Nanocrystalline (nc) materials have high strength but usually low ductility, which has been a major issue for use in structural applications. Deformation twinning has been shown to be effective in simultaneously improving strength and ductility, thus it is critical to understand the mechanisms of deformation twinning so that design nc materials can be designed for superior mechanical properties. However, there is no theoretical model to describe all of the experimental observations on deformation twinning in nc materials, in large part because several factors are not well understood. These include the mechanisms governing the effects of grain size and generalized planar fault energies (GPFEs) on the nucleation of deformation twins and stacking faults and the effect of GPFEs on the spacing of twin boundaries. These issues constitute the basis of the proposed research. Experiments will be done on nanocrystalline fcc Al, Ni, and Cu films with controlled grain sizes and very narrow grain size distributions synthesized using pulsed laser deposition. The films will be deformed by nanoindentation, and subsequently investigated by high-resolution transmission electron microscopy to obtain statistical information on deformation twins and stacking faults as a function of the deformation variables. Nanocrystalline Al, Ni and Cu samples processed by surface mechanical attrition, which produces a range of nano-grain sizes from the sample surface to a depth of several tens of micrometers, will also be investigated. The fundamental mechanisms derived from these experiments will provide a basis for future modeling that can take into account the effects of both grain size and GPFEs.
NON-TECHNICAL SUMMARY: This project will provide insights into the physics of nanomaterials and fundamental knowledge for guiding the design of tough and strong nanomaterials. This is critical for industrial applications such as medical implants, aerospace structures and energy efficient transportation vehicles. The project addresses training of graduate and undergraduate students at NCSU, and collaborations with NC A&T State University to attract minority students into the graduate program at NCSU. The outreach will also involve the annual ASM Materials Camp for High-School seniors and a new Science Saturday Program for high-school and middle school students. These students will be exposed to recent developments in new materials, nanoscale experimental techniques, and correlations with materials properties. The PIs have an established record of research collaboration and supervision of graduate and undergraduate students from NC A&T (a minority institution). The PIs teach a series of courses, through distance education, to students from this minority institution.
Nanocrystalline materials have high strength but usually low ductility, which has been a major issue for their structural applications. Deformation twinning has been shown to be effective in simultaneously improving strength and ductility. Therefore, it is critical to understand the mechanisms of deformation twinning so as to design nc materials for superior mechanical properties. However, our understanding is still limited. In this project, we have studied the grain size effect and alloying element effect on the deformation twinning. It is found that there is an optimum grain size range for deformation twinning. The grain sizes should be made to fall in the range of optimum grain size so that the alloy can produce the best mechanical properties. Adding alloy element is also found to affect the optimum grain size, but this effect is weaker than predicted by the stacking fault effect, which is caused by the fact that the alloy element affect the stacking fault energy and twin fault energy in a different way. We also probed the mechanisms that produce zero-strain twinning, a unique feature for nanocrystalline metals. We have gained enough fundamental understanding on the deformation mechanisms in nanocrystalline metals that we are now ready to develop a comprehensive analytical to model to take into account of the grain size effect, the alloying effect and the generalized planar fault energy effect. The outcome from this project will provide guidance for the design of tough and strong nanomaterials for industrial applications such as medical implants, aerospace structures and energy efficient transportation vehicles.
