Primary Mg2Si phase and Mg2Si/

The microstructure of primary Mg2Si and the interface of Mg2Si/α-Mg modified by Sn and Sb elements in an as-cast Mg-5Sn-2Si-1.5Al-1Zn-0.8Sb (wt.%) alloy were investigated. In the primary Mg2Si phase not only the Si atoms but also the Mg atoms could be substituted by Sn and Sb atoms, resulting in the slightly reduced lattice constant a of 0.627 nm. An OR of Mg2Si phase and α-Mg in the form of [001]Mg2Si∥[011¯1]α, (220)Mg2Si∥(01¯12)α was discovered. Between primary Mg2Si phase and α-Mg matrix two transitional nano-particle layers were formed. In the rim region of primary Mg2Si particle, Mg2Sn precipitates sizing from 5 nm to 50 nm were observed. Adjacent to the boundary of primary Mg2Si particle, luxuriant columnar crystals of primary Mg2Sn phase with width of about 25 nm and length of about 100 nm were distributed on the α-Mg matrix. The lattice constant of the Mg2Sn precipitate in primary Mg2Si particle was about 0.756 nm. Three ORs between Mg2Sn and Mg2Si were found, in which the Mg2Sn precipitates had strong bonding interfaces with Mg2Si phase. Three new minor ORs between Mg2Sn phase and α-Mg were found. The lattice constant of primary Mg2Sn phase was enlarged to 0.813 nm owing to the solution of Sn and Sb atoms. Primary Mg2Sn had edge-to-edge interfaces with α-Mg. Therefore, the primary Mg2Si particle and α-Mg were united and the interfacial adhesion was improved by the two nano-particles layers of Mg2Sn phase.