Precipitation evolution and its correspondence to mechanical properties in Mg-4Sm-xZn-0.4Zr (x=0, 0.3, 0.6, 1.3) (wt.%) alloys are systematically investigated in this work. Precipitation sequences at 200 o C are identified using transmission electron microscopy (TEM), highresolution TEM (HRTEM), and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). A new precipitate β' Z is found to present with the increase of Zn addition. With higher Zn content (> ~1 wt.%), basal γ-series precipitates dominate. The transitionin precipitation sequence agrees wellwith the change of mechanical properties. The microstructure and property evolution in the optimum Mg-4Sm-0.3Zn-0.4Zr alloy is modeled based on obtained experimental data through a combination of classical nucleation and growth model and the CALPHAD (CALculation of PhAse Diagram) simulation. The obtained

a b s t r a c tThe present research presents a series of investigations into phase identification and precipitation sequence in Mg-4Sm-1Zn-0.4Zr 1 alloy, using differential scanning calorimetry (DSC), transmission electron microscopy (TEM) and high angle annular dark field scanning transmission electron microscopy (HAADF-STEM). The precipitation sequence is: super saturated solid solution (S.S.S.S) / solute atom clusters / g 00 / g 0 (/stacking faults) / g. Structure of g 00 has been determined as an ordered hexagonal GP zone, a ¼ 0.556 nm, c ¼ 0.414 nm g 0 is composed of several g 00 layers. Kinetic studies show that quenched-in vacancies play an important role in the formation of solute clusters, while the growth of both precipitates are diffusion controlled. Analysis of microstructure evolution suggests that nucleation of g 0 happens near existing g 00 precipitates.

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