alloy with a lowtemperature holding (at 587 °C). The insets are corresponding FFTs of the nanoparticles (indicated by arrows).
With the temperature of the melts further decreasing and approaching the formation temperature of the iron-rich phases, there will not be any remaining (AlFeSi) clusters that can assist, and iron-rich phases will form exclusively through newly formed clusters. This is the reason why a further low-temperature holding seldom affects the iron-rich particles compared with the naturally solidified samples (see Fig. 4). All of these particles are a-Al8Fe2Si, according to their morphology. If the action temperature of the
Fig. 4. Iron-rich particles (marked by arrows) of Al–12.1Si–0.1Fe alloys, (a and c) without a low-temperature holding, (b) held at 587 °C, (d) held at 600 °C. The insets are corresponding EDS results of particles indicated by white arrows. Al, Si and Fe elements were detected in these particles and oxygen was an artifact from samples and SEM equipment.
Y. Zhang et al. / Journal of Alloys and Compounds 615 (2014) 594–597
(AlFeSi) clusters were not higher than the appearance temperature of the (AlFeSi) clusters in the naturally cooled melts, the remnant (AlFeSi) clusters should affect the formation of iron-rich particles. These remnant (AlFeSi) clusters are perfect nucleating embryos of iron-rich phases, and thus can significantly affect the dimension and crystal structure of iron-rich particles in Al–10Si–xFe (x = 0.1– 1.0) alloys inoculated with (AlFeSi) clusters [21]. This is in stark contrast to the results in Fig. 4. 4. Conclusions In summary, a low-temperature holding (587 °C, 600 °C) can effectively optimize the as-cast microstructures of Al–12.1Si– 0.1Fe alloys. By cooling curve analysis and HRTEM characterization, the (AlFeSi) clusters developing during holding were determined to be responsible for the enhanced nucleation of silicon phase in the Al–12.1Si–0.1Fe alloys. However, (AlFeSi) clusters appear late to enhance nucleation in naturally cooled melts. This is the reason why, without a low-temperature holding, the enhancement effect of iron on silicon nucleation is too weak and often neglected. Acknowledgments We gratefully acknowledge the financial support of National Natural Science Foundation of China (NSFC) Project (No. 51371109). Y.Z. thanks A/Prof. Laure Bourgeois of Monash University for reading the manuscript.
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