*2.2.2 Zinc Zn*

Zinc is added in Mg alloys to enhance the tensile properties. Zinc decreases the weldability property [23]. Above 1% of Zn, it provides strengthening to Mg by solid solution [24]. If zinc is added in higher amount, the Mg alloy presents hot cracking and lower ductility [23]. In addition, micro porosity of sand casted Mg alloys is observed with an Al amount ranging from 2% to 10%. The dissolution of Zn in Mg diminishes its reducing power, improving then its oxidation resistance [25]. The corrosion rate of Mg-Zn alloys decreases with increasing Zn in Mg matrix. For instance, the corrosion rate of Mg-3%Zn alloy is 34.6% lower than that of pure Mg compact. Zhang et al. [26] reported that 6% of Zn diminishes the corrosion rate of the Mg alloy for implant applications.

The micro structural analysis indicates smaller grain size at higher Zn content. The reason for the grain sizes refinement is recognized to close-packed hexagonal structure of both Mg and Zn metals. The diffusion rate of Zn atoms in Mg matrix is fast, favoring an easier diffusion into Mg matrix and forming Mg solid solution or intermetallic compounds. **Figure 2** shows surface scanning micrographs of Mg-Zn alloys with various Zn content. The phases present in Mg-Zn alloys depend on the Zn amount. **Figure 2** shows some white phase along the Mg grain boundary. This white phase increases with higher Zn content, hindering both the movement of the grain boundary and the grain growth [27]. For instance, Mg-3%Zn alloy is mainly composed of α-Mg phase, whereas Mg-4%Zn alloy is composed of α-Mg and MgZn2 phases [27]. The micro-hardness for different Mg-Zn alloys continuously increases with increasing Zn content. For illustration, the micro-hardness HV of Mg-3%Zn alloy is about 45% higher than those of pure Mg samples.

Mg-Zn based alloys are the most popular wrought Mg alloys with good room temperature strength and ductility. Recent researches attempt to develop new type wrought Mg-Zn alloys using the addition of alloying elements including RE (rare earth), non-toxic Ca, Sn and Mn to optimize the properties of Mg-Zn alloys at room and high temperatures. Mg-Zn-Zr (ZK) is the strongest system owing to good strength and elongation at room temperature.

**Figure 2.** *SEM images of Mg-Zn alloys with different Zn contents: (a) 1% Zn and (b) 4% Zn [27].*

The use of Mg-Zn alloys in medical applications as bio degradable materials is one of the research areas [28]. It is well known that the adding element Zn is one of the indispensable trace elements in the human body that promotes the growth, stimulates healing and participates in enzyme synthesis.

## *2.2.3 Manganese*

The alloying element Mn improves the corrosion resistance of Mg alloy and limits the presence of harmful cathodic impurities such as Fe, Ni by the formations of intermetallic compound …. As matter of fact, such impurities lead to galvanizing oxidation of Mg [29]. Mn combines with impurities in order to moderate the corrosion of Mg-Al alloys. In the presence of Al and Fe, adding Mn produces an intermetallic phase Al8(Mn,Fe)5 that moderate the corrosion rate caused by Fe [30]. Until now, the exact Mn content addition required to counter-act the detrimental effect of the Fe impurity are still unknown.

With Al, the limit of solubility of Mn in the solid solution of Mg decreases, the strengthening by solid solution induced by Mn remains limited. That's why, the addition of Mn does not improve the mechanical properties of Mg. When investigated the effect of adding Mn into Mg-Gd alloys, Zhao et al. [31] demonstrated that the strength of alloys gradually increases while the ductility deteriorates. The main reasons are related to the combination of fine-grained strengthening, precipitation strengthening and texture strengthening [31]. In this regard, Cho et al. [32] stated that Mn refines grains of Mg-4Zn-0.5Ca alloy. This occurrence is due to the solute at the S/L interface aggregates in presence of Mn element, resulting different degrees of structural overcooling. Based on the biosafety to the human body, Mn can be accepted by the human body [32].

#### *2.2.4 Ca*

Incorporating Ca in Mg alloys can improve the mechanical properties and the corrosion behavior of Mg-Ca alloys. Ca is considered as an alloying element to develop Mg alloys for biomedical applications owing to their good biocompatibility.

As well known, Ca accelerates the bone growth. Moreover, cytocompatibility evaluation results indicated that Mg-1%Ca alloy induces no toxicity to human cells. In this regard, Li et al. [33] investigated the biodegradability within bone of Mg-Ca alloys with various Ca amounts ranging from from 1 to 20%. It was reported that 20% of Ca content makes Mg alloy very brittle.

Mg-Ca alloys with 0.6–1% Ca were reported to exhibit good mechanical properties and corrosion resistance [34]. The elongation of Mg-Ca alloy samples decreases with rising Ca content.

Previous studies suggested that Mg-Zn-Ca alloy system is a promising candidate for biodegradable implants in biomedical applications. With the increase of Ca content, the yield strength of Mg-Zn-Ca alloy increases. The addition of Ca to Mg-6%Zn alloy inhibits dynamic recrystallization and grain growth. Microstructural results indicate that Mg-Zn-Ca alloys consist of α-Mg matrix and Ca₂Mg₆Zn₃/Mg₂Ca intermetallic phase mainly distributed along grain boundary [35].

High Ca content improves mechanical properties, while it is detrimental to corrosion resistance due to the micro-galvanic corrosion acceleration [36]. Literature data revealed that Mg-5%Zn-1%Ca exhibits excellent corrosion resistance and good biocompatibility.
