Abstract
The intermetallic phase (designated δ phase) in the solution-treated microstructure of commercial magnesium alloy QE22 (Mg–2.5Ag–2.0Nd–0.7Zr, wt%) has been investigated using scanning electron microscopy, electron diffraction, atomic-resolution scanning transmission electron microscopy (STEM), and thermodynamic modeling. In contrast to the previous reports, an orthorhombic structure (space group Cmcm) and a composition of NdAgMg11 are determined. The present experimental data are used to construct a thermodynamic description of the Mg–Ag–Nd system which is embedded in a multicomponent Mg alloy database. Implications on the formation of temperature range and thermal stability of this phase and alloy solidification are discussed based on the calculated Mg–Nd–Ag phase diagram and Scheil solidification paths of alloys. The impact of Ag-replacement in such QE alloys by Zn-addition in the Mg–Ag–Nd–Zn (QEZ) alloy system is elaborated using appropriate thermodynamic simulations to reveal the competition with other intermetallics. These data are suggested to contribute to ICME of magnesium alloys.
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