**2.4. Hydrogen desorption of Ti and Ti‐6Al‐4V**

tions must increase considerably the hydrogen absorption in pure titanium, as well as in the

**Figure 7.** Hydrogen concentration of the hydrogenated samples, before and after implantation [13].

**Table 3.** Gravimetric storage capacity of titanium samples after implantation and hydrogenation [13].

In order to check if the hydrogen absorbed by the metal is forming hydrides, XRD analysis was conducted [13]. **Figure 8** shows the results of the XRD of the implanted and hydrogenated samples, as well as a pure titanium reference. The results show the diffractions of the titanium alpha phase. The titanium dihydride phase (TiH2) is also observed, showing the (100) and (200) lines, which correspond to the higher diffractions. The results also show a decrease in the intensity in the pure titanium (101) reflection as the temperature of hydrogenation increases. This behavior could be the consequence of a structural change or loss of crystallinity produced during implantation, hydrogenation, or both. This figure also shows that the relative intensities of the reflections (100) and (002) in the HCP titanium were modified with the presence of hydrogen. This indicates that during hydrogen absorption a change in crystal orientation was induced. An important result obtained from this figure is that the implanted hydrogen does not produce hydrides in the metal, so the hydride phase is formed during hydrogenation.

**Hydrogenation temperature (°C) Gravimetric storage capacity (wt%)**

300 0.097 ± 0.007 450 0.94 ± 0.066 600 3.77 ± 0.264

220 New Advances in Hydrogenation Processes - Fundamentals and Applications

Ti‐6Al‐4V alloy.

If metal hydrides must considered as reliable fuel cells, it would be mandatory for them to become stable, which means that the materials must be enough stable for not releasing hydrogen when the metal is being stored. For this reason it is important to study the release of hydrogen under conditions of low temperature and atmospheric pressure. Studies were performed to determine the degree of release of hydrogen at atmospheric pressure and room temperature for samples of pure titanium and the Ti‐6Al‐4V alloy. Both materials were first hydrogenated using the same cleaning and hydrogenated experimental details as previously. After that, the samples were kept stored during 4 months; meanwhile their hydrogen desorp‐ tion was measured using the ERDA technique.

The results showed in both materials that hydrogen was naturally released, which means that hydrogen is desorbed from materials, without providing any additional energy to the system. In the case of pure titanium, it was observed that hydrogen concentration decreases slower than the Ti‐6Al‐4V alloy does, showing a decrease of 30% of its total hydrogen concentration, after 130 days that remained stored. Furthermore, the alloy showed a greater decrease in hydrogen content, decreasing up to 40% after 130 days of storage. The decrements in hydrogen after both materials were stored for 1 day is almost null. These results suggest that these materials are good candidates for storing hydrogen, since they are quite stable. In terms of applications to the automotive industry, it means that the vehicle could remain stop during 4 months and it will only lose 30% of its fuel if a titanium cell were used.

#### **2.5. The effects of hydrogen absorption and desorption in Ti and the Ti‐6Al‐4V alloy**

As has been already mentioned, the absorption of hydrogen by metals, such as pure titanium and the Ti‐6Al‐4V alloy, can produce changes in their metallic structures, such as in its hardness.

Hardness measures the resistance of a material when a force is applied to it. In this way, hydrogen is an element that changes hardness when it is introduced into a metal. Hardness is defined as the ratio of the applied load *P* to the indentation area *A*. In this way, Vickers microhardness (*Hv*) follows the relation

$$H\_v = 1854 \frac{P}{d^2} \tag{4}$$

where *d* is the diagonal of the indentation area.

Some works have been carried out in order to study the influence of hydrogen in hardness properties of pure titanium and the Ti‐6Al‐4V alloy [16]. Vickers microhardness tests were carried out on both materials at hydrogenation temperature ranging from 150 to 650°C. The results show that the mechanical properties of the metals were modified when hydrogen is absorbed into the material.

**Figure 9** shows the Vickers microhardness behavior of pure titanium versus hydrogenation temperature. As was mentioned before, hydrogen concentration is increased from 550 to 650°C and after that temperature hydrogen is kept constant. The same behavior is observed when microhardness measures were performed after the metal was activated, as is shown in **Figure 9** (red squares), where pure titanium shows a tendency to become harder as hydrogen increases. This fact is a direct consequence of the hydrogen that remains in interstitial sites inside the metal and was verified when hydrogen is desorb from the material. **Figure 9** also shows two other curves that correspond to the microhardness measures of the titanium after being stored for 4 months (green circles) and 10 months (blue triangles). As was mentioned below, hydrogen is lost with time after the samples had kept stored for several months. The larger hydrogen concentration that correspond to the sample that was hydrogenated at 650°C, show average decreases in the Vickers microhardness of 8 and 27% after being kept stored for 4 and 10 months, respectively. The samples that contained smaller hydrogen concentrations, such as the metals hydrogenated between 150 and 500°C, maintained their microhardness values almost constant with time. In the case of the Ti‐6Al‐4V alloy the scene is repeated, showing an average decrease in hardness of 7 and 20% after 4 and 10 months, respectively. In the same way, the samples with smaller hydrogen amounts did not show a significant loss of micro‐ hardness with time.

The XRD patterns of the hydrogenated samples also showed that the titanium hydride peaks did not present any change in form nor size when hydrogen was released in natural way. This observation can be understood as the loss of hydrogen concentration was only due to the interstitial hydrogen that is released while the samples are kept stored and it is not due to the metal hydride that has formed inside the materials. This result shows that the interstitial hydrogen is the main contributor that increases and decreases hardness in pure titanium and the Ti‐6Al‐4V alloy. In this way, it is corroborated that hydrogen plays an important role in changing the mechanical properties of a material.

**Figure 9.** Vickers microhardness versus hydrogenation temperature for pure titanium samples. A reduction in hard‐ ness as the samples loose hydrogen with time can be observed [16].
