## 4 Discussion
### In Situ Tensile Testing  
The results indicate that the mechanical properties, especially the elongation at break for the in situ tensile test, are reduced dramatically compared to the conventional tensile tests. Since the microstructural constituents of the specimens for the in situ tensile test are identical to the microstructural constituents of the specimens used for the conventional tensile tests, as-built, STA and BASCA, the influences on the mechanical characteristic values obtained in the in situ tensile tests can only be attributed to the specimen geometry (see Figures 4 and 7) and the associated difficulties in mounting the specimens. Due to the use of a laser cutting step, the surface regions are rugged and defects that were previously on the inside will become defects close to the surface. Consequently, the surface quality is negatively affected.  
Figure 14: Change of the failure-critical defect, marked in red, of a BASCA heat-treated Ti-5553 alloy using \(\mathrm{E_{L}}=600\) J/m. Load level: (**a**) 2900 N, (**b**) 2975 N, (**c**) 3050 N, (**d**) 3125 N, (**e**) 3155 N and (**f**) 3170 N.  
Furthermore, during the fabrication and mounting of the flat tensile specimens, some twisting or torsional loading of the specimens may have occurred. This also leads to multiaxial stress in the tensile test, which has a significant influence on the measured values.  
The fracture mechanism of STA heat-treated specimens, which was discussed in Section 3.1, had a major influence on the in situ tests. The very brittle fracture, typical of mechanically highly stressable materials, limits the detection and monitoring of failure-critical defects at early load stages. The first cracks were detected shortly after reaching the YS, suggesting that the strength differences between the hardened matrix and the softer \(\alpha_{\mathrm{p}}\) precipitates, mainly along the grain boundaries (see Figure 8 a,b), were too large. Such a structural arrangement can lead to premature crack nucleation [13; 43]. Furthermore, surface defects, see Figure 10, are regions of high stress concentration, and these can also reduce the mechanical strength [44]. The combination of highly stressed surface defects with brittle material properties and very thin test geometries can lead to a significant influence of notch effects related to these defects, and thus early failure. Inside the second STA specimen tested in situ (see Figure 10), two defects were identified which were in close proximity to each other and to the surface. Consequently, the resulting test cross-section was reduced and thus the range of the stress distribution was very small. This caused the specimens to fail at lower stress and strain levels. It was possible to detect the failure-critical defect of the specimens following the in situ CT analyses made for each load level. The graph in Figure 13 shows an almost constant growth in the defect volume, but only for the final load levels is a major growth along the z-axis can be seen. In Figure 14e,f this fact is seen in the analysis of the \(\mu\)-CT data. This could indicate that the shape change is finished due to geometrical instability despite the application of higher stresses. It can be assumed that the specimen fails because the change in shape of the defect and the plastic deformation of the material cannot compensate these higher stresses, and therefore cracks are nucleated. Hence, the tested specimen fails shortly after the stress is increased to the next level. With these small test sample geometries and their thin wall thicknesses, the ductility of the bcc \(\beta\)-phase is not sufficient to absorb the stresses during the tensile test. BASCA as well as STA heat-treated specimens fail at the boundary between the two present phases (\(\alpha\) and \(\beta\)). Furthermore, defects for such small test object geometries have an increased (bad) influence, due to the bad defect-to-test-volume ratio, on the mechanical strength [45; 46].