Tin-induced dynamic embrittlement of Cu-7% Sn bicrystals with a Sigma 5 boundary

The kinetic model for dynamic embrittlement predicts that the cracking rate is proportional to the diffusivity of the embrittling species along the gr ain boundary. Diffusion-bonded bicrystals of Cu-7% Sn with a Sigma 5 (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-seg regated tin was the embrittling species, were tested at 265 degrees C in va cuum parallel and perpendicular to the tilt axis. Cracking occurred paralle l to the tilt axis, the fast-diffusion direction, by the propagation of a s harp crack at a maximum rate of similar to 2 mu m/ s and at a stress intens ity of less than 3.5 MPa root m. Cracking appeared to be continuous, sugges ting that the tin diffusion occurs in the core of the crack tip. No crackin g occurred perpendicular to the tilt axis, i.e. the slow-diffusion directio n; here, plastic creep occurred with the formation of cavities at the grain boundaries. The hypothesis of a grain-boundary-diffusion process leading t o cracking is supported, and the susceptibility to this type of cracking ap pears to be extremely sensitive to grain boundary structure. (C) 1999 Publi shed by Elsevier Science Ltd. On behalf of Acta Metallurgica Inc. All right s reserved.