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

•The phase equilibria in the Al–Si rich region of the Al–Si–Ti system have been experimentally clarified.•The thermodynamic description for the Al–Si rich region of the Al–Si–Ti system has been developed.•The Ti/Si threshold for excluding τ1 and τ2 in the Al–Si–Ti alloy has been determined.

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.

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