## Results and Discussion
### Microstructure Analysis
The EBSD tests revealed that the as-built state of the Inconel 939 alloy contains numerous subgrains and deformed grains, which carry high amounts of energy from thermal stresses, causing distortions in LPBF-produced structures (Figure 8A,D; Table 4). The HT1 heat treatment did not cause recrystallization, and it consists primarily of subgrains with a reduced amount of deformed grains compared to the as-built state. A reduction in the number of deformed grains should provide an effective reduction in material stresses and distortions (Figure 8B,E; Table 4). The specimen after HT2 heat treatment has a microstructure with a high fraction of recrystallized grains. This suggests that the recrystallization process took place and deformations were eliminated (Figure 8C,F; Table 4). This confirms that the chosen solution treatment temperature produces a recrystallized microstructure. The lack of deformed grains should produce parts that are virtually free of distortion. Grain size varies, with the smallest grains (24.2 um\({}^{2}\)) after HT1 heat treatment and the largest (134.9 um\({}^{2}\)) after HT2 heat treatment (Table 5). At room temperature, the main strengthening mechanism is connected to the density of dislocations and the amount of dislocation-blocking grain boundaries. With the elongated grains in the Z-axis, the amount of grain boundaries across the X-axis is much higher, causing an increase in tensile performance in the X-axis compared to the Z-axis. At elevated temperatures, the effect of grain boundaries changes, and it is easier for dislocations to move across high-density grains, explaining why a switch between the X- and Z-axis tensile performance can be observed. The effect of the grain size difference between HT1 and HT2 is reduced mainly because of the number of precipitations present in both states, which also block dislocation movement. \begin{table}
\begin{tabular}{l c c c} \hline \hline  & **Recrystallized** & **Substructured** & **Deformed** \\ \hline AB & 3.2\% & 44.0\% & 52.8\% \\ \hline HT1 & 3.9\% & 79.0\% & 17.1\% \\ \hline HT2 & 59.6\% & 40.4\% & - \\ \hline \hline \end{tabular}
\end{table} 
Table 4: Recrystallization volume fractions of Inconel 939 for as-built, HT1, and HT2 states (% vol.). Figure 7: SEM micrograph of IN939 HT1 state YAGBSE detector **(A)**; EDS-analyzed fragment **(B)**; EDS maps of elements (**C**); HT2 state YAGBSE detector **(D)**; EDS-analyzed fragment **(E)**; EDS maps of elements **(F)**. Hardness differences between HT1 and HT2 are negligible, indicating that precipitation has a more significant impact on hardness than grain size and recrystallization. The heat-treated specimens experienced oxidation due to the use of a non-inert atmosphere furnace. On both the HT1 and HT2 states, three distinct layers were visible under the SEM microscope. A linear chemical analysis (Figure 9D,H) showed differences between each layer, and all of them were different from the initial alloy. The outermost layer contained a reduced amount of chromium with moderate amounts of nickel, titanium, and cobalt. This layer was present in small amounts on the HT1 state specimens. In the HT2 state specimens, it averaged 11.9 \(\upmu\)m. The next layer showed increased amounts of chromium and titanium with limited nickel and cobalt (Figure 9C,G). This layer was present along the entire edge but varied in thickness, averaging 10.1 \(\upmu\)m for HT1 and 11.5 \(\upmu\)m for HT2; it could be identified as CrO\({}_{3}\) scales. The third layer contained numerous cracks. Both states had nominal amounts of nickel, tantalum, and cobalt with reduced amounts of chromium, while the cracks were titanium- and aluminum-rich. The thickness of this layer averaged 33.5 \(\upmu\)m for the HT1 state and 44.8 \(\upmu\)m for the HT2 state and could be identified as an internal oxidation zone (IOZ). Compared to [PERSON] et al. (2024) [18], we observed an additional layer on top of the CrO\({}_{3}\) scales, which may have been caused by the higher temperatures of our heat treatment compared to 900 degrees Celsius.