Abstract
Binary hypereutectic Al-21.5 wt.% Si alloy was studied by XRD in-situ at 610 °C in the two-phase (liquid + solid) state. Primary β -Si, liquid and α -Al FCC phases were found to co-exist at 610 °C. The α-Al solid solution with FCC lattice parameter 423.8 ± 5 pm was stably present in the hypereutectic melt over a long time soaking, although theoretical equilibrium phase diagram does not allow such combination. The total driving force of nucleation for classical Gibbs-Thomson and modified theories was calculated. The stable existence of α-Al phase was explained as a kinetic effect due to local nucleation of silicon phase and it was shown to be thermodynamically favourable within certain limits of the nuclei size due to local depletion of the liquid phase when nucleation rate exceeds mass transport rate.

Key words
aluminium-silicon alloy, semi-solid state; crystal lattice, heterogeneous nucleation, Gibbs energy.

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