A new type of brittle fracture in a fcc metal bicrystal with intergranular segregation
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I. INTRODUCTION Fracture in polycrystalline solids is usually characterized as transgranular or intergranular. Transgranular fracture may occur in a ductile manner with dimple structure on the fracture surfaces or in a brittle manner as cleavage, quasicleavage, or fatigue fracture. Intergranular fracture is usually observed in environmentally assisted cracking, segregation embrittlement, and creep failure. At ordinary temperatures intergranular fracture usually occurs when grain boundary segregation or adsorption lowers the cohesive energy at the interface, or structure-related slip localization occurs along the interface; it may also result from the environmental effect of oxidized film formation and rupture at the crack tip. Intergranular fracture is usually a process with a lowenergy release rate. Basically, there are two types of intergranular fracture if creep deformation is not involved. One is the tensile decohesion of weakened interatomic bonds on the interface, which produces very smooth fracture surfaces along the grain boundary; dislocation slip is presumably not directly involved in the separation process but may occur profusely in the stressed crack tip region. Another is accomplished by localized plastic flow adjacent to the grain boundary, which involves growth of voids ahead of the crack that have nucleated from preferred sites such as inclusion locations along the interface. The fracture surfaces may be rougher with a microscopically dimpled structure or may appear to be featureless in scanning electron microscopy (SEM) if the scale of the 16
J. Mater. Res. 3 (1), Jan/Feb 1988
http://journals.cambridge.org
shear localization is very small. A mixture of the two types of fractures is possible, which produces fracture surfaces with a mixture of fine scale dimples and decohesion steps. It had been reported 40 years ago that grain boundary segregation of Bi causes intergranular fracture in Cu polycrystalline specimens.' The brittleness was believed due simply to reduced grain boundary cohesion.2 The fracture was found to follow faceted boundaries, and little plastic deformation accompanies fracture, suggesting a tensile-decohesion process of the segregated grain boundaries.3 However, Biggin4 found that even in the embrittled specimen ductile fracture may occur in some regions, and he claimed that high deformation on at least one side of the boundary was necessary for cracking. In a recent work Fraczkiewicz and Biscondi5 showed that Bi segregation also induced intergranular fracture in Cu-Bi bicrystals with [100] tilt symmetrical grain boundaries when the Bi concentration in the bulk is supersaturated. The fracture surfaces were featureless, in that no faceting was observed with a SEM, although the grain boundary surfaces were not always flat. Experimental studies by Russell and Winter6 showed that the intergranular fracture processes in CuBi bicrystals depend strongly on the type and structure of the interfaces. The fracture surfaces of a high 2 number ( 2 is the reciprocal of the coincidence lattice
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