Phase equilibria and solidification characteristics of the Al–Sc–Si alloys

By means of scanning electron microscopy, electron probe microanalyses, and differential scanning calorimetry, 21 alloys in both as-cast and annealed states were investigated to study the phase equilibria and phase transformations of the Al–Sc–Si system in the Al-rich corner. Based on the observed microstructure, the solidification paths of the as-cast alloys were analyzed. In addition, the phase equilibria of the Al–Sc–Si system at 500 °C were determined and phase transition temperatures for representative alloys are measured by means of DSC. For the sake of providing missing thermodynamic data, the enthalpies of formation at 0 K for the compounds ScSi, Sc₅Si₃, and τ (AlSc₂Si₂) were obtained by first-principles calculations. Based on the data from the present work and literature, the thermodynamic descriptions of the Sc–Si and Al–Sc–Si systems were developed. The Al–Sc–Si phase diagram in the entire composition range and the solidification characteristic in the Al-rich corner were then calculated. It is shown that the predicted solidification paths could describe the experimental observations reasonably. Based on the thermodynamic description obtained in this work, solution heat-treated and aged conditions of some Al–Sc–Si alloys are discussed, which suggests that the validated thermodynamic descriptions of this work could be helpful for the microstructure design of the Al–Sc–Si alloys.