CAREER: Understanding Kirkendall Pore Formation and Evolution: Correlating Compositional, Geometrical, and Thermal Influences

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PART 1: NON-TECHNICAL SUMMARY When you wake up in the morning to the smell of bacon or watch food coloring spread in water you are appreciating diffusion. In general terms, diffusion is the movement of things from areas of higher concentration to areas of lower concentration. Even though it is not obvious at room temperature, diffusion is happening in the solids around us as well - just at much slower speeds than what we witness when we smell something in the air or see something spread in water. Because certain types of atoms diffuse at higher rates than others, they can leave behind empty spaces called vacancies. If there are enough vacancies near one another in a material, they can merge and form pores. Typically, scientists try to avoid porosity in a material because it can decrease the mechanical, electrical or thermal properties especially when pores occur near a surface. Interestingly, it was discovered in the 1940's that diffusion in solids could be carefully controlled and as a result, influence where porosity in a material is located. This discovery is called the "Kirkendall effect" and it can be used to intentionally tailor the location of pores in a material so they become beneficial. Hollow structures for example can have several advantages in applications ranging from batteries to biomedical implants. This project supports fundamental research to determine how composition, geometry, and temperature influence the movement of vacancies and the evolution of these "Kirkendall" pores. By understanding the effect of composition, geometry and temperature we can design better materials that either eliminate porosity if harmful to the material or use them to create useful structures if advantageous for a given application. This project also includes a significant educational component that focuses on exposing students to, and engaging them in, materials science across high school, undergraduate and graduate levels. Through both recruiting and outreach activities integrated with this project, broadening participation of traditionally underrepresented groups in STEM will be emphasized to help diversify the engineering careers talent pool for years to come. PART 2: TECHNICAL SUMMARY This CAREER project aims to further understanding and control of vacancy-induced migration via the Kirkendall effect while engaging in multiple education and outreach activities to profoundly impact students in high school, undergraduate study and at the graduate level. The overall research goal is to systematically investigate the compositional, thermal and geometric influences on vacancy migration and Kirkendall pore evolution. Specifically, the three research objectives are to (i) evaluate the effect of Cr additions, (ii) assess the role of radial symmetry and aspect ratio and (iii) determine the magnitude of temperature gradient required for Kirkendall pore coalescence in Ni-Cr-Al-Ti based systems. These objectives are accomplished via experiments involving the fabrication and analysis of diffusion couples and diffusion coated micro-objects via conventional ex-situ metallography and 4-D microstructural characterization enabled by in-situ X-ray tomography. The broader impacts of this work are to help produce a diverse STEM-capable workforce for various career pathways by incorporating central concepts addressed in this project into education and outreach activities that expose and engage students in materials science. The education and outreach activities center on the following three education objectives: (i) expanding a Materials Camp for high school students and teachers (ii) enhancing a “How It’s Made” hands-on undergraduate honors seminar course, and (iii) creating innovative diffusion modules to enable flipped classes for materials science graduate students.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.