Entropy and Phase Transformations in Stable Nanocrystalline Alloys

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Non-Technical SummaryNanocrystalline alloys are next-generation materials that are composed of many very small crystal grains, adhered together at disordered "grain boundaries". These materials have a variety of exceptional properties like extreme strength and wear resistance. They are also beginning to transition from laboratory curiosities to widely-adopted engineering materials thanks to a critically enabling concept: the stabilization of their grain boundaries by adding alloying elements. Traditional alloy science ignores the effect of grain boundaries, but these cannot be ignored in nanocrystalline metals where they are so prominent. Therefore, new alloy science is needed for these materials, with a focus on integrating the interactions between grain boundaries and alloying additions. This project studies the alloy science of nanocrystalline materials, with a special focus on the effects of temperature and maintaining stability at high temperatures. The project uses computer simulations at the atomic scale to explore the stability of various alloys in nanostructured form, and identifies limits to the compositions and conditions under which nanocrystalline structures can be formed. Experiments are conducted to make and test these new theoretical advances, and to produce the first prototype samples of new nanocrystalline alloys. These developments are expected to lead to a predictive scientific toolkit for the future design of new families of nanocrystalline alloys, for use in a wide array of applications ranging from electronics, to machine components, to 3D printing. The research in this project is carried out by undergraduate, graduate, and postdoctoral students at MIT as part of their training in materials science. The project team also engages with industry scientists to focus the work on relevant materials and applications, and to align the research for impact through future technology transfer.Technical SummaryThe intellectual merits of this research program center on resolving the role of entropy in nanostructured alloys. Specifically, the project is developing a full view of configurational and vibrational entropy on grain boundary segregation in polycrystals, through atomistic computations that can separate these two contributions in a system with a full polycrystalline spectrum of grain boundary sites. This information in turn enables a full analysis of the free energy competition between bulk phases and nanostructured states, including details like boundary structural transitions (or complexion transitions) and allotropic phase transformations in the bulk. The overarching goal of the project is to achieve sufficiently quantitative thermodynamic calculations to be able to predict alloy phase diagrams complete with equilibrium and metastable nanocrystalline structures, and then to provide experimental tests of those predictions. The broader impacts of this program comprise a variety of training, outreach and dissemination activities. Undergraduate, graduate, and postdoctoral researchers are trained on topics at the intersection of classical materials science core concepts (alloy thermodynamics, phase equilibria) and new topics in nanoscience (nanostructure stabilization). The research results are published in the open literature and also disseminated widely to industry through the outreach activities of the PI. In particular, industrial interactions are used to guide the research efforts towards relevant materials and temperatures of interest for practical purposes, facilitating future technology transfer.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.