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 alpha-Mg matrix, which are defined as type-A OR (< 001 >(FCC) //[0001](alpha)), type-B OR (< 110 > FCC //[0001](alpha)), and type-C OR (< 111 >(FCC) //[0001](alpha)). It is concluded that the T phase always nucleates at the {100}(FCC) crystallographic of La-rich FCC structures and 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, < 20 (2) over bar3)(alpha), and (10 (1) over bar2)(alpha) zone axes of the alpha-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 alpha-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.

Automated summary

The shapes and orientations of precipitates significantly affect the mechanical properties of precipitation-strengthened magnesium alloys. This study examines the microstructure and precipitation behaviors of an aged Mg-3.0 wt % La alloy using atomic resolution scanning transmission electron microscopy. The main precipitate at the peak-aged stage is a face-centered cubic (FCC) La-enriched solid solution, which evolves into a tetragonal (T) lattice precipitation strengthening phase at the over-aged stage. The crystallographic orientations and shapes of FCC and T precipitates are elaborated on.