TEM study on the microstructure of the Zn-Al-Mg alloy

The microstructure of the 95 wt%Zn-2.3 wt%Al-2.7 wt%Mg bulk alloy used for hot-dip galvanization has been investigated by TEM and SEM. The alloy exhibits a multi-scales and multi-phases microstructure. The dendrite and the granular/lamellar region are observed at hundreds-micron scale, and each has finer details. The dendrite is composed of the coarse hcp-Zn grains, with MgZn2 grains between the branches, and some individual bulk MgZn2. The granular/lamellar region has a complex ternary eutectic morphology, containing the hcp-Zn lamellas, the monotectoid product of Al-bubble wrapping Zn-granule, and the Zn-Mg lamellas, and the dimension of every unit is less than 1 μm. The hcp-Zn phase, in the form of coarse grains and lamellas, is supersaturated, with numerous platelet Al precipitation about tens of nanometers inside, with the orientation relationship of (0 0 1)Al//(0 0 0 1)Zn, and [1 1 0]Al near 112¯0Zn. The Zn-Mg lamella is a MgZn2 matrix with polygon Mg2Zn11 precipitates less than 100 nm and some Mg segregations.