Rotational and translational domains of beta precipitate in aged binary Mg−Ce alloys

Abstract

The structural evolution from β1 (Mg3Ce) to β (Mg12Ce) precipitates, which takes place at the over-aged stage of binary Mg−Ce alloys, are investigated by high-angle annular dark-field scanning transmission electron microscopy. The structural transformation mainly occurs in the {111}β1 crystallographic planes, where the newly formed β lattices exhibit two categories of domain structures, namely rotational and translational domains. The rotational domain is composed of three β domains (βRA, βRB and βRC), which are related by a 120° rotation with respect to each other around the 〈111〉β1 axis of their β1 parent phase. The {111}β1 crystallographic planes can provide four sets of sublattices with the same orientation for an initial nucleation of β lattice. It leads to the formation of four translational β domains (βTA, βTB, βTC and βTD), among which any two differ by a vector of 1/6〈112〉β1. We deduce theoretically that there exist twenty-four β domains during this transition. However, considering the interfacial misfit, only one-third of domains can grow up and eventually forms β ribbon. Furthermore, a majority of β ribbons overlap partially β1 plate, which is beneficial to relax interfacial strain among β, β1 and α-Mg matrix (α/β/β1). The configuration of multiple β domains can effectively regulate interfacial misfit of α/β and β/β1, which are responsible for enhancing the hardness and strength of Mg−Ce alloy. Additionally, this study aims to provide some clues to improve the over-aged performance of magnesium alloys by constructing β domains and optimizing the α/β/β1 interface.

Keywords

• Magnesium alloys
• HAADF-STEM
• Rotational domain
• Translational domain
• Phase transition
• Interfacial misfit