Collaborative Research: Fundamental Investigation of Fatigue Crack Growth Mechanisms in Microstructurally-Stable Nanocrystalline Alloys

微晶尺寸小于100纳米的金属材料（称为纳米晶合金）与具有大得多的晶体尺寸的金属材料相比具有增强的强度和韧性。然而，当暴露于循环外力时，这些晶体倾向于快速生长，限制了它们作为承重结构的实际用途。该奖项支持基础研究，通过应用创新的合金设计和合成方法，提高纳米晶合金的稳定性，并揭示其在循环载荷条件下的行为。此外，该项目将开发一个预测框架，该框架将确定新型纳米晶合金的合适成分，这些合金将在循环载荷下达到或超过传统合金的性能，包括对运输，国防和能源部门至关重要的旋转机械和发动机部件。这一跨学科研究将为少数民族学生、妇女和残疾人提供机会，并将通过创造性的外联活动直接提高公众对科学和工程的兴趣。平均晶粒尺寸低于100 nm的金属由于其上级机械性能而激发了许多研究兴趣。然而，尽管纳米晶合金的承载结构的潜在适用性，很少有研究已经解决了疲劳载荷下的基本微观结构演变，特别是晶粒尺寸对疲劳过程的影响。知识方面的关键差距源于微观结构的不稳定性，即，在循环变形过程中观察到的晶粒生长和织构演变。基于此，本项目的目标是：（1）实现纳米团簇的选择性偏析，以降低晶粒生长的热力学和动力学驱动力，从而可以清楚地了解真实的微观组织晶粒尺寸对显著疲劳生长机制的影响;（2）利用后续知识来协调与纳米晶金属疲劳裂纹扩展行为有关的关键理论和假设。在该项目中量化的作为晶粒尺寸的函数的疲劳裂纹扩展机制具有广泛的科学分支，因为这些知识对于设计和合成具有最佳微观结构的纳米晶合金以及精确的预测模拟具有高度相关性。

Metallic materials with a crystallite size of less than 100 nanometers (known as nanocrystalline alloys) have enhanced strength and toughness compared to those with much larger crystal sizes. These crystals tend to grow rapidly, however, when exposed to cyclic external forces, limiting their practical utility as load-bearing structures. This award supports fundamental research to enable enhanced stability in nanocrystalline alloys and to uncover their behavior under cyclic loading conditions, by applying innovative alloy design and synthesis methods. Further, this project will develop a predictive framework which will identify suitable compositions of novel nanocrystalline alloys that will meet or exceed the performance of conventional alloys under cyclic loads, including rotating machinery and engine components critical to the transportation, defense and energy sectors. This interdisciplinary research will provide opportunities for students from underrepresented minority groups, women, and persons with disabilities, and will directly raise public interest in science and engineering through creative outreach initiatives.Metals with a mean grain size below 100 nm have inspired much research interest owing to their superior mechanical properties as compared to course-grained materials. However, despite the potential applicability of nanocrystalline alloys for load-bearing structures, very few studies have addressed the fundamental microstructural evolution under fatigue loads, specifically, grain size effects on fatigue processes. The critical gap in knowledge has stemmed from microstructural instability, i.e., grain growth and texture evolution observed during cyclic deformation. Motivated by this, the goals of this project are: (1) implementation of selective segregation of nanoclusters to lower the thermodynamic and kinetic driving forces for grain growth, and as a result, a clear understanding of the true microstructural grain size effects on salient fatigue growth mechanisms can be unraveled; and (2) to exploit the subsequent knowledge to reconcile key theories and hypotheses pertaining to fatigue crack growth behavior of nanocrystalline metals. The fatigue crack growth mechanisms as a function of grain size quantified in this project have broad scientific ramifications as this knowledge is highly relevant for designing and synthesizing nanocrystalline alloys with optimal microstructures, and for accurate predictive simulations.