Primary Mg_(2)Si phase and Mg_(2)Si/α-Mg interface modified by Sn and Sb elements in a Mg-5Sn-2Si-1.5Al-1Zn-0.8Sb alloy

The microstructure of primary Mg_(2)Si and the interface of Mg_(2)Si/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 Mg_(2)Si 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 Mg_(2)Si phase and α-Mg in the form of[001]Mg_(2)Si‖[01■1]α,(220)Mg_(2)Si‖(0■12)αwas discovered.Between primary Mg_(2)Si phase and α-Mg matrix two transitional nano-particle layers were formed.In the rim region of primary Mg_(2)Si particle,Mg_(2)Sn precipitates sizing from 5 nm to 50 nm were observed.Adjacent to the boundary of primary Mg_(2)Si particle,luxuriant columnar crystals of primary Mg_(2)Sn phase with width of about 25 nm and length of about100 nm were distributed on the α-Mg matrix.The lattice constant of the Mg_(2)Sn precipitate in primary Mg_(2)Si particle was about 0.756 nm.Three ORs between Mg_(2)Sn and Mg_(2)Si were found,in which the Mg_(2)Sn precipitates had strong bonding interfaces with Mg_(2)Si phase.Three new minor ORs between Mg_(2)Sn phase and α-Mg were found.The lattice constant of primary Mg_(2)Sn phase was enlarged to 0.813 nm owing to the solution of Sn and Sb atoms.Primary Mg_(2)Sn had edge-to-edge interfaces with α-Mg.Therefore,the primary Mg_(2)Si particle and α-Mg were united and the interfacial adhesion was improved by the two nano-particles layers of Mg_(2)Sn phase.