Atomic-scale investigation of precipitate phases in QE22 Mg alloy

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 gamma ", gamma , and delta formed during the isothermal ageing process at 150, 200, 250, and 300 degrees 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 gamma particle, which can be characterized by a separation vector of [1 1 over bar 01] alpha. The delta phase forms in situ from its precursor gamma phase, consequently leading to the formation of three different variants within a single delta particle. The nucleation of the delta phase is strongly related to the domain boundaries of the gamma phase. The formation of the gamma phase may be promoted by its precursor gamma " phase. The similarities in atomic structures of the gamma ", gamma , and delta phases are described and discussed, indicating that transformations between these precipitate phases can be accomplished through the diffusion of added alloying elements.(c) 2023 Published by Elsevier Ltd on behalf of The editorial office of Journal of Materials Science & Technology.