## 3 Results
### High Strain Rate Behaviour of Various Forms of DMLS Ti6 Al4V(ELI) Alloy
#### 3.3.2 Strain Rate Sensitivity  
To get clarity on the effects of strain rate and temperature on the dynamic response of the various microstructures of DMLS Ti6 Al4V(ELI), the variation of true stress with strain rate as a function of temperature was plotted. The effects of strain rate on flow stress at a fixed strain and temperature can be quantified using the strain rate sensitivity parameter \(m\) expressed as:  
\[m=\frac{\partial log\sigma}{\partial log\dot{\varepsilon}} \tag{4}\]  
where, the symbols \(\sigma\) and \(\dot{\varepsilon}\) stand for true stress and true strain rate, respectively. The strain rate sensitivity is therefore, given by the slope of the linear plot of true stress vs. true strain rate in _log-log_ graphs and at a given strain. Such plots for the samples tested in the present study are shown in Figure 11 for a true strain of 0.1. The increase of flow stress with strain rate of the curves presented in Figure 11, shows a linear dependence as evidenced by the correlation coefficients of the plots presented in Table 4. The strain rate sensitivity parameter \(m\) for the three categories of samples at the three test temperatures used for testing here is also shown in the table and for ease of comparison presented in (d) of Figure 11. As seen in Table 4 and Figure 11d the calculated values of strain rate sensitivity \(m\) increase with test temperature for all samples tested. Generally, plastic deformation in alloys is mainly due to slip of dislocations. Figure 11: Plots (**a–c**) true stress vs. true strain rate on a _log-log_ scale at a true strain of 0.1, at different test temperatures and (**d**) the variation of the strain rate sensitivity parameter \(m\) with temperature for samples C, D and E.  
Furthermore, the flow stress of materials consists of a thermal part (that is dependent on strain rate and temperature) and an athermal part (generally regarded as being independent of strain rate and temperature) [32]. The thermal part is controlled by short-range barriers, such as mobile dislocations and the Peierls stress and is sensitive to both temperature and strain rate [32; 33; 34; 35]. High strain rates accelerate the multiplication of these mobile dislocations, leading to the formation of dislocation tangles [33]. On the other hand, high temperatures will supply the dislocations with sufficient thermal energy to help overcome dislocation tangles, thus enhancing the activities of dislocations [33; 34]. Moreover, at high temperatures, edge dislocations of the \(\alpha\)-phase in Ti6 Al4V(ELI), can climb along the direction perpendicular to their slip planes [35]. Thus, the increase in the magnitude of the strain rate sensitivity parameter \(m\) with temperature seen in Table 3 is a sign of an attendant increase in dislocation-activity. The higher value of the strain rate sensitivity parameter at higher deformation temperatures, can also be explained by the increasing activity of the non-basal slip systems as a function of increasing temperature (200 \({}^{\circ}\)C to 500 \({}^{\circ}\)C in this case), in addition to existing slip in the basal slip systems of the hcp structure of this material. The athermal part of the flow stress is controlled by long-range barriers such as grain boundaries and the secondary precipitates, which are generally regarded as being independent of the temperature and strain rate during high strain rate deformation [33; 34]. This implies that the effects of grain size on the activity of dislocations of this material cannot be ignored. A previous study by [PERSON] [36] showed that the effect of grain size on strain rate sensitivity is mainly attributed to the thermally activated dislocation motion, since reducing the grain size leads to a decrease in the spacing of dislocation forests, thereby enhancing the parameter \(m\).