Intrinsic Mechanical Properties of Nanocrystalline Metals

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This renewal award to Johns Hopkins University will support Professor Ma to study new type of bulk nanocrystalline materials prepared by a mechanical attrition process. The dynamically recrystallized nanocrystalline surface layer or in situ consolidated samples would be close to being "ideal" for mechanistic studies of nanocrystalline deformation behavior. Emphasis of this award will be on mechanical property tests with these nanocrystalline materials at different strain rates (including stress relaxation and jump tests) and at different temperatures. There is strong and growing interest in uncovering the novel mechanical behavior of nanocrystalline materials. Molecular dynamics and computer simulations revealed that when the grain size of a metal is brought down to less than 30 nm, the conventional dislocation mechanism is no longer dominant and several unusual deformation mechanisms associated with the large fraction of atoms residing at the abundant grain boundaries would take over. These mechanisms have not been well established experimentally so far. With this award, key parameters such as activation volume, strain rate sensitivity, and temperature dependence would be obtained to conclude on the rate-controlling deformation mechanisms in real nanocrystalline metals. Results from this research would have significant scientific impact, as they would provide much-needed insight into the fundamental structure-property relationship in the nanocrystalline regime. Significant broader impact is also expected because the proposed studies would build the knowledge base that will enable nanomaterials to be used in the technologies of the new century that demand high performances. Students will receive systematic training on these novel materials, and results will be disseminated by the University web site. Another component with broader impact would be the international exchange that this project is expected to foster.