*2.2. The Features of the Mg-Zr master Alloy*

Generally, Mg-Zr master alloy contains a large number of Zr particles, the sizes of which range from the sub-micron level to hundreds of microns. The backscattered electron (BSE) scanning electron microscopy (SEM) images in Figure 1 show the typical microstructural features of different Mg-Zr products available on the market [25–27]. It can be seen that the Zr sizes exhibit a log-normal distribution. However, the number density of each size range has great differences, depending on the preparation parameters.

**Figure 1.** SEM backscattered electron (BSE) images of different Mg-Zr master alloys [25]: (**a**) Mg-30 wt.% Zr; (**b**) Zirmax® Mg-33% Zr, (**c**) AM-CAST® Mg-25 wt.% Zr; and (**d**) Zr particle size distributions [26]. Reprinted with permission from ref. [26]. Copyright 2013 Wiley.

To ensure the high absorptivity of Zr in Mg melt, some aspects should be considered, such as excessive addition of Zr, high alloying temperature, and adequate stirring. In particular, high temperature and adequate stirring accelerate the dissolution of Zr [28]. With the aid of stirring, the dissolution of Zr can be completed within a few minutes in the temperature range 730~780 ◦C [29]. However, if the temperature is above 780 ◦C, the difficulties in melt protection obviously increase, since Mg melt is prone to oxidation and

burning, although protective gas (e.g., CO2 + SF6) can be used. Normally, in industrial practice, temperatures below 780 ◦C are regarded as suitable for Zr alloying.
