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

Precipitates in an Mg-0.99at%Sm (Mg99.01Sm0.99) alloy aged at 200 degrees 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 gamma here, in the alloy aged at 200 degrees C for 4 h have a thin lens-shape with a thickness of 2-5 nm and a diameter of 20-60 nm. The gamma precipitate has an incommensurate structure with an orthorhombic unit cell of a = 2a(0) = 0.64 nm, b = 6a(0)root 3 = 3.334 nm and c = c(0) = 0.52 nm, where a(0) and c(0) are lattice constants of a hexagonal unit of the Mg-rnatrix. In the early stage of aging at 200 degrees C for 0.5 h, isolated structure units forming the gamma structure are dispersed in an Mg hexagonal lattice. By annealing at 200 degrees C for 100h, coarse precipitates of a stable Mg3Sm phase are formed along grain boundaries and inside grains of the Mg-matrix, and wide gamma precipitate-free zones appear around them. [doi: 10.2320/matertrans.M2009046]