*2.3. Settling Behavior of Zr Particles during Alloying*

The settling of Zr particles, forming sediment, is the most serious problem during Zr alloying process. This is due to their having a higher density (~6.52 g/cm3) than liquid Mg (~1.7 g/cm3), leading to the spontaneous settling of Zr particles. The bigger the Zr particle size is within the Mg-Zr master alloy, the faster the rate of settling will be. The settling of Zr reduces the absorption of Zr, causes waste of Zr, and forms a non-uniform microstructure from the top to the bottom of the ingot [3,30]. To compensate for the loss of Zr, the addition of Zr is always as high as 1~3 wt.% in practical operations, which is 2~4 times the nominal Zr composition of Mg alloys [29,31]. Therefore, the expensive Zr waste increases the cost of the Mg alloy.

Figure 2a,b show the experiment and simulation results, respectively, of Zr settling reported by Qian [30]. The settling behavior of Zr particles follows the Stokes law:

$$\mathbf{S} \approx \frac{\mathbf{g} \left(\rho\_{\mathbb{Z}r} - \rho\_{\mathbb{M}\mathbb{g}}\right) d\_{\mathbb{P}}^2}{18\eta\_{\mathbb{M}\mathbb{g}}} t \tag{1}$$

where *S* is settling distance, *g* is gravitational acceleration, *ρZr* is the density of Zr, *ρMg* is the density of Mg melt, *ηMg* is the viscosity of Mg melt, *dP* is the diameter of Zr particle, and *t* is the holding time. It can be seen that larger Zr particles settle more quickly than smaller ones; *S* increases with prolonging time; a higher melt temperature favors quicker settling due to the lower viscosity of Mg melt. These results can guide the industrial operations of the Zr alloying process in order to avoid severe loss of Zr. Generally, the melt should be cast after settling for 15~30 min, in order to prevent continuous settling and the fading of the grain refinement. Moreover, after setting for more than 60 min, the melt can be re-stirred to recover the settled Zr particles [29,32], which makes the casting process more complicated.

**Figure 2.** (**a**) Experimental results for settling; (**b**) predicted settling results of 3 μm Zr particles in pure Mg melt as a function of time and temperature [30]. Reprinted with permission from ref. [30]. Copyright 2001 Elsevier.

Recently, to avoid the settling problem and reduce costs, the in-situ Al2RE nucleating particles were trialed to replace Zr [33–35]. The grain refinement effect of Al2RE is comparable to that of Zr. However, the aging precipitation ability is reduced due to the consumption of RE element following the addition of Al, and the formation of needle-like Al*x*RE*y* phases is detrimental to the mechanical properties.

#### **3. Grain Refinement Mechanisms of Zr**

It is generally accepted that the grain refinement mechanism of Zr is dictated by both soluble and insoluble Zr. With the help of quantitative analysis of ZrS and ZrT, the grain refinement behaviors of Zr can be well estimated [36]. On the one hand, when the Zr content reaches the peritectic point, the peritectic reaction is believed to be the core mechanism of grain refinement [3,4,31,37,38]. On the other hand, a grain refinement effect can also be observed when the Zr content is far below the peritectic point, which is thought to be due to the constitutional supercooling (CS) effect generated by soluble Zr [3,4,31,37,38]. This section will briefly discuss these two aspects.
