Investigation of fundamental creep behavior and mechanisms in a thermally stable nanocrystalline alloy

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Non-technical abstract:Structural materials used by power generation-reliant industries are often processed such that the amount of material imperfections present at small length scales is reduced. This approach has been quite successful; however, it is reaching the limits of what can be achieved, while new demanding applications require even higher material performance. This award supports fundamental research to look for the next generation of high performance materials for high temperature structural applications by using the opposite approach, i.e., to increase the number of imperfections at small, including atomic, length scales in such a way that deformation at high temperatures is impeded rather than enhanced by these imperfections. This requires a complete understanding of composition effects and processing conditions that can produce the high density of imperfections required and to make sure that they are retained during high temperature loading, since high temperatures tend to decrease the number of imperfections in a material. The physical principles that govern the relationship among processing, structure and mechanical properties in materials with such high number of defects as those proposed here are not well understood; hence, they will be studied in this project through a combination of experiments and modeling to produce new scientific knowledge in the field of structural materials. Furthermore, students in science and engineering at both undergraduate and graduate levels will participate in the research and be trained in state-of-the-art techniques to process, test, characterize and model structural material behavior, helping to educate the new generation of engineers and scientists in this field. Finally, the potential to understand how to produce a new generation of structural materials for high temperature applications with better properties than those currently available can have significant impacts and benefits to society. Technical abstract:Metal and alloys with a mean grain size below 100 nm have enhanced static strength compared to coarse-grained alloys and, hence, are very attractive for engineering applications. However, very few studies have addressed the fundamental mechanisms of microstructural evolution during time dependent deformation, limiting their practical use. This critical gap in knowledge has stemmed from the lack of consistency among various experimental conditions (processing methods, testing conditions, etc.) along with the grain growth observed during deformation. This indeterminate understanding hinders our ability to address the current need for advanced load-bearing structural materials in many important technologies where creep loading is common (e.g., power generation, propulsion). Motivated by this, the hypothesis that will be tested is that Ni-Y-Zr is a stable duplex system, wherein both grain boundary segregation (some solid solubility of Zr in Ni) and phase formation (e.g., Ni-Y precipitates) occur, leading to a microstructurally stable nanocrystalline alloy. Samples from the resulting material will be tested and characterized under a variety of loads and temperatures, unraveling true creep behavior and pertaining mechanisms of microstructural effects on creep of stable nanocrystalline materials. Furthermore, identification of these mechanisms will also enhance our understanding of microstructural instability commonly observed in nanocrystal materials at relatively low temperatures during time dependent deformation. In a broader context, the proposed work will lay the groundwork to address gaps in iterative materials design through coordinated modeling and experimental efforts across multiple length scales.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.