Diffusion-Controlled Brittle Fracture of Bicrystals

本文提出利用对称倾斜边界双晶对模型铁铜基合金扩散控制的晶间脆性断裂机制进行实验研究。先前的研究已经在多晶材料上进行，这些研究受到限制，无法对这种现象提出的模型进行定量测试，我们认为这是一种独特而通用的脆性断裂形式。使用双晶的原因如下：首先，裂纹速率应该与脆化元素的晶界扩散率成正比（假设在施加应力的影响下从自由表面扩散，如扩散蠕变）。在双晶中，扩散率可以在给定的方向上保持恒定，而扩散方向可以改变，从而得到一个固定的、可测量的扩散率比率，从而得到裂纹生长速率。其次，断裂的双晶将呈现一个大而平坦的断裂表面，高分辨率的扫描电子显微镜和原子力显微镜可以测量断裂的间隔，这可能是一个循序渐进的过程。裂纹尖端应力场应沿裂纹尖端和裂纹路径保持恒定，至少在试件中心部分保持恒定；假定正是这个应力场驱动了裂纹的扩散，从而推动了裂纹的退聚扩展。这些研究将以一种基本的方式建立扩散控制的晶间脆性断裂机制，这可能是工程材料中常见的，但尚未认识到的破坏形式。＊＊＊

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. ***