## 3 Results and Discussion
### Fracture Behaviour Observations
There are noticeable blocky and wavy steps on the intergranular fracture surfaces of the cracked surface in Figure 17c,d. This is due to the fact that the grain boundaries were stepped before the brittle fracture occurred [93]. The presence of segregation often activates these procedures, which aim to reduce the grain boundary energy to a minimum. It is also possible for slip bands to impinge on boundaries prior to fractures, creating a ledge, and then brittle fracture can occur, leading to steps on fracture surfaces. Figure 17c,d show the surfaces of fractures with slip steps and twin traces, which are caused by deformation that occurs close beyond the crack points. Slip steps are easily identifiable from ledges because, in contrast to other steps,  
Figure 16: Fatigue fractured the surface of the HCF titanium welded joint. they do not align with the steps on the mating areas of opposing fracture surfaces. During solidification, surfaces with inter-grain porosity might take on terrace or step patterns to reduce surface energy, especially in cases when there is not enough liquid to fill the spaces between the grains. The SEM observations of the fatigue-fractured surface for welded austenitic stainless steel joints under the HCF regime are shown in Figure 18. The SEM image in Figure 18a shows that the crack initiated in the base metal, propagated around the fusion region and finally caused failure under overloading conditions, as confirmed by the generation of a shallow dimple in Figure 18c, these are the same observations options obtained from the fractured surface of welded titanium joints. In addition, the SEM analysis shown in Figure 18b confirmed the presence of slip bands on the cracked surfaces, providing evidence that ductile fracture was the mode of failure here. Fatigue slip bands are regions characterised by elevated cyclic slip activity [94]. The slip band undergoes cyclic plastic  
Figure 17: Fatigue fractured surface of LCF titanium welded joint. deformation, which causes surface extrusion and intrusion along the active slip plane traces. Fatigue micro-cracks originate from these surface intrusions. As can be seen in Figure 18c, further SEM fracture analysis of the fractured surface showed a ductile fracture with large, deep dimples in a continuous form. This suggests that a significant amount of energy was used during the fracture process [95], which is typical of local ductile fracture. Micro-voids formed and coalesced along the fracture path, causing ductile dimple rupture. The fractures had faceted patterns that suggested gradual ripping. Additionally, Figure 18c demonstrated the existence of voids. Striations and a few secondary cracks, which are indicative of fatigue failure, were seen in the SEM data in Figure 18d. Striations are caused by micro-depressions that are created in the structure, and the hollow micro-relief of fatigue striations is confirmed by the plane normal to the fracture surface [96]. Striations developed because of a lack of cohesiveness at the limits of the sub-grain, which might have been the starting point for secondary cracking [97]. As a result of micro-stress raisers and material flaws, secondary cracks develop at locations where stresses are already high because of voids [98]. Figure 18: Fatigue–fractured surface of HCF stainless steel welded joint. Upon examining the fractured surfaces of the LCF specimen using SEM, as shown in Figure 19, it was discovered that the surface was fractured with entire cleavage facets. This suggests that brittle fracture is the most common form of failure in the LCF regime. Figure 19a-d clearly show that there are many quasi-cleavage facets on the fracture surface, together with little fatigue striations. A cleavage fracture is a brittle failure of Mode I, characterised by quasi-cleavage, a low release energy, and minimal plastic deformation [99]. In this failure mode, shear loads operate parallel to the crack front and the crack plane. This study examined the failure mechanism of dissimilar Ti and St fatigue samples.