Applicability of bond percolation theory to intergranular stress-corrosion cracking of sensitized AlSl 304 stainless ste
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INTRODUCTION
P H E N O M E N A that promote intergranular (IG) cracking of polycrystalline metals and alloys occur in a wide range of alloy/environment systems. Often, establishment of an IG crack path is a result of the segregation of detrimental impurities to the grain boundaries or removal of beneficial elements from the grain boundaries. Relevant phenomena include temper-, hydrogen-, and graphite-induced embrittlement, tl-91 The intergranular stress-corrosion cracking (IGSCC) of A1-Cu alloys results from the preferential anodic dissolution of Cu-depleted regions between the AI2Cu precipitates at the grain boundaries, twl Similarly, stainless steel becomes susceptible to IGSCC because of carbide precipitation and Cr depletion at the grain boundaries, m] The precise failure mechanisms associated with each of these examples may differ, but common to all cases is grain boundary separation resulting from a compositional difference between the grain boundary and bulk material. For the previously mentioned alloy/environment systems, cracking can occur by a mixture of intergranular and transgranular separation, with the former occurring along the embrittled or "active" grain boundaries only. In the case of equiaxed grains, multiple grain boundaries may be suitably oriented with respect to the principal applied stress so as to satisfy whatever mechanical criteria are necessary for separation. The quantity of embrittled or active grain boundaries will influence the material's performance in a particular environment, regardless of the mechanism of embrittlement. The number of active grain boundaries that exists will determine
M.A. GAUDETT, Graduate Research Assistant, and J.R. SCULLY,. Assistant Professor, are with the Department of Materials Science and Engineering, University of. Virginia, Charlottesville, VA 22903. Manuscript submitted June 23, 1993. METALLURGICAL AND MATERIALS TRANSACTIONS A
whether an IG crack can continue without an alteration of fracture path through a structure. The repetitive linkage of successive active grain boundaries or "active bonds" to form a connected path through a material is a bond percolation phenomenon; t121there is a critical percentage (percolation threshold) of active grain boundaries below which a connected path will not exist. Therefore, it is useful to characterize this threshold for a network of grain boundaries in a polycrystalline material if failure by IG separation is to be better understood. An array of equiaxed grain boundary facets can be represented by a three-dimensional network made up of bonds along the grain boundaries, t13J The shapes of actual grains are by no means regular (there is a distribution of grain sizes), but an array of grains can be represented by identical, space-filling objects of an average size. One such object is Kelvin's tetrakaidecahedron, a space-filling body of minimum interfacial area, consisting of eight hexagonal faces and six square faces. [141 Each of the grain boundaries (bonds) can be active (sensitized, in the case of IGSCC of s
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