Refinement of primary Si in the bulk solidified Al-20Â wt.%Si alloy assisting by high static magnetic field and phosphorus addition

•Magnetic field combined with P addition was firstly studied in Al-20 wt.%Si alloy.•The size of primary Si was refined to 21.1 μm under 1 T with P addition.•The size of primary Si was refined to 15.4 μm under 29 T without P addition.•A unified refinement mechanism under the magnetic field had been proposed.

Effects of high static magnetic field (HSMF) on microstructures and solidification process of Al-20 wt.%Si alloys with and without phosphorus addition (P addition) have been investigated. The size of primary Si (SPS) is refined continuously as the increase of magnetic flux density (B), and the minimum SPS value is about 15.4 μm when B is 29 T without P addition. However, the SPS decreases firstly and then increases as the increasing B combined with P addition, the average SPS value reaches to the minimum about 21.1 μm when B is 1 T. A unified refinement mechanism has been suggested based on the theory of thermodynamic nucleation and kinetic growth under the HSMF. And the influence of P addition on the morphology of primary Si has also been discussed.