Heterogeneous Nucleation of Eutectic Structure in Al-Mg-Si Alloys
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alloys with fine grain structure have received great attention in various fields especially in automotive and aerospace industries due to the advanced properties such as low density and good castability.[1,2] Grain refinement of Al alloys not only can reduce casting defects such as hot tearing and porosity but also can enhance mechanical properties such as strengths and ductility.[3] Many kinds of grain refinement methods were developed, such as thermal control,[4] chemical methods,[5,6] and mechanical method.[7] One of the most successful ways to grain refine Al alloys by chemical method during the casting process is development of Al-Ti-B master alloy system as effective grain refiners.[5,6,8] The master alloys significantly refined a-Al grains by providing extra Ti or B and TiB2 particles as the inoculants for different Al alloys.[9,10] However, all these grain refinement methods are focusing on the refinement of a-Al grains. The other very important factor for the mechanical properties is eutectic structure in the Al alloys. In most of Al alloys, the eutectic structures consist of a-Al phase with one or
ZHONGPING QUE, YUN WANG, and ZHONGYUN FAN are with the BCAST, Brunel University London, Uxbridge, Middlesex UB8 3PH, UK, Contact e-mail: [email protected] Manuscript submitted October 10, 2019.
METALLURGICAL AND MATERIALS TRANSACTIONS A
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.[13–19] 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-5Mg2Si-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 binary eutectic primary Al15(Fe,Mn)3Si2, (Al15(Fe,Mn)3Si2 + a-Al), and t
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