Strengthening precipitate phases in a Mg-4Sm-0.4Zr (wt%) alloy, aged isothermally at 473 K, were characterized using transmission electron microscopy and high-angle annular dark-field scanning transmission electron microscopy. The β″ (D019) precipitates appear in continuous and discontinuous D019 structures, which are designated as β″ (I) and β″ (II) respectively and the β″ (II) is the dominant phase in the peak-aged state. A small number of β' (cbco) precipitates were observed in Mg-Sm-Zr alloy, and these phases have two types of lattice parameters where one is a = 0.64 nm, b = 1.14 nm and c = 0.52 nm, and the other is a = 0.64 nm, b = 2.28 nm and c = 0.52 nm, designated as β L' and β S' respectively. The precipitation sequence in Mg-Sm-Zr alloys can be presented as super-saturated solid solutions (S.S.S.S) → β″ → β' → β1 (fcc) → β (bct), indicating almost Mg-RE (RE = Nd, Sm, Gd, Y, Dy) alloys have the same ageing precipitation sequence. A phase transformation model between β″/β' was proposed based on the experimental results.
- Crystal structure
- High-angle annular dark-field (HAADF)
- Magnesium alloys