Impact of P and Sr on solidification sequence and morphology of hypoeutectic Al-Si alloys: Combined thermodynamic computation and phase-field simulation

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 eutectic morphology was further studied by 3D phase-field simulations. Effective anisotropy functions for the (Si) interface mobility considered Sr-induced internal twinning. Depending on whether subcritical or supercritical P and Sr contents were selected, either a fine lamellar structure, a coarse flaky structure, or the targeted fine fibrous eutectic structure was reproduced. (C) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.