## 3 Results
### Analysis of Deformed Microstructure and Fracture Surfaces
#### 3.4.2 Fractographic Analysis of Failure Surfaces  
Figure 18 presents photographic images of the specimens tested at a strain rate of 2450 s\({}^{-1}\) and at different test temperatures. It is clear from this figure that the specimens failed in a catastrophic manner along a plane oriented at an angle of approximately 45\({}^{\circ}\) to the compression axis. From the observation made in the preceding section, it is plausible to deduce that the specimens fractured because of intensive localized shear flow prompted by the formation of adiabatic shear bands, seen in Figures 14-16, which were also inclined at an angle of approximately 45\({}^{\circ}\). The effects of temperature are also revealed in Figure 18 where at 200 \({}^{\circ}\)C and 500 \({}^{\circ}\)C the specimens were seen to have undergone more plastic deformation in comparison to those deformed at a temperature of 25 \({}^{\circ}\)C. Figure 19 shows the measured height of deformed specimens and corresponding longitudinal plastic strain at a strain rate of 2450 s\({}^{-1}\) and at different temperatures. From Figure 19, the measured deformed height and corresponding longitudinal plastic strain for different specimens are seen to decrease and increase with temperature, respectively. Figure 17: Variation of the average width of adiabatic shear bands as a function of temperature and strain rate for samples C, D and E.  
The observations made from Figures 18 and 19 are consistent with the results presented in Table 2, where ductility of the specimens was seen to increase with increase in temperature. The ductility of samples E and C is seen to be highest and lowest, respectively, judging from the reduction in the height and increase in longitudinal plastic strain of each group of samples at different temperatures. The fracture surfaces of the specimens that were tested at a strain rate of 2450 s\({}^{-1}\) and at different temperatures are presented in Figures 20-22. Figure 19: The measured height of deformed specimens and estimated longitudinal plastic strain at a strain rate of 2450 s\({}^{-1}\) and different temperatures for samples C, D, and E.  
Figure 18: The photographic images of specimens deformed at a strain rate of 2450 s\({}^{-1}\) and at temperatures of 25, 200 and 500 \({}^{\circ}\)C.  
Figure 21: Typical characteristic features on the fracture surfaces of samples D at temperatures of (**a**) 25 \({}^{\circ}\)C, (**b**) 200 \({}^{\circ}\)C, and (**c**) 500 \({}^{\circ}\)C at a magnification of X450. Figure 20: Typical characteristic features on the fracture surfaces of samples C at temperatures of (**a**) 25 \({}^{\circ}\)C, (**b**) 200 \({}^{\circ}\)C, and (**c**) 500 \({}^{\circ}\)C at a magnification of X450. The micrographs of the fracture surfaces of samples C, D and E shown in Figures 20-22, respectively, show elongated dimples in all conditions of testing. The dimples are elongated along the directions of shear. These dimple features suggest that an array of microcracks/microvoids along the ASBs preceded the moment of fracture, with each microcrack/microvoid corresponding to a dimple. After dimples form from nucleated microvoids, their size increase with imposed flow stress, followed by their coalescence with other dimples to form larger dimples. This explains the multiple dimples of varying sizes on each fracture surfaces presented in Figures 20-22. The presence of dimples on a fracture surfaces denotes ductile fracture in contrast to smooth surfaces that are characteristic of cleavage, which is a sign of brittle fracture. It is interesting to note that in every case of the test samples here, the extent of elongation of these dimples was affected by the test temperature. At the temperatures of 200 and 500 \({}^{\circ}\)C the dimples are seen to be more elongated in comparison with ones resulting from the tests conducted at room temperature (25 \({}^{\circ}\)C).