## 3 Results
### High Strain Rate Behaviour of Various Forms of DMLS Ti6 Al4V(ELI) Alloy  
#### 3.3.1 Stress-Strain Characteristics  
High strain rate tests were conducted for a range of temperatures from ambient condition (25 \({}^{\circ}\)C) to 500 \({}^{\circ}\)C. The strain rates were controlled by regulating the pressures  
Figure 5: The average Vickers micro-hardness of various forms of DMLS Ti6 Al4V(ELI) and their variation with heat treatment temperature. Figure 8: The true stress-strain curves (**a–c**) at different strain rates and temperatures, and (**d**) true stress vs. temperature curves at different strain rates and at strains of 0.1 and 0.2, all for samples D.  
Figure 9: The true stress- strain curves (**a–c**) at different strain rates and temperatures, and (**d**) true stress vs. temperature curves at different strain rates and at strains of 0.1 and 0.2, all for samples E.  
A general observation in all curves in Figure 7a-c, Figures 8a-c and 9a-c is that the flow stress increases with increasing strain rate, whereas as shown in Figures 7d, 8d and 9d, it decreases with increase in temperature. Figures 7d, 8d and 9d also show that the flow stresses are generally lower at lower values of strain. These observations clearly show that these three different microstructures of DMLS Ti6 Al4V(ELI) are sensitive to strain, strain rate and temperature. The observed increase in flow stress with increase in strain rate can be attributed to increased generation and interaction of dislocations causing higher instantaneous strain hardening at and beyond yield, thereby increasing flow stress in the material. Whereas, for tests conducted at elevated temperatures, the generation of dislocations is countered by the annihilation of dislocations, a phenomenon referred to as thermal softening, which then leads to a decline in flow stress. It is also apparent in Figures 7-9 that under these high strain rate loading conditions the three different forms of DMLS Ti6 Al4V(ELI) alloys yielded at different levels of stress for the same imposed strain rates and temperatures. This is expected due to the different microstructures of the three groups of samples. Further information on the curves shown in Figures 7-9 and observations made during testing are presented in Table 3.  
strain because the waves generated by these strain rates were not sufficient to load these specimens to failure. It is evident from Equation (2) that the strain experienced by the SHPB test specimen is dependent on the magnitude of the reflected signal amongst the other factors in the equation. As noted earlier, if the duration of buildup of the reflected wave is limited due to low striker velocity the specimen cannot reach the fracture strain. As seen in Figure 10a-c, samples C showed the highest yield stress at any given temperature and strain rate as compared to samples D and E. Samples E showed the lowest yield stress compared to the other two groups of samples for most of the test strain rates and temperatures. As further seen in Figure 10d, the fracture strains that were recorded for samples loaded at the highest strain rate of 2450 s\({}^{-1}\), were lowest at 25 \({}^{\circ}\)C for all samples. The fracture strains of samples E at the highest strain rate were the highest at 25 \({}^{\circ}\)C and 500 \({}^{\circ}\)C and second highest at 200 \({}^{\circ}\)C. The high yield stress in samples C in comparison to samples D and E is attributed to the small average \(\alpha\)-lathe thickness, recorded as 2.5 \(\upmu\)m for samples C, which therefore leads to a short effective slip length during deformation, thus increasing the yield stress and subsequent flow stress. In previous work by the authors [24], the reference microstructures of DMLS Ti6 Al4V(ELI) (as built samples (A) and stress relieved samples (B) were reported to show much higher strength and low fracture strain compared to the heat-treated samples C, D and E at high strain rates.