## 3 Results and Discussion  
### Microstructure and Microtexture Characterisation  
The microstructures and distribution of the \(\alpha\)-grain boundaries in samples C, D and E, that resulted after the heat treatment cycles presented in Table 2, are presented in Figure 4. As seen in Figure 4a,b, the intermediate heat treatment temperature (800 \({}^{\circ}\)C for 2 h), which is far below the \(\beta\) transus temperature (980 \({}^{\circ}\)C), transformed the initial \(\alpha\)'-acicular microstructure of the as-built samples (whose details are published in the authors' work in [PERSON] et al. (2018, 2019)) into a stable mixture of \(\alpha+\beta\)-phase. The \(\beta\)-phase appears in Figure 4a,c,e as bright regions, while the \(\alpha\)-phase appears dark. The \(\beta\)-phase appears in particle-like morphology and is distributed along the grain boundaries of the \(\alpha\)-laths. Coarsening of the \(\alpha\)-laths is observed in Figure 4c,d for the samples that were heat treated at 940 \({}^{\circ}\)C for 2.5 h followed by annealing at 750 \({}^{\circ}\)C with a residence time of 2 h. The \(\beta\)-phase is still embedded along the coarsened \(\alpha\)-laths but has lamellar morphology in contrast to the particle-like shape shown in samples C. Columnar epitaxial prior \(\beta\) grains elongated parallel to the build orientation are visible in Figure 4b,d for samples C and D, respectively. These columnar prior \(\beta\) grains are related to the continuous heating, rapid cooling rate and solidification associated with the DMLS build process. As the laser heats the substrate, the Ti6 Al4V(ELI) powder is melted and rapidly solidifies as the laser moves away. In this process, small equiaxed grains of prior \(\beta\)-phase grow during solidification since the heating temperature usually exceeds the \(\beta\)-transus temperature. The formed equiaxed grains provide ideal nucleation sites for the subsequent melted layer. Therefore, the equiaxed prior \(\beta\) grains orthogonal to the laser scan direction continue to be nucleated and grow epitaxially from the prior \(\beta\) grains in the previous layer, and the process goes on, causing the columnar growth of \(\beta\) grains parallel to the build orientation as shown in Figure 4b,d. Figure 3: The Instron 1342 servo-hydraulic universal test machine (UTM) at CSIR. Since samples E were exposed to heat treatment above the \(\beta\)-transus temperature (1020 \({}^{\circ}\)C for 2 h), the columnar epitaxial prior \(\beta\) grains shown in Figure 4b,d were decomposed and replaced by semi-equiaxed and equiaxed prior \(\beta\) grains shown in Figure 4f. The microstructure of samples E seen in Figure 4e has colonies, each containing numerous parallel \(\alpha\)-laths, with their sizes restricted by the neighbouring \(\alpha\)-colonies. Thick and continuous grain boundary \(\alpha\) is also seen in Figure 4e at prior \(\beta\) grain boundaries having a thickness of about 8 \(\upmu\)m. It is apparent in Figure 4 that the \(\alpha\)-laths were coarser as the heat treatment temperature was increased. A study has shown that the \(\alpha\)-grain sizes in the LPBF Ti6 Al4V parts heat treated at temperatures below the \(\beta\)-transus temperature (980 \({}^{\circ}\)C) were not primarily dependent on the rate of cooling but on the heat treatment temperature in the \(\alpha+\beta\) zone [19]. As the heat temperature treatment increases in this zone, the transformed \(\alpha\)-phase nuclei become scattered, and thus the \(\alpha\)-laths coarsen to a larger extent during cooling before interaction with other neighbouring latths occurs [17; 19]. The cooling rate has a profound effect in LPBF Ti6 Al4V parts when cooling from temperatures above the \(\beta\)-transus temperature, where the much faster cooling rate leads to formation of finer \(\alpha\)-laths and vice versa [31; 32; 33].