Phase Formation and Microstructure Evolution of Al-5Si-0.8Mg Alloys with Different Mn Concentrations

: Mg-containing high-Si aluminum alloys are heat-treatable alloys that are widely used in industry. Substantial attention has been paid to increasing the performance of such alloys by adding a small amount of Mn, which is an effective and common alloying element in aluminum alloys. In the present work, the solidiﬁcation process of Mn-free Al-5Si-0.8Mg alloy and Al-5Si-0.8Mg-(0.45– 1.97)Mn alloys are analyzed by the experimental results combined with thermodynamic calculation. The results showed that α -Al, Si, Mg 2 Si and π (Al 8 Mg 3 FeSi 6 ) are predominant phases in the Mn-free Al-5Si-0.8Mg alloy while the π (Al 8 Mg 3 FeSi 6 ) phase are transformed to α -Al(FeMn)Si phase with the addition of 0.45% Mn. With increasing Mn addition to 0.72%, the L → α -Al was replaced by L → α -Al(FeMn)Si and a primary α -Al(FeMn)Si phase appeared. Further increasing the Mn to 1.97%, the solidiﬁcation reactions remained unchanged. However, the size and number of the primary α -Al(FeMn)Si phase gradually increased, while the divorced eutectic phenomenon of quaternary eutectic structure gradually weakened. Meanwhile, the Mg 2 Si phase in the quaternary eutectic structure gradually transformed from blocky to ﬁne eutectic lamellar, and the quaternary eutectic structure was signiﬁcantly reﬁned. Primary blocky α -Al(FeMn)Si began to form when the Mn content was higher than 0.75%.


Introduction
In recent years, Al-Si alloys have been widely used in engines within the automotive and aerospace industries because of their high specific strength, high wear resistance, high corrosion resistance, low thermal expansion coefficient, low cost and excellent casting performance, making these alloys incomparable to other aluminum alloys [1][2][3][4]. Combinations of Al, Si and other elements is an effective and easy way to modify material performance and expand the use of an alloy. Mn is an effective and common alloying element in aluminum alloys [5,6]. Substantial attention has been paid to increasing the performance of aluminum alloys by adding small amounts of Mn. Qiu et al. [7] found that the addition of Mn could transform the β-Al 5 FeSi phase into the α-Al(MnFe)Si phase in Al-7Si-0.3Mg, and with an increase in Mn content, the morphology of the α-Al(MnFe)Si phase gradually changed from dendritic to rod shaped and finally to a "Chinese script" shape. Huang et al. [8] found that, when Mn was added to spray-deposited Al-25Si-xFe alloy, α-Al 15 (FeMn) 3 Si 2 was formed, which replaced the δ-Al 4 FeSi 2 and β-Al 5 FeSi phases and, subsequently, changed the morphology of the Fe phase in the alloy. Seifeddine et al. [9] studied the Al-9Si-0.3Fe-(0-1.01)Mn alloy and reported that increasing the Mn content together with the solidification rates will change the amount and morphology of Fe-rich intermetallics. At Mn:Fe of 2:1, the formation of needle-like Fe-rich intermetallics are still not totally depleted. At low Mn concentrations but high cooling rates, the formations of Fe-rich intermetallics tend to be suppressed to some extent. However, at higher Mn concentrations, the Chinese-script Fe-rich intermetallics seem to be primary precipitated and embedded within the dendrites. Hwang et al. [10] determined that increasing the Mn content transformed the needle-like β phase into the Chinese script-like α phase in an Al-7Si-3.8Cu-0.5Fe alloy, which improved the strength of the alloy while retaining the same plasticity. It is reported that the tensile strength increased from 328 MPa in Mn-free alloy to the highest 360 MPa with 0.65% Mn, but it decreased to 348 MPa with further addition of Mn (0.85% Mn). Therefore, the addition of Mn can change the morphology of the Fe-containing crystalline phase and improve the mechanical properties of the alloy. This is consistent with relevant research results [11,12]. Lu et al. [13] noted that Mn mainly existed in the form of an Al 7 (MnFeSi) 3 blocky phase (size > 50 microns) in the Al-17Si-2Fe-2Cu-1Ni alloy. Lin et al. [14] observed that when the Mn content reached 0.8%, the α-Al 15 (FeMn) 3 Si 2 phase (size ≈ 50 microns) appeared in the Al-17Si-2Fe-2Cu-1Ni alloy. In addition, the primary micro-scale Mn-containing phase in the Al-9Si-3Cu-(0.8-1.2)Fe-(0.25-0.55)Mn and Al-9Si-3Cu-(0.75-1.62)Fe-(0.55-0.79)Mn alloy studied by S. Ferraro et al. [15] and Timelli et al. [16] was also observed. According to the above research, Mn can also exist in the form of a large alloy phase with a size of several tens of microns in aluminum alloy. On the other hand, Liu et al. [17] and Yan et al. [18] investigated Al-(1.09-1.93)Mg-(0.4-0.58)Si-0.27Mn alloy and Al-0.66Mg-0.85Si-0.1Mn alloy, respectively, and found that the Mn in the ingots mainly existed in the form of insoluble Al 15 (FeMn) 3 Si 2 dispersed phase particles. When Lee et al. [19] studied extruded Al-0.66Mg-0.8Si-1.0Mn alloy and Han et al. [20] studied Al-0.74Mg-0.96Si-0.5Cu-0.6Mn and Al-1.08Mg-1.27Si-0.42Cu-0.6Mn alloy plates, they found that Mn mainly existed in the form of dispersed phase particles in the experimental alloys, effectively pinning dislocations and subgrain boundaries, inhibiting recrystallization during deformation, and improving the strength and plasticity of the alloy.
In conclusion, the current research on the role of Mn in Mg-containing high-Si aluminum alloys have mainly focused on the second phase formed by Mn in Al-Si alloys. The literature shows that Mn mainly exists in the form of a coarse alloy phase (size: tens or even hundreds of microns), a eutectic structure (size: tens of microns) and dispersed phase particles (size: hundreds of nanometers) in aluminum alloys. However, there are few studies on the influence of Mn content on the microstructure of high-Si aluminum alloys, especially on the impact of the variation in Mn content on the solidification process of Al-Si alloys.
In this paper, the transformation of the type, morphology, size and quantity of the second phases in an Al-5Si-0.8Mg alloy ingot with different Mn contents were observed, and the solidification process of the phases and microstructures were analyzed in combination with the phase diagram and thermodynamic analysis. This study provides data for optimizing the composition and improving the overall performance of Mn-containing Al-Si-Mg alloy by controlling the microstructure.

Materials and Methods
The experimental alloys were prepared from commercially pure Al (99.7%), electrolytic grade copper, commercially pure Mg, an Al-30Si master alloy and an Al-10Mn master alloy. These raw materials were melted in an electrical resistance crucible furnace and cast into ingots with a water-cooled copper mold (180 × 100 × 30 mm 3 ). The compositions of the alloy ingots used in the present study are shown in Table 1. Specimens with dimensions of 10 × 10 × 15 mm 3 were cut from the six alloys at the same core position, as shown in Figure 1. The samples for metallurgical observation with OM and SEM are first grinded by sandpapers until to #2000 grit and then mechanical polished with suspensions of 0.5 µm.  The microstructure was observed with an Olympus GX71 optical microscope (Olympus Corporation, Tokyo, Japan), and the observed surface was perpendicular to the direction with the maximum cooling gradient. Phase analysis was performed by an X′ Pert Pro MPD (Panalytical, Eindhoven, Holland) X-ray Diffraction system with the test conditions of Cu Kα rays, scanning rate 2°/min, step size 0.02°, tube voltage 40 kV, and tube current 40 mA. The morphology of the second phases in the alloy was examined with a JSM 6510 scanning electron microscope, equipped with an energy-dispersive spectrometer (JEOL Ltd., Tokyo, Japan). In addition, the solidification simulation function in Pandat software (CompuTherm LLC, Middleton, WI, USA, 2016 version) was used to predict the non-equilibrium solidification processes of the experimental alloys. The Gulliver-Scheil model is used to simulate non-equilibrium solidification calculation.   The microstructure was observed with an Olympus GX71 optical microscope (Olympus Corporation, Tokyo, Japan), and the observed surface was perpendicular to the direction with the maximum cooling gradient. Phase analysis was performed by an X Pert Pro MPD (Panalytical, Eindhoven, Holland) X-ray Diffraction system with the test conditions of Cu Kα rays, scanning rate 2 • /min, step size 0.02 • , tube voltage 40 kV, and tube current 40 mA. The morphology of the second phases in the alloy was examined with a JSM 6510 scanning electron microscope, equipped with an energy-dispersive spectrometer (JEOL Ltd., Tokyo, Japan). In addition, the solidification simulation function in Pandat software (CompuTherm LLC, Middleton, WI, USA, 2016 version) was used to predict the non-equilibrium solidification processes of the experimental alloys. The Gulliver-Scheil model is used to simulate non-equilibrium solidification calculation. refined. Finally, as the Mn content increased from 1.09% to 1.97%, the number of blocky alloy phases increased, but the eutectic lamellar structure did not change significantly. 21, 11, x FOR PEER REVIEW 4 of 10 lamellar structure of the alloy ingot is further refined. Finally, as the Mn content increased from 1.09% to 1.97%, the number of blocky alloy phases increased, but the eutectic lamellar structure did not change significantly.  Figure 3 shows the X-ray diffraction (XRD) patterns of Al-5Si-0.8Mg ingots free of Mn and those with 0.45% Mn, 0.72% Mn, 1.09% Mn, and 1.97% Mn. The figure shows that there are mainly four types of alloy phases existing in Al-5Si-0.8Mg ingots without Mn: α-Al, Si phase, π phases (Al8Mg3FeSi6) and Mg2Si phases. The structure of π (Al8Mg3FeSi6) is hexagonal P-62m space group with the lattice parameter of a as 0.66 nm and c as 0.79 nm. When Mn was added to the Al-5Si-0.8Mg alloys, the intensity of the diffraction peak of the π phase (Al8Mg3FeSi6) weakened and nearly disappeared. Moreover, diffraction peaks of the α-Al(FeMn)Si phases appeared in the XRD patterns of the alloy ingots. With the further increase in Mn contents, the number of diffraction peaks of the α-Al(FeMn)Si phases increased, and the intensity of those peaks increased. The intensity of the Mg2Si phase diffraction peaks also increased with increasing Mn. Accord-  Figure 3 shows the X-ray diffraction (XRD) patterns of Al-5Si-0.8Mg ingots free of Mn and those with 0.45% Mn, 0.72% Mn, 1.09% Mn, and 1.97% Mn. The figure shows that there are mainly four types of alloy phases existing in Al-5Si-0.8Mg ingots without Mn: α-Al, Si phase, π phases (Al 8 Mg 3 FeSi 6 ) and Mg2Si phases. The structure of π (Al 8 Mg 3 FeSi 6 ) is hexagonal P-62m space group with the lattice parameter of a as 0.66 nm and c as 0.79 nm. When Mn was added to the Al-5Si-0.8Mg alloys, the intensity of the diffraction peak of the π phase (Al 8 Mg 3 FeSi 6 ) weakened and nearly disappeared. Moreover, diffraction peaks of the α-Al(FeMn)Si phases appeared in the XRD patterns of the alloy ingots. With the further increase in Mn contents, the number of diffraction peaks of the α-Al(FeMn)Si phases increased, and the intensity of those peaks increased. The intensity of the Mg 2 Si phase diffraction peaks also increased with increasing Mn. According to the above results, there are mainly α-Al, Si phases, α-Al(FeMn)Si phases and Mg 2 Si phases existing in Mncontaining Al-5Si-0.8Mg alloy ingots. The structure of the α-Al(FeMn)Si phase is bcc with Im 3 space group with the lattice parameter of a as 1.25 nm.  Scanning electron microscopy (SEM) images of Al-5Si-0.8Mg alloy ingots without Mn and those with 0.45% Mn and 1.09% Mn are shown in Figure 4, and the energy-dispersive spectroscopy (EDS) results of the various alloy phases (indicated by arrow and number) in Figure 4 are summarized in Table 2. According to the XRD results in Figure 3 and the EDS analysis results in Table 2, the gray needle-flake phases in the Al-5Si-0.8Mg alloy were eutectic Si phases (Figure 4a, point 1). The results show that there are inhomogeneous black blocky or skeletal Mg2Si phases (Figure 4a, point 2) and bright white needle-like or blocky π phases (Al8Mg3FeSi6) (Figure 4a, point 3) in the Al-5Si-0.8Mg alloy without Mn. When Mn was added to the experimental alloy, there were rarely any π phases (Al8Mg3FeSi6) existing in the alloy. Instead, the Mn-containing alloys had α-Al(FeMn)Si phases that are a few microns to a few dozen microns in size (Figure 4b, point 4 and point 5). These results are consistent with the XRD results. The eutectic structure of the alloy was refined. When the Mn content reached 1.09%, massive α-Al(FeMn)Si phases that are several tens of microns thick appear in the alloy (Figure 4a,  point 6). In addition, the black Mg2Si phases in the alloy changed from medium-sized blocky or skeletal morphologies to a smaller uniformly distributed pin-shaped morphology, for which the number increased significantly.  Scanning electron microscopy (SEM) images of Al-5Si-0.8Mg alloy ingots without Mn and those with 0.45% Mn and 1.09% Mn are shown in Figure 4, and the energy-dispersive spectroscopy (EDS) results of the various alloy phases (indicated by arrow and number) in Figure 4 are summarized in Table 2. According to the XRD results in Figure 3 and the EDS analysis results in Table 2, the gray needle-flake phases in the Al-5Si-0.8Mg alloy were eutectic Si phases (Figure 4a (Figure 4a, point 6). In addition, the black Mg 2 Si phases in the alloy changed from medium-sized blocky or skeletal morphologies to a smaller uniformly distributed pin-shaped morphology, for which the number increased significantly.  Scanning electron microscopy (SEM) images of Al-5Si-0.8Mg alloy ingots without Mn and those with 0.45% Mn and 1.09% Mn are shown in Figure 4, and the energy-dispersive spectroscopy (EDS) results of the various alloy phases (indicated by arrow and number) in Figure 4 are summarized in Table 2. According to the XRD results in Figure 3 and the EDS analysis results in Table 2, the gray needle-flake phases in the Al-5Si-0.8Mg alloy were eutectic Si phases (Figure 4a, point 1). The results show that there are inhomogeneous black blocky or skeletal Mg2Si phases (Figure 4a, point 2) and bright white needle-like or blocky π phases (Al8Mg3FeSi6) (Figure 4a, point 3) in the Al-5Si-0.8Mg alloy without Mn. When Mn was added to the experimental alloy, there were rarely any π phases (Al8Mg3FeSi6) existing in the alloy. Instead, the Mn-containing alloys had α-Al(FeMn)Si phases that are a few microns to a few dozen microns in size (Figure 4b, point 4 and point 5). These results are consistent with the XRD results. The eutectic structure of the alloy was refined. When the Mn content reached 1.09%, massive α-Al(FeMn)Si phases that are several tens of microns thick appear in the alloy (Figure 4a,  point 6). In addition, the black Mg2Si phases in the alloy changed from medium-sized blocky or skeletal morphologies to a smaller uniformly distributed pin-shaped morphology, for which the number increased significantly.

Effect of Mn Contents on the Solidification Behavior of the Al-5Si-0.8Mg Alloy
The solidification curves of experimental alloys under non-equilibrium (Gulliver-Scheil model) condition were calculated by Pandat software, and the results were shown in Figure 5 and also summarized in Table 3. It can be seen that there are Al, Si, Al 8 Mg 3 FeSi 6 and Mg 2 Si phases precipitating from liquid phase during the solidification of Mn-free Al-5Si-0.8Mg alloy. However, in addition to the Al, Si, Al 8 Mg 3 FeSi 6 and Mg 2 Si phases, there is also α-Al(FeMn)Si precipitating from the liquid with the addition of 0.45% Mn. Additionally, the model calculation showed that the mass fraction of Al 8 Mg 3 FeSi 6 phase was only 0.001%. This means that the Al 8 Mg 3 FeSi 6 phase almost disappeared and the eutectic α-Al(FeMn)Si formed in Al-5Si-0.8Mg alloy when the Mn content increased to 0.45%. Additionally, the primary α-Al(FeMn)Si phase appeared in the alloy with the addition of 0.72% Mn. The type of alloy phases remained unchanged as the Mn content gradually increased to 1.97%, which was completely consistent with the XRD results in Figure 3. According to the metallograph of the Al-5Si-0.8Mg-(0-1.97)Mn alloys in Figure 2, the solidification curves calculated by the thermodynamics in Figure 4 and the research results of Backerud [21], the solidification reactions of the six alloys with different Mn were concluded and are shown in Figure 5a-f and Table 3, respectively. In the solidification of Mn-free Al-5Si-0.8Mg alloy, the α-Al dendrite, (α-Al+Al 5 FeSi) binary eutectic, (α-Al+Si+Al 5 FeSi) ternary eutectic, (α-Al+Si+Al 8   In conclusion, the addition of Mn changed the composition and morphology of the Fe phase in Al-5Si-0.8Mg alloy. The Al 8 Mg 3 FeSi 6 phase almost disappeared and the eutectic α-Al(FeMn)Si formed in Al-5Si-0.8Mg alloy when the Mn content increased to 0.45%. At the same time, when the Mn content was up to 0.72%, the primary α-Al(FeMn)Si phase occurred during the solidification of Al-5Si-0.8Mg alloy, and the solidification reaction temperature of primary α-Al(FeMn)Si phase gradually increased with the increasing Mn content. It can be observed from Figures 2 and 5 that there were significant differences in the morphology of alloy phases among Al-5Si-0.8Mg alloys with different Mn contents. No obvious (α-Al + Si + Mg 2 Si + Al 8 Mg 3 FeSi 6 ) quaternary eutectic structure was observed in the Mn-free Al-5Si-0.8Mg alloy, because most of the solution Mg atom in the alloy firstly formed the Al 8 Mg 3 FeSi 6 phase, and there was a relatively small number of remaining Mg atoms to form Mg 2 Si. During the reaction of L→α-Al + Si + Mg 2 Si + Al 8 Mg 3 FeSi 6 in Mn-free Al-5Si-0.8Mg alloy, α-Al and Si phases attached on the existing α-Al and Si phases to nucleate and precipitate, and finally formed a very small amount of divorced blocky eutectic Mg 2 Si (as shown in the dashed area in Figure 4a).
When the amount of Mn was 0.45%, the model calculation result showed that the mass fraction of Al 8 Mg 3 FeSi 6 phase was only 0.001%, which means that the Al 8 Mg 3 FeSi 6 phase was almost negligible in Al-5Si-0.8Mg-0.45Mn alloy. No Al 8 Mg 3 FeSi 6 phase can be observed in the microstructure (Figure 2b) or be analyzed in the XRD testing results ( Figure 3). Additionally, there is no (α-Al + Si + Mg 2 Si + Al 8 Mg 3 FeSi 6 + α-Al(FeMn)Si) quintet eutectic structure but (α-Al + Si + Mg 2 Si + α-Al(FeMn)Si) quaternary eutectic structure can be observed in Al-5Si-0.8Mg-0.45Mn alloy (as shown in the dashed area in Figure 4b). In the meantime, the eutectic α-Al(FeMn)Si appeared in Al-5Si-0.8Mg alloy when the Mn content increased to 0.45%. Therefore, the calculation result is consistent with the experimental observations.
When the content of Mn reached 0.72%, primary α-Al(FeMn)Si occured by L→α-Al(FeMn)Si, and the formation of the primary α-Al(FeMn)Si phase relatively increased the concentration of solution Mg atom at the interface between liquid phase and α-Al(FeMn)Si phase, which was conducive to the formation of (α-Al+Si+Mg 2 Si+α-Al(FeMn)Si) quaternary eutectic structure. Therefore, a large number of lamellar (α-Al+Si+Mg 2 Si+α-Al(FeMn)Si) quaternary eutectic structure was observed in the alloy, as shown in the