4.3.4. Ca

Chang et al. [68] showed that the grain size of squeeze-cast Mg-*x*Ca-0.6Zr (*x* = 0.2~1.2) alloy was finer than that of squeeze-cast Mg-0.6Zr alloy. They concluded that Ca was able to increase the effect of grain refinement with Zr. The reason for this was explained from a thermodynamics perspective. Before the addition of Ca, the Mg melt reacts preferentially with O2, forming MgO inclusions that envelope the Zr particles. This effect increases the difficulty of the dissolution of Zr. After the addition of Ca, the formation of MgO is suppressed due to CaO possessing a lower standard free energy than MgO. Accordingly, the dissolution of Zr particles becomes more efficient, and the interface wettability between Zr particles and the Mg matrix improved. Therefore, the combined addition of Zr and Ca shows a better grain refinement effect.

However, the effect of CS was neglected in [68]. According to StJohn et al. [4], the GRF value for Ca is 11.94, which is the third highest among all solute elements in Mg alloy (the first is Fe, at 52.56, and the second, Zr, at 38.29). Thus, the high *GRF* of Ca may be another reason for the increase in the grain refinement effect of Mg-Ca-Zr alloy.

#### 4.3.5. Zn

Zn has been reported to show contradictory influences on the effect of grain refinement with Zr in Mg-Zn-Zr alloy, depending on the Zn content. According to Hildebrand et al. [69] and Li et al. [70], there exists a critical Zn content (3~4%) below which the grain size is gradually refined, but beyond which the grain size coarsened. The reasons for this have been well explained. When Zn content does not exceed 3~4%, the soluble Zr content increases with increasing Zn content, resulting in a finer grain size [69]. When Zn content exceeds 4%, Zn and Zr form the stable compounds ZnZr2 or Zn2Zr3 [79], decreasing the soluble Zr content and resulting in a coarser grain size. However, in partial divergence from Hildebrand et al. [69], Li et al. [70] attributed the grain coarsening effect at higher Zn contents to the "dendrite coherency theory". A large constitutional undercooling caused by higher Zn content leads to instability of the *S-L* interface. The sharp tip of the dendrite increases the growth rate of the dendritic grains, thus leading to coarser grains.

In addition, higher Zn contents have been reported to relieve the grain poisoning effect of Zr on AZ91 alloy. Jafari et al. [71] showed that the grain size of AZ93 alloy was not coarsened when 0.25~0.9% Zr was added, which was ascribed to the higher content of Zn (3%). Compared with lower Zn content (1%), higher Zr content can improve the Zr solubility in Mg alloy, thus improving the grain refinement effect.

## **5. Methods of Improving the Grain Refinement Efficiency of Zr**

As is well known, Mg-Zr master alloy with the largest number density of fine Zr particles (1~5 μm) exhibits the best refinement ability [26,31,61]. Therefore, the Zr particle size distribution in the Mg-Zr master alloy needs to be controlled to be as fine and as uniform as possible. To achieve this aim, a new Mg-Zr master alloy with fine Zr particles [14,80] has been developed, and pre-treatments [27,81–87] have been conducted to modify the Mg-Zr master alloys already available. In addition, melt treatment can be conducted to reduce the settling of Zr during the melting of the Mg alloy, thereby improving the Zr grain refinement efficiency [41,49,88–94]. This section briefly summarizes these methods.
