*4.2. Effect of Cooling Condition*

Generally, increasing the cooling rate (*T˙*) results in a decrease in grain size. For example, Liu et al. [62] showed that the grain size of Mg-Zr binary alloys (0.3, 0.7 or 1.2Zr) decreased with increasing cooling rate from sand mold to Cu mold. Sun et al. [63] found

that the relationship between the grain size of Mg-Y-Zr alloy (1~9Y, 0.1~0.5Zr) and 1/ *Q* . *T* could be linearly fitted at a cooling rate ≤80 ◦C/s (Figure 9 shows Mg-1Y-Zr alloy as an example of this phenomenon). Yang et al. [51] showed that, compared with traditional gravity casting, HPDC improved the percentage of activated Zr nuclei from 1~2% to 48.78% in the Mg-0.1Zr alloy, and resulting in finer grains.

**Figure 8.** (**a**) The relationship between grain size (*dgs*) and total Zr content (ZrT) for Mg-5Gd-1.5Y alloy refined using different Mg-Zr master alloys. (**b**) The distribution of Zr nuclei in Mg-5Gd-1.5Y alloy with ZrT contents of 0.46, 0.55 and 0.59, inoculated by master alloys A, B and C, respectively [26]. The microstructure and Zr particle size distributions of the Mg-Zr master alloys are shown in Figure 1. Reprinted with permission from ref. [26]. Copyright 2013 Wiley.

**Figure 9.** Effect of cooling rate (*T˙*) on the grain size (*dgs*) of Mg-1Y-Zr alloy [63]. Reprinted with permission from ref. [63]. Copyright 2020 Springer.

Nevertheless, grain coarsening has also been observed under some cases of high cooling rate and steep thermal gradient [58,63]. According to Figure 9, by Sun et al. [63], grain coarsening of the Mg-Y-Zr alloy (1~9Y, 0~0.5Zr) occurred at a cooling rate 160 ◦C/s (with rapid water-quenching). This was due to the reduction in size of the CS zone, decreasing the likelihood of further nucleation. Yang et al. [50] showed that the grain coarsening effect occurred in HPDC Mg-1Zr alloy, in contrast to HPDC pure Mg. The grain size increased abnormally from 6.7 to 18.9 μm following the addition of 1% Zr. This change was attributed to the competition between the native MgO and the Zr particles. Without the addition of Zr, the high number density of the in-situ MgO particles permitted them to act as nucleation sites, due to a the low misfit with Mg (5.46%), leading to a finer grain size. However, with the addition of Zr, the Zr particles acted as nucleation sites first, thereby suppressing the activation of the in-situ MgO particles. More importantly, the number density of the Zr particles was lower than that of the in-situ MgO particles. Therefore, the grain size of Mg-1Zr was coarser than that of pure Mg.

## *4.3. Effect of Alloying Elements*

Contrary effects can be found depending on the elements employed. On the one hand, the elements Al [64,65], Mn, Si, Fe [29,66], Sn, Ni, Co and Sb [29] inhibit the grain refinement ability of Zr, which is referred to as the poisoning effect. This is mainly due to the reactions with Zr forming stable compounds that can no longer act as a nucleant [4,13,29]. Beryllium (Be) also poisons Zr, but via a different mechanism, which is probably the formation of a new coating on the Zr particle surface, thus reducing the potency of the Zr [67]. On the other hand, the elements Ca [68] and Zn [69–71] enhance the grain refinement ability of Zr, mainly via the mechanism increasing the soluble Zr content. This section summarizes the effects of these elements.
