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

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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.