Thermodynamic investigation on phase formation in the Al-Si rich region of Al-Si-Ti system

The thermodynamic condition for phase formation in the Al-Si-Ti system is crucial for understanding the reason behind deteriorations of grain refinement and castability of Al-Si-Ti alloys. Seven Al-Si-Ti alloys annealed between 550 and 650 °C were used to clarify the inconsistencies of phase equilibria in the Al-Si rich region. The existences of τ₁ (Zr₃Al₄Si₅-type tetragonal), τ₂ (ZrSi₂-type orthogonal) and the three-phase equilibrium of τ₁ + α-Al + Si were confirmed. Combined with a critical review of literature data, a CALPHAD-type thermodynamic description was developed for the phase equilibria in the Al-Si rich region of the Al-Si-Ti system between 550 and 900 °C. Meanwhile, edge-to-edge matching analysis according to the crystal structures of τ₁ and τ₂ shows that their nucleation efficacies for α-Al are both weaker than that of Ti(Al,Si)₃. From the calculated phase equilibrium of L + τ₂ + Ti(Al,Si)₃ at 727 °C and the local equilibrium principle, τ₂ would form on the surface of TiAl₃ particle and deteriorate its grain refinement efficiency for α-Al. From the calculated liquidus projection, Ti(Al,Si)₃ is the primary phase within 0-10.3 wt.% Si and 0.08-0.30 wt.% Ti. To avoid the formation of τ₁ and τ₂ for retaining good grain refinement efficiency and castability, a Ti/Si threshold was determined by thermodynamic calculations.