Year of Publication

2018

Degree Name

Doctor of Philosophy (PhD)

Document Type

Doctoral Dissertation

College

Engineering

Department

Chemical and Materials Engineering

First Advisor

Dr. T. John Balk

Abstract

High entropy alloys (HEA) are a relatively new group of alloys first introduced in 2004. They usually contain 5 to 6 different principle elements. Each of these elements comprise 5-35 at. % of the chemical composition of the alloy. There is a growing interest in the research community about the development of these alloys as well as their engineering applications. Some HEAs have interesting properties that have made them well suited for higher temperature applications, particularly refractory uses, while some have been shown to maintain their mechanical properties even at cryogenic temperatures.

Initially, the HEA research was focused on developing alloys with equiatomic compositions as it was believed that the single phase HEA would only form at such composition ratios. However, further research have found multiple HEAs with non-equiatomic chemical compositions. A major question that needs to be answered at this point is how to identify these non-equiatomic single phase alloy systems. Unlike the conventional alloys, the HEAs do not have a base element as a solvent, which complicates the identification of new alloy systems via conventional development techniques. To find a potential HEA, alloy development techniques of both exploratory and computational natures are being conducted within the community. Even though multiple HEAs have been successfully identified and fabricated by these techniques, in most cases they require extensive experimental data and are relatively time consuming and expensive. This study proposes a thin film combinatorial approach as a more efficient experimental method in developing new HEA alloy systems.

In order to study HEA systems with different crystal structures, nominal HEA compositions were selected, including: CoFeMnNiCu in order to achieve face centered cubic (FCC) HEA, OsRuWMoRe to obtain hexagonal closed packed (HCP) and VNbMoTaW in an attempt to form a body centered cubic (BCC) crystal structure. Thin film samples were fabricated by simultaneous magnetron sputtering of the elements onto silicon wafer substrates. The arrangement of the sputtering targets yielded a chemical composition gradient in the films which ultimately resulted in the formation of various phases. Some of these phases exhibited the desired single-phase HEA, albeit with non-equiatomic chemical compositions. Bulk samples of the identified HEA compositions were prepared by arc melting mixtures of the metals. Microstructure of both thin film samples and bulk samples were characterized via scanning electron microscopy (SEM), focused ion beam (FIB) and energy dispersive x-ray spectroscopy (EDX). The crystal structures of the samples were studied by X-ray diffraction (XRD) and electron backscattered diffraction (EBSD) technique. Applying nano-indentation technique, the mechanical properties of some of the samples were screened over the composition gradient as well.

By applying this combinatorial thin film approach, single-phase FCC, HCP and BCC HEAs were detected and successfully produced in bulk form. Additionally, screening the properties of the compositionally gradient thin films, as well as their chemical composition and crystal structure, provided a thorough understanding of the phase space. This experimental approach proved to be more efficient in identifying new alloy systems than conventional exploratory development methods.

Digital Object Identifier (DOI)

https://doi.org/10.13023/etd.2018.407

Funding Information

Kentucky Science and Engineering Foundation (KSEF-148-502-15-363)

Included in

Metallurgy Commons

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