Tin-induced dynamic embrittlement of Cu–7% Sn bicrystals with a Σ5 boundary

The kinetic model for dynamic embrittlement predicts that the cracking rate is proportional to the diffusivity of the embrittling species along the grain boundary. Diffusion-bonded bicrystals of Cu–7% Sn with a Σ5 (031)/[100] symmetrical tilt boundary were used to test the model and to examine the mechanism of the cracking process. The bicrystals, in which surface-segregated tin was the embrittling species, were tested at 265°C in vacuum parallel and perpendicular to the tilt axis. Cracking occurred parallel to the tilt axis, the fast-diffusion direction, by the propagation of a sharp crack at a maximum rate of ∼2 μ m/s and at a stress intensity of less than 3.5 MPa √m. Cracking appeared to be continuous, suggesting that the tin diffusion occurs in the core of the crack tip. No cracking occurred perpendicular to the tilt axis, i.e. the slow-diffusion direction; here, plastic creep occurred with the formation of cavities at the grain boundaries. The hypothesis of a grain-boundary-diffusion process leading to cracking is supported, and the susceptibility to this type of cracking appears to be extremely sensitive to grain boundary structure.