Metastable precipitate phases in Mg-9.8 wt%Sn alloy

Aberration-corrected scanning transmission electron microscopy has been used to characterize precipitates in a binary Mg-9.8 wt%Sn alloy aged in the temperature range 100-250 degrees C. In contrast to the traditional view that precipitation in Mg-Sn based alloys involves exclusive formation of the equilibrium phase beta (Mg2Sn, face-centered cubic), we found the formation of a GP zone and a metastable phase, designated beta', prior to the equilibrium beta phase. The GP zone has a plate morphology with its habit plane parallel to the basal plane of the matrix phase. The beta' phase also has a plate morphology with its habit plane parallel to the magnesium basal plane. It has a L1(2) structure (space group Pm (3) over barm, lattice parameter a = 0.453 nm) and a Mg3Sn composition. The orientation relationship between the beta' precipitate and the magnesium matrix is such that (11 (1) over bar)(beta')//(0001)(alpha) and [011](beta')//[2 (11) over bar0](alpha). The beta' phase forms from GP zones and the magnesium matrix. Its formation is similar to the classical hcp -> fcc transformation that involves generation and motion of Shockley partial dislocations on (0002)(alpha). With prolonged ageing, some beta' precipitates transform in situ to the beta phase.(C) 2017 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.