9634718 McMahon It is proposed to carry out an experimental study of the mechanisms of diffusion-controlled intergranular brittle fracture in model iron-and copper-based alloys using bicrystals with symmetrical tilt boundaries. Previous research has been carried out on polycrystalline materials, and these suffer from limitations that preclude a quantitative test of the model proposed for this phenomenon, which we believe is a unique and generic form of brittle fracture. The reasons for using bicrystals are the following: First, the cracking rate should be proportional to the grain-boundary diffusivity of the embrittling element (which is hypothesized to diffuse in from a free surface under the influence of an applied stress, as in diffusion creep). In a bicrystal the diffusivity can be held constant in a given direction, and the diffusion direction can be varied so as to obtain a fixed and measurable ratio of diffusivities, and therefore crack-growth rates. Second, the fractured bicrystal will present a large flat fracture surface on which high-resolution scanning electron microscopy and atomic-force microscopy measurements can be made of the intervals of cracking in what is likely a step- wise process. Third, the crack-tip stress field should be constant along the crack tip and along the crack path, at least in the central portion of the specimen; it is this stress field that is postulated to drive the diffusion and therefore the crack extension by decohesion. %%% These research will establish in a fundamental way the mechanisms of diffusion-controlled intergranular brittle fracture, which could be a common, but unrecognized form of failure in engineering materials. ***
