Hierarchical Length-Scale Influence on Crack Propagation in In-Situ Microlamellar Composites

9974013材料的Dravid断裂提出了一个经典的微观结构-性能相关性问题，其中裂纹扩展受到几乎所有微观结构长度尺度的影响：从界面处复杂的原子键合到裂纹轨迹的几何形状到界面。 陶瓷微层状复合材料（microlaminates）的断裂也不例外，该项目解决了微观结构长度尺度的两个极端之间的复杂相互作用：宏观残余应力和纳米级界面现象。 该研究项目建立在先前的工作基础上，这些工作提供了对界面原子结构的深入了解以及用于分析模型氧化物系统的微层定向凝固共晶（DSE）中的微观结构和残余应力的工具/技术的开发，例如，氧化镍-立方氧化锆。 DSE呈现出两相交替单晶薄片的均匀微观结构（约1-2微米厚），两相之间具有明确的取向关系。 在宏观尺度上，通过改变氧化镍与氧化钴和氧化锰的固溶体系列的热膨胀系数，以及通过改变立方氧化锆的热膨胀系数，可以改变氧化镍-氧化锆系统中DSE中残余应力的符号、大小和位置，也可以改变立方氧化锆中稳定剂（氧化钇和氧化钙）及其在立方相场中的用量。 在合适的温度和时间下，通过在还原气氛下选择性地化学还原氧化镍/氧化钴材料，将实现DSE的纳米尺度定制。然后，部分还原的氧化物DSE将包含夹在脆性掺杂的氧化镍和立方氧化锆相之间的薄（1-50 nm）金属层。微观结构定制方法的成功和有效性将通过广泛的表征进行评估，包括原子尺度成像，光谱学，相对界面能，通过单晶X射线衍射和面向对象的有限元模拟来确定残余应力张量。裂纹扩展行为的现象学在定制的DSE将被探讨，以评估残余应力，界面现象和它们可能的协同作用的影响。 提出了通过原位SEM和TEM应变实现裂纹轨迹的实时、实时和高分辨率观测，这将极大地促进聚焦离子束技术在特定位置微加工方面的新发展。 残余应力，弹性失配，界面结构和晶体学各向异性DSE之间的可能复杂的相互作用将分析的上下文中的裂纹扩展的分析，唯象和理论模型的微层压板。建议的研究预计将提供相当深入的微层压复合材料中的裂纹扩展行为的微观结构的长度尺度的影响，具有相当大的影响，许多其他材料系统，表现出微观结构成分之间的长度尺度的相互作用。 PI与美国和欧洲的研究人员进行了广泛的合作。 参与该项目的学生将有机会在这些合作者的实验室工作。*

9974013DravidFracture of materials presents a classic microstructure-property correlation problem wherein crack propagation is affected by virtually all microstructural length-scales: from intricate atomic bonding at interfaces to the geometry of crack trajectory to the interfaces. Fracture of ceramic microlamellar composites (microlaminates) is no exception, and the this project addresses the complex interplay between two extremes of microstructural length-scales: macroscopic residual stress and nanoscale interfacial phenomena. This research project builds on prior work that provided insight into interface atomic structure and development of tools/techniques for analysis of microstructure and residual stresses in microlamellar directionally solidified eutectics (DSEs) of model oxide systems, e.g., nickel oxide-cubic zirconia. DSEs present a uniform microstructure of alternate single-crystal lamellae of the two phases (~ 1-2 micrometer thick) with well-defined orientation relations between the two phases. At the macroscopic length-scale, the sign, magnitude, and location of residual stresses in DSEs in the nickel oxide-zirconia system ill be altered by changing the thermal expansion coefficient of nickel oxide by forming solid solution series with cobalt oxide and manganese oxide, as well as that of cubic zirconia by using various stabilizers (yttria and calcia) and their amount within the cubic phase field. Nanoscale tailoring of DSEs will be achieved by selective chemical reduction of nickel oxide/cobalt oxide materials under reducing atmosphere at appropriate temperature and duration. The partially reduced oxide DSEs would then contain thin (1-50 nm) metallic layers sandwiched between brittle doped nickel oxide and cubic zirconia phases. The success and efficacy of the microstructural tailoring approach will be evaluated by extensive characterization including atomic-scale imaging, spectroscopy, relative interface energies, to determination of residual stress tensor via single crystal x-ray diffraction and object oriented finite element simulations. The phenomenology of crack propagation behavior across tailored DSEs will be probed to assess the influence of residual stresses, interfacial phenomena and their possible synergy. Real-space, real-time and high resolution observations of crack trajectory via in-situ SEM and TEM straining are proposed that will be greatly facilitated by the new development in site-specific microfabrication by focused ion beam technique. The probable complex interplay among residual stresses, elastic mismatch, interface structure and crystallographic anisotropy in DSEs will be analyzed in the context of analytical, phenomenological and theoretical models of crack propagation across microlaminates. %%%The proposed research is expected to provide considerable insight into the influence of microstructural length-scales on crack propagation behavior in microlaminate composites, with considerable implications for many other materials systems that exhibit length-scale interplay among microstructural constituents. The PI has extensive collaborations with researchers both here in the US and in Europe. The students working on this project will have a chance to work in the laboratories of these collaborators.***