TECHNICAL: Nanocrystalline materials have yet to realize their potential as engineering materials because of processing limitations. The challenge is to develop large-scale processing methods that can produce bulk nanostructured metals and alloys free of processing defects. The objective of the research is to develop strategies to stabilize nanoscale microstructures during consolidation of powder particulates at elevated temperatures. This is motivated by the fact that processing methods which can be scaled up for large volume production of nanoscale materials typically start out with powder particulates that must be consolidated into bulk form. These methods include mechanical attrition of powders that make large-size particulates with an internal nanocrystalline grain structure or chemical reactions that produce nanoscale powder particles. These methods have great versatility in producing a variety of alloy and multiphase systems. Their major drawback is the need to consolidate the powders into bulk form, attaining theoretical density and complete interparticle bonding, without significantly coarsening the nanoscale microstructure. The experimental approach to be used in this research emphasizes a systematic study of grain growth and the kinetic and thermodynamic factors that influence it in selected metals alloy prepared by mechanical attrition. Two model systems based on bcc Fe and fcc Ni will be selected for the research. Grain growth studies have shown fundamentally different behavior for nanocrystalline samples of these two metals, with the activation energies for grain growth being close to either lattice diffusion (Fe) or grain boundary diffusion (Ni). The possibility to stabilize nanocrystalline microstructures is explored by using alloy additions to reduce the grain boundary energy (thermodynamic basis) or limit the grain boundary mobility by pinning (kinetic basis). Equilibrium soluble and immiscible elements and second-phase oxide dispersion additives are added to the base metals to study the effectiveness of their ability to stabilize the microstructure. Guided by the grain growth studies, consolidation of powders will be carried out by sinter forging. The effectiveness of the consolidation processes is investigated using mechanical property tests suited to laboratory-scale sample sizes. The ductility and fracture surfaces should reveal the presence of processing defects. Analysis and simulation modeling is conducted to identify and optimize the thermodynamic and kinetic mechanisms that inhibit grain growth. NONTECHNICAL: The educational impact of the proposed research includes participation in the Kenan Fellows for Curriculum and Leadership Development program at North Carolina State University. A principal investigator acts as a mentor for a local K-12 teacher who serves a two-year fellowship in which he/she will carry out research and bring up-to-date knowledge of science, engineering, and technology into the classroom. Undergraduate students will share in research experience through participation in the REU program sponsored by the NSF.
