Dynamic embrittlement - time-dependent quasi-brittle intergranular fracture at high temperatures

With the focus on intergranular crack propagation of Ni-base superalloys at high temperatures, a failure mechanism is discussed that seems to be due to a generic phenomenon: dynamic embrittlement. Under the influence of a tensile stress, an embrittling species is moved into the grain boundaries by grain-boundary diffusion, giving rise to local decohesion ahead of an intergranular crack tip. A very similar crack propagation behaviour was observed for S-induced intergranular cracking of low-alloy steels, Cu-Cr alloys, and for Sn-induced cracking of Cu-Sn model alloys, where the embrittling element coming from the bulk segregates at the grain boundary, as well as for O-induced cracking of Cu-Be and Ni-base alloys, where the embrittling element comes from the atmosphere. A qualitative steady-state model of the dynamic embrittlement mechanism suggests that the dynamic embrittlement damage zone lies in the nanometre range ahead of the crack tip. Mechanical testing of grain-boundary-engineering-type processed and bicrystalline specimens revealed strong interactions between the crystallographic misorientation relationship and the dynamic embrittlement mechanism, whereby special grain boundaries with a high fraction of coincident lattice sites seem to have a particularly high resistance against quasi-brittle intergranular fracture by dynamic embrittlement.