Doctoral Dissertations

Date of Award

8-2024

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Dustin Gilbert

Committee Members

Lisa Debeer-Schmitt, Mashiid Ahmadi. Yungfei Gao, Peter Liaw, Dustin Gilbert

Abstract

High Entropy Alloys (HEAs) are a class of material which is well-known for their high-temperature mechanical strength and corrosion resistance. In these materials, the entropy of mixing is used to encourage alloying between elements which are typically immiscible, sometimes described as alloying beyond the Hume-Rothery Rules. Specifically, the entropy of mixing is increased by including multiple elements on a single lattice site, increasing the free energy cost associated with phase separation. In the HEA community, this ‘high entropy effect’ is typically associated with alloy containing five or more elements. Achieving alloying between immiscible elements generally means the atomic size and/or electronegativity distribution can be large, resulting in extreme local environments, which contribute to strengthening of the alloy for mechanical applications and can also result in unique functional properties. The focus of my current work is electrodeposited HEA nanowires which are used to prepare a low-density metamaterial similar to an aerogel. The other project includes microstructure and spinodal decomposition of bulk HEA, AlxCoCrFeNi. The low-density materials have been shown to have impressive mechanical properties and present opportunities for energy, health, and catalytic technologies. Low density metamaterials combined with the high entropy effect of these nanowires create customizable materials associated with highly tough and corrosion resistance characteristics. The five elements in these nanowires, Co, Cr, Cu, Fe and Ni, were chosen for their magnetic responses as well as high toughness found in similar CrCoNi alloys. Magnetic and thermal properties of these materials were measured. Future work focuses on strengthening the material by sintering the wires and measuring mechanical and thermal properties in the interconnected solid. The AlxCoCrFeNi samples were fabricated with arc melting and have shown higher magnetic responses with higher paramagnetic content. This has been looked at with SANS for a closer look at the microstructure and to help see possible phrase separations from the spinodal decompositions.

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