Solute segregation and precipitation in a creep-resistant Mg–Gd–Zn alloy

Solute segregation and the precipitate phases in a creep-resistant Mg–1Gd–0.4Zn–0.2Zr (at.%) alloy aged isothermally at 250 and 200 °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 γ″ and γ′, with γ″ as the key strengthening phase. The equilibrium phase γ is not detected for ageing up to 1000 h at 250 °C. The γ″ phase has an ordered hexagonal structure (space group P6¯2m, a = 0.560 nm, c = 0.444 nm) and an atomic composition of approximately Mg70Gd15Zn15, and forms as (0 0 0 1)α plates with a thickness of a single unit cell height. The γ′ phase has a disordered hexagonal structure (space group P3¯m1, a = 0.321 nm, c = 0.781 nm) and an atomic composition of about MgGdZn. The γ′ phase also forms as thin (0 0 0 1)α plates, but it has a much larger aspect ratio. The formation of the γ′ phase involves a large shear strain, and it often forms in twin-related plates to self accommodate this shear strain.