A new type of brittle fracture in a fcc metal bicrystal with intergranular segregation

Abstract

A new type of brittle fracture was discovered in a Cu–Bi alloy bicrystal with a random misorientation interface. Macroscopically, fracture occurred along the interface with a low-energy release rate of order 50 Jm₋₂. Microscopically, the crack apparently propagated in a zigzag manner such that a cleavagelike fracture was produced, which consists of two sets of smooth {110} facets and one set of {100} facets with serrations corresponding to their intersections with {111} slip planes. Facet sizes are typically of order 1 by 5 μm or smaller. The very smooth facets and low-energy absorption suggest that a decohesion separation process has occurred along {110} planes, and the serrations suggest that substantial dislocation slip accompanied fracture of the {100} facets. The fracture surface morphology may be explained as the result of a similar morphology of Bi-induced microscopic faceting of the bicrystal interface or, instead, may result from fracture propagating in the vicinity of, but not quite along, the interface. Evidence is presented that favors the latter so that this is a rapid, cleavagelike, semiintergranular fracture that has not been observed in fee metals or alloys when an environment effect is not involved. Its faceted features resemble those of transgranular stress corrosion cracks in certain fee metals.