Heterogeneous Nucleation of Eutectic Structure in Al-Mg-Si Alloys

The microstructure of Al-5Mg-2Si-0.4Mn-0.7Fe alloy solidified at different conditions were examined. Different kinds of eutectic structures such as (Al15(Fe,Mn)3Si2 + α-Al), (Mg2Si + α-Al) and (FIMCs + Mg2Si + α-Al) were selected due to the different primary phases formation. The phase relationships between the phases within the eutectic structures, and the phase relationships between the primary phases and the eutectic leading phases were investigated. A well-defined orientation relationship (OR) between α-Al15(FeMn)3Si2 and Mg2Si was examined.

more kinds of intermetallic compounds (IMCs). The eutectic spacing, eutectic grain size, the size and morphology of the IMCs play very important roles in determining the mechanical properties. As reported, [11,12] the eutectic structures in Al alloys were difficult to control and unpredictable due to the composition segregation. Therefore, to achieve high mechanical properties for Al alloys, the refinement of eutectic structure is as important as the a-Al grain refinement. For example, as one of the unavoidable impurities in Al alloys, Fe usually forms large-scale Fecontaining IMCs. [13][14][15][16][17][18][19] These FIMCs deteriorate the mechanical properties especially the toughness when they have the large plate-like or needle-like shapes.
Recent research [20] indicated that the solidification sequence affected the formation of FIMCs in the eutectic structure. Therefore, a better understanding on nucleation of the eutectic in Al alloys should be brought to the attention for controlling the eutectic structure. In this study, we aimed to investigate the nucleation of the leading phase of eutectic structure during the solidification process of Al alloys. The microstructure variation due to the leading phase changes under different solidification conditions was studied as well.
The alloy used in this study had a composition of 5.66 ± 0.6 Mg, 2.65 ± 0.2 Si, 0.44 ± 0.2 Mn, 0.67 ± 0.2 Fe and balanced Al (all compositions are in wt pct unless specified otherwise). The phase diagram of the Al-5Mg-2Si-0.4Mn-0.7Fe alloy was calculated by Pandat software with Sheil model. The liquidus for the alloy was 638.34°C, and the main solidification sequence is primary Al 15 (Fe,Mn) 3 Si 2 , binary eutectic (Al 15 (Fe,Mn) 3 Si 2 + a-Al), and ternary eutectic (a-Al+Al 13 Fe 4 +Mg 2 Si).
Pure Al (> 99.86 pct), Pure Mg (> 99.95 pct), Al-50 pct Si, Al-20 pct Mn and Al-38 pct Fe master alloys were used to prepare the alloy with the above nominal composition. The standard TP-1 test [21] was used to assess the solidified microstructure of the alloy under a fixed solidification condition with a cooling rate of 3.5 K/s. The prepared alloy melt was isothermally held at 750°C for 30 minutes in an electric resistance furnace, and then cast into the TP-1 mould. The pouring temperature for the TP-1 test of the alloy melt was 660°C and 690°C.
The microstructure characteristics of the samples were examined using a Zeiss field emission gun (FEG) Supra 35 scanning electron microscope (SEM). To investigate the 3-dimensional morphology of the IMCs, samples were deep-etched in an etchant of an aqueous solution containing 10 to 15 vol pct HCl for 1 to 3 minutes. After dissolution of the Al matrix in 1 to 2 mm thick, the remaining was treated with completely but gently cleaning in an ethanol bath to get rid of the etching solution. The EBSD measurements were made using the Zeiss Crossbeam 340 FIB instrument. The scanning step ZHONGPING  size ranged from 0.05 to 0.2 lm. Thin foils for highresolution transmission electron microscopy (HRTEM) examinations were prepared from the slices of different samples which were cut into 3 mm diameter discs and finally grounded to a thickness of less than 60 lm, followed by ion-beam-thinning using a Gatan precision ion polishing system (PIPS) at an energy of 3 to 5.0 kV and an incident angle of 3 to 6 deg. TEM examination was performed on a JEOL 2100F microscope equipped with EDS facility operated at an accelerating voltage of 200 kV. Figure 1(a) demonstrates a microstructure consisting of the primary FIMC with compacted morphology, the binary eutectic (BE) (FIMC + a-Al) and/or binary eutectic (Mg 2 Si + a-Al) with Chinese script morphology, the primary a-Al and the ternary eutectic (TE) (FIMCs + Mg 2 Si + a-Al) when casted at 690°C. It demonstrates that the BE-(FIMC + a-Al) and BE-(Mg 2 Si + a-Al) structures are coarse and only a small amount TE-(FIMCs + Mg 2 Si + a-Al) can be examined. The eutectic spacing of BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al) and BE-(Mg 2 Si + a-Al) were quantified to be 12.2 ± 1.0 and 2.0 ± 0.2 lm. The measurement for the BE structure with Chinese script morphology was done according to the secondary dendritic arm spacing of the intermetallic compounds. All the phases were identified with the SEM and EBSD analysis. The FIMCs in the primary and binary eutectic structures were identified as Al 15 (Fe,Mn) 3 Si 2 . The black phase in the Chinese script BE structure was identified as Mg 2 Si. The small amount white phase in the TE structure was identified as major Al 13 Fe 4 mixed with some Al 15 (Fe,Mn) 3 Si 2 by TEM analysis. Figure 1(b) shows that no primary FIMCs can be observed, and the primary a-Al is dominating. The structure surrounding the primary a-Al dendrites is the TE-(FIMCs + Mg 2 Si + a-Al) with a very fine spacing (0.62 ± 0.05 lm) which is much smaller than that of the Chinese script BE structures. The intermetallic compounds in these TE structure presents the needlelike morphology. The white IMC is identified as Al 13 Fe 4 , and the black IMC is identified as Mg 2 Si.
The 3D morphology shown in Figure 2(a) demonstrates that the Chinese script Al 15 (Fe,Mn) 3 Si 2 phase connected and grown from the compacted primary Al 15 (Fe,Mn) 3 Si 2 phase, with the latter being identified as the nucleation substrate for the Al 15 (Fe,Mn) 3 Si 2 phase in BE structure. [20] The eutectic associated to the BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al) is the Chinese script BE-(Mg 2 Si + a-Al). Figure 2(b) shows 3D morphology of the TE structure in the a-Al inter-dendritic zone. After deep etching, the a-Al and Mg 2 Si phase in the TE structure are almost gone. The remaining phase in the TE structure is the Al 13 Fe 4 phase. It shows that the TE-Al 13 Fe 4 phase is fine and has needle-like morphology.
The orientation relationships between the primary phases (Al 15 (Fe,Mn) 3 Si 2 or a-Al) and its surrounding eutectic structures are examined by EBSD. Our previous work [20] has proved that the Chinese script BE-Al 15 (Fe,Mn) 3 Si 2 was nucleated from the connected compacted primary Al 15 (Fe,Mn) 3 Si 2 . Therefore, in this study, we focus on the relationship between the primary a-Al and the associated BE structures.   Figure 3(c) with the crystal lattice of a-Al in primary dendrites and TE structure shows that the a-Al phase in the TE structure has the same orientation as the a-Al phase in the associated primary a-Al dendrites.
Interface between Al 15 (Fe,Mn) 3 Si 2 and Mg 2 Si was further examined by TEM, with an example being shown in Figure 4. Si + a-Al) and TE-(a-Al + Al 13 Fe 4 + Mg 2 Si). As we reported, [20] the BE structure Chinese script Al 15 (Fe,Mn) 3 Si 2 was nucleated and grown from the primary Al 15 (Fe,Mn) 3 Si 2 due to the lowest lattice misfit (0 pct) [22] between the solid and the substrate. In addition, the EBSD results in Figure 3(c) revealed that the a-Al in BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al) does not have the same orientation as any associated primary a-Al. Therefore, the BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al) is less possible to be nucleated on the surrounding primary a-Al with the a-Al as the leading phase. The BE-Al 15 (Fe,Mn) 3 Si 2 can be considered to be nucleated on the primary Al 15 (Fe,Mn) 3 Si 2 as the leading phase of the BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al). The other work [22] has reported that the primary Al 15 (Fe,Mn) 3 Si 2 particles can act as the nucleation substrates for the following a-Al. The primary a-Al phase can be considered to be formed before the formation of BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al) and after the formation of primary Al 15 (Fe,Mn) 3 Si 2 particles in this study. The other interesting result is that the large-scale Chinese script BE-(Mg 2 Si + a-Al) is always associated with the BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al). The 3D SEM image (Figure 2(a)) shows that the BE-Mg 2 Si is connected firmly with the BE-Al 15 (Fe,Mn) 3 Si 2 . The TEM results shown in Figure 4 revealed a well-defined orientation relationship (OR) between the Al 15 (Fe,Mn) 3 Si 2 and Mg 2 Si. According to the previous study, [20] Al 15 (Fe,Mn) 3 Si 2 is {1 1 0} faceted. The examined interface (Figure 4   (Al 15 (Fe,Mn) 3 Si 2 + a-Al) was suppressed correspondingly. This is possible due to the lack of the potent nucleation substrate for the BE-Al 15 (Fe,Mn) 3 Si 2 and BE-Mg 2 Si. In this case, the remaining liquid in the a-Al inter-dendritic zone formed the TE-(FIMCs + Mg 2 Si + a-Al). The EBSD map shown in Figure 3(C) revealed that the a-Al in TE-(FIMCs + Mg 2 Si + a-Al) has the same orientation with the surrounding primary a-Al, which indicates that the TE-(FIMCs + Mg 2 Si + a-Al) nucleates on the surrounding primary a-Al with the a-Al leading phase.
In addition, the morphology and eutectic spacing of the eutectic structures are found to depend on the leading phase. When the IMCs (FIMCs or Mg 2 Si) were selected as the leading phase, the eutectic structure grew into large size structure, such as Chinese script BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al) and BE-(Mg 2 Si + a-Al) (Figures 1 and 2). When a-Al was selected as the leading phase, both the IMC and the eutectic structure was refined. (Figures 1 and 2).
In this study, BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al) nucleated and grew from the primary Al 15 (Fe,Mn) 3 Si 2 phase with the BE-Al 15 (Fe,Mn) 3 Si 2 as the leading phase in Al-5Mg-2Si-0.4Mn-0.7Fe alloy when casted at 690°C. The BE-(Mg 2 Si + a-Al) nucleated on the Al 15 (Fe,Mn) 3 Si 2 phase in BE-(Al 15 (Fe,Mn) 3 Si 2 + a-Al) structure with the Mg 2 Si as the leading phase in Al-5Mg-2Si-0.4Mn-0.7Fe alloy when casted at 690°C. The TE-(FIMCs + Mg 2 Si + a-Al) nucleated on the surrounding primary a-Al. The a-Al was the leading phase for the TE structure in Al-5Mg-2Si-0.4Mn-0.7Fe alloy when casted at 660°C. The EPSRC is gratefully acknowledged for providing financial support under Grant EP/N007638/1.

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