Author ORCID Identifier
Date Available
5-11-2025
Year of Publication
2024
Degree Name
Doctor of Philosophy (PhD)
Document Type
Doctoral Dissertation
College
Engineering
Department/School/Program
Materials Science and Engineering
First Advisor
Thomas John Balk
Abstract
High Entropy Alloys (HEAs), also known as Multiprincipal Element Alloys (MPEAs), represent a paradigm shift in alloy design. Unlike traditional practices that limit compositions to the edges of phase diagrams, HEAs exploit equiatomic mixing of five or more elements at the center of phase diagrams to maximize configurational entropy. This unconventional approach exponentially expands the spectrum of potential alloy compositions, rendering conventional experimental methods inadequate for exploring this extensive compositional space.
In response to these challenges, the scientific community has shown a growing interest in high-throughput techniques, encompassing both computational and experimental domains. Particularly, Refractory High Entropy Alloys (RHEAs) have arisen as a focal point of interest. RHEAs hold promise in uncovering the optimal synergy between strength and ductility within their expansive compositional landscape. However, the scarcity of quality tensile property datasets for RHEAs has presented significant barriers. While experimental techniques have been proposed for phase and mechanical property screening, there remains a critical void in high-throughput methods for assessing the intrinsic ductility of alloy compositions. To address this void, this thesis presents an innovative high-throughput experimental approach to expedite the screening process and complement computational strategies.
In the first part of this thesis, a novel approach is introduced, building upon the established technique of thin film fragmentation testing used in evaluating the stretchability of flexible electronics. The current study demonstrates that by isolating the factors influencing crack onset strain (COS), a crucial measure of intrinsic ductility can be obtained. By carefully controlling process parameters, thin film samples of Nb, Mo, Ta, and W were fabricated with comparable thicknesses and residual stress levels. COS values were compared to bulk ductility, paving the way for a rapid and cost-effective high-throughput screening method in exploring refractory alloy design spaces.
The second segment undertakes combinatorial thin film screening within the VNbMoTaW system, known for its exceptional high-temperature strength. In pursuit of comparative ductility, in situ fragmentation testing of thin films and indentation fractography of arc melted samples were conducted on various alloy compositions, revealing the superior intrinsic ductility of VNb2TaW and V2NbTaW alloys. The research discusses five theories guiding RHEA design and found that the χ-parameter theory exhibits a stronger correlation with the experimental observations. This represents the first experimental evidence supporting the χ-parameter's efficacy in predicting intrinsic ductility in RHEAs.
The third part of this study delves into a detailed analysis of the in situ resistance method used in the previous sections. Typically employed to determine fracture strain in flexible electronics, this method was extended to track crack evolution beyond the fracture strain. A "gradient plot" analysis technique was introduced, enabling the monitoring of crack initiation, multiplication, and the onset of buckle formation through resistance measurements alone. This technique simplifies the tracking of crack evolution, potentially extracting additional properties such as crack density and fracture toughness.
In summary, this study extends an established characterization technique from the realm of flexible electronics into alloy designing, offering a promising tool for rapidly identifying alloy compositions with optimized strength and ductility. With this new technique, this study provides the first experimental evidence of the χ-parameter's applicability as a predictive tool for intrinsically ductile RHEAs. It also pushes the boundaries of in situ resistance method in thin film fragmentation testing.
Digital Object Identifier (DOI)
https://doi.org/10.13023/etd.2024.164
Funding Information
This work was supported by grant DE-SC0019402 from the U.S. Department of Energy, Office of Science in 2018 Also backed by DOE EPSCoR funding and additional support provided by the College of Engineering through EMC
Recommended Citation
Tuhser, Taohid Bin Nur, "ADVANCING COMBINATORIAL SCREENING OF REFRACTORY HIGH ENTROPY ALLOYS THROUGH INTRINSIC DUCTILITY ASSESSMENT VIA THIN FILM FRAGMENTATION TESTING" (2024). Theses and Dissertations--Chemical and Materials Engineering. 161.
https://uknowledge.uky.edu/cme_etds/161
Included in
Metallurgy Commons, Other Materials Science and Engineering Commons, Structural Materials Commons
