Heterogeneous nucleation of Mg2Si and Mg2(Si,Sn) on Mg3Sb2 nucleus in Mg containing Si alloys

Abstract The modification effects of Sb on the morphologies of reinforcements Mg 2 Si and Mg 2 (Si,Sn) in Mg-4Si and Mg-5Sn-xSi(x = 1,4) alloys were investigated and the ideal adhesion works (W ad ) between substrate (Mg 3 Sb 2 ) and crystallization phases (Mg 2 Si, Mg 2 Sn and Mg 2 (Si,Sn)) were calculated using first principles calculations. The results indicate that the primary α-Mg matrix is significantly refined by the modification of Sb. For the eutectic mixture, the arm spacing of the intermetallics of Mg 2 Si or/and Mg 2 (Si,Sn) is decreased. For the primary Mg 2 Si or/and Mg 2 (Si,Sn) which in coarse angular bulk forms without any modification was rounded and distributed uniformly on the α-Mg matrix after modification. The primary Mg 2 Si in octahedrons morphology without modification grows in tetrakaidecahedron mode in Sb modified Mg-4%Si alloys. The clearly straight edges and the sharp corners of the primary Mg 2 Si become ambiguity, and tend to being spheroidized. Primary Mg 2 Si, Mg 2 Sn and Mg 2 (Si,Sn) were refined because Mg 3 Sb 2 precipitated during solidification could act as their nuclei. When some Sn atoms replace equivalent Si from Mg 2 Si phase, the interfaces adhesion work (W ad ) values between substrate and new crystal will increase. Moreover, the interface cohesive strength of Sb-Mg-Si(/Sn/Si-Sn) or Sb-Mg-Mg-Si(/Sn/Si-Sn) will be stronger than the others.