Investigation of structure, morphology and orientation of precipitates in Mg−Bi alloy

In Mg−0.8 at.% Bi binary alloys,the structure, the morphology and the orientation of precipitates were characterized by high-resolution transmission electron microscopy (HRTEM) and high-angle annular dark detector field scanning transmission electron microscopy (HAADF−STEM) techniques. The main precipitate is the α-Mg3Bi2 phase with hexagonal structure (P3¯m1). The morphology of α-Mg3Bi2 precipitate is classified into three types, rod-shaped, cuboid-shaped and polyhedral, which are related closely with orientation relationships (ORs) between α-Mg3Bi2 precipitate and αMg matrix. For the first two types of morphologies, precipitate and αMg matrix can maintain the following ORs: 112¯0Mg3Bi2//0001Mg and 11¯00Mg3Bi2//11¯00Mg. In contrast, another set of ORs between polyhedral α-Mg3Bi2 and αMg matrix can be determined as: 112¯0Mg3Bi2//112¯0Mg and 0003Mg3Bi2//11¯01Mg. The morphology of α-Mg3Bi2 precipitates is discussed particularly based on the interfacial misfit mechanism.