Temperature-induced anomalous structural changes of Al-12wt.%Sn- 4wt.%Si melt and its influence on ascast

Temperature-induced anomalous structural changes of Al-12wt.%Sn- 4wt.%Si melt and its influence on ascast structure Wang Zhiming 1, *Geng Haoran 2, Zhou Guorong 2, Guo Zhongquan 1, and Teng Xinying 2 (1. Key Laboratory for Liquid-Solid Structural Evolution and Processing of Materials (Ministry of Education), Shandong University, Jinan 250061, China; 2. School of Materials Science and Engineering, University of Jinan, Jinan 250022, China) Abstract: The temperature dependence of the viscosity of liquid Al-12wt.%Sn-4wt.%Si was studied with a hightemperature viscosity apparatus. Anomalous changes of viscosity of the melt were found at 1,103 K and 968 K in the cooling process, which indicates anomalous structural changes of the melt. It is calculated that the anomalous structural change is associated with an abrupt decrease of atomic clusters size and activation energy in the melt. According to the temperature of the anomalous structural changes, melt heat treatment process (quenching from superheat to pouring temperature) was performed on Al-12wt.%Sn-4wt.%Si melt prior to pouring, aimed to keep the small atomic clusters from higher temperature to lower pouring temperature. The results suggest that relatively small atomic clusters at the pouring temperature in the melt could generate a deep under-cooling of nucleation in the subsequent solidifi cation process, and refi ne the as-cast structure. After being quenched from superheating to pouring temperature, the relatively small atomic clusters, especially the Si-Si clusters in the melt will grow to equilibrium state (relatively big atomic clusters) with holding time, resulting in the prominent coarsening of the Si morphology in the as-cast structure. Key words: melt heat treatment; viscosity; Al-Sn-Si; structural change CLC number: TG111.4 Document code: A Article ID: 1672-6421(2010)02-138-05 For several decades, a lot of attention has been paid to the studies of the structure of liquid metal. As a result, temperature-induced structural changes have been found in some liquid metals [1-3]. The temperature dependence of the structure of liquid metal has been interpreted in terms of some structural units (atomic clusters), which exist after melting and disappear with the increase of temperature [4]. Hence, as an important parameter, superheating has been found to play an important role in the foundry field [5-7]. In addition, as heredity phenomena always exist between melt and the subsequent as-cast metal [8], heat treatment on the melt is considered as an important process. Referring to the temperature dependent characteristic of structure in liquid alloys, some researchers have applied heat treatment to liquid alloys, expecting to modify the microstructure of castings. Without any addition of modifying elements, Bian Xiufang et al. modified the structure of Al- *Geng Haoran Male, born in 1954, Ph. D, professor, graduated from Shandong University. Research interests: smelting and casting of nonferrous metal. Up to now his five scientific books and 140 research papers have been published, and 50 of which were indexed by SCI or EI. E-mail: mse_genghr@ujn.edu.cn Received: 2009-04-18; Accepted: 2010-02-20 13wt.%Si alloy by applying thermal-rate treatment process to the melt [9]. Wang Jun et al. found that melt thermal treatment can significantly refine the solidification structure of hypoeutectic Al-Si alloys [10]. Zu Fangqiu et al. have discussed the effect of overheating on as-cast structure in terms of temperature-induced melt structure transition [11-13]. With excellent tribological properties, Al-Sn-Si alloys are gradually being applied to replace the Al-based bearing materials which contain contaminating Pb element. However, low strength is still an obstacle for the wide-spread use of this type of alloy. Always, reinforcing elements such as Cu and Mg are added to this kind alloy to promote the mechanical properties. But a new problem is found as the corrosion resistance and plasticity of the alloy decrease with the addition of these elements. In our work, based on the temperature dependent characteristic of the structure of liquid Al-12wt.%Sn-4wt.%Si alloy, melt heat treatment process was applied to the original melt, in order to investigate the heredity between the liquid structure and the as-cast structure. 1 Experimental procedures The Al-12wt.%Sn-4wt.%Si alloy was prepared from aluminum (99.9 wt.%), tin (99.9 wt.%) and single crystal silicon (99.99 wt.%) in a SiC crucible using an electric 138

May 2010 resistance furnace. The viscosity of liquid Al-12wt.%Sn- 4wt.%Si alloy was measured by an oscillating-vessel method, using a high-temperature viscosity apparatus. The experimental sample was placed in a vessel which is set in oscillation about a vertical axis. Samples were heated up to 1,273 K and held for 1 h. After that, the melts were cooled to the given temperature to be held for 1 h and then the vessel was oscillated around the vertical axis. The resulting oscillation was gradually damped on account of frictional energy absorption and dissipation within the liquid. The viscosity was calculated from the measured decrement and time period of the oscillations. The data were measured three times at each desired temperature, and the mean data were used. The viscosity measurement of this apparatus ranges from 0.1 to 10 mpa s, with a measurement precision of ±5%. The temperature measurement ranges from room temperature to 1,773 K. All the measurements were carried out under the protection of Ar gas. In this paper, the melt heat treatment process was carried out as follows. The alloy was first heated to the superheating temperature and held, then was quenched to pouring temperature by stirring with a copper rod (22 mm in diameter) which is coated with alumina. The melt was held in another furnace at pouring temperature as soon as it was quenched to the vicinity of pouring temperature. The temperature of the melt should be held stably in the vicinity of pouring temperature for several minutes, otherwise, this quenching process has failed and should be repeated. Finally, as soon as the melts were poured into the thermo-analysis cup the freezing curves were recorded by a paperless recorder. The remaining melts were poured into a permanent mold. During the whole process, a laser temperature tester was used to realize a real-time temperature measurement. Microstructures of the casting were observed using S-2500 scanning electron microscopy (SEM) and an energy dispersion spectrometer (EDS). 2 Results and discussions Research & Development 2.1 Anomalous structure changes of Al-12wt.%Sn-4wt.%Si melt The temperature dependence of the viscosity for liquid metal can be well described by the Arrhenius formula [14] : η = Aexp(E/RT) (1) Where, A = h/v m, h is Planck constant, V m is the volume of clusters; η denotes kinematic viscosity; R is the gas constant, E represents activation energy, and T is the Kelvin temperature. Equation (1) can be written as a linear equation: E 1 lnη= ln A + (2) R T Which shows that ln η and 1/T are in a linear relationship. Figure 1(a) shows the temperature dependence of viscosity for liquid Al-12wt.%Sn-4wt.%Si alloy in the cooling process. It is shown that the viscosity changes anomalously at 1,103 K and 968 K. The relationship between lnη and 1/T presented in Fig. 1(b) is divided into three approximately linear parts, 933 K 968 K (low temperature range), 998 K 1,103 K (moderate temperature range) and 1,138 K 1,223 K (high temperature range). During the entire measurement, no external factor except temperature is introduced to influence the viscosity, so only the temperature-induced structural change of the melt is considered to be responsible for the anomalous changes of viscosity. That is, the melt may have different structures in the three different temperature ranges. Il inskii et al. [15] made a hypothesis on the structure of liquid matter that a liquid near the melting point consists of atomic clusters and a proportion of free atoms filling the space between clusters. The number of clusters and their size are related to temperature. In this work, the volume of clusters in the melt at the three different temperature ranges can be approximately calculated by V m = h/a, the value of A is obtained thorough linear fitting of experimental data (lnη, 1/T). At the same time activation energy E is also calculated. The values of V m and E are presented in Table 1. As can (a) The relationship between η and T (b) The relationship between lnη and 1/T Fig. 1: The temperature dependence of viscosity of Al-12wt.%Sn-4wt.%Si melt Table 1: The values of V m and E in Al-12wt.%Sn-4wt.%Si melt High temperature range (1,138 1,223 K) Moderate temperature range (998 1,103 K) Low temperature range (933 968 K) V m (10-25 cm 3 ) E (kj mol -1 ) V m (10-25 cm 3 ) E (kj mol -1 ) V m (10-25 cm 3 ) E (kj mol -1 ) 7.93 6.76 10.84 10.03 16.54 14.07 139

be seen in Table 1, in three different temperature ranges, the melt has different atomic cluster s size and activation energy. In the low temperature range, the melt has the biggest atomic cluster s size and highest activation energy. The temperature induced structural changes are considered to take place at 1,103 K and 968 K. It is known that for tin with covalent bonds in solid state, the residual covalent bonds, namely the short range order, still exist in the liquid [16, 17]. Si-Si covalent bonds are also proved to exist in liquid Al-Si alloys through the study of structure factors [18]. X-ray diffraction investigation on liquid tin showed that the temperature-induced structural transformation occurred near1,073 K and 1,473 K [19]. Wang Li found that the viscosity of pure tin changed discontinuously at 1,110 K, which is considered as an indication of liquid structural phase transition [20]. The coordination number of liquid Al-13wt.%Si alloy changes suddenly in the temperature range from 1,048 K to 1,148 K, indicating a structural transformation accompanied by the dissolving of Si-Si clusters into aluminum bulk melt [9]. In our work, the anomalous structure change of Al-12wt.%Sn- 4wt.%Si melt is associated with abrupt changes of atomic cluster size and activation. The anomalous structure change temperature 1,103 K is in approximate accordance with that of structural transformations in liquid tin and Al-13wt.%Si referred to above. It may suggest that, at 1,103 K some Si-Si and Sn-Sn covalent bonds form and grow abruptly in the Al- 12wt.%Sn-4wt.%Si melt. Another anomalous structure change of Al-12wt.%Sn-4wt.%Si melt occurs at 968 K (118 K above the equilibrium Al-Si eutectic temperature), leading to a sharp increase of viscosity and atomic cluster s size. It is considered that, at this temperature different types of atoms may start to associate and form bigger atomic clusters. 2.2 The effect of melt heat treatment on as-cast structure Since the structure of liquid Al-12wt.%Sn-4wt.%Si differs in different temperature ranges, it is sensible to perform heat treatment process on the melt to investigate the relationship between the melt structure and the as-cast structure. Two superheating temperatures, 1,053 K and 1,160 K, were selected in the heat treatment processes in terms of the temperature ranges for different melt structure discussed above. To make a comparison, a melt heated to 963 K was poured directly without melt heat treatment. The results are shown in Fig. 2, which indicate that the melt heat treatment process does affect the morphology of the phases in the solidified alloy. The melt heat treatment process starting at 1,160 K has a more powerful refinement effect than that starting at 1,053 K. The white tin crystals existing on α-(al) grain boundaries are changed from coarse grains to refined ones. The silicon phases are also refined as shown in Fig.3. During the solidification of Al-Sn-Si alloys, the primary α-(al) phase separates out from the liquid first; then the binary Al Si eutectic reaction, L- α(al) + Si, takes place; and the Al- (a) (b) (c) (a) The melt was poured directly at 963 K (unprocessed) (b) The melt was poured after quenched from 1,053 K to 967 K and held for 3 min at about 963 K (c) The melt was poured after quenched from 1,160 K to 968 K and held for 4 min at about 963 K Fig. 2: Effect of melt heat treatment process on as-cast microstructure of Al-12wt.%Sn-4wt.%Si (a) (b) (a) The melt was poured directly at 963 K (unprocessed) (b) The melt was poured after quenched from 1,160 K to 968 K and held for 4 min at about 963 K Fig. 3: Effect of melt heat treatment process on the morphology of silicon in as-cast microstructure 140

May 2010 Sn eutectic reaction, L- α(al) + Sn, occurs finally [21]. Figure 4 shows the influence of the melt heat treatment on freezing curves for Al Si eutectic solidification of Al- 12wt.%Sn-4wt.%Si melt. It is obvious that the melt heat treatment process results in decreases in the nucleation temperature T n, the mass nucleation temperature T m and the Research & Development maximum recalescence temperature T g. The equilibrium temperature of the Al-Si eutectic reaction T e is 850 K according to the phase diagram. The Al-Si eutectic undercooling magnitude T e ( T e = T e -T g ) is increased from 33.2 K to 52.2 K by the melt heat treatment from 1,160 K to 968 K. (a) The melt was poured directly at 963 K (unprocessed) (b) The melt was poured after quenched from 1,160 K to 968 K and held for 4 min at about 963 K Fig. 4 Effects of the melt heat treatment process on freezing curves for Al Si eutectic solidification of the Al-12wt.%Sn-4wt.%Si melt As analyzed above, in liquid Al-12wt.%Sn-4wt.%Si alloy the Si-Si atomic clusters in the low temperature range are bigger than that in the two higher temperature ranges. When the Al-12wt.%Sn-4wt.%Si melt is quenched, the small atomic clusters in the melt at the superheating temperature are possibly retained to the pouring temperature in a nonequilibrium state, because the atomic clusters can not grow as fast as the quenching cools the temperature. The small atomic clusters are not as easy as the big ones to evolve to nucleus during solidification. So the treated melt, containing smaller atomic clusters, will yield a deeper under-cooling in the subsequent solidification. Under the effect of deeper undercooling, a great number of potential nucleation substrates are activated to enhance nucleation, resulting in a subsequent fine solid structure. It must be stressed that all the melts were soon controlled stably in the vicinity of the pouring temperature after being quenched, so there was almost no influence of pouring temperature difference on the results. 2.3 The influence of holding time on nonequilibrium equilibrium structure transition of melt at pouring temperature in the melt heat treatment process After the quenching from 1,160 K to pouring temperature, the temperature of the melt was controlled in the vicinity of pouring temperature by proper holding. But a long holding time will cause the obvious structure transition of the melt from nonequilibrium to equilibrium state at pouring temperature, which may certainly influence the final as-cast structure. As shown in Fig. 3 (b) and Fig. 5, with the increase of holding time, the morphology of the eutectic silicon clearly changes in the as-cast structure. As the holding time is extended to 25 min, the silicon phase grows from needle-like crystal to feather crystals in the as-cast structure. The relative long time of 4 min at pouring temperature, the Sn phase is relative fine in the as-cast structure. However, after 15 min, it coarsens and no longer changes obviously in size with long holding time as shown in Fig. 5. (a) (b) (c) Fig. 5: Effect of holding time at pouring temperature on the as-cast structure of Al-12wt.%Sn-4wt.%Si alloy after the melt heat treatment from 1,160 to 968 K: 15 min (a), 25 min (b) and 40 min (c) 141

When it was quenched from 1,160 K to 963 K, the melt is in a thermodynamic non-equilibrium state. The subsequent transition to equilibrium state is a growing process for atomic clusters. So, the diffusion dynamic process cannot be ignored. The relationship between the diffusion coefficient and viscosity for the melt follows the Stokes-Einstein equation [22] : D = k B T/(6πaη) (3) Where, D represents the diffusion coefficient, k B the Boltzman s constant, T the temperature, a the radius of the solute, and η the viscosity of the solvent. The Si atoms can be thought of as a solute in the melt. Thus, the diffusion coefficient for them varies directly with temperature and inversely with viscosity. At low pouring temperature, the viscosity of the melt is high and the diffusion coefficient is correspondingly low. Based on that, the growth of small Si-Si clusters to equilibrium size in the quenched melt at pouring temperature may depend on diffusion, that is, on time with known D according to Fick s law of diffusion. The Si-Si atomic clusters grow at pouring temperature with longer holding time, resulting in the precipitation of coarse and dendrite crystals during the subsequent solidification. However, after a holding time of 15 min, the size of Sn crystals in the as-cast structure no longer depends on time. This may indicate that the Sn-Sn atomic clusters grow to equilibrium state earlier than the Si-Si clusters. This may correlate to the higher concentration of Sn atoms than Si atoms in the melt. 3 Conclusions Through the study on the temperature dependence of viscosity of the Al-12wt.%Sn-4wt.%Si melt, anomalous changes of viscosity of the Al-12wt.%Sn-4wt.%Si melt were found at 1,103 K and 968 K in the cooling process, which indicate the abnormal structural changes of the melt. 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