Electron beam irradiation induced metastable phase in a Mg−9.8 wt%Sn alloy

Abstract Aberration-corrected scanning transmission electron microscopy has been used to study a novel metastable phase, designated as β′′ phase, induced to form by electron beam irradiation in a Mg−9.8 wt.%Sn alloy. This phase is spherical in three dimensions, having a D019 structure with the lattice parameters of a = 0.642 nm, c =0.521 nm and space group of P63/mmc. Its chemical formula is Mg3Sn, like the β′ metastable precipitate phase. The orientation relationship between the β′′ phase and the α-Mg matrix is such that [2 1 ¯ 1 ¯ 0]β′′ // [2 1 ¯ 1 ¯ 0]α and (0001)β′′ // (0001)α. Its formation involves solely the ordering of Sn atoms in the solid solution magnesium matrix. First-principles calculations indicate that the formation of the β′′ phase is energetically favored.