PSI - Issue 14

4

Rajesh Sharma et al. / Procedia Structural Integrity 14 (2019) 738–745 Author name / Structural Integrity Procedia 00 (2018) 000–000

741

Fig. 4. (a) SEM fractograph showing close view of fracture surface and (b) corresponding EDS spectra.

3.3. Microstructural characterization The optical microstructures of cartridge case along longitudinal direction at different locations marked from 1 to 6 (Fig. 1) are shown in Fig. 5. The locations 1 and 6 are marked near to shoulder and head regions, respectively. The fracture surface region is marked at location 5.The microstructure of location 1 (Fig. 5a) exhibits a single phase consisting of equiaxed grains and annealing twins. The microstructure at location 2 (Fig. 5b) is similar to that of the location 1 with relatively finer grain size. The microstructure at location 3 is quite fine in comparison to those of locations 1 and 2 (Fig.5c). The locations 4 to 6 exhibit deformed microstructures consisting of elongated grains along with fine equaixed grains. The microstructures of location 5 adjacent to fracture region are shown in Fig. 6. The microstructure is heavily deformed consisting of curved flow lines pointing towards fracture end. The optical microstructures at locations 1, 2, 3 and 4 shows the presence of intergranular and transgranular cracks (Fig.7) along with other microstructural features. The crack is propagating from inner to outer surface of the cartridge case. In addition, large numbers of corrosive pits are present which act as initiation for cracks. The primary cracks along with branching observed in Fig. 7 (b and c) resemble typical feature of stress corrosion cracking. The BSE SEM microstructure in as polished condition is shown in Fig. 8a. The outer surface exhibits thin coating layer. The coating layer is discontinuous at a few locations. The SE SEM microstructure also reveals the presence of thin coating layer along with deformation flow lines (Fig. 8b). The magnified view of the Fig. 8b displays the presence of pits on inner surface (Fig.8c). Large numbers of particles are present on the surface near fracture. The SEM microstructure consisting of a particle along with EDS pattern is shown in Fig. 9. The EDS pattern displays the presence of mainly Cu and Zn elements. It appears that these particles are small broken pieces of cartridge case which are separated during fracture and deposited on surface again during firing.