Analysis of Intermetallic Particles in Mg-12wt.%Zn Binary Alloy using Transmission Electron Microscopy

Abstract: The fundamental microstructure characterization of intermetallic compounds in Mg-12wt.%Zn binary alloy including the speculative Mg21Zn25 phase is presented. The alloy was thoroughly analysed using transmission electron microscopy techniques to make an unambiguous crystallographic identification of all intermetallic compounds in the α-Mg matrix. The intermetallic compounds that were found in the microstructure and their fine details were analysed in bright-field and high-angle annular dark field imaging. Their crystal structures and orientation relationships were inspected using selected area electron diffraction, convergent beam electron diffraction, precession-assisted electron diffraction tomography and high-resolution transmission electron microscopy supported by in-situ heating.
Three distinct intermetallic particles with sizes ≥ 1 μm, ~ 100 nm and ~ 5 nm were found in the α-Mg matrix and identified as Mg21Zn25 (trigonal structure with the View the MathML source space group and lattice parameters a = 2.578 nm and c = 0.876 nm), Mg51Zn20 (orthorhombic structure with the Immm space group and lattice parameters a = 1.408 nm, b = 1.449 nm, c = 1.403 nm) and MgZn2 (hexagonal structure with the P63/mmc space group and lattice parameters a = 0.522 nm, c = 0.857 nm), respectively. The structure of the Mg21Zn25 and MgZn2 phases was confirmed based on the precession-assisted electron diffraction tomography data, and the existence of Mg21Zn25 is discussed in detail. MgZn2 nanoparticles have the preferential shape and orientation relationship toward the α-Mg matrix. Mg51Zn20 nanoparticles with sizes of 10-50 nm were also discovered in the Mg21Zn25 particles, which form a eutectic compound Mg21Zn25 + Mg51Zn20.