## 3 Results and Discussion
### Interstitials Content  
Interstitials (O, N, H) are particularly critical for Ti-6 Al-4V. As mentioned before, these elements are highly soluble, even at relatively low temperature, and directly impact on the alloy mechanical properties, for example reducing ductility [14; 46]. To estimate the influence of multiple elements at the same time, it is common to evaluate the oxygen equivalent concentration \(\left[\text{O}\right]_{\text{EQ}}\)[48]:  
\[\left[\text{O}\right]_{\text{EQ}}=\left[\text{O}\right]+2\left[\text{N} \right]+\frac{2}{3}\left[\text{C}\right] \tag{2}\]  
Another study [49] implemented Equation (2), taking in account a greater coefficient for [N] and assessed in addition the influence of Fe on \(\left[\text{O}\right]_{\text{EQ}}\) as:  
\[\left[\text{O}\right]_{\text{EQ}}=\left[\text{O}\right]+2.77\left[\text{N} \right]+0.1\left[\text{Fe}\right] \tag{3}\]  
Figure 4: c/a ratio and cell volume for all the conditions considered.  
In this study, only O and N were considered because the other elements concentrations included in Equations (2) and (3) resulted negligible. Moreover, the second equation was used since it takes in account a greater coefficient for [N], resulting in higher \(\left[\text{O}\right]_{\text{EQ}}\). This possible overestimation was considered acceptable due to precautionary purposes. An oxygen and nitrogen evaluation were performed on the samples in all the building conditions and on the powder used for deposition, in the virgin state. The [O], [N] and [O]\({}_{\text{EQ}}\) results are reported in Figure 5.  
As expected, [O] and [N] were found to be lowest in the powder. The deposition led to an increase in the concentration of both elements in all the conditions. The ArS samples revealed a high interstitial concentration, providing a [O] of 0.132% and a [N] of 0.104%. These led to an [O]\({}_{\text{EQ}}\) value of 0.42%, markedly higher than the 0.22% threshold [20], resulting in a brittle sample. The results for SE-CB and SE-HB samples were close, especially for oxygen (respectively 0.0924% and 0.0956%). Nitrogen pick-up was more severe in SE-HB samples, which showed a [N] of 0.0205%, whilst in SE-CB samples a concentration of 0.0109% was reached. However, in both cases, the values of [O]\({}_{\text{EQ}}\) were lower than the 0.22% threshold (0.1226% for SE-CB and 0.1523% for SE-HB).  
The high oxygen/nitrogen pick-up detected can also explain two main results:  
* the differences in microstructure between ArS samples (\(\alpha^{\prime}\) martensitic microstructure and bimodal \(\alpha+\beta\)) and the other conditions (completely \(\alpha^{\prime}\) martensitic microstructure). In fact, [PERSON] et al. [50] proved that Ms (martensite start) is substantially unaffected by oxygen content increases, when relatively low concentrations are involved (\(\approx\) 0.2%). However, T\({}_{\beta}\) (beta transus temperature) markedly rises, even at very low oxygen content. This different behaviour might be a possible explanation for the formation of a relevant amount of \(\beta\) phase in ArS samples. In fact, both O and N act in titanium alloys as \(\alpha\) phase stabilizers [14]. These elements cause basically an increase in the temperature range between T\({}_{\beta}\) and Ms, favouring the \(\beta\)\(\rightarrow\)\(\alpha+\beta\) transformation;
* the bigger cell that ArS provided as a result of the XRD analysis. In fact, as previously mentioned, O and N fit well in the octahedral vacancy of the h.c.p. lattice of Ti-\(\alpha\), distorting the cell and resulting in a larger volume.  
Figure 5: Oxygen and nitrogen concentration evaluated for the samples built in all the considered environment and relative [O]\({}_{\text{EQ}}\) evaluation. The 0.22% threshold is highlighted in red.