Diffusion-Controlled Brittle Fracture of Bicrystals

9634718麦克马洪提出了利用具有对称倾斜边界的双晶对模型铁基和铜基合金进行扩散控制沿晶脆断机制的实验研究。以前的研究是在多晶材料上进行的，这些材料受到限制，无法对为这一现象提出的模型进行定量测试，我们认为这是一种独特的通用脆性断裂形式。使用双晶体的原因如下：首先，裂纹率应该与脆化元素的晶界扩散系数成正比(假设脆化元素在施加应力的影响下从自由表面扩散进来，如扩散蠕变)。在双晶中，扩散系数可以在给定的方向上保持恒定，并且可以改变扩散方向，以获得固定的和可测量的扩散系数比率，从而获得裂纹扩展速率。其次，断裂的双晶将呈现一个巨大的平坦的断裂面，在上面可以用高分辨率扫描电子显微镜和原子力显微镜测量裂纹的间隔，这可能是一个逐步的过程。第三，沿着裂纹尖端和裂纹路径，至少在试件的中心部分，裂纹尖端应力场应该是恒定的；正是这种应力场被假定为驱动扩散，从而通过退粘来驱动裂纹扩展。这些研究将从根本上建立扩散控制的沿晶脆性断裂机制，这可能是工程材料中一种常见但未被认识的失效形式。***

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. ***