**4. Discussion**

*Magnesium - The Wonder Element for Engineering/Biomedical Applications*

alloys with hydrogenation time. The white rhombi and the reverse triangles are shown as *d*max and *d*min, respectively. The black circles are shown as *d*av and the vertical lines are expressed as standard deviation. Focusing on *d*min, *d*min = 0 means that the surface was not completely covered with MgH2(sur). Therefore, paying attention to the value of *d*min, it is possible to estimate the time when entire surface is covered with MgH2(sur), and the time describes as time of halt (*τ*h). *τ*h was 32.4 ks for Mg and AZ31, 64.8 ks for AZ61, 129.6 ks for AZ91, and *τ*h increased with increasing Al concentration. Next, looking at *d*av, it was found that *d*av grew greatly

*Relation between average value of dav and concentration of Al in Mg after τh.*

*Relation between thickness of MgH2(sur) and hydrogenation time of (a) Mg, (b) AZ31, (c) AZ61, and (d) AZ91.*

**110**

**Figure 9.**

**Figure 8.**

#### **4.1 Formation of MgH2 in Mg core**

After hydrogenating Mg and AZ alloys, two types of MgH2 were formed: on the surface, MgH2(sur) and in the unreacted Mg core, MgH2(int). MgH2(int) formed only along Mg grain boundary, and did not form in Mg crystal grain. It is known that the diffusion of H atoms in Mg made the grain boundary a preferential route [20]. Therefore, the reason why MgH2(int) preferentially formed on grain boundary would be one factor of the fast diffusion of H atoms at Mg grain boundary. In addition, MgH2 preferentially formed with existence of defect as nanocrystallize Mg showed fast hydrogenation and dehydrogenation [3]. This observation also supports the reason why MgH2(int) formed on Mg grain boundary.

As shown in **Figure 6**, MgH2(int) grew for adjacent Mg crystal grain or only on one crystal grain. In addition, some Mg grain boundaries did not form MgH2(int). Some orientation relationship between Mg and MgH2 was reported as (0001)Mg// (001)MgH2, [−1–120]Mg//[001]MgH2 [21] and (−2110)Mg//(12–1)MgH2, [0001]Mg// [101]MgH2 [22]. Because interface energy between new phase and mother phase was low in orientation relationship, solid-solid transformation easily proceeded in orientation relationship with a small lattice mismatch. On the other hand, hydrogenation rate of Mg increased with formation of (0002) crystal texture [23, 24]. Form above results, nucleation and growth of MgH2(int) would also depend on adjacent Mg crystal orientation. However, influence of Mg crystal texture for MgH2 formation had other formation of MgO factor [24], and particle shape of MgH2(int) was ellipsoidal. Therefore, consideration with complex effects for formation of MgH2(int) should be needed such as orientation relationship, precipitates on Mg grain boundary, interface energy with adjacent Mg grains, and flux of H atom and so on.

## **4.2 Relationships between particle size of MgH2(int) and Al concentration**

Before *τ*h, MgH2(sur) formed in granular form and spread and the growth rate of average thickness was larger than that after *τ*h. After *τ*h, the growth of MgH2(sur) apparently halted. These growth rate change was same in MgH2(int). These results be attributed to extremely low diffusion rate of H in MgH2 when compared to that in Mg [12, 13]. When the whole surface was covered with MgH2(sur), the supply rate of H to unreacted internal metallic Mg or AZ significantly decreased, halting the growth of MgH2(sur) and MgH2(int).

Some studies have reported that the amount of MgH2 formed depends on Gibbs free energy (Δr*G*) of MgH2 from Mg and H2 gas [16, 25, 26]. The microstructure of MgH2(sur) at initial state was formed as granular and scattered. Concerning this result, the nucleation rate of MgH2(sur) was low and the nucleation rate would depend on absolute value of Δr*G*. Applying low value of Δr*G*, growth of MgH2(int) proceeded because nucleation rate of MgH2(sur) was low and *τ*h is shift to longer time. However, when high value of Δr*G* was applied, the nucleation rate of MgH2(sur) increased and immediately the surface covered with thin MgH2(sur) rapidly and the particle size of MgH2(int) was explained to be small size.

The average particle size of MgH2(int) increased with increasing Al concentration. As shown in **Figure 10**, the growth rate of MgH2(int) decreased when the surface was covered with MgH2(sur). The thickness of MgH2(sur) decreased with increasing Al concentration. From these results, the reasons why the particle size of MgH2(int) decreased with increasing Al concentration would be shifting of long time *τ*h and small diffusion distance of H in MgH2 at high Al content. Therefore, the amount of supplying H increased and large size of MgH2(int) formed with increasing Al concentration.
