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

Abstract

Abstract The modification effects of Sb on the morphologies of reinforcements Mg2Si and Mg2(Si,Sn) in Mg-4Si and Mg-5Sn-xSi(x = 1,4) alloys were investigated and the ideal adhesion works (Wad) between substrate (Mg3Sb2) and crystallization phases (Mg2Si, Mg2Sn and Mg2(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 Mg2Si or/and Mg2(Si,Sn) is decreased. For the primary Mg2Si or/and Mg2(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 Mg2Si 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 Mg2Si become ambiguity, and tend to being spheroidized. Primary Mg2Si, Mg2Sn and Mg2(Si,Sn) were refined because Mg3Sb2 precipitated during solidification could act as their nuclei. When some Sn atoms replace equivalent Si from Mg2Si phase, the interfaces adhesion work (Wad) 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.