Comprehensive study of phase transformation in age-hardening of Mg-3Nd-0.2Zn by means of scanning transmission electron microscopy

Effects of thermal history on precipitation behavior and precipitation kinetics in Mg-3Nd-0.2Zn (wt.%) alloy were studied. Microstructure analysis of the aged alloys revealed the presence of new phases in the microstructure in addition to the old description: Super saturated solid solution (SSSS) → G. P. zones → β″ → β′/β1 → β, reported for Mg-Nd system. In this regard, the sequence of precipitation for the studied alloy was identified as: Super saturated solid solution (SSSS) → Clusters of atoms → G. P. zones (I, II, III) → β′ → β2 → β1/γ′ → β. The formation of clusters of solute atoms in this system was confirmed by high angle annular dark field scanning transmission electron microscopy (HAADF-STEM) and atom probe tomography analysis. In addition, new metastable phases designated as G. P. zones (I, II, III) and β2 were identified in the microstructure of the aged alloys by means of high resolution HAADF-STEM. The β2 phase was found to be a transition phase in transformation of β′ to β1. Furthermore, the microstructure analysis revealed that the structure of β1 phase is indeed a body centered tetragonal structure in spite of face centered cubic structure that has been reported for this phase in the literature.