Nucleation in Solid Metallic Solutions with Steep Composition Gradients

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TECHNICAL SUMMARY: When intermetallic phases form through diffusional mixing of elements steep chemical gradients can inhibit nucleation, often leading to the formation of nonequilibrium or metastable phases. The proposed effort applies experimental and computational tools to increase our understanding of solid-state nucleation within these steep composition gradients. More specifically, a unique combination of nanocalorimetry, in situ DTEM, in situ TEM, and critical concentration gradient theory is used to identify and predict which phases form first and at what temperatures in a model, layered material system (Ni-Al) under both slow (10^1 K/s) and fast heating rates (10^5 K/s). The Ni-Al samples will be deposited on nanocalorimeters with a range of bilayer thicknesses (10 to 150 nm) and average compositions (0.1 to 0.9). In addition, inputs to the concentration gradient model will be enhanced to improve predictions by calculating interface energies for the relevant phases and by quantifying the rates and paths of interdiffusion in the Ni-Al bilayer samples. With these enhancements in hand, heating rate, bilayer thickness, and average composition will be varied experimentally and computationally to assess the effectiveness of this concentration gradient model in predicting which phase nucleates first. NON-TECHHNICAL SUMMARY: The proposed effort is designed to enhance our understanding of how compounds form during the fabrication of solid components such as metallic and ceramic protective coatings, porous materials and MEMS devices. Unique experimental and computational tools will be used and a series of experiments will be performed to assess the ability of a critical concentration gradient model to predict the first phase to nucleate. Improving our models and understanding of nucleation will help future researchers develop improved materials for a variety of industries including aerospace, automobile and microelectronics. The PI will involve at least one female high school student from Western High School in Baltimore. The high school student will be teamed with a PhD student on this project, who has already mentored undergraduate students at Hopkins. Summer salary will be provided for the student through the proposed budget. The PI also plans to recruit one undergraduate student to join the project during the school year and efforts will be made to recruit under-represented groups such as women and minorities. Lastly, the PI will promote entrepreneurial activities on the Hopkins Campus by overseeing the introduction of professional development modules for PhD students and Postdocs through the Center for Leadership Education that he directs. He will also draw on his own entrepreneurial experience to promote similar efforts on campus by helping to grow an incubator and an entrepreneurial culture.