## 2 Materials and Methods  
### Ti-5553  
The titanium alloy Ti-5553 is a metastable \(\beta\)-titanium alloy. These alloys can reach very high mechanical strengths compared to other titanium alloys. A comparison between different titanium (alloys) can be seen in Table 1. In addition to higher mechanical strength, metastable \(\beta\)-alloys show increased ductility, fracture toughness and cold deformation behaviour compared to the most used Ti alloy, Ti-6 Al4V [24; 25; 26]. These properties are attributed to the ductile \(\beta\)-matrix and the strengthening phase of \(\alpha\) precipitations [14]. Furthermore, it must be mentioned that the powder bed temperatures, required in the EB-PBF process, can be lowered to 660 \(\pm\) 12 \({}^{\circ}\)C compared to the other materials. In Figure 1 an EB-PBF-manufactured Ti-5553 microstructure can be seen. The \(\beta\)-matrix shows a bcc structure compared to the hcp-structured \(\alpha\)-phase [13]. Materials with bcc structures have more active slip planes and are much more deformable compared to hcp structures [28]. The microstructure constituent that can most strongly influence the mechanical properties is the \(\alpha\) precipitations. These can be precipitated, depending on the temperature, as primary \(\alpha_{\text{p}}\) (T \(>\) 600 \({}^{\circ}\)C) or secondary \(\alpha_{\text{s}}\) (T \(<\) 600 \({}^{\circ}\)C) phases [13]. \(\alpha_{\text{p}}\) precipitations are usually lager in size, compared to \(\alpha_{\text{s}}\), due to their increased exposure times at high temperatures and, therefore, enhanced diffusion time [15]. Firstly, these precipitate along grain boundaries and afterwards inside grains. \(\alpha_{\text{s}}\) precipitate from a nucleation point and eventually form Wittmannstatt microstructures. These precipitates are very small and have a needle-like shape. Due to the decreased times at high temperature the growth is severely limited. If the Ti-5553 alloy is kept at a high temperature for a longer time, these precipitates grow and rearrange themselves into a triangular shape [29]. Large and dense precipitations (mainly \(\alpha_{\text{p}}\)) along grain boundaries lead to a decreased fracture toughness and deteriorate the mechanical properties [14]. Large precipitations can deform under slipping and shearing stress [11; 15]. Furthermore, [PERSON] et al. has shown that \(\alpha_{\text{s}}\) precipitations of 5-10 \(\mu\)m can also deform under a twining mechanism [21]. Overall, it can be said that large \(\alpha_{\text{p}}\) precipitations strengthen the material but decrease the ductility. Due to the small size of the \(\alpha_{\text{w}}\) precipitates, dislocations can be favourably accumulated, which can significantly increase the material's strength [30; 31]. By using different heat treatments and therefore changing the fractions, morphology and size of phases, tailor-made properties can be achieved [15]. A dense population of \(\alpha\) precipitations directly affects the stability of the \(\beta\)-matrix. The \(\beta\) stabilising elements (Mo, V and Cr) diffuse into the matrix, and therefore the concentration of these is enriched there [15]. For the Ti-5553 alloy, two main heat treatments are currently used. The solution-treated and aged (STA) treatment is used \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Alloy** & \(\mathbf{T_{\beta}}\) in \({}^{\circ}\)C & **YS in MPa** & **UTS in MPa** & **Young Modulus in GPa** \\ \hline Pure titanium & 882 & 140 & 100–145 & 110 \\ Ti-6 Al4V & 995 & 800–1100 & 900–1200 & 110–140 \\ Ti-10V-2 Fe-3 Al & 800 & 1000-1200 & 1000-1400 & 110 \\ Ti-5 Mo-5V-5 Al-3 Cr & 845 & 1250 & 1400 & 110 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Mechanical properties of different Ti alloys [13; 27]. Figure 1: As-built microstructure of an EB-PBF Ti-5553 using \(\text{E}_{\text{L}}=\) 600 J/m. for high mechanical strength and the beta-annealed, slow-cooled and aged (BASCA) treatment is used for maximum fatigue and fracture toughness performance [32].