Non-Isothermal Viscoplasticity in Metals

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Metals tend to creep or deform permanently under constant mechanical stress at elevated temperatures leading to failure. Based on data obtained from material testing at elevated but constant temperatures, it has been generally assumed that the higher the operating temperatures the shorter the lifetime of metallic applications. This award supports fundamental research to understand how varying temperature with time towards higher values could enhance creep performance of metals. This new knowledge will address the need to increase operating temperatures in key industrial sectors, such as aerospace and electricity generation, to improve efficiency and reduce environmental impact, without compromising safety. Insights from this project will ultimately be used by metallurgists to design materials that are better suited for higher thermo-mechanical loading. This award also supports an attractive educational platform for a diverse group of high school, undergraduate and graduate students, including female and minority students, through exposure to STEM topics and participation in laboratory research.The underlying hypothesis of this project is that the lattice misfit between phases in Nickel-based superalloys under certain non-isothermal loadings plays an essential role in enhancing creep performance at elevated temperatures. The project, therefore, rests on the transformative paradigm that “hotter can be longer” depending on how the coherency stresses evolve. To test this hypothesis, the lattice misfit evolution will be tracked in new temperature/stress regimes by in situ X-ray diffraction under synchrotron radiation. In situ results will be used to correlate, at the macroscale, the non-isothermal mechanical responses. Furthermore, discrete dislocation dynamics simulations will be carried out to gain further insight into dislocation/precipitate interactions depending on the microstructural state and lattice misfit. These simulations will help identify and quantify competing mechanisms, e.g., climb/glide and self-interaction/precipitate hardening effects, which will qualitatively help explain experimental results.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.