Effect of Si, Co, Fe contents and cooling condition on the microstructure of Al–Si–Co(–Fe) alloys

In this paper, several Al–Si–Co and Al–Si–Co–Fe alloys were prepared by varying the contents of Si, Co and Fe, solidified via fast and slow–cooling conditions. In the ternary Al–xSi–yCo (x = 6, 12, 18; y = 2, 4) alloys, the Co–rich phase is Al9Co2–type with a certain Si content, which can be marked as (Al,Si)9Co2. The Al9Co2–type phase exhibits developed dendrites and block–like morphologies under fast and slow cooling conditions, respectively. In the quaternary Al–xSi–4Co–yFe (x = 6, 12, 18; y = 2, 4) alloys, the formed intermetallic compound is Al13Fe4–type, containing a certain amount of Si and Co. It exhibits fine and coarse dendritic morphologies under fast and slow–cooling conditions, respectively. With the increase of Si content and Fe: Co ratio in the Al–xSi–4Co–yFe alloys, the Si content and Fe: Co ratio in the Al13Fe4–type phase increase synchronously. The hardness of Al–xSi–4Co–yFe alloys were tested, and it was found that the fast–cooling alloys have higher hardness than the slow–cooling ones, while the value of fast–cooling Al–12Si–4Co–4Fe alloy is the highest. Besides, comparing with Al9Co2, the Al13Fe4 phase has a much higher tendentiousness to precipitate from Al–Si–Co–Fe alloys. This work may be referred for the control of Co– and Fe–rich phases in Al–Si alloys, with the concept of altering element contents and solidification conditions.


Introduction
Al-Si alloys have been developed for decades and widely used to produce cylinder blocks, cylinder heads and valve lifters, since they possess good castability, low thermal-expansion coefficient, high wear resistance, good corrosion resistance and improved mechanical properties [1][2][3]. A series of transitional metal elements are usually introduced into Al-Si alloys to improve the strength and heat resistance ability [4][5][6]. Among them, Fe is one of the most common impurities and it may be introduced into Al-Si alloys either from the low-purity alloying materials, or through the contamination of unprotected ferrous crucible, tools or equipment [7][8][9]. The effect of Fe in Al-Si alloys is two-sided. On the one hand, a certain amount of Fe is reported beneficial for enhancing the elevated-temperature strength in multicomponent Al-Si alloys, making it one of the best candidates for application in automotive and diecasting industries [6,[10][11][12]. On the other hand, since the solubility of Fe in Al is relatively low, the Fe usually forms intermetallic phases such as θ-Al 13 Fe 4 , β-Al 5 FeSi and α-Al 15 (Fe,Mn) 3 Si 2 , etc, with the preferred morphology of plate-like, needle-like and Chinese script [13,14]. Nowadays, with the development of characterization instruments and technologies, a series of researches by using X-radiographic imaging or synchrotron X-ray tomographic to study the morphology and growing models of Fe-rich phases can be referred [15][16][17][18][19]. For instance, Bjurenstedt et al [15] reported that the α-Al (FeMnCr)Si intermetallic compound nucleates primarily on surface oxides, and the continued growth yielded both rhombic dodecahedrons and elongated rod-like morphologies, by employing X-radiographic imaging and deep-etching methods. Song et al [18] found that the three-dimensional morphology of Fe-rich phases changes from interconnected plate-like to separated hollow polyhedron with increasing Mn content, according to the results of synchrotron X-ray tomography.
Among various Fe-rich phases in Al-Si alloys, β-AlSiFe phase is quite easy to precipitate, which is harmful for the mechanical properties due to its flake-like morphology [20,21]. To modify the unfriendly morphology of β-Al 5 FeSi, the concept of overheating the melt, increasing cooling rate and adding neutralized elements have been proven efficient [22][23][24][25][26][27]. For instance, Lan et al [28] reported that increasing the cooling rate will not only refine the microstructure but also promote the transformation of β-Al 9 Fe 2 Si 2 to α-Al 8 Fe 2 Si in Al-4.8Fe-3Si alloy. For the concept of adding neutralizers to modify Fe-rich phase, the most used elements include Mn, Cr and Co [29][30][31][32]. Zhang et al [33] found that the addition of Mn and Cr in Al-7Si-0.5Fe melt can increase the precipitation temperatures of Fe-rich phase, resulting in variation of crystal structures from monoclinic γ-AlFeSi to cubic α-AlFeMnCrSi phases. Simultaneously, the morphology of Fe-rich phase changes from platelike to cubic shape with the addition of Mn and Cr. Wang et al [34] reported that the addition of Mn in 6082Al alloy will modify β-AlFeSi to α-Al(FeMn)Si, as a result, the yield strength can be obviously enhanced. However, comparing to Mn and Cr, the behavior of Co element and its interaction behavior with Fe in Al-Si alloys can be rarely referred.
In this paper, Al-6Si, Al-12Si and Al-18Si alloys were selected as the base alloys. Ternary Al-Si-2Co and Al-Si-4Co alloys were prepared to clarify the size, morphology and structure of Co-rich intermetallic compounds, and the casting procedure was designed through fast and slow-cooling conditions. On this basis, different content of Fe was introduced and a series of Al-xSi-4Co-yFe (x = 6, 12, 18; y = 2, 4) alloys were prepared. The interaction behavior between Co and Fe in formed intermetallic compounds was assessed and discussed.

Experimental
The raw materials used in this work include pure Al ingot (99.9 wt%, all compositions quoted in this work are in wt% unless otherwise stated), pure crystalline Si (99.9%), pure Co and Fe ingots (99.9%), provided by Shandong Al & Mg Melt Technology Co. Ltd. Several Al-xSi-yCo (x = 6, 12, 18; y = 2, 4) and Al-xSi-4Co-yFe (x = 6, 12, 18; y = 2, 4) alloys were prepared through the following procedures. Firstly, Al and Si were melted in a claybonded graphite crucible by high frequency induction furnace to 1200°C, then Co (or 'Co and Fe') was added in the melt. After the materials were totally melted, machine stirring was applied for 30 s with the speed of 180 r min −1 . Then, the liquid was poured into an iron chill mold ( figure 1(a), the solidification rate is ∼100°C·s −1 ) or a pre-heated graphite mold ( figure 1(b), the solidification rate is ∼2°C·s −1 ), i.e., namely via a fast-cooling and slow-cooling process, respectively.
The samples were cut from center of the as-cast alloys, then were machined to proper size. After that, they were grinded by SiC metallographic sandpaper and polished using MgO turbid liquid (5%) by a polishing machine. In order to observe the three-dimensional (3D) morphologies of the intermetallic compounds, the fracture surface was observed. Also, the Al matrix of selected samples was dissolved in a solution with 10 vol.% of NaOH. The extracted sediments were collected by a centrifugal extractor, and rinsed with distilled water and ethanol.
The microstructures of the as-cast alloys were observed by a Hitachi SU-70 field emission scanning electron microscopy (SEM) operated at 15 kV, linking with an energy dispersive X-ray spectrometry (EDS) attachment. Phase identification was conducted by X-ray diffraction (XRD) using a Rigaku D/max-rB diffractometer, with Cu Kα radiation (k = 1.5406 Å) at 40 kV and 100 mA. The diffraction patterns were recorded and phase calibration was based on the database of Joint Committee Powder Diffraction Standards [35]. Besides, thermodynamic calculation was conducted using Pandat software based on the thermodynamic database in [36].

Results and discussion
3.1. Effect of Si and Co contents on the microstructure of Al-Si-Co alloys Figure 2 shows SEM microstructures of the as-cast Al-6Si-2Co, Al-12Si-2Co and Al-18Si-2Co alloys via fastcooling condition. In the Al-6Si-2Co alloy, except for Al matrix, there form two kinds of eutectics with different contrast (figures 2(a), (b)). The grey worm-like eutectic phase is Si, while the bright phase is supposed to be Corich, according to the inserted EDS spectrum. In the Al-12Si-2Co alloy, it was found that much finer eutectic Si was achieved, and the worm-like Co-rich phase distributes among eutectic Si particles, as clearly shown in figures 2(c) and (d). When the Si content is increased to 18%, i.e., in the Al-18Si-2Co alloy, it was surprised to find that no primary Si particles appear in the microstructure. The Co-rich phase is still worm-like, while the Si phase exhibits dendritic (figures 2(e), (f)).
Increasing the Co content to 4% and via similar fast-cooling procedure, the Al-6Si-4Co, Al-12Si-4Co and Al-18Si-4Co alloys were prepared, as shown in figure 3. As labelled in figure 3(a), it was clearly seen that the Corich phase forms dendritic morphology, with quite fine dendrite arms. In the Al-12Si-4Co alloy, the dendritic Co-rich intermetallic compound is much coarser than that in Al-6Si-4Co alloy, and the eutectic Si is also modified as relatively fine. Increasing the Si content to 18%, the Co-rich dendrites in Al-18Si-4Co alloy are further coarsened. Therefore, comparing figure 2 with figure 3, it can be concluded that the increase of Co content in the alloys results in the morphological change of Co-rich intermetallic compounds, except for the increase of its volume fraction.
The Al-12Si-2Co and Al-12Si-4Co alloys were selected to conduct mapping analysis by EDS. From the distributions of Al, Si and Co elements (figures 4(a), (b)), the eutectic Si and Co-rich phases can be easily separated. Figures 4(c) and (d) show 3D morphologies of the Co-rich intermetallic compounds, with fine and coarse dendrites arms, respectively. It clearly indicates that the direction of second dendrite arm is perpendicular to the main arm, exposing with crystallographic planes, indicating that the Co-rich dendrite has given growing direction during solidification process. This morphology is just like the α-AlFeSi type dendrites, as reported in our previous work [37,38]. Figure 4(e) shows the chemical content of the Co-rich intermetallic compound in Al-12Si-4Co alloy by EDS, and a certain amount of Si element was detected in the Co-rich intermetallic compound. Figure 5 shows XRD patterns of the above-mentioned Al-xSi-yCo (x = 6, 12, 18; y = 2, 4) alloys. By comparing the diffraction peaks with the database of Joint Committee Powder Diffraction Standards [35], the formed phases were successfully labelled. Except for Al and Si, the Al 9 Co 2 (PDF# 06-0699) phase was detected in all of these samples. As a certain amount of Si was detected in the phase (figure 4(e)), therefore, the Co-rich intermetallic compound can be marked as (Al,Si) 9 Co 2 , i.e., some Al atoms are replaced by Si atoms in the crystal lattice of Al 9 Co 2 , since they have similar atom size. This phenomenon can be also referred in the formation of Zr/Ti-rich intermetallic compounds in Al-Si-Zr/Ti alloys [39,40].

Effect of cooling condition on the microstructure of Al-Si-Co alloys
To reveal the effect of cooling condition on the formation of Co-rich intermetallic compounds in Al-xSi-yCo (x = 6, 12, 18; y = 2, 4) alloys, these six alloys solidified via slow-cooling rate were also characterized by SEM, as shown in figure 6. In the slow-cooling Al-6Si-2Co and Al-12Si-2Co alloys (figures 6(a), (b)), except for Al matrix, only eutectic Si and Co-rich intermetallic compound was found. Besides, the eutectic Si is not wormlike but flake-like, as it is usually be in hypoeutectic Al-Si alloys [41]. However, in the slow-cooling Al-18Si-2Co alloy, primary blocky Co-rich particles occur together with flake-like eutectic Si, as marked in figure 6(c), which is quite different from the fast-cooling alloy ( figure 2(e)). For the Al-xSi-4Co (x = 6, 12, 18) alloys, primary Co-rich intermetallic compounds were obviously observed, as labelled in figures 6(d)-(f). In these alloys, the eutectic Si phase also exhibits flake-like, quite different from the worm-like morphology in the fastcooling alloys (figures 1 and 2). Figure 6(g) shows the EDS mapping of the Al-12Si-4Co alloy (figures 6(e)), and (h) is the EDS spectrum of Co-rich phase. The detected Al, Si and Co contents are quite similar to the result in figure 4(e). Furthermore, the XRD analyses of these alloys are shown in figure 7, from which the Co-rich phase is also identified to be Al 9 Co 2 .
The experimental results above indicate that the change of cooling rate does affect the morphology of eutectic Si and Co-rich phases. For instance, in the fast-cooling Al-18Si-2Co alloy ( figure 1(e)), no blocky  primary Si was found, while it exhibits dendritic shape. This morphology is commonly occurred in Al-Si alloys when the undercooling level is relatively high during solidification process [42]. Furthermore, take the fast-and slow-cooling Al-6Si-4Co alloys for example, although both dendritic Al 9 Co 2 particles form, but the dendrite arms are quite fine in the fast-cooling Al-6Si-4Co alloy than the slow-cooling sample. Therefore, it is deduced that the Al 9 Co 2 phase is quite sensitive to the undercooling during solidifying process, i.e., a high undercooling level leads to a fast-growing speed along preferred orientations. As a result, the crystal is obviously refined.
To explain the forming mechanism of the microstructure characteristics, the equilibrium Al-xSi-2Co and Al-xSi-4Co phase diagrams were calculated by the software and database of Pandat, as shown in figure 8. For the samples mentioned above, the phases in the alloys via slow-cooling should be close to the phase diagrams theoretically, as the cooling rate is quite low. As can be seen in figures 8(a) and (b), the Al 9 Co 2 phase in Al-6Si-4Co alloy is the primary phase, while the primary phase is α-Al in Al-6Si-2Co alloy. The theoretical phase types in these two alloys are well corresponded with the experimental results (figures 6(a), (d)). However, according to the phase diagram of Al-18Si-4Co, the Al 9 Co 2 phase is not the primary phase. This finding is different from the SEM microstructure in figure 6(f). There is no doubt that proper explaining needs to be further provided.

Effect of Fe content and cooling condition on the microstructure of Al-Si-Co-Fe alloys
As mentioned earlier, the elements of Co and Fe have similar chemical properties, since they are next to each other in the periodic table, and they usually occur together in Al-Si alloys. Based on the experimental results above, 2% and 4% of Fe were further introduced into Al-xSi-4Co (x = 6, 12, 18) alloys, respectively. Under these two conditions, the Co: Fe ratio was designed as 2: 1 and 1: 1, respectively. Figure 9 shows the SEM microstructures of the fast-cooling and slow-cooling Al-xSi-4Co-2Fe (x = 6, 12, 18) alloys, and figure 10 shows the corresponding microstructures of the fast-cooling and slow-cooling Al-xSi-4Co-4Fe (x = 6, 12, 18) alloys. In all of these alloys, dendritic Co-rich intermetallic compounds were found, and the size of Co-rich intermetallic compounds in slow-cooling alloys are much coarser that those in the fast-cooling alloys. Besides,  in the fast-cooling Al-6Si-4Co-2Fe alloy, the dendrite is quite fine, similar to the morphology occurred in Al-6Si-4Co alloy ( figure 2(a)).
EDS analysis was carried out on the Co-rich intermetallic compounds in these twelve alloys, and the statistical element contents are listed in table 1. Besides, the fast-cooling Al-xSi-4Co-2Fe and Al-xSi-4Co-4Fe (x = 6, 12, 18) alloys were selected and tested by XRD ( figure 11). From the EDS and XRD results, several laws can be concluded: 1) The formed Fe-and Co-rich phase has the crystal structure of Al 13 Fe 4 (PDF# 50-0797), with a certain amount of Al, Si, Fe and Co. As mentioned above, Al and Si atoms have similar size and so does Fe and Co, therefore, the atoms can replace each other in the crystal lattice of Al 13 Fe 4 . 2) With the increase of Si or Fe in the Al-Si-Co-Fe alloys, the detected content of Si or Fe in the Al 13 Fe 4 -type phase also increases, e.g., the Si content increases from 5.3% to 7.0% and 10.8% in the fast-cooling Al-xSi-4Co-2Fe alloys when x varies from 6 to 12 and 18.
3) The ratio of Fe: Co in the Al 13 Fe 4 -type phase is close to 2.0 and 1.0 (in at.%, corresponding to 1.9 and 0.9 in wt%) in Al-xSi-4Co-2Fe and Al-xSi-4Co-4Fe (x = 6, 12, 18) alloys, which is quite close to the original ratio between Co and Fe content in the alloys, i.e., 2: 1 and 1: 1 (in wt%). Therefore, it indicates that the more Fe (or Co) in the alloys, the more Fe (or Co) in the Al 13 Fe 4 -type phase. Besides, take the Al-xSi-4Co-2Fe alloys for example, although the Co content is much higher than Fe, the in situ formed intermetallic compound is Al 13 Fe 4 -type but not Al 9 Co 2 -type, indicating the Al 13 Fe 4 phase has a much higher tendentiousness to precipitate than Al 9 Co 2 from Al-Si-Co-Fe alloys.
The phase diagrams of Al-xSi-4Co-2Fe and Al-xSi-4Co-4Fe alloys were also calculated by the software and database of Pandat, as shown in figure 12. As can be clearly seen, the phase diagrams of Al-xSi-4Co-2Fe and Al-xSi-4Co-4Fe are quite similar. The theoretically formed intermetallic compounds in all of these alloys are  Al 13 Fe 4 , which has good agreement with the experimental results. It further confirms the deduction above, i.e., the element Fe is more preferred to combine with Al to form Al 13 Fe 4 -type phase than Co to form Al 9 Co 2 -type phase, although the content of Co in the Al-xSi-4Co-2Fe alloys is higher than Fe.
The hardness of the fast-and slow-cooling Al-xSi-4Co-2Fe and Al-xSi-4Co-4Fe (x = 6, 12, 18) alloys were tested, as shown in figure 13. It was found that the hardness values of fast-cooling samples are higher than the slow-cooling ones. As shown in figures 9 and 10, the fast-cooling alloys have fine Al 13 Fe 4 -type dendrites with a higher volume fraction, thus contributing to enhanced hardness values. Besides, with the increase of Si content in the slow-cooling alloys, the hardness increases synchronously. However, among the fast-cooling alloys, the   Al-12Si-4Co-4Fe alloy has the highest hardness. Compared the Al-12Si-4Co-4Fe alloy with Al-18Si-4Co-2/ 4Fe alloys, it can de deduced that the Al 13 Fe 4 -type intermetallic compounds has more contribution on the hardness value than Si phase. The above results indicate that the element Co does have modifying performance on Fe-rich phase in Al-Si-Fe alloys, similar to Mn and Cr [29][30][31][32]. Comparing with Fe [33], the Co-rich phase in Al-Si alloys is more preferred to exhibit finer dendritic shape at a certain content. Also, the addition of Co element in Al-Si alloys is much easier to lead to a high undercooling, resulting in the modification of both eutectic/primary Si phase and Co-rich intermetallic compounds. Comparing with Mn and Cr [32,38], the element Co is also preferred to form finer Fe-rich dendrites. Therefore, Co element may be chosen as an alternative neutralizer for modifying Fe-rich phase in Al-Si alloys, especially when Mn and Cr elements fail to achieve a satisfied effect.
Besides, it should be mentioned that, on the one hand, the total content of Fe and its modifier (Mn, Co, Cr etc) in Al-Si alloys is usually less than 1 wt%, as the formed intermetallic compounds may be coarse at a high content level. Under this condition, rapid cooling method such as melt spinning is suggested to be used, just as the results shown in figures 2, 3 and 9(a). On the other hand, the ratio between Fe and its modifier is also a key parameter affecting the structure and morphology of the Fe-rich intermetallic, which should be taken into consideration when designing multiple Al-Si-Fe-Co alloys.

Conclusions
In this work, several Al-Si-Co and Al-Si-Co-Fe alloys were prepared to investigate the effect of Si, Co, Fe element contents and cooling rate on the microstructures and phase compositions of the alloys. Several findings can be concluded: (1) The Co-rich phase is Al 9 Co 2 in both fast and slow-cooling Al-xSi-yCo (x = 6, 12, 18; y = 2, 4) alloys. A certain of Si participates in this phase, and the chemical formula can be marked as (Al,Si) 9 Co 2 , due to the replacement of Si atoms on Al atoms in Al 9 Co 2 crystal.
(2) The Al 9 Co 2 phase is eutectic in Al-xSi-2Co (x = 6, 12, 18) alloys, while primary Al 9 Co 2 phase may form when the Co content is 4%. The Al 9 Co 2 phase is quite sensitive to the undercooling during solidifying process. A fast-cooling rate will result in developed Al 9 Co 2 dendrites as well as modified eutectic Si particles.
(3) The addition of 2% and 4% Fe in Al-xSi-4Co (x = 6, 12, 18) alloys lead to the formation of Al 13 Fe 4 -type phases containing a certain amount of Al, Si, Fe and Co, under both fast and slow-cooling conditions. With the increase of Si content in the alloys, the Si content in the Al 13 Fe 4 -type phase increases simultaneously.
(4) The ratios of Fe: Co in Al 13 Fe 4 -type phases are similar to the original ratio of Fe: Co in Al-xSi-4Co-2Fe and Al-xSi-4Co-4Fe (x = 6, 12, 18) alloys, indicating the Fe and Co atoms have a preferred tendency to replace each other in Al 13 Fe 4 . Besides, the Al 13 Fe 4 phase has a much higher tendentiousness to precipitate than Al 9 Co 2 , even in Al-xSi-4Co-2Fe alloys.