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Even small amounts of Phosphorus and Strontium strongly affect the microstructure of hypoeutectic Al–Si alloys. P is an unavoidable trace element in commercial Al-alloys which causes formation of AlP particles as potent nucleation sites for eutectic (Si). Sr, in contrast, is purposely added to modify the morphology of eutectic (Si) towards fine coral-like fibers. It is hypothesized that Sr does not only alter the growth kinetics of (Si), but additionally prevents detrimental (Si) nucleation due to neutralization of AlP particles by Al₂Si₂Sr formation. This presumes that both AlP and Al₂Si₂Sr precipitate prior to (Si). Using a newly assessed thermodynamic database for the Al–Si–Sr–P system, critical P and Sr thresholds for pre-silicon formation of AlP and Al₂Si₂Sr were evaluated and mapped under equilibrium and Scheil conditions. The competitive precipitation of AlP, Al₂Si₂Sr and (Si) and its impact on the evolution of the …