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The high strength of magnesium alloy WE54 (Mg–5wt.% Y–2wt.% Nd–2wt.% RE) is achieved through aging at 150–250°C, during which precipitation of intermediate phases β₁ and β′ and equilibrium phase β takes place. In order to understand the microstructure evolution of β₁ phase and its effects on β′ and β precipitation, a phase field model of β₁ precipitation is developed. Model inputs, including lattice parameters, precipitate–matrix orientation relationship, elastic constants and free energy data, are obtained from experimental characterization, ab initio calculations and thermodynamic databases. Through computer simulations, the equilibrium shape and spatial distribution of and stress field around β₁ precipitates are quantitatively determined. The results show that the {11¯00} rather than {0001} habit plane of β₁ is determined by the coherency elastic strain energy minimization (e.g., the habit plane is close …