**3. Conclusions**

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

hardness.

microhardness (*Hv*) follows the relation

absorbed into the material.

hardness with time.

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

222 New Advances in Hydrogenation Processes - Fundamentals and Applications

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 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

> = <sup>2</sup> 1854 *<sup>v</sup> <sup>P</sup> <sup>H</sup>*

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

**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‐

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

*<sup>d</sup>* (4)

The potential for storing hydrogen in metal hydrides is an area of material research that must be kept active. This chapter showed that pure titanium and the alloy Ti‐6Al‐4V are good candidates as hydrogen storage materials, which can increase their storage capacities after ion implantation. Ion implantation improves activation easing the diffusion of hydrogen through the whole material. It was also noted that the surface quality of the metal is important during hydrogenation in order to get better results during hydrogen absorption.
