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 Mg21Zn25 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≥1μm, ~100nm and ~5nm were found in the α-Mg matrix and identified as Mg21Zn25 (trigonal structure with the R3¯c space group and lattice parameters a=2.578nm and c=0.876nm), Mg51Zn20 (orthorhombic structure with the Immm space group and lattice parameters a=1.408nm, b=1.449nm, c=1.403nm) and MgZn2 (hexagonal structure with the P63/mmc space group and lattice parameters a=0.522nm, c=0.857nm), 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 10–50nm were also discovered in the Mg21Zn25 particles, which form a eutectic compound Mg21Zn25+Mg51Zn20.