Analysis of intermetallic particles in Mg–12 wt.%Zn binary alloy using transmission electron microscopy

Abstract The fundamental microstructure characterization of intermetallic compounds in Mg–12 wt.%Zn binary alloy including the speculative Mg 21 Zn 25 phase is presented. The alloy was thoroughly analyzed 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 analyzed 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 Mg 21 Zn 25 (trigonal structure with the R 3 ¯ c space group and lattice parameters a  = 2.578 nm and c  = 0.876 nm), Mg 51 Zn 20 (orthorhombic structure with the Immm space group and lattice parameters a  = 1.408 nm, b  = 1.449 nm, c  = 1.403 nm) and MgZn 2 (hexagonal structure with the P6 3 /mmc space group and lattice parameters a  = 0.522 nm, c  = 0.857 nm), respectively. The structure of the Mg 21 Zn 25 and MgZn 2 phases was confirmed based on the precession-assisted electron diffraction tomography data, and the existence of Mg 21 Zn 25 is discussed in detail. MgZn 2 nanoparticles have the preferential shape and orientation relationship toward the α-Mg matrix. Mg 51 Zn 20 nanoparticles with sizes of 10–50 nm were also discovered in the Mg 21 Zn 25 particles, which form a eutectic compound Mg 21 Zn 25  + Mg 51 Zn 20 .