Solute segregation and the precipitate phases in a creep-resistant Mg-1Gd-0.4Zn-0.2Zr (at.percent) alloy aged isothermally at 250 and 200 deg C have been examined using a three-dimensional atom probe and high-angle annular dark-field scanning transmission electron microscopy. Co-segregation of Gd and Zn atoms is detected in the as-quenched condition, and this contributes to the large hardness increase in this condition. Precipitation during isothermal ageing involves the formation of metastable phases gamma" and gamma', with gamma" as the key strengthening phase. The equilibrium phase gamma is not detected for ageing up to 1000 h at 250 deg C. The gamma" phase has an ordered hexagonal structure (space group P6-bar 2m, a = 0.560 nm, c = 0.444 nm) and an atomic composition of approximately Mg_(70)Gd_(15)Zn_(15), and forms as (0001)_(alpha) plates with a thickness of a single unit cell height. The gamma' phase has a disordered hexagonal structure (space group P3-bar ml, a = 0.321 nm, c = 0.781 nm) and an atomic composition of about MgGdZn. The gamma' phase also forms as thin (0001)_(alpha) plates, but it has a much larger aspect ratio. The formation of the gamma' phase involves a large shear strain, and it often forms in twin-related plates to self accommodate this shear strain.
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