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Investigation of equilibrium and non-equilibrium solidification of Al-2, 2Mg-2, 1Li alloy

The investigation of equilibrium and non- equilibrium solidification of aluminum (Al) alloy containing 2, 2 wt. % magnesium (Mg) and 2, 1 wt.% lithium (Li) was conducted in order to estimate the influence of chemical composition and thermodynamic parameters on microstructure development. Calculations of solidification sequence under equilibrium and non-equilibrium conditions were obtained using Computer Aided Thermodynamic Diagram Calculation (CALPHAD). Equilibrium calculations covered transformations and precipitations in liquid and solid state. Calculations in non-equilibrium conditions, based on Scheil-Gulliver solidification, covered only diffusion based processes in liquid state, while solid state processes were not covered due to the low diffusion rate. Developed microstructure of Al-2, 2Mg-2, 1Li alloy was analyzed using metallographic and thermal analysis, respectively. Equilibrium solidification begins with transformation of αAl dendritic network followed by diffusion based solid state precipitations. Stabile AlLi (δ) phase precipitates first as a result of reduced solubility of Li in αAl and a high Li/Mg ratio. Ternary Al2LiMg (T) phase precipitates from Mg rich balked αAl. Equilibrium solidification ends with the precipitation of Al8Mg5 (β) phase. Predication under non-equilibrium conditions covered the precipitation of αAl, stabile δ and ternary T phase directly from the Liquid. Precipitation of β phase was not predicated due to the low diffusion rate. However, variations in the solidification sequence were identified by microstructural analysis. According to microstructural analysis, solidification of Al-2, 2Mg-2, 1Li alloy begins with transformation of αAl dendritic network followed by the precipitation of metastable Al3Li (δ’) phase inside the grains of αAl. Metastable precipitation is caused by reduced solubility of Li in αAl. Reduced solubility of Li in αAl is additionally affected by the Mg. During the rest of solidification, δ’ phase is used as a nuclei for precipitation of both δ and T phase. Nucleation and growth of δ and T phase causes the formation of precipitation free zones (PFZ) near grain boundaries. Solidification ends with the precipitation of irregular and course β phase at grain boundaries. The Al-2, 2Mg-2, 1Li alloy investigation indicated a significant influence that the correlation of chemical composition and thermodynamic parameters can have on the solidification sequence. This correlation caused variations between equilibrium and non-equilibrium solidification.

Al-2, 2Mg-2, 1Li alloy, equilibrium and non-equilibrium solidification, microstructure

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