## 3 Results
### Analysis of Deformed Microstructure and Fracture Surfaces  
#### 3.4.1 Analysis of Deformed Surfaces  
As previously indicated in Table 2, not all the dynamic test conditions resulted in failure of specimens and only the samples that were tested at a strain rate of 2450 s\({}^{-1}\) failed. As the specimens that were tested at strain rates of 750 and 1500 s\({}^{-1}\) at all test temperatures did not fail, they form a good basis for analysing the deformation characteristics of the various forms of DMLS Ti6 Al4V(ELI) alloy. Figure 13 shows the quintessential deformation features of these samples, observed under low magnification in an optical microscope. Figure 12: Viscous drag stress as a function of (**a**) strain rate for different grain sizes and (**b**) grain size at different strain rates. Figure 13: Typical deformation features on cross sections through the surfaces of samples (**a**) C, (**b**) D, and (**c**) E observed under an optical microscope (the red arrows indicate the compression axis). It is apparent from the micrographs in Figure 13 that the deformed surfaces of various samples of DMLS Ti6 Al4V(ELI) are characterised by bright bands and in some places the bright bands are replaced by dark elongated features. The bright bands are known as adiabatic shear bands (ASBs), while the dark elongated features are cracks propagating along the ASBs. The ASBs are seen in Figure 13 to be narrow and to extend through the grains across the deformed surfaces at an angle of approximately 45\({}^{\circ}\) to the compression axis, which is the expected angle of maximum shear in cases of uniaxial loading. Generally, plastic work during high strain rate deformation of materials is invariably transformed into thermal energy, which leads to a rise in the local temperature in a material. Because the thermal conduction of Ti6 Al4V(ELI) is very poor, while its strain hardening as seen in Figures 7-9 is low (the plastic deformation curves with relatively low gradients), the alloy is sensitive to strain localization in the conditions of dynamic deformation [43]. The thermal softening effect due to adiabatic or quasi-adiabatic deformation processes at high strain rate lowers the strength of materials during plastic deformation. As the strain increases, the thermal softening effect outweighs the strengthening effect due to strain hardening and shear instability occurs. Basically, if the stress in the primary shear zone is surpassed by the stress in the bulk material, a shear localization event takes places forming narrow shear bands [44]. Once formed, the ASBs become sites of subsequent failure by rapid nucleation and growth of cracks within them as seen in Figure 13. The ASBs in the three forms of DMLS Ti6 Al4V(ELI) at different strain rates and temperatures were studied further at higher magnification in a SEM, to give the SEIs presented in Figures 14-16. Figure 14: Secondary electron images showing adiabatic shear bands on sections of deformed surfaces of samples C at different strain rates and temperatures (note the different magnification at 750 s\({}^{-1}\) and 25 \({}^{\circ}\)C). The red arrows indicate the compression loading axis. It is clear from observation of Figures 14-16 that the microstructures within the adiabatic shear bands are different from that of the bulk material, a difference thought to have arisen during the formation of these adiabatic shear bands. It is known from literature [44; 45] that microstructural changes such as dynamic recovery, recrystallization and phase transformation do occur during formation of ASBs. However, a direct evidence of these microstructural changes could not be pinpointed from the SEIs presented in the three figures. The most appropriate method to study the microstructural changes along the ASBs is the EBSD technique. As previously mentioned, the formation of ASBs precedes failure mechanisms such as nucleation and propagation of cracks. As seen in these figures, well-developed ASBs in some cases are accompanied by cracks which propagate within and along the bands.