Diffusion-Controlled Brittle Fracture of Bicrystals

小行星9634718 本文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中扩散控制的沿晶脆性断裂机制进行实验研究。 以前的研究已经进行了多晶材料，这些遭受的限制，排除了定量测试的模型提出的这种现象，我们认为这是一个独特的和通用的形式的脆性断裂。 使用双晶的原因如下：首先，开裂速率应该与脆化元素的晶界扩散率成比例（假设在施加应力的影响下从自由表面扩散，如扩散蠕变）。 在双晶体中，扩散率在给定方向上可以保持恒定，并且扩散方向可以变化，以便获得固定且可测量的扩散率比率，从而获得裂纹生长速率。 第二，断裂的双晶体将呈现大的平坦断裂表面，在该断裂表面上，高分辨率扫描电子显微镜和原子力显微镜测量可以在可能的逐步过程中对开裂的间隔进行。 第三，裂纹尖端应力场应沿着裂纹尖端和沿着裂纹路径是恒定的，至少在试样的中心部分是恒定的;正是这种应力场被假定为驱动扩散，从而通过脱粘驱动裂纹扩展。 这些研究将从根本上建立扩散控制的沿晶脆性断裂机制，这可能是一种常见的，但未被识别的形式，在工程材料的故障。 ***

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. ***