The Structures of Precipitates in an Mg-0.5 at%Nd Age-Hardened Alloy Studied by HAADF-STEM Technique

The crystal structures and microstructures of precipitates formed in an Mg-0.5 at%Nd alloy aged at certain temperatures ranging between 170 degrees C and 250 degrees C are studied in detail by high-angle annular detector dark-field scanning transmission electron microscopy. The precipitation sequence can be presented as Mg-solution -> GP-zone -> beta' (orthorhombic) -> beta(1) (fcc). At the early stage of aging (170 degrees C for 2 h), tine precipitates of planar GP-zones appear in parallel to (100), planes, with a thickness of sub-nm and a length of 5-15 nm (the subscript letter of m denotes matrix). With an advance of aging. the GP-zones increasingly grow larger and combine with the neighbours, thus making themselves further prolonged along the directions addressed above. When reaching at the top-stage of aging (170 degrees C for 100 h), the alloy additionally allows the beta'-phase to coexist, taking the form of lens-shape with a thickness of 2-5 nm and a diameter of 5-15 nm. The beta'-phase has an orthorhombic structure (Mg7Nd) with a = 0.64 nm, b = 1.1 nm, and c = 0.52 nm, which is coherently connected to the matrix. At the stage of over-aging, both the GP-zones and the beta'-phase disappear and instead coarse precipitates of the stable beta(1)-phase (Mg3Nd; fcc) are formed with particular crystallographic relations of [001](m) // [110](p) and [110](m) // [112](p) (the subscript letter of p denotes precipitate). [doi:10.2320/matertrans.M2011163]