The characterization and nanomechanical properties of microstructurally complex systems

Since the dawn of civilization, the use of metals has played an integral role in the evolution of human society. Over the years, and with the introduction of new engineering and science, we have learned how to combine metals to create new metallic systems. We have expanded our understanding of dealloying and chemical reactions, and, in doing so, we created nanoporous metals. Our use of metals has evolved from basic alloys such as bronze and steel to more complex alloys such as multi-principal element alloys. Nanoporous gold is an advanced metallic system that can be created through the dealloying process. Nanoporous gold has a high surface area-to-volume ratio, which, combined with its material properties, makes it a good candidate for catalytic, sensing, and capacitance applications. These applications may require the nanoporous gold to be exposed to elevated temperatures. As elevated temperatures affect nanoporous gold’s structure and ligament size, this work studies the coarsening of nanoporous gold. In order to test the mechanical behavior of nanoporous gold via nanoindentation, the optimal ratio of the indent spacing to indent depth (d/h) must be found. It was found that for nanoporous gold with different microstructures, the optimal d/h ratio was 10. The coarsening of nanoporous gold was performed in-situ with an SEM heating stage to note any changes within the microstructure. The rate of growth was found to be higher than predicted by previous work due to the influence of the electron beam while coarsening. Multi-principal element alloys have high-temperature resistance, increased harness, and complex microstructures that make them useful in numerous applications, such as jet engines, submarines, and projectiles. Their complex microstructure can result in multi-phase compositions that have various mechanical behaviors which influence the overall bulk mechanical behavior. This work will focus on the mechanical behavior of W-Mo-Fe-Ni alloys and the individual phases found within this system. This dissertation explores the characteristics and mechanical behavior of newer, complex metallic systems such as nanoporous gold and multi-principal element alloys

Dynamic Mechanical Properties and Microstructure of an (Al0.5CoCrFeNi)0.95Mo0.025C0.025 High Entropy Alloy

The dynamic mechanical properties and microstructure of the (Al0.5CoCrFeNi)0.95Mo0.025C0.025 high entropy alloy (HEA) prepared by powder extrusion were investigated by a split Hopkinson pressure bar and electron probe microanalyzer and scanning electron microscope. The (Al0.5CoCrFeNi)0.95Mo0.025C0.025 HEA has a uniform face-centered cubic plus body-centered cubic solid solution structure and a fine grain-sized microstructure with a size of about 2 microns. The HEA possesses an excellent strain hardening rate and high strain rate sensitivity at a high strain rate. The Johnson–Cook plastic model was used to describe the dynamic flow behavior. Hat-shaped specimens with different nominal strain levels were used to investigate forced shear localization. After dynamic deformation, a thin and short shear band was generated in the designed shear zone and then the specimen quickly fractured along the shear band