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Nanocrystalline alloys can be stabilized through selective grain boundary segregation of specific solute element additions. Increasing attention is being paid to ternary and higher order systems, where complex interactions govern segregation. To efficiently study the large composition spaces of such systems, we apply a high-throughput combinatorial technique revealing nanocrystalline stability through composition-grain-size maps. We compare two systems with distinct binary and ternary alloy interactions: In Pt–AuAg both binaries are expected to be stable, whereas in Pt–AuPd the Pt–Pd binary is unstable and Au-induced co-segregation of Pd was previously reported. For ternary Pt–AuAg we find excellent thermal stability throughout. The Pt–AuPd system, by contrast, divides into an unstable regime, where Pd solute dominates and precipitates, and a stable regime, where Au solute dominates and retains Pd in the grain boundary. Overall, by combining current theory and the introduced combinatorial approach, stable multicomponent nanocrystalline composition spaces can be rapidly determined., NSF (Grant DMR 1606914), DOE (Grant DE-SC0020180)

In nanocrystalline alloys, a range of configurations c an have low energies when solute atoms have favorable interactions with interfaces. Whereas binary nanostructured alloys have been well studied, here we lay groundwork for the computational thermodynamic exploration of alloy configurations in multicomponen t nanocrystalline alloys. Multicomponent nanostructured systems are shown to occupy a vast space, with many topological possibilities not accessible in binary systems, and where the large majority of interesting configurations will be missed by a regular solution approximation. We explore one interesting ternary case in which the first alloying element stabilizes grain boundaries, and the second forms nano-sized precipitates., United States. Army Research Office (grant W911NF-14-1-0539), Massachusetts Institute of Technology. Institute for Soldier Nanotechnologies, National Defense Science and Engineering Graduate (NDSEG) Fellowship

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Materials Science and Engineering, 2018., Cataloged from PDF version of thesis., Includes bibliographical references (pages 129-135)., The development of stable nanocrystalline binary alloys, which possess a large volume fraction of grain boundaries at elevated temperatures, is a promising route to high yield strength materials. Previous studies have focused on alloying by selecting solute elements that segregate at grain boundaries to stabilize the nanostructure. A selection criterion has been established for designing stable binary nanocrystalline materials. This thesis explores the extension of this concept to the design of multicomponent nanostructured systems. In contrast to the simplicity of a binary system where not many topological possibilities are accessible, multicomponent nanostructured systems are shown to occupy a vast space where the large majority of interesting configurations will be missed by a regular solution approximation. This thesis describes research to develop a conceptual basis for the thermodynamic properties of multicomponent nanocrystalline alloys, and to design interesting ternary configurations not accessible in binary systems. The conditions necessary to achieve the desired nanostructure configurations are developed in a model that takes solute interactions into consideration. Based on the model, we performed a systematic case study on one alloy system expected to exhibit nanocrystalline stability: Pt-Pd-Au. As a control, two binary systems (Pt-Au, Pt-Pd) were produced for comparison. While a uniform distribution of Pd is observed in binary Pt-Pd alloys at 400 °C, the results from scanning transmission electron microscopy (STEM) reveal that Pd segregation behavior was induced by the Au grain boundary segregation in the ternary system at 400 °C. Our work on induced co-segregation behavior of Pt-Pd-Au alloy is just a simple example of solute interaction in nanocrystalline alloys. Our approach more generally presents a new design framework to control the complex configurations possible in nanocrystalline materials by alloying element selection., by Wenting Xing., Ph. D.