**2. Materials and methods**

Pure Mg manufactured by Rare Metallic Co., Ltd. and Mg- (3, 6, 9) mass%, Al-1 mass%, Zn alloy (hereinafter referred to as AZ31, AZ61, AZ91) manufactured by Sankyo Aluminum Co., Ltd. were used as samples. **Table 1** shows the composition of the samples.

As pretreatment, these samples were cut into prismatic shapes (8 mm in width, 5 mm in length, 3 mm in thickness) and then annealed, evacuated to 2 × 10<sup>−</sup><sup>4</sup> Pa in a stainless steel container, and replacing it with Ar gas (nominal purity 99.9999 vol%) was repeated several times and then heated under an Ar atmosphere of 3 kPa at a temperature of 723 K and a treatment time for 32.4 ks. For AZ61, annealed material (annealed) and 10% cold-rolled sample (cold-rolled) were subjected to hydrogenation (129.6 ks), respectively. An observation of the cross-sectional structure was carried out using an optical microscope (OM). Mg alloys are known to produce MgH2 at low temperature by applying severe plastic deformation [17]. **Table 2** shows the crystal grain sizes of annealed Mg and AZ31, AZ61, and AZ91 alloys.


**105**

ses here after.

**Table 2.**

fraction (EBSD) analyzer.

nitrogen gas (99.99%).

calculation on average.

*Magnesium-Based Materials for Hydrogen Storage: Microstructural Properties*

Mg 250 (70) AZ31 34 (3.4) AZ61 36 (4.8) AZ91 40 (3.5)

For hydrogenation of Mg alloys, these samples after annealing was drypolished with SiC abrasive paper (# 320) in Ar gas circulation-type glove box (dew point −55 to −70°C, oxygen concentration 1 ppm or less), dry polished and immediately sealed in a stainless steel reaction vessel with a gasket made of stainless steel with Ag plating. In this reaction vessel, a sheath-type thermocouple was installed inside the container and the temperature in the vicinity of the samples was set. In order to generate MgH2, hydrogenation was carried out at a hydrogen pressure of 3.61 MPa and a temperature of 673 K using a Siebert's type apparatus, and the nominal purity of the hydrogen gas used was 99.99999 vol%. The time was maintained in the range of 0–259.2 ks, and it is shown in parenthe-

**Material Grain size,** *d***/μm (standard deviation)**

The X-ray diffraction (XRD) instrument was used for the identification of MgH2(sur). X-ray source of CuKα with the tube voltage of 40 kV and the tube current of 15 mA was used. Microstructure of MgH2 was judged with presence or absence of charge up by using of scanning electron microscope (SEM). Crystal structure of MgH2(int) was investigated using an electron beam backscatter dif-

Samples for microstructural observation and analysis were cut at a position of 4 mm which is half in the width direction and roughly polished using SiC polishing paper (# 320 → # 800 → # 1000) followed by an Al2O3 suspension (0.05 μm) or SiO2 suspension (0.04 μm). Dehydrated methanol was used as the extension liquid. The sample after polishing was promptly washed with an ultrasonic washing machine in dehydrated methanol. After washing, the sample was dried using pure

Three-dimensional analysis of MgH2(sur) and MgH2(int) was carried out based on data constructed by a continuous serial sectional method, and an optical microscope (OM) was used for microstructural observation. Information at the position and observation depth was obtained by driving a pyramidal indentation into the sample using a Vickers hardness instrument. ImageJ was used for geometrical

**Figure 1** described quantitative analysis method for obtaining information on the thickness of the MgH2(sur) layer. In the analysis, five or more images are selected and the MgH2 was observed as a dark area. The mean thickness (*d*av) of MgH2(sur) was calculated by dividing the area of each observed MgH2(sur) by the length of MgH2(sur) on the surface. Next, the shape of the surface is extracted. The surface line was translated in the direction of the inside. The maximum thickness of MgH2(sur) was evaluated as the translated length at point of the surface line parted from MgH2(sur) particle. The minimum thickness (*d*min) was evaluated at the first point of intersection between the shape line and Mg. The average particle size of MgH2(int) is calculated by calculating the area of MgH2(int), and the value of the diameter calculated when assuming MgH2(int) as a spherical

analysis and the construction of the three-dimension image [18, 19].

*DOI: http://dx.doi.org/10.5772/intechopen.88679*

*Grain size of annealed Mg and AZ31, AZ61, and AZ91 alloys.*

#### **Table 1.**

*Composition of Mg and AZ31, AZ61, AZ91 alloys.*

*Magnesium-Based Materials for Hydrogen Storage: Microstructural Properties DOI: http://dx.doi.org/10.5772/intechopen.88679*


**Table 2.**

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

MgH2 = 1.1 × 10−20 m2

attractive for handling, safety, and applying large module.

**Material Composition (mass%)**

s<sup>−</sup><sup>1</sup>

on these characteristics, powder Mg forms core-shell-type structure hydride, MgH2 as a shell and unreacted Mg remains in the core [14, 15] making progress of completely hydrogenation difficult. On the other hand, the hydrogen partial pressure has a great influence on the progress of the hydrogenation. When the hydrogen partial pressure is high, since MgH2 quickly covers the Mg powder surface, hydrogenation halts and the amount of hydride concentration decreases markedly, whereas when the hydrogen partial pressure is low, the time until MgH2 covers the Mg surface extends, then the hydride concentration increases [16]. Therefore, to accomplish the efficient hydrogenation, the process of hydro-

As a result of investigation aiming at efficient hydrogenation, some curious microstructural characteristics were obtained, that is shell of MgH2 and core Mg in addition to MgH2 in the core Mg. In the following, MgH2 on the surface layer named as MgH2(sur) and that in Mg core named as MgH2(int). MgH2(int) formed in the Mg core is distinguished from MgH2(sur). The particle size of MgH2(int) in Mg<sup>−</sup><sup>6</sup> mass%, Al-1 mass%, Zn alloy was larger than that in pure Mg. This result shows the grain size of MgH2(int) would be in correlation with Al concentration. Therefore, in this study, the influence of Al concentration in Mg on formation of MgH2(int) is clarified. To reveal the mechanism of MgH2(int) formation, coarse-type Mg-based hydrogen storage materials will be developed. Bulky hydrogen storage materials are

Pure Mg manufactured by Rare Metallic Co., Ltd. and Mg- (3, 6, 9) mass%, Al-1 mass%, Zn alloy (hereinafter referred to as AZ31, AZ61, AZ91) manufactured by Sankyo Aluminum Co., Ltd. were used as samples. **Table 1** shows the composition

As pretreatment, these samples were cut into prismatic shapes (8 mm in width,

stainless steel container, and replacing it with Ar gas (nominal purity 99.9999 vol%) was repeated several times and then heated under an Ar atmosphere of 3 kPa at a temperature of 723 K and a treatment time for 32.4 ks. For AZ61, annealed material (annealed) and 10% cold-rolled sample (cold-rolled) were subjected to hydrogenation (129.6 ks), respectively. An observation of the cross-sectional structure was carried out using an optical microscope (OM). Mg alloys are known to produce MgH2 at low temperature by applying severe plastic deformation [17]. **Table 2** shows the crystal grain sizes of annealed Mg and AZ31, AZ61, and AZ91 alloys.

**Al Zn Mn Si Fe Cu Ni Ca**

Mg 0.0013 0.0024 0.012 0.022 0.0016 0.0031 0.0003 0.01

AZ31 2.80 0.80 0.32 0.022 0.003 0.003 <0.002 AZ61 6.26 0.63 0.26 0.015 0.005 0.001 0.0007 AZ91 8.70 0.82 0.22 0.007 0.005 <0.002 0.002

5 mm in length, 3 mm in thickness) and then annealed, evacuated to 2 × 10<sup>−</sup><sup>4</sup>

(305 K) in MgH2 [12, 13]. Based

Pa in a

(300 K) in Mg and *D*<sup>H</sup>

genation should be revealed.

**2. Materials and methods**

of the samples.

**104**

**Table 1.**

*Composition of Mg and AZ31, AZ61, AZ91 alloys.*

*Grain size of annealed Mg and AZ31, AZ61, and AZ91 alloys.*

For hydrogenation of Mg alloys, these samples after annealing was drypolished with SiC abrasive paper (# 320) in Ar gas circulation-type glove box (dew point −55 to −70°C, oxygen concentration 1 ppm or less), dry polished and immediately sealed in a stainless steel reaction vessel with a gasket made of stainless steel with Ag plating. In this reaction vessel, a sheath-type thermocouple was installed inside the container and the temperature in the vicinity of the samples was set. In order to generate MgH2, hydrogenation was carried out at a hydrogen pressure of 3.61 MPa and a temperature of 673 K using a Siebert's type apparatus, and the nominal purity of the hydrogen gas used was 99.99999 vol%. The time was maintained in the range of 0–259.2 ks, and it is shown in parentheses here after.

The X-ray diffraction (XRD) instrument was used for the identification of MgH2(sur). X-ray source of CuKα with the tube voltage of 40 kV and the tube current of 15 mA was used. Microstructure of MgH2 was judged with presence or absence of charge up by using of scanning electron microscope (SEM). Crystal structure of MgH2(int) was investigated using an electron beam backscatter diffraction (EBSD) analyzer.

Samples for microstructural observation and analysis were cut at a position of 4 mm which is half in the width direction and roughly polished using SiC polishing paper (# 320 → # 800 → # 1000) followed by an Al2O3 suspension (0.05 μm) or SiO2 suspension (0.04 μm). Dehydrated methanol was used as the extension liquid. The sample after polishing was promptly washed with an ultrasonic washing machine in dehydrated methanol. After washing, the sample was dried using pure nitrogen gas (99.99%).

Three-dimensional analysis of MgH2(sur) and MgH2(int) was carried out based on data constructed by a continuous serial sectional method, and an optical microscope (OM) was used for microstructural observation. Information at the position and observation depth was obtained by driving a pyramidal indentation into the sample using a Vickers hardness instrument. ImageJ was used for geometrical analysis and the construction of the three-dimension image [18, 19].

**Figure 1** described quantitative analysis method for obtaining information on the thickness of the MgH2(sur) layer. In the analysis, five or more images are selected and the MgH2 was observed as a dark area. The mean thickness (*d*av) of MgH2(sur) was calculated by dividing the area of each observed MgH2(sur) by the length of MgH2(sur) on the surface. Next, the shape of the surface is extracted. The surface line was translated in the direction of the inside. The maximum thickness of MgH2(sur) was evaluated as the translated length at point of the surface line parted from MgH2(sur) particle. The minimum thickness (*d*min) was evaluated at the first point of intersection between the shape line and Mg. The average particle size of MgH2(int) is calculated by calculating the area of MgH2(int), and the value of the diameter calculated when assuming MgH2(int) as a spherical calculation on average.

**Figure 1.**

*Method of quantitative analysis for average (dav), maximum (dmax) and minimum (dmin) thickness values of MgH2(sur) from SEM-BSE images.*

## **3. Results**

**Figure 2** shows the XRD pattern of hydrogenated Mg, AZ31, AZ61, AZ91 alloys for 1.8 ks. In all the samples, formation of MgH2 was observed. These samples were not polished, therefore the information from XRD was mainly from MgH2(sur). The peaks from Mg were shifted to higher angle side with Al concentration due to substitutional solid solution formation. On the other hand, in the peak pattern of MgH2, obvious peak shifts were not observed with increase of Al concentration.

**Figure 3** shows cross-sectional OM images of annealed and cold rolled AZ61 alloy. The annealed and cold rolled AZ61 exhibited both MgH2(sur) (white arrow) and MgH2(int) (black arrow). There were no difference on MgH2(sur) between annealed and cold rolled AZ61 alloy as *d*av was 9 μm (*d*max: 22 μm, *d*min: 2 μm) in annealed and *d*av was 8 μm (*d*max: 22 μm, *d*min: 2 μm) in cold rolled condition. However, particle size of MgH2(int) was lager in cold rolled condition than in

**Figure 2.** *XRD profiles of hydrogenated Mg, AZ31, AZ61 and AZ91 alloys at 673 K in 3.61 MPa H2 for 1.8 ks.*

**107**

**Figure 4.**

*Magnesium-Based Materials for Hydrogen Storage: Microstructural Properties*

*Cross-sectional optical micrographs of hydrogenated AZ61 in 3.61 MPa H2 at 673 K for 129.6 ks (a) annealed and (b) cold-rolled samples. Open and solid arrows point out MgH2(sur) and MgH2(int), respectively.*

annealed condition as mean diameter was 15 μm (maximum: 38 μm, minimum: 3 μm) in annealed condition and mean diameter was 53 μm (maximum: 125 μm,

*SEM-EBSD analyzed images of AZ31 hydrogenated at 673 K in 3.61 MPa H2 for 64.8 ks. (a) SEM-BSE image,* 

**Figure 4** shows the SEM image and results of EBSD analysis of hydrogenated AZ31. **Figure 4** (a) shows the backscattered electron image (BSE), (b) shows the phase map in the same area with (a), (c) is image quality (IQ ) map, and (d) is an inverse pole figure (IPF). In phase map (b), Mg was shown in red and MgH2 was shown in green color. Comparing (a) and (b), dark area in BSE (a) corresponds to MgH2 area in phase map (b). As focused on IQ map (c), image quality of MgH2 area was lower than that of Mg area. The decrease of quality at MgH2 was due to high volume change between Mg to MgH2 about 32% as the result of residual strain and/

The MgH2(int) observed in **Figures 3** and **4** was surrounded with metallic Mg, and the MgH2(int) has not interface with hydrogen gas at glance. Therefore,

minimum: 14 μm) in cold rolled condition.

*(b) phase map, (c) IQ map, and (d) IPF.*

or nonconducting material; MgH2 tends to charge up.

*DOI: http://dx.doi.org/10.5772/intechopen.88679*

**Figure 3.**

*Magnesium-Based Materials for Hydrogen Storage: Microstructural Properties DOI: http://dx.doi.org/10.5772/intechopen.88679*

#### **Figure 3.**

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

**Figure 2** shows the XRD pattern of hydrogenated Mg, AZ31, AZ61, AZ91 alloys for 1.8 ks. In all the samples, formation of MgH2 was observed. These samples were not polished, therefore the information from XRD was mainly from MgH2(sur). The peaks from Mg were shifted to higher angle side with Al concentration due to substitutional solid solution formation. On the other hand, in the peak pattern of MgH2, obvious peak shifts were not observed with increase of Al concentration. **Figure 3** shows cross-sectional OM images of annealed and cold rolled AZ61 alloy. The annealed and cold rolled AZ61 exhibited both MgH2(sur) (white arrow) and MgH2(int) (black arrow). There were no difference on MgH2(sur) between annealed and cold rolled AZ61 alloy as *d*av was 9 μm (*d*max: 22 μm, *d*min: 2 μm) in annealed and *d*av was 8 μm (*d*max: 22 μm, *d*min: 2 μm) in cold rolled condition. However, particle size of MgH2(int) was lager in cold rolled condition than in

*XRD profiles of hydrogenated Mg, AZ31, AZ61 and AZ91 alloys at 673 K in 3.61 MPa H2 for 1.8 ks.*

*Method of quantitative analysis for average (dav), maximum (dmax) and minimum (dmin) thickness values of* 

**106**

**Figure 2.**

**3. Results**

*MgH2(sur) from SEM-BSE images.*

**Figure 1.**

*Cross-sectional optical micrographs of hydrogenated AZ61 in 3.61 MPa H2 at 673 K for 129.6 ks (a) annealed and (b) cold-rolled samples. Open and solid arrows point out MgH2(sur) and MgH2(int), respectively.*

**Figure 4.**

*SEM-EBSD analyzed images of AZ31 hydrogenated at 673 K in 3.61 MPa H2 for 64.8 ks. (a) SEM-BSE image, (b) phase map, (c) IQ map, and (d) IPF.*

annealed condition as mean diameter was 15 μm (maximum: 38 μm, minimum: 3 μm) in annealed condition and mean diameter was 53 μm (maximum: 125 μm, minimum: 14 μm) in cold rolled condition.

**Figure 4** shows the SEM image and results of EBSD analysis of hydrogenated AZ31. **Figure 4** (a) shows the backscattered electron image (BSE), (b) shows the phase map in the same area with (a), (c) is image quality (IQ ) map, and (d) is an inverse pole figure (IPF). In phase map (b), Mg was shown in red and MgH2 was shown in green color. Comparing (a) and (b), dark area in BSE (a) corresponds to MgH2 area in phase map (b). As focused on IQ map (c), image quality of MgH2 area was lower than that of Mg area. The decrease of quality at MgH2 was due to high volume change between Mg to MgH2 about 32% as the result of residual strain and/ or nonconducting material; MgH2 tends to charge up.

The MgH2(int) observed in **Figures 3** and **4** was surrounded with metallic Mg, and the MgH2(int) has not interface with hydrogen gas at glance. Therefore,

#### **Figure 5.**

*Three-dimensional microscopic images obtained from optical micrographs of cold rolled and hydrogenated AZ61 for 129.6 ks (a) a sample of optical micrograph, (b) samples of inverted black and white micrographs, and three-dimensional images (c) 0°, (d) 20°, (e) 40°, and (f) 60°.*

three-dimensional analysis was conducted at the area shown in **Figure 5**. **Figure 5** taken from cold rolled and hydrogenated AZ61 sample show (a) part of OM micrograph, (b) MgH2(sur) and MgH2(int) extracted as ROI area contrast inverted, and construction of three-dimensional image rotated from 0 to 60° ((c) 0°, (d) 20°, (e) 40°, and (f) 60°). From **Figure 5c**–**f**, the MgH2(int) was not in contact with hydrogen gas and MgH2(sur).

Focusing on MgH2(sur), homogeneous hydride layer does not growth from surface and structure with variations in thickness was spread out in a planar manner with respect to the surface. The white arrow point of MgH2(sur) was grown abnormally like as heteroepitaxially, however size of the part of MgH2(sur) was same size with surrounding MgH2(int), since the part of MgH2(sur) would be formed by collision between MgH2(sur) and MgH2(int) at near surface.

**109**

**Figure 7.**

*0–259.2 ks.*

**Figure 6.**

*Magnesium-Based Materials for Hydrogen Storage: Microstructural Properties*

**Figure 6** shows an OM micrograph of hydrogenated Mg and a schematic image.

Focusing on MgH2(int), the shape of MgH2(int) was granular and precipitated on the Mg grain boundary. In addition, two types of MgH2(int) were observed as growth to two adjacent Mg grains (at pointed two grains) and on only one side of

**Figure 7** shows the SEM-BSE images of the MgH2(sur) of hydrogenated Mg, AZ31, AZ61, and AZ91 alloys for 1.8–259.2 ks. Focusing on images of hydrogenated in relatively short time, for example at 1.8 ks, MgH2(sur) formed with granular and dotted and that was not formed uniformly like a layer at the time when the entire surface was not covered with hydride. With increase of hydrogenation time, the granular hydride, MgH2(sur), grew in the direction parallel to the surface and formed a rough layer. For images of samples exposed to relatively long hydrogenation time, the thickness of MgH2(sur) decreased with an increase in Al concentration. **Figure 8** shows the thickness of MgH2(sur) of Mg, AZ31, AZ61, and AZ91

*DOI: http://dx.doi.org/10.5772/intechopen.88679*

Mg grain. They were indicated in the inset image.

*SEM-BSE image of Mg hydrogenated at 673 K in 3.61 MPa H2 for 129.6 ks.*

*SEM-BSE images at MgH2(sur) of Mg, AZ31, AZ61 and AZ91 after hydrogenation at 673 K in 3.61 MPa H2 for* 

### *Magnesium-Based Materials for Hydrogen Storage: Microstructural Properties DOI: http://dx.doi.org/10.5772/intechopen.88679*

**Figure 6** shows an OM micrograph of hydrogenated Mg and a schematic image. Focusing on MgH2(int), the shape of MgH2(int) was granular and precipitated on the Mg grain boundary. In addition, two types of MgH2(int) were observed as growth to two adjacent Mg grains (at pointed two grains) and on only one side of Mg grain. They were indicated in the inset image.

**Figure 7** shows the SEM-BSE images of the MgH2(sur) of hydrogenated Mg, AZ31, AZ61, and AZ91 alloys for 1.8–259.2 ks. Focusing on images of hydrogenated in relatively short time, for example at 1.8 ks, MgH2(sur) formed with granular and dotted and that was not formed uniformly like a layer at the time when the entire surface was not covered with hydride. With increase of hydrogenation time, the granular hydride, MgH2(sur), grew in the direction parallel to the surface and formed a rough layer. For images of samples exposed to relatively long hydrogenation time, the thickness of MgH2(sur) decreased with an increase in Al concentration. **Figure 8** shows the thickness of MgH2(sur) of Mg, AZ31, AZ61, and AZ91

**Figure 7.**

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

*Three-dimensional microscopic images obtained from optical micrographs of cold rolled and hydrogenated AZ61 for 129.6 ks (a) a sample of optical micrograph, (b) samples of inverted black and white micrographs,* 

three-dimensional analysis was conducted at the area shown in **Figure 5**. **Figure 5** taken from cold rolled and hydrogenated AZ61 sample show (a) part of OM

micrograph, (b) MgH2(sur) and MgH2(int) extracted as ROI area contrast inverted, and construction of three-dimensional image rotated from 0 to 60° ((c) 0°, (d) 20°, (e) 40°, and (f) 60°). From **Figure 5c**–**f**, the MgH2(int) was not in contact with

Focusing on MgH2(sur), homogeneous hydride layer does not growth from surface and structure with variations in thickness was spread out in a planar manner with respect to the surface. The white arrow point of MgH2(sur) was grown abnormally like as heteroepitaxially, however size of the part of MgH2(sur) was same size with surrounding MgH2(int), since the part of MgH2(sur) would be formed by

*and three-dimensional images (c) 0°, (d) 20°, (e) 40°, and (f) 60°.*

collision between MgH2(sur) and MgH2(int) at near surface.

hydrogen gas and MgH2(sur).

**108**

**Figure 5.**

*SEM-BSE images at MgH2(sur) of Mg, AZ31, AZ61 and AZ91 after hydrogenation at 673 K in 3.61 MPa H2 for 0–259.2 ks.*

**Figure 8.** *Relation between thickness of MgH2(sur) and hydrogenation time of (a) Mg, (b) AZ31, (c) AZ61, and (d) AZ91.*

**Figure 9.**

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

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

**111**

*Magnesium-Based Materials for Hydrogen Storage: Microstructural Properties*

with increasing hydrogenation time before *τ*h, whereas the growth rate drastically decreased after the time of *τ*h. These results indicate that the growth of MgH2(sur)

*Relation between average diameter of MgH2(int) and hydrogenation time of Mg, AZ31, AZ61 and AZ91.*

To reveal the influence of Al concentration on thickness of MgH2(sur), the relationship between average of *d*av after *τ*h and Al concentration is shown in **Figure 9**. As shown in **Figure 9**, it was found that the thickness of MgH2(sur) decreased with the increase of Al concentration, and the decrease ratio is remarkable from 0 to 3

Changes in the growth rate of MgH2(int) are described as follows. **Figure 10** shows the average particle size of MgH2(int) of Mg, AZ31, AZ61, and AZ91 alloys with hydrogenation time. As with the changes of MgH2(sur), the growth rate of MgH2(int) was high over short time and low on longer hydrogenation time side. The time of infection point was almost correlated with *τ*h as 3.6 ks for Mg and AZ31, 64.8 ks for AZ61, 129.6 ks for AZ91. **Figure 10** shows the relationship between the average particle size of MgH2(int) after *τ*h and Al concentration. The average particle size of MgH2(int) increased with increase of Al concentration, contrary to

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.

halted when sample surface was covered with MgH2(sur).

mass% Al than from 6 to 9 mass% Al.

the change of thickness of MgH2(sur).

**4.1 Formation of MgH2 in Mg core**

**4. Discussion**

**Figure 10.**

*DOI: http://dx.doi.org/10.5772/intechopen.88679*

*Magnesium-Based Materials for Hydrogen Storage: Microstructural Properties DOI: http://dx.doi.org/10.5772/intechopen.88679*

**Figure 10.** *Relation between average diameter of MgH2(int) and hydrogenation time of Mg, AZ31, AZ61 and AZ91.*

with increasing hydrogenation time before *τ*h, whereas the growth rate drastically decreased after the time of *τ*h. These results indicate that the growth of MgH2(sur) halted when sample surface was covered with MgH2(sur).

To reveal the influence of Al concentration on thickness of MgH2(sur), the relationship between average of *d*av after *τ*h and Al concentration is shown in **Figure 9**. As shown in **Figure 9**, it was found that the thickness of MgH2(sur) decreased with the increase of Al concentration, and the decrease ratio is remarkable from 0 to 3 mass% Al than from 6 to 9 mass% Al.

Changes in the growth rate of MgH2(int) are described as follows. **Figure 10** shows the average particle size of MgH2(int) of Mg, AZ31, AZ61, and AZ91 alloys with hydrogenation time. As with the changes of MgH2(sur), the growth rate of MgH2(int) was high over short time and low on longer hydrogenation time side. The time of infection point was almost correlated with *τ*h as 3.6 ks for Mg and AZ31, 64.8 ks for AZ61, 129.6 ks for AZ91. **Figure 10** shows the relationship between the average particle size of MgH2(int) after *τ*h and Al concentration. The average particle size of MgH2(int) increased with increase of Al concentration, contrary to the change of thickness of MgH2(sur).
