Hydrogen-induced Material Degradation: Brittle Decohesion Versus Plastic Flow Localization

拟议工作的主要目标是使用完全集成的、结合应用力学/建模和材料科学/微结构的方法，以定量的方式查明导致模型和与工业相关的工程材料氢致降解的基本机制。该方法将定量、大规模的数值模拟与新颖的实验和现场成像相结合，以确定局部断裂事件中涉及的显著物理微观机制和特征微结构尺度。由于这种断裂事件是随机的，所以分析本质上是统计的，针对真实的微观结构而制定的，并基于可操作的物理微观机制。这项工作的主要成果将是为延性和脆性金属材料中与氢有关的断裂建立基于物理的工程标准，其中氢降解的主要机制，特别是脱粘和剪切局部化是活跃的。由于它们的工业意义，这些材料包括低强度钢和超高强度钢以及Ni3Al高温金属间化合物。第二个结果是对学生进行材料科学方面的培训，但这是在基于力学的材料行为分析的背景下进行的。从事该项目的研究生和本科生将接触到材料科学和应用力学文化；因此，他们将获得现代断裂研究人员所需的基本能力。由于这项研究主要集中在具有实际工程意义的结构材料上，学生们正在以跨学科的方式接受关于结构工程主体的教育。达到这些目标将为许多与氢脆相关的紧迫科学问题提供答案。这包括(I)脆性脱粘与氢辅助剪切局部化机制的相对重要性，(Ii)氢增强局域塑性促进局域断裂的过程，(Iii)平衡与非平衡脱粘理论的相关性，以及(Iv)溶质杂质在不同材料微观结构中的作用和潜在的协同作用。这项工作还对材料在恶劣环境条件下的一般操作产生了重大的技术影响。此外，它代表了一种能够在任何环境中成功应用先进材料(如金属间化合物)的技术，因为人们对这些材料中的环境辅助局部断裂事件的标准知之甚少。

The prime objective of the proposed work is to discern in a quantitative fashion the fundamental mechanisms responsible for the hydrogen-induced degradation of both model and industrially-relevant engineering materials using a fully integrated, combined applied mechanics/modeling and materials science/microstructural approach. The approach combines quantitative, large-scale numerical simulations with novel experimentation and in situ imaging to identify the salient physical micro-mechanisms and characteristic microstructural size-scales involved in the local fracture events. Since such fracture events are stochastic, the analysis is statistical in nature, formulated for real microstructures and based on the operative physical micro-mechanisms. The principal outcome of this work will be the establishment of physically based engineering criteria for hydrogen-related fracture in both ductile and brittle metallic materials, where the primary mechanisms of hydrogen degradation, specifically decohesion and shear localization, are active. Because of their industrial significance, these materials include low and ultrahigh strength steels and a Ni3Al high-temperature intermetallic. A second outcome will be the training of students in materials science, but in the context of mechanics-based analyses of material behavior. Graduate and undergraduate students who work on the project are exposed to both the materials science and applied mechanics cultures; thereby, they will acquire an essential capability required of modern researchers in fracture. Since the study focuses largely on structural materials of real engineering significance, students are being educated in an interdisciplinary way on the mainstay of structural engineering.%%%Meeting these goals provides answers to numerous pressing scientific issues associated with hydrogen embrittlement. These include (i) the relative significance of brittle decohesion versus hydrogen-assisted shear localization mechanisms, (ii) the process by which hydrogen enhanced localized plasticity promotes localized fracture, (iii) the relevance of equilibrium vs. non-equilibrium decohesion theories, and (iv) the role, and potential synergism, of solute impurities in varying material microstructures. The work also has a significant technological impact on the general operation of materials under severe environmental conditions. Moreover, it represents an enabling technology for the successful application of advanced materials such as intermetallics in any environment, since so little is known about the criteria for environmentally assisted local fracture events in these materials.