Investigation of fundamental creep behavior and mechanisms in a thermally stable nanocrystalline alloy

非技术摘要：依赖发电的工业使用的结构材料通常经过加工，以减少小长度尺度上存在的材料缺陷量。这种方法相当成功；然而，它正在达到可以实现的极限，而新的苛刻应用需要更高的材料性能。该奖项支持基础研究，通过使用相反的方法来寻找下一代用于高温结构应用的高性能材料，即增加小尺寸（包括原子）长度尺度上的缺陷数量，从而阻碍而不是增强这些缺陷在高温下的变形。这需要完全了解成分效应和加工条件，这些条件可以产生所需的高密度缺陷，并确保它们在高温加载期间保持不变，因为高温往往会减少材料中的缺陷数量。在具有如此多缺陷的材料中，控制加工、结构和机械性能之间关系的物理原理尚未得到很好的理解；因此，本项目将通过实验和建模相结合的方式对它们进行研究，以产生结构材料领域的新科学知识。此外，本科和研究生阶段的理工科学生将参与研究，并接受最先进技术的培训，以处理、测试、表征和模拟结构材料的行为，帮助教育该领域的新一代工程师和科学家。最后，了解如何生产新一代具有比现有材料更好性能的高温应用结构材料的潜力可以对社会产生重大影响和效益。技术摘要：与粗晶合金相比，平均晶粒尺寸小于100纳米的金属和合金具有更高的静态强度，因此在工程应用中非常有吸引力。然而，很少有研究解决了时间依赖变形过程中微观结构演变的基本机制，限制了它们的实际应用。这一关键的知识缺口源于各种实验条件（加工方法、测试条件等）与变形过程中观察到的晶粒生长缺乏一致性。这种不确定的理解阻碍了我们解决当前在许多重要技术中对先进承重结构材料的需求，这些技术中蠕变载荷很常见（例如，发电、推进）。基于此，将被验证的假设是Ni- y -Zr是一个稳定的双相体系，其中晶界偏析（Zr在Ni中的固溶性）和相形成（例如Ni- y沉淀）都会发生，从而导致微观结构稳定的纳米晶合金。所得材料的样品将在各种载荷和温度下进行测试和表征，揭示稳定纳米晶材料蠕变的真实蠕变行为和微观结构影响的相关机制。此外，这些机制的识别也将增强我们对纳米晶体材料在相对低温下随时间变形过程中常见的微观结构不稳定性的理解。在更广泛的背景下，提出的工作将奠定基础，通过协调建模和跨多个长度尺度的实验工作来解决迭代材料设计中的差距。该奖项反映了美国国家科学基金会的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

Non-technical abstract:Structural materials used by power generation-reliant industries are often processed such that the amount of material imperfections present at small length scales is reduced. This approach has been quite successful; however, it is reaching the limits of what can be achieved, while new demanding applications require even higher material performance. This award supports fundamental research to look for the next generation of high performance materials for high temperature structural applications by using the opposite approach, i.e., to increase the number of imperfections at small, including atomic, length scales in such a way that deformation at high temperatures is impeded rather than enhanced by these imperfections. This requires a complete understanding of composition effects and processing conditions that can produce the high density of imperfections required and to make sure that they are retained during high temperature loading, since high temperatures tend to decrease the number of imperfections in a material. The physical principles that govern the relationship among processing, structure and mechanical properties in materials with such high number of defects as those proposed here are not well understood; hence, they will be studied in this project through a combination of experiments and modeling to produce new scientific knowledge in the field of structural materials. Furthermore, students in science and engineering at both undergraduate and graduate levels will participate in the research and be trained in state-of-the-art techniques to process, test, characterize and model structural material behavior, helping to educate the new generation of engineers and scientists in this field. Finally, the potential to understand how to produce a new generation of structural materials for high temperature applications with better properties than those currently available can have significant impacts and benefits to society. Technical abstract:Metal and alloys with a mean grain size below 100 nm have enhanced static strength compared to coarse-grained alloys and, hence, are very attractive for engineering applications. However, very few studies have addressed the fundamental mechanisms of microstructural evolution during time dependent deformation, limiting their practical use. This critical gap in knowledge has stemmed from the lack of consistency among various experimental conditions (processing methods, testing conditions, etc.) along with the grain growth observed during deformation. This indeterminate understanding hinders our ability to address the current need for advanced load-bearing structural materials in many important technologies where creep loading is common (e.g., power generation, propulsion). Motivated by this, the hypothesis that will be tested is that Ni-Y-Zr is a stable duplex system, wherein both grain boundary segregation (some solid solubility of Zr in Ni) and phase formation (e.g., Ni-Y precipitates) occur, leading to a microstructurally stable nanocrystalline alloy. Samples from the resulting material will be tested and characterized under a variety of loads and temperatures, unraveling true creep behavior and pertaining mechanisms of microstructural effects on creep of stable nanocrystalline materials. Furthermore, identification of these mechanisms will also enhance our understanding of microstructural instability commonly observed in nanocrystal materials at relatively low temperatures during time dependent deformation. In a broader context, the proposed work will lay the groundwork to address gaps in iterative materials design through coordinated modeling and experimental efforts across multiple length scales.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.