Showing papers by "Kenji Hiraga published in 2009"

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 99.01 Sm 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 α = 2a 0 = 0.64 nm, b˙=. 6a 0 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-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 100h, coarse precipitates of a stable Mg 3 Sm phase are formed along grain boundaries and inside grains of the Mg-matrix, and wide γ precipitate-free zones appear around them.

Stabilization of Stacking Faults and a Long Period Stacking Phase Dispersed in α-Mg Crystalline Grains of Mg-0.7 at%Zn-1.4 at%Y Alloy

Abstract: Mg-0.7 at%Zn-1.4 at%Y alloys annealed at low temperatures after quenching in water from 520°C were studied by high-resolution transmission electron microscopy (HRTEM) and high-angle annular detector dark-field scanning transmission electron microscopy (HAADF-STEM). Stacking faults, thin bands of a 14H-type long period stacking (LPS) phase and relatively thick bands of LPS were precipitated in α-Mg crystalline grains by annealing at 300°C, 400°C and 500°C, respectively. The precipitation of stacking faults, LPS phase and a supersaturated solid solution without any precipitates were reversibly transformed by annealing at low temperatures. It can be concluded that the stacking faults and LPS phase are stabilized by the segregation of Zn and Y from a supersaturated solid solution.