## 3 Results and Discussion
### Microstructure and Phase Identification  
The optical micrographs of all the conditions investigated in this study (ArS, SE-CB, and SE-HB) are provided in Figure 2. All samples showed prior-\(\beta\) grains, characterized by a columnar morphology, ranging from few microns to the sample height in size. These grains developed during the deposition due to the highly directional cooling, typical of this process, which involves epitaxial growth phenomena [33; 34]. Banding, a typical feature of Ti-6 Al-4V processed by DED, as reported for example by [PERSON] et al. [35], was also observed in all cases. For all conditions, the corresponding microstructure was mainly \(\alpha^{\prime}\) martensite, easily recognizable from its needle-like \(\pm\)45\({}^{\circ}\) structure [36], as generally occurring in processes that involve very rapid cooling (>410 \({}^{\circ}\)C/s) [37]. In these cooling conditions in fact the diffusionless \(\beta\)  
Figure 2: Optical micrographs of samples built in conditions: protective argon flow (ArS) (**a**,**b**), sealed environment using a non-preheated platform (SE-CB) (**c**,**d**) and sealed environment using a preheated platform (SE-HB) (**e**,**f**). transformation is more kinetically favoured than the usual, diffusion-driven \(\beta\!\rightarrow\!\!\alpha\!+\!\beta\) transformation, typical of duplex titanium alloys such as Ti-6 Al-4V [38]. However, a closer inspection of the high magnification micrographs highlights that the microstructure of the ArS samples (Figure 2b) shows different features in addition to the typical martensitic microstructure of samples produced in a sealed environment (Figure 2d,f). The ArS samples present, near to the martensitic structure, lamellae arranged in a \(\pm 30^{\circ}\)/60\({}^{\circ}\) fashion, or basket-weave, which are typical of the more stable \(\alpha+\beta\) Widmanstatten microstructure, possibly resulting from \(\alpha^{\prime}\) needles decomposition [39]. So, it is possible to summarize the resulting microstructures as:  
* ArS: \(\alpha^{\prime}\) martensitic microstructure and bimodal \(\alpha+\beta\);
* SE-CB: Completely \(\alpha^{\prime}\) martensitic microstructure;
* SE-HB: Completely \(\alpha^{\prime}\) martensitic microstructure. To further analyse phase composition and to confirm the hypothesis made on the basis of microstructures, the XRD patterns of materials processed in the three conditions are reported in Figure 3. All these patterns appear quite similar, exhibiting all the peaks related to the Hexagonal Close Packed (HCP) phase \(\alpha/\alpha^{\prime}\)[36; 40]. A complete distinction between the thermodynamically stable \(\alpha\)-phase from the metastable \(\alpha^{\prime}\) martensite just by means of X-ray pattern analysis is very complex, due to the common HCP lattice [40; 41]. In addition, in SE-CB and SE-HB conditions, a shift of the \(\alpha/\alpha^{\prime}\) peaks towards higher angles was also noted, probably due to different internal stresses and/or dislocation densities, caused by the different cooling path [36; 42]. Overall, no significant differences between SE-CB and SE-HB were noted in terms of XRD patterns. A body centred cubic (BCC) \(\beta\)-phase, instead, was clearly distinguishable exclusively in ArS, as can be noted from the (110) peak [43]. This was the only peak available to evaluate the \(\beta\) phase due to the overlapping of the other peaks [44]. Detecting a non-negligible amount of \(\beta\)-phase confirms the aforementioned hypothesis of a small amount of \(\alpha+\beta\) phases detected in ArS samples from micro  
Figure 3: XRD patterns for all the deposition conditions considered. The appearance of \(\alpha+\beta\) phases only in ArS samples was probably correlated to the process environment and consequent oxygen/nitrogen pick-up. A more in-depth analysis was conducted by calculating the cell parameters of the \(\alpha\)-phase, to better evaluate the differences occurring among all the conditions considered.