Effect of La Addition on Solidification Behavior and Phase Composition of Cast Al-Mg-Si Alloy

• Main Authors: Vladislav Deev, Evgeny Prusov, Pavel Shurkin, Ernst Ri, Svetlana Smetanyuk, Xizhang Chen, Sergey Konovalov
• Format: Article
• Language: English
• Published: MDPI AG 2020-12-01
• Series: Metals
• Subjects: cast aluminum alloy; Al-Mg-Si-La system; lanthanum; Mg₂Si; eutectic modification; microstructure
• Online Access: https://www.mdpi.com/2075-4701/10/12/1673

The current study focusses on the phase composition, solidification path, and microstructure evaluation of gravity cast Al-4Mg-0.5Si-<i>x</i>La aluminum alloy, where <i>x</i> = 0, 0.1, 0.25, 0.5, 0.75, and 1 wt.% La. A computational CalPhaD approach implemented in Thermo-Calc software and scanning electron microscopy technique equipped with electron microprobe analysis (EMPA) was employed to assess its above-mentioned characteristics. The thermodynamic analysis showed that the equilibrium solidification path of La-containing Al-Mg-Si alloys consists of only binary phases LaSi₂ and Mg₂Si precipitation along with <i>α</i>-Al from the liquid and further solid-state transformation of this mixture into <i>α</i>-Al + Al₁₁La₃ + Mg₂Si + Al₃Mg₂ composition. Scheil–Gulliver simulation showed a similar solidification pathway but was accompanied by an increase in the solidification range (from ~55 °C to 210 °C). Furthermore, microstructural observations were congruent with the calculated fraction of phases at 560 °C and related to <i>α</i>-Al + LaSi₂ + Mg₂Si three-phase region in terms of formation of La-rich phase having both eliminating effect on the eutectic Mg₂Si phase. Quantitative EMPA analysis and elemental mapping revealed that the La-rich phase included Al, La, and Si and may be described as Al₂LaSi₂ phase. This phase shows a visible modifying effect on the eutectic Mg₂Si phase, likely due to absorbing on the liquid/solid interface.