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Al composites fabricated by stir casting process
Thermochimica Acta 641 (2016) 29–38
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Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal analysis of in-situ Al2 O3 /SiO2(p) /Al composites fabricated by stir casting process Kang Wang a , Wenfang Li a,∗ , Jun Du a , Lu Yang a , Peng Tang b a b
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China School of Materials Science and Engineering, Guangxi University, Nanning 530004, China
a r t i c l e
i n f o
Article history: Received 24 May 2016 Received in revised form 8 August 2016 Accepted 22 August 2016 Available online 24 August 2016 Keywords: Aluminum matrix composites (AMCs) SiO2 /Al in-situ reaction Thermal analysis Dendrite arm spacing Solidification behavior
a b s t r a c t The in-situ fabricated Al2 O3 /SiO2(p) /Al composites were produced by adding different quantities of SiO2 particles (viz. 0, 3, 6 and 9 wt.%) into molten commercial purity Al at 750 ◦ C. Newtonian thermal analysis was conducted to investigate the solidification behaviors of these molten composites prepared by the stir casting process. The microstructure and phase identification were achieved via optical microscope, SEM (equipped with EDS) and X-ray diffraction. Results showed the dendrite arm spacing of ␣-Al grain decreased with the increase of SiO2 addition. The SEM image showed the special duplex phase with Al2 O3 layer formed around SiO2 particle in Al matrix. The Si concentration in Al matrix increased with increasing in the SiO2 addition, resulting in the decrease of liquidus temperature of the melts. Compared with the purity Al melt, one more stage related to the exothermic Al-Si phase transformation was revealed in the cooling curves of these molten composites. Cooling rate of the purity Al melt was lower than that of the melts with SiO2 particles addition. The difference in solidification behaviors of these melts was ascribed to the effect of SiO2 /Al reaction on the effective heat capacity and different amounts of latent heat released from phase transformation process. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In-situ fabricated aluminum matrix composites (AMCs) are deemed as the novel materials for their light weight, high specific strength and excellent wear resistance properties [1,2]. They have been gaining wide acceptance in aerospace and automotive industry in recent years. In order to achieve even better mechanical properties, ceramic particles with high elasticity modulus (such as SiC and Al2 O3 ) are added to the metals or alloys. Among them, the Al-Mg alloys are usually used as the matrix of SiC(p) /Al-Mg, Al2 O3(p) /Al-Mg and Al2 O3 /SiO2(p) /Al-Mg composites due to their feasibility of insitu reaction at ceramic/metal interface [3]. Lai et al. [4] and Eustathopoulos et al. [5] suggested the in-situ reaction at the interface of different materials could enhance their wettability. This enhancement not only promoted the mechanical properties of the composites, but also facilitated the incorporation of ceramic particles in molten Al [6]. Schultz et al. [7] indicated the Al2 O3 could
∗ Corresponding author. E-mail address: mewfl[email protected] (W. Li). http://dx.doi.org/10.1016/j.tca.2016.08.009 0040-6031/© 2016 Elsevier B.V. All rights reserved.
react with the Mg in molten Al-Mg alloy. Shi et al. [8] reported that the wettability of SiC particle with SiO2 layer surrounded was better than that without SiO2 layer surrounded. It was resulted from the reduced contact angel at the reactive SiO2 /Al interface. Mohammadpour et al. [9] indicated the defects at SiCp /Al interface in the AMCs could be eliminated by the interfacial reaction taken place among Al, SiO2 and Mg. Nowadays, researchers pay their attention on the SiO2 /Al insitu reaction which takes place in different fabricate process of the composites [10,11]. Zhu et al. [10] produced the in-situ fabricated SiO2 /Al composites using powder metallurgical process. In this case, the SiO2 powder could completely react with Al at about 1200 ◦ C. However, the high temperature and complexity of powder metallurgical method limits the mass production for these composites. Stir casting has been regarded as a commercial route for incorporating SiO2 particles into Al melts because it can be achieve at a lower temperature [12]. Some researchers reported that the contact angle at SiO2 /Al interface below 1000 ◦ C was smaller than 90◦ [13,14], which suggested the incorporation of SiO2 particles in molten Al was feasible. In our previous study [15], it has been proven that the zircon tailing sand (SiO2 content: 78 wt.%) can react with purity Al melt. Since the added particles and products from SiO2 /Al in-situ reaction will change the physical properties of the
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K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
Table 1 Chemical composition of commercial purity Al. Element
Si
Fe
Mn
Al
Content (wt.%)
0.02
0.05
0.02
Balance
molten composites, it is necessary to further study the effect of SiO2 additions on their solidification behaviors. The in-situ reaction in molten AMCs is favorable to the formation of reinforcements, whilst the products from the reaction might have a significant effect on modifying the microstructure of the composite’s matrix. However, to the best of our knowledge, only a few reports [10,16,17] were focus on the relationship among the solidification behavior, phase transformation and microstructure in AMCs. The DSC testing was often provided to detect the phase transformation in the alloy by using a very little piece of sample [18,19]. But this way was taken as the laboratiorial testing method because the samples were tested under an equilibrium condition. Moreover, the samples remelted in the DSC analyser could not reflect the solidification feature of the melt obtained right after the stir casting process. The cooling curve thermal analysis (CCTA) has been extensively used to detect the occurrence of different phases in the solidification history of molten alloys [20]. Compared with the DSC technique, the CCTA method is more suitable to monitor the phase transformation of the molten alloys and AMCs after their fabricate processes [21,22]. Wu et al. [16] investigated the cooling curve of SiC/Al-Si-Mg composites. The AMCs in this study were immediately poured into a mould for CCTA analysis after the stirring operation. This method was also conducted to analyzed the cooling curves and solidification behaviors of Al2 O3 /Al-Mg composites by Petric et al. [17]. In this study, the SiO2 particles were incorporated into the molten Al by stir casting process. The commercial purity Al was used as the composite matrix without adding any alloying elements. Effect of SiO2 quantity on the solidification behavior of the molten composite was studied by the CCTA method. 2. Experiment and method 2.1. Preparation and thermal analysis for molten AMCs The SiO2 particles (with a purity of 99.9%) were supplied from a quartz manufacturer (Dongjing Quartz Company, Jiangsu Province, P. R. China). The morphologies and size distribution of these particles are shown in Fig. 1. The average size of them is about 103 m (tested by the laser size instrument: BACKMAN COULTER LS-13-320). Table 1 shows the chemical composition of commercial purity Al used as the matrix of the composites. The commercial purity Al ingot (1.5 kg) was cleaned by water, dried and placed inside a graphite crucible. It was molten in an electrical furnace at 750 ± 5 ◦ C. An electric motor with graphite stirrer assembled was used for generating a vortex in molten Al. Both the crucible and stirrer were coated with Mica coating. In order to avoid the reaction of Al melts and atmospheric air, the argon gas was supplied into the furnace at a constant flow rate of 2 lpm. Meanwhile, C2 Cl6 (0.01 kg) was added to the molten Al as the degassing agent, then the oxides on the surface of the melts were thoroughly skimmed. A predetermined quantity of SiO2 particles were previously preheated at 700 ◦ C for 1 h in a muffle furnace. Thereafter, these particles were slowly fed into the molten Al with the help of stirring process. In order to make the particles obtain enough drag force from the vortex, and also to avoid an accidental spilling out of the Al melts from the crucible, the rotating speed of the impeller was set at 500 rpm. The stirring operation was lasted for 10 min after all of the particles were fed into the melt.
As soon as the stirring process was completed, the oxides on the surface of the melt were thoroughly removed once again. When the temperature of the melt decreased to 700 ± 5 ◦ C, it was immediately decanted to a graphite container with a capacity of 40 mL. The thermal analysis process is exhibited in Fig. 2. A K-type thermocouple was placed in the center of the container and immersed into the position of 25 mm from the bottom of the container. The thermocouple and the container were all fixed on a test stand to avoid any vibration. The temperature-time curve was recorded at a dynamic rate of 5 Hz/Ch using NI Thermocouple 9212 Series Module which was connected with NI cDAQ-9171 system. The room temperature was controlled at 25 ± 2 ◦ C. 2.2. Solidification history characterization Temperature-time data was obtained when the temperature of the melt was decreased to 500 ◦ C. Investigation of the cooling curve was conducted on the basis of Newtonian thermal analysis (NTA) method. This method assumed the absence of thermal gradients within the melt during its cooling procedure. The temperature-time curve and its first- and second- derivatives were plotted to study the phase transformation during solidification process. The first- derivative curve reflects the latent heat released from the phase transformation course. The second- derivative of the curve is used for determining the onset point (t0 ) and the end point (tE ) of the course. In this theory, the energy balance in the thermal analysis system can be expressed as [23]: Qc + Qs = Qa
(1)
where Qc is the heat flow transferred from the heat source to the surrounding, Qs is the latent heat released from solidification process and Qa is the accumulated heat flow in the testing system. In consideration of the volumetric heat flows, Eq. (1) can be written as: dQex dfs dT + LF = Cv dt dt dt
(2)
where Qex the heat flux transferred from the metal melt to the surrounding container, LF the volumetric latent heat of fusion, fs the solid fraction, dfs /dt the solidifying rate, CV the volumetric heat capacity of the melts system and dT/dt the temperature rate. If the system is cooled without phase transformation, Eq. (2) can be reduced to: dT 1 dQex = Cv dt dt
(3)
Rearrangement of Eqs. (2) and (3) is shown as follow: Qs = Cv
dQex dT − dt dt
dT − ZN) = Cv ( dt
(4)
where ZN is a baseline used to describe the cooling rate of the melt without any phase transformation. The QS in Eq. (4) is used to detect the amount of latent heat by measuring the cooling rate difference between the heat releasing caused by phase transformation and the baseline. The solidification history of the molten composite is based on the latent heat measurement. According to Eq. (4), latent heat released from the solidification process is written as:
te
LF =
Cv ( t0
dT − ZN)dt dt
(5)
K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
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Fig. 1. Morphologies and size distribution of SiO2 particles.
Fig. 2. Thermal analysis set up with graphite mould and K-type thermocouple.
The t0 and te in Eq. (5) represent for the onset and the end of the solidification course, respectively. During solidification process, the solid fraction at time t is determined as: fs =
1 LF
t
Cv ( t0
dT − ZN)dt dt
(6)
In this equation, the baseline (ZN) can be evaluated based on references [23,24]: ZN = ZNL (1 − fs ) + ZNs fs
(7)
where ZNL and ZNS are related to the cooling behavior of liquid and solid phase, respectively. These two parameters are determined by the cooling rate at the onset (dT(t0 )/dt) and end point (dT(te )/dt) of the solidification process, respectively. The volumetric heat capacity (Cv ) of the solid-liquid system during solidification process is evaluated from the following equation [23]: Cv = L CpL (1 − fs ) + s CpS fs
(8)
where and Cp signifies density and heat capacity, respectively. The subscript ‘L’ and ‘S’ represents for ‘liquid’ and ‘solid’, respectively. The Cv in Eq. (8) will change with the fraction solid of the melt during the cooling process. During the stir casting process, the Si and Al2 O3 would generate by the in-situ reaction of SiO2 /Al [25]: 4Al(l) + 3SiO2(s) = 2Al2 O3(s) + 3Si(s)
(9)
Before solidification, there are four phases (i.e., Al(l) , SiO2(s) , Si(s) and Al2 O3(s) ) exist in the molten Al2 O3 /SiO2(p) /Al composites. Therefore, the effective heat capacity of the melt is defined as [16]: n=4
CP =
i Vi Cp−i
i=1
c
(10)
where i and Vi are the density and volume fraction of the materials in molten composites, respectively. The variable ‘i’ given in Eq. (10) represents for different materials (viz. Al (i = 1), SiO2 (i = 2), Si (i = 3),
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Table 2 Physical parameters for thermal analysis.
Table 3 Thermal analysis parameters of the molten composites.
Materials
Density (kg/m3 )
CP (kJ/(kg ◦ C) [26]
Samples
Sample-1
Sample-2
Sample-3
Sample-4
Al
2380 (liquid) 2700 (solid) 2500 2328 3965
0.880
Cooling Rate (◦ C/s) TN (◦ C) t0 (s) tE -t0 (s) Stage-1 (◦ C) Stage-2 (◦ C)
0.6 670.7 52 410 660.6 –
1.3 674.5 21 350 653.9 566.2
0.8 664.5 51 356 650.4 576.8
0.8 669.1 39 451 648.4 576.2
SiO2 Si ␣-Al2 O3
0.966 0.703 0.750
Al2 O3 (i = 4)) in the system. The c given in Eq. (10) is the effective density of the melt. It is evaluated by the following equation:
n=4
c =
Vi i (11)
i=1
According to Eq. (11), the volume fraction of Si in the matrix of the composite can be estimated by using EDS testing. Then, the fraction of the other reactants and product, i.e., Al, SiO2 and Al2 O3 , can be evaluated on the basis of Eq. (9). All the thermophysical parameters related to thermal analysis are summarized in Table 2 [26]. Numerical integration algorithm [23,24] was carried out to determine the baseline on the basis of Eqs. (3)–(11). The algorithm was executed by using MATLAB software. Cooling rates (CR) of the molten purity Al and composites were determined by the slopes of the cooling curves before the onsets of the nucleation point [20]: CR =
TP − TN ◦ ( C/s) |tP − tN |
(12)
where TP and TN represent the pouring temperature and nucleation temperature, respectively. The tP and tN symbolize for the time of pouring the molten composite and the nucleation point of the melt, respectively. 2.3. Microstructure identification After thermal analysis, the solidified samples were cut and machined out for metallographic analysis. Metallographic specimens were fabricated via standard grinding and polishing. Thereafter, these specimens were etched on their polished surface by using 0.5% HF. The acid etching process was lasted for 15 s. The optical microscope (Leica DMI300-M) was utilized to observe the distribution of reinforcements in Al matrix. Dendrite arm spacing (DAS) of the ␣-Al grains were statistically measured via ImagePro Plus software. The average value of a total 100 measurements under five different view fields was reported for each sample. The constituents of these specimens were characterized with X-ray diffraction (XRD). The morphologies of the phases in the matrix were identified by SEM equipped with EDS. The EDS testing was also used to estimate the Si content in the matrix of the composites. An average value of Si wt.% was obtained from five measurements at different regions of a composite matrix. 3. Results 3.1. Cooling curves of the molten composites Fig. 3 shows the cooling curves of the molten commercial purity Al and composite samples. The first- and second- derivative curves and the baselines are also plotted in the corresponding graph. Cooling rates of the melts with 0 wt.%, 3 wt.%, 6 wt.% and 9 wt.% SiO2 additions are calculated based on Eq. (12). They are mentioned in Table 3. The cooling curve plotted in Fig. 3a shows a wide stage at about 660 ◦ C. It is related to the liquidus of purity Al. The first- derivative of the cooling curve in this diagram reveals a wide peak. This peak
is caused by the latent heat released from the solidification of ␣Al. Since the commercial purity Al (with about 0.1% contaminants) used in this study is not pure enough, a very small fluctuation in this first- derivative curve is revealed (labeled with a green arrow in the enlarged diagram: ‘nearby tE ’). The cooling curves in Fig. 3b–d show one more narrow stage at about 570 ◦ C. These stages are associated with the Al-Si phase transformations. The Si atoms in molten Al are from the in-situ reaction of SiO2 /Al. Compared with Fig. 3a, the first- derivative curves in Fig. 3b–d display one more sharp peak, which indicate the increase of the amount of latent heat caused by the Al-Si phase transformation. The onset and end points of the solidification process are labeled as ‘t0 ’ and ‘tE ’ in Fig. 3, respectively. Details of the curves nearby of these two points are also plotted by reducing the ranges of the time axes. Determination of these two points are achieved by using the peaks and valleys displayed in second- derivative curves. When the second derivative curve radically shifts upward and firstly intercepts the zero line, the intersection is determined as the onset point of ␣-Al nucleation. The nucleation point of ␣-Al (TN ) is deemed as the onset of solidification process (at t0 ) [27]. The first- derivative curve which sharply intercepts the zero line is defined as Tmin point. The Tmin point in the cooling curve signifies the temperature slightly rises up again, whilst the wide stage in the cooling curve is formed. The end of the solidification course (at tE ) is determined by the last time of the second- derivative curve swiftly goes up and intercepts the zero line. The solidification onset (t0 ) and its time duration (tE -t0 ) of these samples are given in Table 3. The melts with 0 wt.% 3 wt.%, 6 wt.% and 9 wt.% SiO2 particles addition are named as Sample-1, Sample2, Sample-3 and Sample-4, respectively. The temperature of the onset points for the two stages (represented by Stage-1 and Stage-2, respectively) are also summarized in this table. 3.2. Solidification course of the molten composites The solidification histories of these molten composites are plotted in Fig. 4. The fraction solid curve of Sample-1 is very smooth, which suggests no significant reaction has taken place during the solidification process of ␣-Al. Inflection points are found in the fraction solid curves of Sample-2, Sample-3 and Sample-4. It indicates phase transformations except for the ␣-Al solidification have taken place during cooling process of the melt. In Fig. 4, the range between the solidification onset and the inflection point is taken as the solidification period of ␣-Al grains. Since the SiO2 /Al reaction will produce Si atoms in Al melt, inflection point in this curve represents the onset of Al-Si phase transformation. Obviously, the fraction solid of ␣-Al decreases with the promotion of SiO2 quantity in molten Al. As far as the molten composites, their solidification time durations are extended with the increase of SiO2 addition. 3.3. Microstructure identification The optical microstrutures of these fabricated samples are given in Fig. 5. Fig. 5a shows the purity Al matrix with no SiO2 particles addition. Fig. 5b–d display the microstructures of these composite samples with 3 wt.%, 6 wt.% and 9 wt.% SiO2 particles addition,
K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
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Fig. 3. Cooling curves and their first- and second- derivatives of Al melts with different additions of SiO2 particles: (a) 0 wt%, (b) 3 wt.%, (c) 6 wt.%, (d) 9 wt.%.
respectively. It is obvious that area fraction of the particles in the matrix increases with increasing in the SiO2 addition. The distribution of particles in these composites are relatively uniform. Besides, it is noted that grain size in the matrix of these samples are different. Frequency histograms of the DAS values for purity Al and composites’ matrix are plotted in Fig. 6. Mean DAS values determined by Gauss fitting curves [28] are 125.33 m, 83.76 m, 57.23 m and
45.38 m for Sample-1, Sample-2, Sample-3 and Sample-4, respectively. It reveals the downward trend with the increasing addition of SiO2 particles. The results of XRD testing are shown in Fig. 7. The XRD patterns of SiO2 and the commercial pure Al applied in this research are colored with black and red in this diagram, respectively. The curve in blue is the XRD pattern of Al2 O3 /SiO2(p) /Al composite (Sample-4).
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K. Wang et al. / Thermochimica Acta 641 (2016) 29–38 Table 4 Estimation of Si concentrations in the matrix of Al2 O3 /SiO2(p) /Al composites.
Fig. 4. Fraction solid curves of purity Al with different additions of SiO2 particles: (Sample-1) 0 wt.%; (Sample-2) 3 wt.%; (Sample-3) 6 wt.% and (Sample-4) 9 wt.%.
The peaks related to Al matrix, SiO2 particles, Si and Al2 O3 are separately labeled with different symbols. It suggests that the reactants and products of SiO2 /Al reaction are existed in the composite. Fig. 8 shows the morphologies of the phases in Al2 O3 /SiO2(p) /Al composites. Fig. 8a shows some particles surrounded with layers are distributed in the matrix. The EDS result indicates the particles and the layers are SiO2 and Al2 O3 , respectively. This special duplex phase proves the SiO2 /Al in-situ reaction has taken place at the particle/matrix interface. The micrograph of the composite with higher magnification is provided in Fig. 8b. In Al matrix, the line scanning pattern shows a sudden increase of Si intensity with plunged Al intensity (denoted by the black arrow in Fig. 8b). It proves the Si atoms are precipitated on Al matrix during the solidification course. The rectangle marked in Fig. 8b is the district for estimation of Si concentration in Al matrix. Five areas with SiO2 particles excluded for each sample have been measured and given in
Samples
Sample-1
Sample-2
Sample-3
Sample-4
Si (wt.%)
–
0.80 ± 0.10
1.27 ± 0.12
2.01 ± 0.15
Table 4. The errors of these measurements are also provided in this table. The microstructure with a magnification of 50000 x is given in Fig. 8c. The homogeneously dispersed bright particles is shown in this image. The EDS testing result indicates these fine particles are Al2 O3 phase. Fig. 8d provides more details of the special duplex phase. According to the elements distribution in line scanning pattern, this structure is divided into three regions. The Region-1 in this graph is determined as the composite’s matrix. It is noted the aluminum intensity in this region is relatively high, whilst the silicon element can also be detected. Region-2 is identified as the Al2 O3 layer. High intensity of Al and O elements is detected in this area, but the intensity for Si element is very weak. Region-3 is confirmed as the SiO2 particle. High intensity of Si and O and low intensity of Al element are found in this region. These findings suggest the SiO2 /Al in-situ reaction under the current fabricate process is incompleted. 4. Discussion 4.1. Effect of the in-situ reaction on microstructure During the stir casting process, the SiO2 /Al in-situ reaction has been identified according to Eq. (9). The standard Gibbs free energy (GT ) of this reaction can be calculated based on the equation as follow [25]: GT = −617977 + 79.3T
(13)
where T represents the temperature of the fabricate process. The GT value at 750 ◦ C is about −558.5 kJ/mol. Negative standard Gibbs free energy of this reaction suggests the formation of Al2 O3 and Si phases are thermodynamically possible [29]. However, the
Fig. 5. Optical microstructures of Al2O3/SiO2(p)/Al composites with different additions of SiO2 particles: (a) 0 wt.%, (b) 3 wt.%, (c) 6 wt.% and (d) 9 wt.%.
K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
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Fig. 6. Distribution of the dendrite arm spacing on the matrix of Al2O3/SiO2(p)/Al composites with different SiO2 additions: (a) 0 wt.%; (b) 3 wt.%; (c) 6 wt.%; (d) 9 wt.%.
Fig. 7. XRD patterns of the SiO2 particles, Al matrix and Al2O3/SiO2(p)/Al composite.
Al2 O3 layers formed between the particles and Al matrix will hinder the further reaction of SiO2 and Al melt. Because there exists no reactive wetting at the Al2 O3 /Al liquid interface below 1000 ◦ C, and the contact angel at this interface is larger than 90◦ [30,31]. It leads to the phenomenon that SiO2 particles can not be completely consumed (as presented in Fig. 8a and d). Besides, Fig. 8b shows an increase of Si concentration in Al matrix (denoted by an arrow in the line scanning pattern). It proves the Si atoms penetrate into the alumina layer and diffuse into the melt after the SiO2 /Al reaction. This finding is also supported by the kinetic model of SiO2 /Al
in-situ reaction proposed by Zhu et al. [25]. The alumina particles displayed in Fig. 8c are mainly resulted from the effect of the stirring process. The Al2 O3 particles might be peeled off from the Al2 O3 layers by the interaction between particles and the melt. The decreasing trend of DAS value in Fig. 6 is associated with the SiO2 /Al in-situ reaction. Vugt et al. [32] suggested the DAS was sensitive to the cooling rate and composition of the melt. As seen from Table 3, the liquidus stages (Stage-1) of these melts decrease with the increase of particles addition. From Table 4, the silicon content in Al matrix increases with increasing in the SiO2 addition. In fact, the higher particles addition provides more SiO2 surface to react with Al and generates more Si atoms in the melt. It results in the decline in liquidus stage due to the colligative property of Al-Si dilute solution. In this case, the lower liquidus stage provides greater undercooling to the system, which further increases the nucleation rate of ␣-Al grains. Hence, the larger addition of SiO2 particles leads to the reduction of DAS value. The Al2 O3 phase is the other product of SiO2 /Al reaction. To date, in our knowledge scope, no references pointed out the Al2 O3 particles could facilitate the nucleation of ␣-Al grain during solidification course. In contrary, Wu et al. [33] suggested the ␣-Al could not nucleate at the alumina substrate because of the contact angle at their interface is larger than 90◦ . Meanwhile, alumina could not be the heterogeneous nucleation substrate for Si phase. Because their lattice mismatch parameter was too large [33]. The line scanning result in Fig. 8d shows the relatively low intensity of Si in Al2 O3 layer. Because the diffusion ability of Si atoms is weaken with the increased thickness of Al2 O3 layer and the decreasing temperature during solidification course [34].
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K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
Fig. 8. Microstructure identification of the Al2O3/SiO2(p)/Al composite. (a) Duplex phase: SiO2 particles surrounded with Al2O3 layer; (b) identification for Si phase in Al matrix; (c) fine Al2O3 particles in Al matrix; (d) Line scanning for the duplex phase.
4.2. Effect of SiO2 additions on solidification behavior Solidification patterns of the molten composites are influenced by the additions of SiO2 particles and different quantities of the products from the in-situ reaction. As seen from Fig. 3, the shapes
of the cooling curves are totally different. The increase of particles addition in the melt leads to the reduction of Al liquid in per unit volume. Therefore, during the solidification course, the amount of latent heat released from the formation of ␣-Al grain in the molten composites are less than that of molten purity Al.
K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
Table 3 shows the solidification time duration (tE -t0 ) of molten commercial purity Al is longer than that of molten composites with 3 wt.% and 6 wt.% SiO2 particles. But solidification time duration of Sample-4 (with 9 wt.% SiO2 addition) is longer than that of molten commercial purity Al. Meanwhile, it is noted the time durations of the stages at about 570 ◦ C increase with increasing in the amount of SiO2 particles. These results suggest the solidification rates of these melts are different in mushy zone. According to the heat transfer model [35], energy balance of the system can be expressed as: hAs (T∞ − T )dt = mCp dT
(14)
where h is the heat transfer coefficient, As is the surface area of the molten alloy, m is the mass of these melt, and Cp is the effective heat capacity which depends on the total amount of latent heat released from the system during the cooling process; the T∞ and T is the temperature of environment and the melt, respectively. This equation can also be taken as the equilibrium between ‘Heat transfer into the environment during dt’ and ‘The decrease in the energy of the system during dt’. During the differential time interval dt, the temperature of the system decreases by a differential amount dT. In order to know the effect of latent heat releasing on the solidification rate, Eq. (14) is rearranged: dT hAs (T∞ − T ) = mCp dt
(15)
The previous study [16] indicated the change of heat transfer coefficient (h) for AMCs was not obvious, even though the addition of ceramic particles in molten Al reached 20 wt.%. Therefore, the solidification rate of the melt in the mushy zone mainly depends on the change of Cp which is correlated with the total of latent heat released from the phase transformation and temperature decrease of all the materials in the melt. As far as the experimental results, higher Si concentration in molten Al is caused by the greater SiO2 addition. It further results in the greater amount of latent heat released from the Al-Si phase transformation during solidification process. In this period, the effective Cp value in the system with higher SiO2 addition is greater, leading to the decrease of solidification rate. Thus it makes the possibility for Sample-4 possessing the longer time duration of the Al-Si transformation stage (Stage-2) and solidification course than those of the other samples. Moreover, the cooling rates of the four samples are varied before solidification. During this period, the effective heat capacity of the melt is one of the main factors influencing the cooling rate. From Table 2, the heat capacity of SiO2 , Si and Al2 O3 are different from that of purity Al. Wu et al. [16] measured the cooling curves of SiC/A359 composites. This report suggested the cooling rate of the molten alloy decreased with the increase of SiC particles addition. It was because the heat capacity of SiC particles was larger than that of the base alloy. In the SiC/A359 system, the reinforcement particles would not react with the molten Al alloy. But in the present research, the reaction between the reinforcement and molten matrix could take place during the fabricate process. The products from the reaction (i.e., Si atoms and Al2 O3 phase) are existed in the molten Al before solidification. According to Eq. (11), the effective heat capacity of the molten composite depends on the fraction and heat capacity of each phase in the system. Since the heat capacity of the products are lower than that of purity Al and SiO2 particles, it is possible to result in the decrease of effective heat capacity of the molten composites after SiO2 /Al reaction. Therefore, as shown in Table 3, the cooling rates of the molten composites are higher than that of the pure Al melt. Compared with Sample-2, the lower cooling rates of Sample-3 and Sample-4 are due to the complex effect of the increased amount of SiO2 particles and the different quantities of the products from in-situ reaction.
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5. Conclusion In this study, the Newtonian thermal analysis was originally conducted on Al2 O3 /SiO2(p) /Al composites fabricated by using stir casting method. The conclusion derived from this research is given below:
1) The dendrite arm spacing decreased to 45.38 m when the SiO2 addition increased to 9 wt.%. It was resulted from the effect of higher Si concentration and greater undercooling level of the melt. 2) The special duplex phase in the composite indicated the reaction of SiO2 /Al at 750 ± 5 ◦ C was incomplete. 3) The cooling curves showed liquidus temperature of the composite decreased with the increase of SiO2 addition. It was due to the higher Si concentration resulted from SiO2 /Al reaction in molten Al. 4) Since the volume fraction of molten Al decreased with the increase of SiO2 addition, solidification time durations of ␣-Al in the composites were shorter than that of molten purity Al. But the total solidification time durations of the composite samples were varied due to the effect of Al-Si exothermic reaction. 5) Before solidification, cooling rates were varied with the additions of particles in purity Al melt. This phenomenon was mainly due to the difference in effective heat capacity of the melts.
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Contents lists available at ScienceDirect
Thermochimica Acta journal homepage: www.elsevier.com/locate/tca
Thermal analysis of in-situ Al2 O3 /SiO2(p) /Al composites fabricated by stir casting process Kang Wang a , Wenfang Li a,∗ , Jun Du a , Lu Yang a , Peng Tang b a b
School of Materials Science and Engineering, South China University of Technology, Guangzhou 510640, China School of Materials Science and Engineering, Guangxi University, Nanning 530004, China
a r t i c l e
i n f o
Article history: Received 24 May 2016 Received in revised form 8 August 2016 Accepted 22 August 2016 Available online 24 August 2016 Keywords: Aluminum matrix composites (AMCs) SiO2 /Al in-situ reaction Thermal analysis Dendrite arm spacing Solidification behavior
a b s t r a c t The in-situ fabricated Al2 O3 /SiO2(p) /Al composites were produced by adding different quantities of SiO2 particles (viz. 0, 3, 6 and 9 wt.%) into molten commercial purity Al at 750 ◦ C. Newtonian thermal analysis was conducted to investigate the solidification behaviors of these molten composites prepared by the stir casting process. The microstructure and phase identification were achieved via optical microscope, SEM (equipped with EDS) and X-ray diffraction. Results showed the dendrite arm spacing of ␣-Al grain decreased with the increase of SiO2 addition. The SEM image showed the special duplex phase with Al2 O3 layer formed around SiO2 particle in Al matrix. The Si concentration in Al matrix increased with increasing in the SiO2 addition, resulting in the decrease of liquidus temperature of the melts. Compared with the purity Al melt, one more stage related to the exothermic Al-Si phase transformation was revealed in the cooling curves of these molten composites. Cooling rate of the purity Al melt was lower than that of the melts with SiO2 particles addition. The difference in solidification behaviors of these melts was ascribed to the effect of SiO2 /Al reaction on the effective heat capacity and different amounts of latent heat released from phase transformation process. © 2016 Elsevier B.V. All rights reserved.
1. Introduction In-situ fabricated aluminum matrix composites (AMCs) are deemed as the novel materials for their light weight, high specific strength and excellent wear resistance properties [1,2]. They have been gaining wide acceptance in aerospace and automotive industry in recent years. In order to achieve even better mechanical properties, ceramic particles with high elasticity modulus (such as SiC and Al2 O3 ) are added to the metals or alloys. Among them, the Al-Mg alloys are usually used as the matrix of SiC(p) /Al-Mg, Al2 O3(p) /Al-Mg and Al2 O3 /SiO2(p) /Al-Mg composites due to their feasibility of insitu reaction at ceramic/metal interface [3]. Lai et al. [4] and Eustathopoulos et al. [5] suggested the in-situ reaction at the interface of different materials could enhance their wettability. This enhancement not only promoted the mechanical properties of the composites, but also facilitated the incorporation of ceramic particles in molten Al [6]. Schultz et al. [7] indicated the Al2 O3 could
∗ Corresponding author. E-mail address: mewfl[email protected] (W. Li). http://dx.doi.org/10.1016/j.tca.2016.08.009 0040-6031/© 2016 Elsevier B.V. All rights reserved.
react with the Mg in molten Al-Mg alloy. Shi et al. [8] reported that the wettability of SiC particle with SiO2 layer surrounded was better than that without SiO2 layer surrounded. It was resulted from the reduced contact angel at the reactive SiO2 /Al interface. Mohammadpour et al. [9] indicated the defects at SiCp /Al interface in the AMCs could be eliminated by the interfacial reaction taken place among Al, SiO2 and Mg. Nowadays, researchers pay their attention on the SiO2 /Al insitu reaction which takes place in different fabricate process of the composites [10,11]. Zhu et al. [10] produced the in-situ fabricated SiO2 /Al composites using powder metallurgical process. In this case, the SiO2 powder could completely react with Al at about 1200 ◦ C. However, the high temperature and complexity of powder metallurgical method limits the mass production for these composites. Stir casting has been regarded as a commercial route for incorporating SiO2 particles into Al melts because it can be achieve at a lower temperature [12]. Some researchers reported that the contact angle at SiO2 /Al interface below 1000 ◦ C was smaller than 90◦ [13,14], which suggested the incorporation of SiO2 particles in molten Al was feasible. In our previous study [15], it has been proven that the zircon tailing sand (SiO2 content: 78 wt.%) can react with purity Al melt. Since the added particles and products from SiO2 /Al in-situ reaction will change the physical properties of the
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K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
Table 1 Chemical composition of commercial purity Al. Element
Si
Fe
Mn
Al
Content (wt.%)
0.02
0.05
0.02
Balance
molten composites, it is necessary to further study the effect of SiO2 additions on their solidification behaviors. The in-situ reaction in molten AMCs is favorable to the formation of reinforcements, whilst the products from the reaction might have a significant effect on modifying the microstructure of the composite’s matrix. However, to the best of our knowledge, only a few reports [10,16,17] were focus on the relationship among the solidification behavior, phase transformation and microstructure in AMCs. The DSC testing was often provided to detect the phase transformation in the alloy by using a very little piece of sample [18,19]. But this way was taken as the laboratiorial testing method because the samples were tested under an equilibrium condition. Moreover, the samples remelted in the DSC analyser could not reflect the solidification feature of the melt obtained right after the stir casting process. The cooling curve thermal analysis (CCTA) has been extensively used to detect the occurrence of different phases in the solidification history of molten alloys [20]. Compared with the DSC technique, the CCTA method is more suitable to monitor the phase transformation of the molten alloys and AMCs after their fabricate processes [21,22]. Wu et al. [16] investigated the cooling curve of SiC/Al-Si-Mg composites. The AMCs in this study were immediately poured into a mould for CCTA analysis after the stirring operation. This method was also conducted to analyzed the cooling curves and solidification behaviors of Al2 O3 /Al-Mg composites by Petric et al. [17]. In this study, the SiO2 particles were incorporated into the molten Al by stir casting process. The commercial purity Al was used as the composite matrix without adding any alloying elements. Effect of SiO2 quantity on the solidification behavior of the molten composite was studied by the CCTA method. 2. Experiment and method 2.1. Preparation and thermal analysis for molten AMCs The SiO2 particles (with a purity of 99.9%) were supplied from a quartz manufacturer (Dongjing Quartz Company, Jiangsu Province, P. R. China). The morphologies and size distribution of these particles are shown in Fig. 1. The average size of them is about 103 m (tested by the laser size instrument: BACKMAN COULTER LS-13-320). Table 1 shows the chemical composition of commercial purity Al used as the matrix of the composites. The commercial purity Al ingot (1.5 kg) was cleaned by water, dried and placed inside a graphite crucible. It was molten in an electrical furnace at 750 ± 5 ◦ C. An electric motor with graphite stirrer assembled was used for generating a vortex in molten Al. Both the crucible and stirrer were coated with Mica coating. In order to avoid the reaction of Al melts and atmospheric air, the argon gas was supplied into the furnace at a constant flow rate of 2 lpm. Meanwhile, C2 Cl6 (0.01 kg) was added to the molten Al as the degassing agent, then the oxides on the surface of the melts were thoroughly skimmed. A predetermined quantity of SiO2 particles were previously preheated at 700 ◦ C for 1 h in a muffle furnace. Thereafter, these particles were slowly fed into the molten Al with the help of stirring process. In order to make the particles obtain enough drag force from the vortex, and also to avoid an accidental spilling out of the Al melts from the crucible, the rotating speed of the impeller was set at 500 rpm. The stirring operation was lasted for 10 min after all of the particles were fed into the melt.
As soon as the stirring process was completed, the oxides on the surface of the melt were thoroughly removed once again. When the temperature of the melt decreased to 700 ± 5 ◦ C, it was immediately decanted to a graphite container with a capacity of 40 mL. The thermal analysis process is exhibited in Fig. 2. A K-type thermocouple was placed in the center of the container and immersed into the position of 25 mm from the bottom of the container. The thermocouple and the container were all fixed on a test stand to avoid any vibration. The temperature-time curve was recorded at a dynamic rate of 5 Hz/Ch using NI Thermocouple 9212 Series Module which was connected with NI cDAQ-9171 system. The room temperature was controlled at 25 ± 2 ◦ C. 2.2. Solidification history characterization Temperature-time data was obtained when the temperature of the melt was decreased to 500 ◦ C. Investigation of the cooling curve was conducted on the basis of Newtonian thermal analysis (NTA) method. This method assumed the absence of thermal gradients within the melt during its cooling procedure. The temperature-time curve and its first- and second- derivatives were plotted to study the phase transformation during solidification process. The first- derivative curve reflects the latent heat released from the phase transformation course. The second- derivative of the curve is used for determining the onset point (t0 ) and the end point (tE ) of the course. In this theory, the energy balance in the thermal analysis system can be expressed as [23]: Qc + Qs = Qa
(1)
where Qc is the heat flow transferred from the heat source to the surrounding, Qs is the latent heat released from solidification process and Qa is the accumulated heat flow in the testing system. In consideration of the volumetric heat flows, Eq. (1) can be written as: dQex dfs dT + LF = Cv dt dt dt
(2)
where Qex the heat flux transferred from the metal melt to the surrounding container, LF the volumetric latent heat of fusion, fs the solid fraction, dfs /dt the solidifying rate, CV the volumetric heat capacity of the melts system and dT/dt the temperature rate. If the system is cooled without phase transformation, Eq. (2) can be reduced to: dT 1 dQex = Cv dt dt
(3)
Rearrangement of Eqs. (2) and (3) is shown as follow: Qs = Cv
dQex dT − dt dt
dT − ZN) = Cv ( dt
(4)
where ZN is a baseline used to describe the cooling rate of the melt without any phase transformation. The QS in Eq. (4) is used to detect the amount of latent heat by measuring the cooling rate difference between the heat releasing caused by phase transformation and the baseline. The solidification history of the molten composite is based on the latent heat measurement. According to Eq. (4), latent heat released from the solidification process is written as:
te
LF =
Cv ( t0
dT − ZN)dt dt
(5)
K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
31
Fig. 1. Morphologies and size distribution of SiO2 particles.
Fig. 2. Thermal analysis set up with graphite mould and K-type thermocouple.
The t0 and te in Eq. (5) represent for the onset and the end of the solidification course, respectively. During solidification process, the solid fraction at time t is determined as: fs =
1 LF
t
Cv ( t0
dT − ZN)dt dt
(6)
In this equation, the baseline (ZN) can be evaluated based on references [23,24]: ZN = ZNL (1 − fs ) + ZNs fs
(7)
where ZNL and ZNS are related to the cooling behavior of liquid and solid phase, respectively. These two parameters are determined by the cooling rate at the onset (dT(t0 )/dt) and end point (dT(te )/dt) of the solidification process, respectively. The volumetric heat capacity (Cv ) of the solid-liquid system during solidification process is evaluated from the following equation [23]: Cv = L CpL (1 − fs ) + s CpS fs
(8)
where and Cp signifies density and heat capacity, respectively. The subscript ‘L’ and ‘S’ represents for ‘liquid’ and ‘solid’, respectively. The Cv in Eq. (8) will change with the fraction solid of the melt during the cooling process. During the stir casting process, the Si and Al2 O3 would generate by the in-situ reaction of SiO2 /Al [25]: 4Al(l) + 3SiO2(s) = 2Al2 O3(s) + 3Si(s)
(9)
Before solidification, there are four phases (i.e., Al(l) , SiO2(s) , Si(s) and Al2 O3(s) ) exist in the molten Al2 O3 /SiO2(p) /Al composites. Therefore, the effective heat capacity of the melt is defined as [16]: n=4
CP =
i Vi Cp−i
i=1
c
(10)
where i and Vi are the density and volume fraction of the materials in molten composites, respectively. The variable ‘i’ given in Eq. (10) represents for different materials (viz. Al (i = 1), SiO2 (i = 2), Si (i = 3),
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K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
Table 2 Physical parameters for thermal analysis.
Table 3 Thermal analysis parameters of the molten composites.
Materials
Density (kg/m3 )
CP (kJ/(kg ◦ C) [26]
Samples
Sample-1
Sample-2
Sample-3
Sample-4
Al
2380 (liquid) 2700 (solid) 2500 2328 3965
0.880
Cooling Rate (◦ C/s) TN (◦ C) t0 (s) tE -t0 (s) Stage-1 (◦ C) Stage-2 (◦ C)
0.6 670.7 52 410 660.6 –
1.3 674.5 21 350 653.9 566.2
0.8 664.5 51 356 650.4 576.8
0.8 669.1 39 451 648.4 576.2
SiO2 Si ␣-Al2 O3
0.966 0.703 0.750
Al2 O3 (i = 4)) in the system. The c given in Eq. (10) is the effective density of the melt. It is evaluated by the following equation:
n=4
c =
Vi i (11)
i=1
According to Eq. (11), the volume fraction of Si in the matrix of the composite can be estimated by using EDS testing. Then, the fraction of the other reactants and product, i.e., Al, SiO2 and Al2 O3 , can be evaluated on the basis of Eq. (9). All the thermophysical parameters related to thermal analysis are summarized in Table 2 [26]. Numerical integration algorithm [23,24] was carried out to determine the baseline on the basis of Eqs. (3)–(11). The algorithm was executed by using MATLAB software. Cooling rates (CR) of the molten purity Al and composites were determined by the slopes of the cooling curves before the onsets of the nucleation point [20]: CR =
TP − TN ◦ ( C/s) |tP − tN |
(12)
where TP and TN represent the pouring temperature and nucleation temperature, respectively. The tP and tN symbolize for the time of pouring the molten composite and the nucleation point of the melt, respectively. 2.3. Microstructure identification After thermal analysis, the solidified samples were cut and machined out for metallographic analysis. Metallographic specimens were fabricated via standard grinding and polishing. Thereafter, these specimens were etched on their polished surface by using 0.5% HF. The acid etching process was lasted for 15 s. The optical microscope (Leica DMI300-M) was utilized to observe the distribution of reinforcements in Al matrix. Dendrite arm spacing (DAS) of the ␣-Al grains were statistically measured via ImagePro Plus software. The average value of a total 100 measurements under five different view fields was reported for each sample. The constituents of these specimens were characterized with X-ray diffraction (XRD). The morphologies of the phases in the matrix were identified by SEM equipped with EDS. The EDS testing was also used to estimate the Si content in the matrix of the composites. An average value of Si wt.% was obtained from five measurements at different regions of a composite matrix. 3. Results 3.1. Cooling curves of the molten composites Fig. 3 shows the cooling curves of the molten commercial purity Al and composite samples. The first- and second- derivative curves and the baselines are also plotted in the corresponding graph. Cooling rates of the melts with 0 wt.%, 3 wt.%, 6 wt.% and 9 wt.% SiO2 additions are calculated based on Eq. (12). They are mentioned in Table 3. The cooling curve plotted in Fig. 3a shows a wide stage at about 660 ◦ C. It is related to the liquidus of purity Al. The first- derivative of the cooling curve in this diagram reveals a wide peak. This peak
is caused by the latent heat released from the solidification of ␣Al. Since the commercial purity Al (with about 0.1% contaminants) used in this study is not pure enough, a very small fluctuation in this first- derivative curve is revealed (labeled with a green arrow in the enlarged diagram: ‘nearby tE ’). The cooling curves in Fig. 3b–d show one more narrow stage at about 570 ◦ C. These stages are associated with the Al-Si phase transformations. The Si atoms in molten Al are from the in-situ reaction of SiO2 /Al. Compared with Fig. 3a, the first- derivative curves in Fig. 3b–d display one more sharp peak, which indicate the increase of the amount of latent heat caused by the Al-Si phase transformation. The onset and end points of the solidification process are labeled as ‘t0 ’ and ‘tE ’ in Fig. 3, respectively. Details of the curves nearby of these two points are also plotted by reducing the ranges of the time axes. Determination of these two points are achieved by using the peaks and valleys displayed in second- derivative curves. When the second derivative curve radically shifts upward and firstly intercepts the zero line, the intersection is determined as the onset point of ␣-Al nucleation. The nucleation point of ␣-Al (TN ) is deemed as the onset of solidification process (at t0 ) [27]. The first- derivative curve which sharply intercepts the zero line is defined as Tmin point. The Tmin point in the cooling curve signifies the temperature slightly rises up again, whilst the wide stage in the cooling curve is formed. The end of the solidification course (at tE ) is determined by the last time of the second- derivative curve swiftly goes up and intercepts the zero line. The solidification onset (t0 ) and its time duration (tE -t0 ) of these samples are given in Table 3. The melts with 0 wt.% 3 wt.%, 6 wt.% and 9 wt.% SiO2 particles addition are named as Sample-1, Sample2, Sample-3 and Sample-4, respectively. The temperature of the onset points for the two stages (represented by Stage-1 and Stage-2, respectively) are also summarized in this table. 3.2. Solidification course of the molten composites The solidification histories of these molten composites are plotted in Fig. 4. The fraction solid curve of Sample-1 is very smooth, which suggests no significant reaction has taken place during the solidification process of ␣-Al. Inflection points are found in the fraction solid curves of Sample-2, Sample-3 and Sample-4. It indicates phase transformations except for the ␣-Al solidification have taken place during cooling process of the melt. In Fig. 4, the range between the solidification onset and the inflection point is taken as the solidification period of ␣-Al grains. Since the SiO2 /Al reaction will produce Si atoms in Al melt, inflection point in this curve represents the onset of Al-Si phase transformation. Obviously, the fraction solid of ␣-Al decreases with the promotion of SiO2 quantity in molten Al. As far as the molten composites, their solidification time durations are extended with the increase of SiO2 addition. 3.3. Microstructure identification The optical microstrutures of these fabricated samples are given in Fig. 5. Fig. 5a shows the purity Al matrix with no SiO2 particles addition. Fig. 5b–d display the microstructures of these composite samples with 3 wt.%, 6 wt.% and 9 wt.% SiO2 particles addition,
K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
33
Fig. 3. Cooling curves and their first- and second- derivatives of Al melts with different additions of SiO2 particles: (a) 0 wt%, (b) 3 wt.%, (c) 6 wt.%, (d) 9 wt.%.
respectively. It is obvious that area fraction of the particles in the matrix increases with increasing in the SiO2 addition. The distribution of particles in these composites are relatively uniform. Besides, it is noted that grain size in the matrix of these samples are different. Frequency histograms of the DAS values for purity Al and composites’ matrix are plotted in Fig. 6. Mean DAS values determined by Gauss fitting curves [28] are 125.33 m, 83.76 m, 57.23 m and
45.38 m for Sample-1, Sample-2, Sample-3 and Sample-4, respectively. It reveals the downward trend with the increasing addition of SiO2 particles. The results of XRD testing are shown in Fig. 7. The XRD patterns of SiO2 and the commercial pure Al applied in this research are colored with black and red in this diagram, respectively. The curve in blue is the XRD pattern of Al2 O3 /SiO2(p) /Al composite (Sample-4).
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K. Wang et al. / Thermochimica Acta 641 (2016) 29–38 Table 4 Estimation of Si concentrations in the matrix of Al2 O3 /SiO2(p) /Al composites.
Fig. 4. Fraction solid curves of purity Al with different additions of SiO2 particles: (Sample-1) 0 wt.%; (Sample-2) 3 wt.%; (Sample-3) 6 wt.% and (Sample-4) 9 wt.%.
The peaks related to Al matrix, SiO2 particles, Si and Al2 O3 are separately labeled with different symbols. It suggests that the reactants and products of SiO2 /Al reaction are existed in the composite. Fig. 8 shows the morphologies of the phases in Al2 O3 /SiO2(p) /Al composites. Fig. 8a shows some particles surrounded with layers are distributed in the matrix. The EDS result indicates the particles and the layers are SiO2 and Al2 O3 , respectively. This special duplex phase proves the SiO2 /Al in-situ reaction has taken place at the particle/matrix interface. The micrograph of the composite with higher magnification is provided in Fig. 8b. In Al matrix, the line scanning pattern shows a sudden increase of Si intensity with plunged Al intensity (denoted by the black arrow in Fig. 8b). It proves the Si atoms are precipitated on Al matrix during the solidification course. The rectangle marked in Fig. 8b is the district for estimation of Si concentration in Al matrix. Five areas with SiO2 particles excluded for each sample have been measured and given in
Samples
Sample-1
Sample-2
Sample-3
Sample-4
Si (wt.%)
–
0.80 ± 0.10
1.27 ± 0.12
2.01 ± 0.15
Table 4. The errors of these measurements are also provided in this table. The microstructure with a magnification of 50000 x is given in Fig. 8c. The homogeneously dispersed bright particles is shown in this image. The EDS testing result indicates these fine particles are Al2 O3 phase. Fig. 8d provides more details of the special duplex phase. According to the elements distribution in line scanning pattern, this structure is divided into three regions. The Region-1 in this graph is determined as the composite’s matrix. It is noted the aluminum intensity in this region is relatively high, whilst the silicon element can also be detected. Region-2 is identified as the Al2 O3 layer. High intensity of Al and O elements is detected in this area, but the intensity for Si element is very weak. Region-3 is confirmed as the SiO2 particle. High intensity of Si and O and low intensity of Al element are found in this region. These findings suggest the SiO2 /Al in-situ reaction under the current fabricate process is incompleted. 4. Discussion 4.1. Effect of the in-situ reaction on microstructure During the stir casting process, the SiO2 /Al in-situ reaction has been identified according to Eq. (9). The standard Gibbs free energy (GT ) of this reaction can be calculated based on the equation as follow [25]: GT = −617977 + 79.3T
(13)
where T represents the temperature of the fabricate process. The GT value at 750 ◦ C is about −558.5 kJ/mol. Negative standard Gibbs free energy of this reaction suggests the formation of Al2 O3 and Si phases are thermodynamically possible [29]. However, the
Fig. 5. Optical microstructures of Al2O3/SiO2(p)/Al composites with different additions of SiO2 particles: (a) 0 wt.%, (b) 3 wt.%, (c) 6 wt.% and (d) 9 wt.%.
K. Wang et al. / Thermochimica Acta 641 (2016) 29–38
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Fig. 6. Distribution of the dendrite arm spacing on the matrix of Al2O3/SiO2(p)/Al composites with different SiO2 additions: (a) 0 wt.%; (b) 3 wt.%; (c) 6 wt.%; (d) 9 wt.%.
Fig. 7. XRD patterns of the SiO2 particles, Al matrix and Al2O3/SiO2(p)/Al composite.
Al2 O3 layers formed between the particles and Al matrix will hinder the further reaction of SiO2 and Al melt. Because there exists no reactive wetting at the Al2 O3 /Al liquid interface below 1000 ◦ C, and the contact angel at this interface is larger than 90◦ [30,31]. It leads to the phenomenon that SiO2 particles can not be completely consumed (as presented in Fig. 8a and d). Besides, Fig. 8b shows an increase of Si concentration in Al matrix (denoted by an arrow in the line scanning pattern). It proves the Si atoms penetrate into the alumina layer and diffuse into the melt after the SiO2 /Al reaction. This finding is also supported by the kinetic model of SiO2 /Al
in-situ reaction proposed by Zhu et al. [25]. The alumina particles displayed in Fig. 8c are mainly resulted from the effect of the stirring process. The Al2 O3 particles might be peeled off from the Al2 O3 layers by the interaction between particles and the melt. The decreasing trend of DAS value in Fig. 6 is associated with the SiO2 /Al in-situ reaction. Vugt et al. [32] suggested the DAS was sensitive to the cooling rate and composition of the melt. As seen from Table 3, the liquidus stages (Stage-1) of these melts decrease with the increase of particles addition. From Table 4, the silicon content in Al matrix increases with increasing in the SiO2 addition. In fact, the higher particles addition provides more SiO2 surface to react with Al and generates more Si atoms in the melt. It results in the decline in liquidus stage due to the colligative property of Al-Si dilute solution. In this case, the lower liquidus stage provides greater undercooling to the system, which further increases the nucleation rate of ␣-Al grains. Hence, the larger addition of SiO2 particles leads to the reduction of DAS value. The Al2 O3 phase is the other product of SiO2 /Al reaction. To date, in our knowledge scope, no references pointed out the Al2 O3 particles could facilitate the nucleation of ␣-Al grain during solidification course. In contrary, Wu et al. [33] suggested the ␣-Al could not nucleate at the alumina substrate because of the contact angle at their interface is larger than 90◦ . Meanwhile, alumina could not be the heterogeneous nucleation substrate for Si phase. Because their lattice mismatch parameter was too large [33]. The line scanning result in Fig. 8d shows the relatively low intensity of Si in Al2 O3 layer. Because the diffusion ability of Si atoms is weaken with the increased thickness of Al2 O3 layer and the decreasing temperature during solidification course [34].
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Fig. 8. Microstructure identification of the Al2O3/SiO2(p)/Al composite. (a) Duplex phase: SiO2 particles surrounded with Al2O3 layer; (b) identification for Si phase in Al matrix; (c) fine Al2O3 particles in Al matrix; (d) Line scanning for the duplex phase.
4.2. Effect of SiO2 additions on solidification behavior Solidification patterns of the molten composites are influenced by the additions of SiO2 particles and different quantities of the products from the in-situ reaction. As seen from Fig. 3, the shapes
of the cooling curves are totally different. The increase of particles addition in the melt leads to the reduction of Al liquid in per unit volume. Therefore, during the solidification course, the amount of latent heat released from the formation of ␣-Al grain in the molten composites are less than that of molten purity Al.
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Table 3 shows the solidification time duration (tE -t0 ) of molten commercial purity Al is longer than that of molten composites with 3 wt.% and 6 wt.% SiO2 particles. But solidification time duration of Sample-4 (with 9 wt.% SiO2 addition) is longer than that of molten commercial purity Al. Meanwhile, it is noted the time durations of the stages at about 570 ◦ C increase with increasing in the amount of SiO2 particles. These results suggest the solidification rates of these melts are different in mushy zone. According to the heat transfer model [35], energy balance of the system can be expressed as: hAs (T∞ − T )dt = mCp dT
(14)
where h is the heat transfer coefficient, As is the surface area of the molten alloy, m is the mass of these melt, and Cp is the effective heat capacity which depends on the total amount of latent heat released from the system during the cooling process; the T∞ and T is the temperature of environment and the melt, respectively. This equation can also be taken as the equilibrium between ‘Heat transfer into the environment during dt’ and ‘The decrease in the energy of the system during dt’. During the differential time interval dt, the temperature of the system decreases by a differential amount dT. In order to know the effect of latent heat releasing on the solidification rate, Eq. (14) is rearranged: dT hAs (T∞ − T ) = mCp dt
(15)
The previous study [16] indicated the change of heat transfer coefficient (h) for AMCs was not obvious, even though the addition of ceramic particles in molten Al reached 20 wt.%. Therefore, the solidification rate of the melt in the mushy zone mainly depends on the change of Cp which is correlated with the total of latent heat released from the phase transformation and temperature decrease of all the materials in the melt. As far as the experimental results, higher Si concentration in molten Al is caused by the greater SiO2 addition. It further results in the greater amount of latent heat released from the Al-Si phase transformation during solidification process. In this period, the effective Cp value in the system with higher SiO2 addition is greater, leading to the decrease of solidification rate. Thus it makes the possibility for Sample-4 possessing the longer time duration of the Al-Si transformation stage (Stage-2) and solidification course than those of the other samples. Moreover, the cooling rates of the four samples are varied before solidification. During this period, the effective heat capacity of the melt is one of the main factors influencing the cooling rate. From Table 2, the heat capacity of SiO2 , Si and Al2 O3 are different from that of purity Al. Wu et al. [16] measured the cooling curves of SiC/A359 composites. This report suggested the cooling rate of the molten alloy decreased with the increase of SiC particles addition. It was because the heat capacity of SiC particles was larger than that of the base alloy. In the SiC/A359 system, the reinforcement particles would not react with the molten Al alloy. But in the present research, the reaction between the reinforcement and molten matrix could take place during the fabricate process. The products from the reaction (i.e., Si atoms and Al2 O3 phase) are existed in the molten Al before solidification. According to Eq. (11), the effective heat capacity of the molten composite depends on the fraction and heat capacity of each phase in the system. Since the heat capacity of the products are lower than that of purity Al and SiO2 particles, it is possible to result in the decrease of effective heat capacity of the molten composites after SiO2 /Al reaction. Therefore, as shown in Table 3, the cooling rates of the molten composites are higher than that of the pure Al melt. Compared with Sample-2, the lower cooling rates of Sample-3 and Sample-4 are due to the complex effect of the increased amount of SiO2 particles and the different quantities of the products from in-situ reaction.
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5. Conclusion In this study, the Newtonian thermal analysis was originally conducted on Al2 O3 /SiO2(p) /Al composites fabricated by using stir casting method. The conclusion derived from this research is given below:
1) The dendrite arm spacing decreased to 45.38 m when the SiO2 addition increased to 9 wt.%. It was resulted from the effect of higher Si concentration and greater undercooling level of the melt. 2) The special duplex phase in the composite indicated the reaction of SiO2 /Al at 750 ± 5 ◦ C was incomplete. 3) The cooling curves showed liquidus temperature of the composite decreased with the increase of SiO2 addition. It was due to the higher Si concentration resulted from SiO2 /Al reaction in molten Al. 4) Since the volume fraction of molten Al decreased with the increase of SiO2 addition, solidification time durations of ␣-Al in the composites were shorter than that of molten purity Al. But the total solidification time durations of the composite samples were varied due to the effect of Al-Si exothermic reaction. 5) Before solidification, cooling rates were varied with the additions of particles in purity Al melt. This phenomenon was mainly due to the difference in effective heat capacity of the melts.
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