*2.2.6 Rare earths RE elements*

The rare Earths elements are common incorporating elements in Mg alloys. A large number of rare-earth elements have been investigated and proven to successfully refine the crystals and to enhance the creep and the corrosion resistance at elevated temperatures required for automobile engineering. Due to their high solubility in Mg, RE elements strengthen Mg alloys either by solid solution strengthening or precipitation hardening mechanisms [36]. For instance, the addition of RE elements to Mg-Al alloys accelerate the formation of the thermally stable (Mg,Al)xREy phases to improve the high temperature mechanical properties of wrought Mg alloys.

Depending on the chemical composition, various amounts of precipitates can be observed. **Figure 3** exhibits small precipitates containing Mg, Zn and O in Mg-1.5Zn (**Figure 3a**), while precipitates in the RE-containing alloys consist of Mg, Zn and RE elements [27]. The amount of precipitates of Mg-1.5Zn-0.2Gd alloys are more than Mg-1.5Zn alloy, leading to smaller grain size of this alloy. Compared with other rare earth elements, Gd shows higher solubility in Mg, allowing simultaneous solid solution hardening and precipitation strengthening, enhancing then the thermal stability of the microstructure in Mg alloys [27]. Yang et al. [39] pointed out that Gd acts as grain refinement and grain boundary strengthening. In this regard, Liu et al. [40] reported that Mg-1.5Zn-0.2Gd alloy exhibits higher plasticity with the elongation of 27%. Meanwhile, Gd simultaneously reduces the toughness [36]. These Mg alloy systems containing large amounts of rare-earth for solid solution strengthening result high costs for many practical applications.

#### **Figure 3.**

*Microstructural (SEM) of Precipitates in Mg alloy (a) Mg-1.5%Zn; (b) Mg-1.5%Zn-0.2%Gd [27].*
