Direct observation of beta″ precipitate with a three-dimensional periodic structure in binary Mg–La alloy

Abstract

It is a long-debated issue whether or not β′′ forms in magnesium-rare earth alloys. In this study, after solid solution and aging treatment, the binary Mg-3.0 wt% La alloys were characterized using atomic-resolution scanning transmission electron microscopy. A large number of β′′ metastable precipitate was captured successfully along [0001]α and 112¯0α directions. The [0001]α observation reveals that the inchoate β′′ is composed of single row of linked hexagonal rings. Under further aging, the expansion direction of the hexagonal ring is not limited to 112¯0α direction and the β′′ can reach a thickness of 5cβ′′ along [0001]α, demonstrating that the β′′ forms a three-dimensional periodic structure and the β′′ phase can form in binary MgLa alloy. Meanwhile, a planar defect with an ABCBA stacking fault can be formed usually in the interface between β′′ phase and α-Mg matrix, which contributes to alleviate the distortion of adjacent α-Mg matrix. Keywords: Magnesium alloy, Scanning transmission electron microscopy, Metastable precipitate, Three-dimensional periodic structure, Stacking fault