Study on mechanism of refining and modifying in Al–Si–Mg casting alloys with adding rare earth cerium

Figure 1 shows the XRD results of the 0% Ce, 0.1% Ce, 0.2% Ce alloys. As shown in figure 1, it can be seen that the alloys contain matrix α-Al(∀), eutectic Si(●). However, the diffraction peaks of Ce-containing compounds and Fe-rich phases were not found. It is due to lower content of Ce-containing compounds and from table 1 can be seen that the mass percentage content of Fe is almost less than 1% in the Al alloy, this results in less Fe-rich phases in the alloy. In order to determine the Fe-rich phases which present in the experimental alloys, the Fe-rich phases were further analyzed by SEM and TEM.

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Figure 2 shows the OM microscopy of experimental alloys with different Ce contents. As shown in figure 2(a), it is mainly composed of α-Al and eutectic Si in Ce-free Al alloy. It can be found that the α-Al is in the form of course and irregular dendrites, and the eutectic Si is along the α-Al branches. Also, the grain boundaries are distributed in a semi-continuous network, and the range of eutectic of α-Al + Si is large. However, comparing to figure 2(a), after adding rare earth cerium, as shown in figures 2(b) and (c), it can be seen that α-Al dendrite arms become narrowing, the size of α-Al is smaller, and the range of α-Al + Si eutectic area is reduced. By comparing figures 2(b) and (c), as increasing of Ce contents, it can be found that the degree of grain refinement has hardly changed, and the size of eutectic Si does not change significantly.

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Figure 3 is the grain size and proportional histogram of α-Al obtained by using Software image J to statistically count 10 metallographic diagrams of each of the three experimental alloys. As shown in figure 3, the α-Al grain size distribution in the experimental alloy without rare earth Ce addition is uneven, the minimum size is 20 μm, the maximum can reach 170 μm, and the average grain size is 80.44 μm. After adding 0.1% rare earth Ce in the experimental alloy, α-Al is refined, the grain size distribution of α-Al becomes more uniform, its size is concentrated between 20–60 μm, and the average grain size is reduced to 47.21 μm. When the addition amount of rare earth Ce in the experimental alloy increases to 0.2%, the grain size of α-Al tends to increase, the grain size of α-Al is concentrated in the range of 30–70 μm, and the average grain size increases slightly to 52.15 μm, but it is still refined compared with the experimental alloy without rare earth Ce added. Therefore, when 0.1% rare earth Ce is added to the experimental alloy, the refining effect of α-Al grains is the best.

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Figure 4 is the SEM micrograph of the deep etched experimental alloy eutectic Si. As shown in figure 4(a), in the experimental alloy without Ce, there is a single piece of flake eutectic Si. As shown in figures 4(b) and (c), after adding rare earth Ce to the experimental alloy, eutectic Si exhibits a fibrous structure.

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To further determine the composition and structure of the second phase in the experimental alloy, the alloy was tested by EDS in this experiment. Figure 5 is the SEM micrographs and EDS mapping of the experimental alloy without rare earth addition. As shown in figure 5(a), there are bright white needle-like phases and dark gray flaky or massive phases in the experimental alloys. From the Mapping results in figures 5(b)–(e), it can be seen that the bright white needle-like phases are mainly composed of Al, Fe, and Si element enrichment, combined with the point scanning results of point A in table 2, it is determined that the phase is β-AlFeSi phase; the dark gray flaky phase is mainly enriched by Al, Si, Mg, and Fe (Si: Mg: Fe ≈ 6:3:1) combined with point B in table 2. From the point scan results, it was determined that the phase was the π-Al₈Si₆Mg₃Fe phase.

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Figure 6 is the SEM and EDS pictures of the Ce-containing experimental alloy. As shown in figure 6(a), a dark grey elongated phase and a bright bulky phase are present in the experimental alloys. Figure (b)–(f) are the Mapping results of the box-selected area in figure 6(a). It can be seen that the gray strip-like phase is mainly enriched by Al, Si, Mg and Fe elements. Combined with the point scanning results of point C (Si: Mg: Fe ≈ 6:3:1) shown in table 3, This phase was determined to be the π-Al₈Si₆Mg₃Fe phase. At the same time, the bright bulk phase is mainly enriched by Al, Si, and Ce elements, and from the point scan results of point D, the atomic percentage content ratio of Ce and Si is about 1:2, confirming that this phase is AlCeSi₂ phase [21, 26].

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As everyone knows, DSC is used to determine the changes that occur during coagulation, and the increase in subcooling is related to the refinement of tissue. Figure 7 shows the DSC curves of the three groups of experimental alloys. The heating and cooling rates were both 10 °C min⁻¹. In the DSC Endothermic curve, peak 1 corresponds to the melting (solidification) of α-Al, and peak 2 corresponds to the binary eutectic reaction Al + Si → L (L → Al + Si). The DSC curve is processed by the definition of Mueller [27], that is, the initial melting temperature (the intersection of the steep tangent at the beginning of the melting endothermic curve and the baseline) is T_M, and the initial nucleation temperature of the eutectic reaction is T_N, then the degree of supercooling ΔT = T_M−T_N. The T_M and T_N values of eutectic Si in three groups of experimental alloys were obtained, and their respective ΔT values were calculated. The results were shown in table 4.

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It can be seen from table 3 that the addition of rare earth Ce in the experimental alloy reduces the initial nucleation temperature of the binary eutectic reaction and increases the degree of undercooling of the eutectic reaction at 0.65 °C,1.78 °C, and 1.66 °C, the undercooling of the experimental alloys with 0.1%Ce and 0.2%Ce additions increased by 1.13 °C and 1.11 °C, respectively, compared to the experimental alloys without rare earth Ce addition. The results show that the addition of Ce element leads to the increase of supercooling, refinement of microstructure.

To further understand the existence form of rare earth Ce in the experimental alloy, and to explore its refinement and modification mechanism of the experimental alloy, TEM analysis of the alloy was carried out in this experiment. Figure 8 is a TEM photograph of the rare earth phase in the experimental alloy. As shown in figure 8(a), there is a black massive rare earth phase with a size of about 4 μm in the experimental alloy. Figure 8(b) shows the diffraction spot (SAED) of the black bulk phase. After calibration, it is found that the black bulk phase is the AlCeSi₂ phase, hexagonal system, and the crystal band axis is [0–21], which is consistent with the EDS results. The ratio of Si and Ce content is consistent.

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Figure 9 is a TEM photograph of the experimental alloy eutectic Si. As shown in figure 9(a), there is an elongated eutectic Si with a concave–convex interface in the alloy, and the length is about 10 μm. Figure 9(b) is a high-resolution photo of a position on the strip-shaped phase in figures 9(a) and(c) is the Fourier-transformed diffraction spot (SAED) of the red-framed region in figure 9(b). As shown in figure 9(b), there are a large number of parallel stripes on the surface of eutectic Si, which correspond to the parallel blind lines in the diffraction spots in figure 9(c), and the diffraction spots are clear and sharp [0–11]. Figures 9(d)–(f) are the Mapping results of the elongated phase. The elongated phase is mainly enriched by Si element, and it can be observed that Ce element is solid dissolved on the eutectic Si.

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Figure 10 is a transmission photograph of the iron-rich phase in the experimental alloy after adding rare earth Ce. As shown in figure 10(a), the alloy has two gray strip-like phases with a length of about 3 μm and a white bulk phase with a length of about 1 μm. Figure 10(c) shows the diffraction spot (SAED) at one point on the gray strip-like phase. After calibration, it is concluded that the gray strip-like phase is the Al₈Si₆Mg₃Fe phase [19, 28], and the crystal band axis is [−1210]. Combined with the Mapping results in figures 10(d)–(g), it is found that Al, Si, Mg, and Fe are abundantly enriched in the gray elongated phase, which verifies that the elongated phase is Al₈Si₆Mg₃Fe. By checking the PDF card, it can be seen that the Al₈Si₆Mg₃Fe phase belongs to the hexagonal crystal system, which is a kind of π-Fe. As shown in figure 10(e), there is an obvious enrichment of rare earth Ce on Al₈Si₆Mg₃Fe, which indicates that Ce element can be solid-dissolved on Al₈Si₆Mg₃Fe, and it can also be observed that rare-earth Ce can be solid-dissolved on α-Al. It can be seen from figure 10(h) that only Si element is enriched on the white bulk phase in figures 10(a), (e), (d) that the white bulk phase is eutectic Si and it is determined.

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At present, the refining effect of rare earths is mainly reflected in two aspects, one is to promote the α-Al grain-shaped nucleus, and the other is to inhibit the growth of α-Al after nucleation. Heteromorphic nucleus theory holds that when the lattice type of the crystal nucleus in the cast metal is consistent with the α-Al matrix and the mismatch is less than 5%, it can be used as a particle of heterogeneous nuclei and plays a role in refining grains [29].

AlCeSi₂ belongs to the hexagonal crystal system, [0–21] crystal belt axis, and its crystal structure is composed of CeSi₂ (AlB₂) and Al₂CeSi₂ (La₂O₃) [30], lattice constant a = 0.417 Å. The Al matrix is a face-centered cubic structure with a lattice constant a = 0.405 Å. The following formula [31] is used to calculate the lattice mismatch degree of the two:

$$\begin{eqnarray*}&&\delta =\displaystyle \frac{\left|{a}_{s}-{a}_{n}\right|}{{a}_{n}}\times 100 \% \end{eqnarray*}$$

where a_s is the lattice parameter of the α-Al phase; a_n is AlCeSi₂ The lattice parameters of the phase; after calculation, the lattice mismatch between the α-Al phase and the AlCeSi₂ phase is 2.96%, less than 5%, which can theoretically be used as the heterogeneous nucleation core of α-Al to refine the effect of the grain. In the solidification process of Al–Si–Mg experimental alloy, a small amount of AlCeSi₂ phase is generated at 598 °C [26], and the initial nucleation temperature of the binary eutectic reaction is 564.87 °C from table 4, indicating that the AlCeSi₂ phase is preferred to the eutectic Si nucleation, eutectic Si can rely on the growth of AlCeSi₂ phase, the formation of AlCeSi₂ phase can consume part of Si elements, AlCeSi₂ inhibits the growth of eutectic Si, thereby refining eutectic Si. At the same time, the solubility of Ce in Al and Si is extremely small, and Ce will be enriched to the front of the solid–liquid interface to hinder the growth of surrounding α-Al grains (figure 9(f)), thus playing a role in refining the structure.

At the same time, the impurity-induced twinning theory believes that the metamorphism of eutectic Si is closely related to the adsorption of impurity elements, and the impurity atoms adsorbed on the {111} close-packed surface of eutectic Si can cause stacking faults of Si atoms to form growth steps, resulting in eutectic Si metamorphism [32]. The high density of cross-step stacking faults observed in the high-resolution image of figure 9(b) corresponds to the parallel awn lines in the selected area diffraction pattern. The stacking fault is formed by the staggered arrangement of atoms in the stacking, which can be regarded as a twin with a thickness of one atom, and the rare earth Ce is distributed in the eutectic Si (figure 9(f)), indicating that Ce, as a surface-active element, can be adsorbed on eutectic Si as an impurity element. In addition, the ratio of impurity element atomic radius to Si atomic radius (r (modifier): r (silicon)) also has an important influence on the modification of eutectic Si. When r (modifier): r (silicon) = 1.646, the modification effect of eutectic Si is the best [33]. In this experiment, the ratio of the atomic radius (0.181 nm) of rare earth Ce to that of Si (0.117 nm) is 1.547, which is close to 1.646, which indicates that rare earth Ce can have a metamorphic effect on eutectic Si.

Since the solid solubility of Fe in the Al-Si alloy is very low, Therefore, the existence of Fe in the aluminum alloy is basically in the form of reacting with Al and Si in the molten aluminum to form an Al-Fe-Si iron-rich phase. The two-dimensional morphology of β-AlFeSi is coarse needle-like, and π-Al₈Si₆Mg₃Fe generally occurs in Al-Si alloys with Mg, and its morphology is usually block or Chinese character. It can be seen from figure 10(e) that the rare earth Ce can be solid-dissolved into the iron-rich phase and enriched at its tip, and the rare earth Ce will enter the iron-rich phase to change the composition of the iron-rich phase. At the same time, Ce will be adsorbed on the tip of the iron-rich phase, hindering its growth, resulting in changes in the morphology and distribution of the iron-rich phase, and passivation of its tip.