On Shapes and Orientations of Precipitation Strengthening Phases in an Aged Mg–La Alloy

The shapes and orientations of precipitates have important influences on the mechanical properties of precipitation-strengthened magnesium alloys. In this work, the microstructure and precipitation behaviors of an aged Mg-3.0 wt % La alloy are investigated by atomic resolution scanning transmission electron microscopy. The main precipitate product at the peak-aged stage is a category of La-enriched solid solution with a face-centered cubic (FCC) lattice, which will subsequently evolve into another precipitation strengthening phase with a tetragonal (T) lattice at the over-aged stage. The crystallographic orientations and shapes of FCC and T precipitates are elaborated on. Distinctively, La-rich FCC structures can maintain three types of orientation relationships (ORs) with the α-Mg matrix, which are defined as type-A OR (⟨001⟩FCC //[0001]α), type-B OR (⟨110⟩FCC //[0001]α), and type-C OR (⟨111⟩FCC //[0001]α). It is concluded that the T phase always nucleates at the {100}FCC crystallographic planes of La-rich FCC structures and gradually grows up by consuming La-rich solid solution. Based on the conversion from FCC to T lattices, the occurred T precipitates can be divided into four classifications whose fourfold rotational axes are parallel to the c-axis, basal plane, ⟨202̅3⟩α, and ⟨101̅2⟩α zone axes of the α-Mg matrix, respectively. At the same time, there exist six sets of crystallographic ORs (type-A1, type-A2, type-A3, type-B1, type-B2, and type-C1) between the T phase and α-Mg matrix. In view of the interfacial matching, the shapes of the existing T phase involve the sphere, prismatic, and basal plates, whose effects on the mechanical properties are also predicted theoretically.