Influence of P and RE modification on the microstructure and mechanical properties of hypereutectic Al-21Si alloy have been investigated. The results reveal that the coarse bulky and strip primary Si in the alloy can be greatly refined

and eutectic Si is converted from coarse needle into fibre or short bar with modification

which can improve the mechanical properties of the alloy. With 0.08% P and 0.6% RE addition

compared with ones without modification

the tensile strength at room and high temperature of the alloy is increased from 236.2 MPa to 287. 6 MPa

and from 142.5 MPa to 210 MPa

increasing by 22% and 47%

respectively

and the elongation is increased by 56% and 42%

respectively

showing the desirable ambient and high temperature mechanical properties. The modification of primary Si is controlled by heterogeneous nucleation mechanism induced by AlP particles

and the modification of eutectic is resulted form the creation of twin grain from RE absorption. In addition

the strengthening mechanism of the alloy was described.