Strength (ability to bear mechanical load) and toughness (ability to resist fracture) are desired in civil and transportation infrastructure, manufacturing and processing, and bio-medical implants to make them stronger, safer, and lighter and energy efficient. Unfortunately, strength and toughness in metals are mutually exclusive - a fundamental challenge that remains to be resolved because the mechanism that controls strength in metals is inversely coupled with toughness. This research project aims to introduce a new type of mechanism, where atomic scale vacancies interact with existing defects to induce decoupling of strength and toughness. Such vacancy-defect interaction is not well understood because in conventional metals, vacancies do not play dominant roles. In addition, these mechanisms are strongly influenced by the internal structure and existing defect distribution in the metal as well as temperature. The experiments will be performed inside high-resolution microscopes, so the role of different deformation mechanisms can be seen in real time as strength and toughness are controlled and measured. The scientific outcome of this research will be new knowledge in atomic scale vacancy - defect interactions, which will be demonstrated through technological innovation in metals that are both strong and tough. Graduate and undergraduate students will be trained in crosscutting disciplines of metallurgy, mechanics, nanofabrication and microscopy to solve this highly coupled research problem. Academic outreach activities will be performed to attract the next generation workforce towards materials science & engineering.
2. Technical Abstract
Strength of metals originates from their capability of suppressing dislocation motion or plastic deformation. Toughness, on the other hand, requires large amount of plastic work before fracture. Because of such conflict, they are mutually exclusive. The current paradigm is to optimize, or in other words, compromise between strength and toughness. Contrarily, this research aims to maximize strength and toughness through synergistic multi-scale defect interactions (0D: vacancy, 1D: dislocations and 2D: grain/twin boundaries). Here, dislocation confinement in nano-crystalline metals is exploited to achieve high strength. At the same time, point defects (vacancies) are generated at ambient conditions, which facilitate diffusion-based plasticity without unlocking the dislocations. The net result is the unique co-existence of suppressed dislocation activities and pronounced diffusional plasticity without any change in grain size. This research is based on the hypothesis that it is possible to have both dislocation-based strength and diffusion-based deformability in the same grain at the same time and at ambient temperature. A unique material processing technique with temperature-current-stress synergy is proposed to generate the required vacancy concentration. This leads to a transformative concept; a three-way interaction among three dimensions of defects to maximize both strength and toughness. The nanoscale grain boundaries will lead to very high strength by impeding dislocation motion. The lack of ductility arising from small grains is removed by vacancies through diffusional plasticity in the grain boundaries. Finally, scalability of the proposed concept in meso and macroscopic manufacturing processing will be studied for feasibility in real life applications.
