A novel tetragonal precipitate with multiple orientations in Mg−La alloy: An atomic scale investigation using HAADF−STEM

Abstract During the process of precipitation in aged magnesium rare earth (Mg−RE) alloys, the subsequent structural transformation of the β ′ ′ precipitate is still controversial. In this study, the sequential precipitates of the β ′ ′ were investigated in Mg−3.0 wt.% La binary alloys by atomic resolution high-angle annular dark-field scanning transmission electron microscopy (HAADF−STEM). Distinguished from the β ′ , the β ′ ′ precipitate in Mg−La alloy is transformed preferentially into a La-rich FCC structure with the two crystallographic orientations to α-Mg matrix, and then evolved into a hitherto unreported tetragonal phase. It is confirmed that the crystal structure of tetragonal precipitate has a symmetry of space group I 4 / mmm and lattice parameters of a = 0.426 nm and c = 1.080 nm. Meanwhile, there exist three categories of orientation relationships (Type-І, Type-П and Type-Ш) between the tetragonal precipitate and α-Mg matrix in the microstructure. Based on it, the tetragonal precipitate exhibits three typical precipitation morphologies, which are elucidated reasonably according to the interfacial misfit degree. The aim of this study is to provide some efficient subsequent precipitation strengthening phases of β ′ ′ precipitate.