Fatigue fracture at copper bicrystal interfaces: fractography

The fracture surfaces of intergranular cracks in copper bicrystals with different misorientations, subjected to different mechanical pretreatments and cracking directions, were studied by scanning electron microscopy (SEM). The dislocation structures present before the crack formed and propagated were characterized by transmission electron microscopy (TEM). It was found that they could be either loop patches and persistent slip bands (PSB’s) (single slip) or maze and cell structures (multiple slip) depending on the misorientation and the pretreatment. The results showed that these structures influenced the shape of the crack front, which could be aligned close to specific crystallographic directions or straight depending on the prior structure. The crack propagation direction also influenced the morphology of the crack front, macroscopically and microscopically, due to differences in the multiplicity of slip. These results suggest that, for a ductile metal, the fatigue cracking behavior depends mostly on hardening and overall dislocation structure, even when the crack is propagating intergranularly.