Atomic-scale investigation of precipitate phases in QE22 Mg alloy

Xiaojun Zhao, Zhiqiao Li, Aiping Zhang, Longlong Hao, Houwen Chen, Jian Nie

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Precipitation-hardenable commercial Mg alloy QE22 (Mg-2.5Ag-2.0Nd-0.7Zr, wt.%) has excellent mechanical properties, but precipitates in this alloy have not been well understood. In this work, precipitate phases γ'', γ, and δ formed during the isothermal ageing process at 150, 200, 250, and 300°C have been characterized using atomic-resolution high-angle annular dark-field scanning transmission electron microscopy and atomic-scale energy-dispersive X-ray spectroscopy. The morphology, crystal structure, and orientation relationship of these precipitate phases have been determined. Domain boundaries usually exist in a single γ particle, which can be characterized by a separation vector of [1
01]α. The δ phase forms in situ from its precursor γ phase, consequently leading to the formation of three different variants within a single δ particle. The nucleation of the δ phase is strongly related to the domain boundaries of the γ phase. The formation of the γ phase may be promoted by its precursor γ'' phase. The similarities in atomic structures of the γ'', γ, and δ phases are described and discussed, indicating that transformations between these precipitate phases can be accomplished through the diffusion of added alloying elements.
Original languageEnglish
Number of pages40
JournalJournal of Materials Science and Technology
Publication statusAccepted/In press - 19 Sept 2023

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