## 3 Results  
### Microstructural Features of Heat-Treated DMLS Ti6 Al4V(ELI) Samples  
The secondary electron images (SEIs) and EBSD orientation distribution maps of DMLS Ti6 Al4V(ELI) specimens that were exposed to the heat treatment strategies shown in Figure 2, are presented in Figure 4.  
Samples C were heat treated at 800 \({}^{\circ}\)C followed by furnace cooling (FC). During this heat treatment process, the well-known fine acicular \(\alpha^{\prime}\) microstructure associated with as-built DMLS parts [24,25] transformed to an equilibrium mixture of \(\alpha\) and \(\beta\) phases as shown in Figure 4a. The average width of \(\alpha\)-lathes for samples C measured about 2.5 \(\upmu\)m. Samples D, that were exposed to duplex annealing (940 \({}^{\circ}\)C/FC followed by 750 \({}^{\circ}\)C/FC) had a coarse bi-phasic microstructure as seen in Figure 4c. The average width of the \(\alpha\)-lathes in this microstructure was about 6 \(\upmu\)m, which was an increase of about 140% in width from the value recorded for samples C. The shape of the \(\alpha\)/\(\beta\) grains in samples D was lamellae-like in contrast to the partial particle-like morphology of samples C.  
The samples E were heat treated at a temperature above the \(\alpha\)\(\rightarrow\)\(\beta\) transformation temperature and consisted of typical Widmanstatten microstructure with large \(\alpha\) colonies as seen in Figure 4e. These colonies contained several parallel \(\alpha\) plates where their sizes were limited by the adjacent \(\alpha\) colonies. The width of the \(\alpha\) lathes within these colonies range between 7-15 \(\upmu\)m, with an average width determined through the line-intercept method of 9 \(\upmu\)m. A much thicker grain boundary \(\alpha\) can be observed at prior \(\beta\) boundaries having a thickness of about 10 \(\upmu\)m (Figure 4e).  
It is important to note that the \(\beta\)-phase is shown in Figure 4a,c,e as brighter regions that are rich in vanadium, while the regions that are low in vanadium content (\(\alpha\)-phase) appear darker. The fraction of \(\beta\)-phase for the bulk microstructure was determined in the  
Figure 4: (**a**,**c**,**e**) Secondary electron images and (**b**,**d**,**f**) EBSD orientation maps of (**a**,**b**) samples C, (**c**,**d**) samples D, and (**e**,**f**) samples E, respectively. The inserts in (**b**,**d**,**f**) are keys for the inverse pole figures (IPF).  
authors previous work [28] through the X-ray diffraction (XRD) method as 3.6%, 6.4% and 6.6% for samples C, D and E, respectively.  
The crystallographic orientations of the \(\alpha\)-lathes in samples C, D and E are illustrated by the EBSD orientation maps in Figure 4b,d,f. The prior \(\beta\)-grains elongated along the \(X\)-build direction are distinct in samples C (Figure 4b) and samples D (Figure 4d), while in samples E (Figure 4e) the columnar grains are seen to have disappeared. The decomposition of the prior \(\beta\)-grains in samples E is a result of heat treatment at a temperature above the \(\alpha\)\(\rightarrow\)\(\beta\) transformation temperature. The IPF shown in the insert of Figure 4b,d,f has a colour code related to specific orientations: the colours red, green and blue represent the family of planes \(\{0001\}\), \(\{11\overline{2}0\}\) and \(\{1\overline{1}00\}\), respectively, parallel to the surface X-Y. Samples C and D exhibit random texture of the \(\alpha\)-lathes, however, the crystallographic orientations of some \(\alpha\)-lathes tend to repeat within the same prior \(\beta\)-grains as indicated by repetition of the same colour of \(\alpha\)-grains. As noted in Figure 4f, the colonies of \(\alpha\)-lathes within the equiaxed and semi-equiaxed morphologies in samples E, consist of crystals with a similar orientation, shown by the same colour.