## 3 Results and Discussion
### Strain Hardening  
Figures 8-10 show the effect of microstructure, strain rate, and temperature, respectively, on the strain hardening of the LPBF Ti6 Al4V(ELI) alloy. It is important to note that the strain-hardening behaviour of polycrystalline materials involves an intricate interaction of numerous microstructural factors such as grain boundaries, misorientation, and dislocations [44]. Strain hardening is also intrinsically coupled with other aspects such as the development of preferred lattice orientations, the formation of new sub-grains, the annihilation of dislocations, and the formation of local shear zones [45]. All these factors influence the various stages of strain hardening. It is seen in Figures 8-10 that the test data for the three different forms of LPBF Ti6 Al4V(ELI) show at least two of the three known stages of strain hardening. Stage I exhibits a sharp decline in the rate of strain hardening, which is ascribed to the multiplication of dislocations, with an increase in plastic strain at a decreasing rate and therefore a piling-up of dislocations at grain boundaries, resulting in an increase in flow stress [46; 47]. Immediately after this stage, the strain hardening rate decreases to another region with a constant value of strain hardening and is referred to as stage II. The relatively steady state in this region is due to a prevailing balance between the accumulation and annihilation of dislocations. Stage III, which exhibits a sudden decrease in strain hardening, is generally not very clear, especially at tests conducted at elevated temperatures, as the applied loads are normally withdrawn at low strains, probably before the occurrence of the intense localization of shear. The sharp increase and decrease in strain hardening rate observed in these test conditions conducted at elevated temperatures here was probably due to the sudden withdrawal of the deforming force during the test and not the propertiesof the material. Nevertheless, stage III strain hardening is clearly visible for tests conducted at ambient temperature, as shown in Figures 8a,d,g and 9a,d,g, all at room temperature. From Figure 8, at all test conditions of temperature and strain rate, the strain hardening rate of samples C in stage I was generally higher than that of samples D and E, except at a temperature of 200 \({}^{\circ}\)C and a strain rate of 0.001s\({}^{-1}\) (Figure 8f), which can be considered an outlier in this case. These observations are attributed to the fine microstructure and therefore the extended network of grain boundaries in samples C, which would suggest a high rate of accumulation and a pile-up of dislocations. The strain hardening curves of samples D and E are nearly overlapping in stage I, while in stage II, the curves for the three forms of LPBF Ti6 Al4V (ELI) are nearly overlapping in most of the testing conditions. In Figure 9a, the strain hardening rate in stage I of samples C at 25 \({}^{\circ}\)C at the lower strain rate of 0.001 s\({}^{-1}\) is higher than those of the other two strain rates. The effect seems to diminish at high test temperatures as the strain hardening curves of samples C at different strain rates are overlapping, as seen in Figure 9b,c. Figure 8: Effect of microstructure on the strain hardening of LPBF Ti6 Al4V (ELI) at different temperatures and strain rates. The effects of strain rate on strain hardening for samples D and E were generally insignificant, as in most cases, the curves were overlapping, as seen in Figure 9d-i. At a test temperature of 200 \({}^{\circ}\)C for samples D and E, stage I and II strain hardening at the middle strain rate were higher and lower, respectively, than for the other two strain rates. This is likely to have been due to a testing error of the Geeble machine that caused less banding together of the obtained stress-strain curves, as shown in Figure 5b,c.