## 3 Results and Discussion
### Flow Properties at Different Strain Rates and Temperatures
\begin{table}
\begin{tabular}{c c c c c c c c} \hline \hline \multirow{2}{*}{**Samples**} & \multirow{2}{*}{**Strain Rate**} & \multicolumn{2}{c}{**25 \({}^{\circ}\)C**} & \multicolumn{2}{c}{**200 \({}^{\circ}\)C**} & \multicolumn{2}{c}{**500 \({}^{\circ}\)C**} \\ \cline{3-8}  & & \(\sigma_{\mathbf{s}}\) & \(\sigma_{\mathbf{ucs}}\) & \(\sigma_{\mathbf{s}}\) & \(\sigma_{\mathbf{ws}}\) & \(\sigma_{\mathbf{s}}\) & \(\sigma_{\mathbf{s}}\) & \(\sigma_{\mathbf{ucs}}\) \\  & & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** & **(MPa)** \\ \hline \multirow{4}{*}{C} & 0.1 & 1164 & 1237 & 1011 & 1114 & 738 & 842 \\  & 0.005 & 1133 & 1220 & 968 & 1132 & 738 & 819 \\  & 0.001 & 1040 & 1177 & 956 & 1132 & 738 & 816 \\  & 0.1 & 995 & 1146 & 842 & 1076 & 608 & 796 \\ \multirow{4}{*}{D} & 0.005 & 969 & 1145 & 841 & 1074 & 575 & 724 \\  & 0.001 & 935 & 1145 & 794 & 1024 & 575 & 700 \\  & 0.1 & 946 & 1190 & 797 & 1027 & 526 & 773 \\ \multirow{4}{*}{E} & 0.005 & 940 & 1184 & 740 & 1011 & 517 & 726 \\  & 0.001 & 903 & 1184 & 740 & 967 & 517 & 726 \\ \hline \hline \end{tabular}
\end{table} 
Table 3: Some mechanical properties of different microstructures of LPBF Ti6 Al4V (ELI). Figure 6: Comparison of yield stress and ultimate compressive stress at different strain rates and temperature for samples C, D, and E.  
Studies have shown that polycrystalline materials exhibit a weak strain rate-dependent response at low strain rates, but this increases at high strain rate regimes (10\({}^{-2}\) s\({}^{-1}\)) [32; 40] It has been argued that deformation at low strain rates is controlled by dislocation forests and/or the interaction of dislocations with grain boundaries or precipitates. On the other hand, deformation at high strain rates is attributed to viscous drag forces slowing down the movement of dislocations and increasing plastic strain flow stress, in what is commonly referred to as strain rate hardening [41]. Further details on the strain hardening of different microstructures of LPBF Ti6 Al4V(ELI) are covered in Section 3.3. At a given test condition, the yield stress and compressive stress of samples C are higher than those of samples E and D, while those of samples E are lower than those of samples D, as shown in Figure 5d-f. The yield strength of a material can be decomposed into two components: (a) the thermal part, which is dependent on strain rate and temperature, and (b) the athermal part, which is insensitive to these two factors [32; 41; 42]. The athermal component of yield stress is normally influenced by long-range obstacles such as far-field dislocation forests and grain boundaries [42]. Furthermore, the influence of grain boundaries on yield stress can be rationalized in terms of the Hall-Petch relationship [43], where small grains result in higher yield stress and vice versa. Moreover, the mechanical properties of titanium alloys with lamellar microstructures such as those presented in Figure 3 are influenced by effective slip length (the shortest distance dislocations can travel before encountering obstacles), which is obviously the thickness of the \(\alpha\)-laths. This explains the trends seen in Figure 5 and Table 3 that the yield stress decreased from samples C, with an average grain size of 2.5 \(\upmu\)m, through to samples D and E, with average grain sizes of 6 \(\upmu\)m and 9 \(\upmu\)m, respectively. Another factor that could support the observation made in Figure 5d-f and Table 3 is the density and misorientation of grain boundaries in samples C, D, and E, shown in Figure 7. A dense large network of grain boundaries, in any order of the samples from the highest to lowest, would suggest an increase in hardness and therefore strength in the same order. The trend of Vickers's micro-hardness of these samples, in order from the highest to the lowest, was reported in [PERSON] et al. [39] for samples C, E, and D. The average initial dislocation densities of these different forms of LPBF Ti6 Al4V(ELI) also showed a similar trend, as reported in [PERSON] et al. [39].