Fatigue fracture at copper bicrystal interfaces: fractography

The fracture surfaces of intergranular cracks in copper bicrystals with dif ferent misorientations, subjected to different mechanical pretreatments and cracking directions, were studied by scanning electron microscopy (SEM). T he dislocation structures present before the crack formed and propagated we re characterized by transmission electron microscopy (TEM). It was found th at they could be either loop patches and persistent slip bands (PSB's) (sin gle slip) or maze and cell structures (multiple slip) depending on the miso rientation and the pretreatment. The results showed that these structures i nfluenced the shape of the crack front, which could be aligned close to spe cific crystallographic directions or straight depending on the prior struct ure. 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 dislocat ion structure, even when the crack is propagating intergranularly. (C) 1999 Elsevier Science S.A. All rights reserved.