The influence of various Mg contents on the microstructures, mechanical properties and thermal expansion behaviors of P-modified Al–50Si alloys

In this work, the evolutions on the microstructure, mechanical performance as well as thermal expansivity of hypereutectic Al–50Si alloys with different Mg contents were investigated in detail. Besides that, the growth mechanism of the five-fold branch-like primary Si crystal was also explained in the view of thermodynamics. The results indicated that the size of primary Si crystal and the thermal expansivity showed upward trends with the increase of Mg content but the mechanical property presented a downward trend. Moreover, as the Mg content rose, the morphologies of primary Si crystals in the P-modified Al–50Si–xMg (x = 0, 5, 10 wt%) alloy changed from octahedron-like to plate-like and fivefold branch-like. The reason of these adverse phenomena was attributed to the formation of Mg₃P₂ phase caused by the reaction between Mg element and AlP compound, which will generate a poisoning effect and worsen the performances of the alloys.