Doctoral Dissertations

Date of Award

8-2020

Degree Type

Dissertation

Degree Name

Doctor of Philosophy

Major

Materials Science and Engineering

Major Professor

Peter K. Liaw

Committee Members

Peter K. Liaw, Hahn Choo, Yanfei Gao, Hairong Qi

Abstract

The objectives of this proposed study are to (1) design and develop single BCC phase refractory high-entropy alloys (HEAs) for the high-temperature applications, (2) investigate the deformation mechanisms of refractory HEAs, (3) improve an integrated approach, coupling focused experiments and theoretical modeling, to design, discover, and develop HEAs, and (4) understand the alloy design-microstructure-property-performance links underlying the mechanical behavior of refractory HEA systems for gas-turbine applications

A traditional alloy system generally includes one or two principal elements that form the matrix with other additional elements, e.g., iron or aluminum alloys, to strengthen some specific properties, such as strength and corrosion resistance. As a novel class of multicomponent materials, HEAs have recently stimulated the intensive study because of their large composition space and great properties. The definition of HEAs could be based on their chemical compositions or configurational entropy. According to their compositions, HEAs are typically composed of four or more principal elements in an equal or near-equal atomic percent between 5 to 35 atomic percent (at. %), which can crystallize as a single phase. According to their entropies, HEAs are defined as alloys that have configurational entropies larger or equal to 1.5R, where R is the ideal gas constant, no matter whether they are a single phase or multi-phases. A majority of HEAs reported so far have multiple phases (second phases, nanoparticles, element segregations and etc.), rather than a single solid-solution phase. However, specially-designed HEAs can possess single-phase structures, such as body-centered-cubic (BCC), face-centered-cubic (FCC), and hexagonal-close-packed (HCP) structures These single-phase and multiple-phase HEAs have desirable materials properties, such as high strength, reasonable ductility, high hardness, good fatigue and corrosion resistance, high thermal conductivity, low coefficient of thermal expansion, and great toughness for gas turbines. Thus, it is of critical importance to develop a fundamental knowledge and understanding of the mechanical characteristics at elevated temperatures. The microstructure, and elastic/plastic deformation behavior of the refractory HEAs will be investigated, using the integrated experimental (transmission-electron microscopy/scanning-transmission-electron microscopy, in-situ neutron diffraction, and atom-probe tomography) and theoretical (first-principles calculations, and crystal-plasticity finite-element modeling) approaches.

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