*2.2.1 Aluminum*

Mg alloys including Al are called Mg-Al binary alloys. Al is the most widely used alloying element of Mg alloys as structural materials [16]. Indeed, Al improves the tensile strength, the ductility and the castability of Mg alloys at temperature not exceeding 120 °C [17]. If the amount of Al varies from 1 to 9%, the grains size of Mg-Al alloys decreases, increasing accordingly the microhardness [18]. Zheng et al. [18] demonstrated that the average grain size of Mg-Al alloy falls from 3097 μm to 111 μm if Al amount increases from 1% to 9%. **Figure 1** shows the influence of the addition of Al on the size of α-Mg dendrites for Mg-Al binary alloys. It can be seen that with 1% of Al to pure the structure of Mg, α-Mg dendrites are transformed from columnar to equiaxial (**Figure 1**). With higher Al amount, α-Mg dendrites become more developed and the dendrite arms become finer [18]. According to Mg-Al binary alloy phase diagram [19], α-Mg dendrite firstly precipitates during the solidification process, and then expels the redundant Al solute. In fact, this precipitation releases latent heat, causing an increase of the melt temperature ahead of the solidification interface. It decreases the degree of super-cooling, and suppresses the growth of α-Mg dendrites, leading to the grain refinement [18]. When Al exceeds 2%, an eutectic microstructure involving α-Mg and Mg17All2 will be formed along the grain boundaries. The Mg17Al12 phase improves the corrosion potential and reduces the corrosion rate [20]. However, the Mg17Al12 phase has a relatively low melting temperature (437 °C), giving arise to the microstructure instability over 120 °C. This occurrence related to the grain boundary sliding explains the degradation of the mechanical properties of Mg-Al alloys at elevated temperatures [21]. Moreover, the micro-hardness of the α-Mg matrix increases from 36.3 HV to 50.1 HV. This

**Figure 1.** *Size and morphology of* α*-Mg dendrites of Mg-Al binary alloys with various Al amount [18].*

### *Alloying Elements of Magnesium Alloys: A Literature Review DOI: http://dx.doi.org/10.5772/intechopen.96232*

increase is recognized to solution strengthening caused by Al solubilized in the α-Mg matrix.

The most popular alloys are Mg-Al-Zn (AZ) and Mg-Al-Mn (AM) as Zn improves the ambient temperature mechanical properties and Mn enhances the creep resistance of alloys [22].
