EAGER: Controlling Microstructure for Strong and Damage Tolerant Nanocrystalline Metals

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Nanocrystalline metals and alloys (polycrystals with grain sizes less than ~100 nm) offer a suite of appealing mechanical properties for structural applications, including high strength and hardness, enhanced fatigue resistance, and tribological robustness. These virtues derive from the large fraction of material that resides at the interfaces between neighboring crystals, known as grain boundaries. For these materials, this high fraction of interfacial volume can cause deleterious effects such as thermal instability and relatively poor damage tolerance. Many present and future applications of nanocrystalline metals such as robust coatings, electrical interconnects, micro- and nano-electro-mechanical systems, and soft magnets subject these materials to extreme mechanical duress, which can activate microstructural transformation and alter the beneficial materials properties. This EArly-concept Grant for Exploratory Research (EAGER) award supports research centered on the concept that control of grain boundary chemistry in nanocrystalline alloys can be used to tailor the thermal and mechanical stability of nanocrystalline materials against grain boundary migration in extreme service environments. Control over this behavior can allow for unprecedented control of damage tolerance, thus enabling a novel and inexpensive structural materials design strategy.In this research program, the investigators aim to control grain boundary chemistry in nanocrystalline alloys as a means to encode the onset of thermally- and mechanically-driven grain boundary migration under service conditions. In cases where extreme mechanical environments are encountered (e.g. at stress concentrations such as crack tips), stress-driven grain boundary migration can be triggered to respond to damage, endowing the material with damage tolerance. This dynamic material response is predicated on local stress triggers that drive microstructure transition and dissipate energy to mitigate catastrophic failure, allowing for both strength and toughness. The research will be accomplished via the following scientific and technical goals: (a) identify and characterize the mechanisms that lead to mechanically-induced grain boundary migration and grain growth, (b) identify and characterize the manner in which these mechanisms are influenced by grain boundary chemistry, (c) identify elements that will segregate to grain boundary and modulate thermal and stress-driven grain boundary migration, (d) synthesize nanocrystalline alloys with tailored grain boundary chemistry, and (e) perform material characterization and quantitative in situ mechanical testing.