Characterization of Precipitates in Mg-Sm Alloy Aged at 200°C, Studied by High-Resolution Transmission Electron Microscopy and High-Angle Annular Detector Dark-Field Scanning Transmission Electron Microscopy

Abstract

Precipitates in an Mg-0.99 at%Sm (Mg_99.01Sm_0.99) alloy aged at 200°C were studied by the combination of high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Fine precipitates of a meta-stable phase, which is called γ here, in the alloy aged at 200°C for 4 h have a thin lens-shape with a thickness of 2–5 nm and a diameter of 20–60 nm. The γ precipitate has an incommensurate structure with an orthorhombic unit cell of a=2a₀=0.64 nm, b\\fallingdotseq6a₀\\sqrt3=3.334 nm and c=c₀=0.52 nm, where a₀ and c₀ are lattice constants of a hexagonal unit of the Mg-matrix. In the early stage of aging at 200°C for 0.5 h, isolated structure units forming the γ structure are dispersed in an Mg hexagonal lattice. By annealing at 200°C for 100 h, coarse precipitates of a stable Mg₃Sm phase are formed along grain boundaries and inside grains of the Mg-matrix, and wide γ precipitate-free zones appear around them.