Temperature-Dependent Structures and Atomic Mixing Behaviors in High-Entropy Alloys

115 Pages·2016·4.62 MB·English

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Doctoral Dissertations Graduate School
5-2016
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Louis J. Santodonato
University of Tennessee - Knoxville, [email protected]
Follow this and additional works at: https://trace.tennessee.edu/utk_graddiss
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Santodonato, Louis J., "Temperature-Dependent Structures and Atomic Mixing Behaviors in High-Entropy
Alloys. " PhD diss., University of Tennessee, 2016.
https://trace.tennessee.edu/utk_graddiss/3743
This Dissertation is brought to you for free and open access by the Graduate School at TRACE: Tennessee
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[email protected].
To the Graduate Council:
I am submitting herewith a dissertation written by Louis J. Santodonato entitled "Temperature-
Dependent Structures and Atomic Mixing Behaviors in High-Entropy Alloys." I have examined the
final electronic copy of this dissertation for form and content and recommend that it be
accepted in partial fulfillment of the requirements for the degree of Doctor of Philosophy, with a
major in Materials Science and Engineering.
Peter K. Liaw, Major Professor
We have read this dissertation and recommend its acceptance:
Jaime Fernandez-Baca, David J. Keffer, James R. Morris
Accepted for the Council:
Carolyn R. Hodges
Vice Provost and Dean of the Graduate School
(Original signatures are on file with official student records.)
Temperature-Dependent Structures and Atomic Mixing Behaviors in High-
Entropy Alloys

A Dissertation Presented for the
Doctor of Philosophy
Degree
The University of Tennessee, Knoxville

Louis J. Santodonato
May 2016
Copyright ©2016 Louis J. Santodonato
All rights reserved

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Dedication

To my loving and supportive wife, Marla J. Santodonato, who shares my hopes and dreams.

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Acknowledgements

I am grateful for the encouragement and guidance of my advisor, Prof. Peter K. Liaw,
who always aims high, and teaches his students to do the same. I owe many thanks for the
members of my thesis committee, Profs. Jaime Fernandez-Baca, David Keffer, and Jamie
Morris, for their guidance, collaboration, and diligent efforts to thoroughly review and improve
my research.

Oak Ridge National Laboratory (ORNL) has sponsored my studies, through their
Educational Assistance Program, which I am thankful to have benefitted from. I have many
colleagues and mentors from ORNL to thank, especially Dr. Ken Herwig, who was my
supervisor at the Spallation Neutron Source for several years, and encouraged me to go back to
school and get my PhD. My career took some difficult turns as a working student, and just when
things seemed to be falling apart, I found a new home at ORNL’s High Flux Isotope Reactor
(HFIR). There I joined the Neutron Imaging Team, led by Dr. Hassina Bilheux, who has been
incredibly supportive of my scientific aspirations, and mentors my transition to a scientific
career. Others at the HFIR have gone out of their way to help me, including Lakeisha Walker
and Kathy Bailey (my supervisor), and I am grateful to work with such a fun and supportive
group.

I am very fortunate to be able to collaborate with so many excellent scientists at ORNL,
including Drs. Hassina Bilheux, Jean Bilheux, Lisa DeBeer Schmitt, Mikhail Feygenson, Ken
Littrel, Jeorg Neuefeind, Chad Parish, and Ray Unocic. Collaborators and mentors from other
institutions include Profs. Karen Dahmen and Yang Zhang (University of Illinois at Urbana-
Champaign), Dr. Michael Gao (National Energy Technology Laboratory), Drs. Dan Miracle and
Oleg Senkov (U.S. Air Force Research Laboratory), Dr. Zhi Tang (Alcoa), Dr. Thanh Tran (U.S.
Naval Surface Warfare Research Center), and Dr. Richard Weber (Argonne National Laboratory,
and Materials Development, Inc.)
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Abstract
The goal of the present dissertation is to advance the fundamental understanding of the
atomic mixing behavior in a new class of alloys, known as high-entropy alloys (HEAs), and
provide new methods to develop HEAs for practical applications. The HEA design strategy is
based upon the influence of configurational entropy of mixing, which serves as a driving force
for the formation of disordered solid-solution structures in certain alloys. In particular, alloys
containing multiple elements have a tendency to form stable, disordered structures, sometimes
with exceptional engineering properties. Despite the tendency toward structural disorder, HEAs
usually have some degree of structural order. To date, rigorous descriptions and quantifications
of the structural order are lacking. In the literature, the HEA structures are usually categorized in
an all-or-nothing “ordered” versus “disordered” fashion. Furthermore, the HEA development
efforts are mainly aimed at achieving disordered solid-solution structures. The present Ph.D.
study develops new experimental and theoretical techniques for understanding and controlling
structural order in HEAs. These new techniques are actually extensions of well-established
approaches, such as the Bragg-Williams modeling of binary alloys, Monte Carlo simulations of
atomic mixing, complementary neutron and X-ray scattering studies, and microscopy studies.
The results reveal a persistent entropy effect, even when the HEAs undergo ordering transitions.
which are quite complex and counterintuitive compared to the more familiar ordering behaviors
in binary alloys. The conclusion is he HEA design strategy may be understood and applied in a
broader fashion, with the goal of producing new engineering alloys, suitable for applications
where high strength, toughness, and corrosion resistance at elevated temperatures are crucial.

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Table of Contents
Chapter 1: Introduction ................................................................................................................... 1
1.1 Objective and Motivation ................................................................................................. 2
1.2 High-Entropy Alloys ........................................................................................................ 3
1.3 Integrated Experimental and Theoretical Framework ...................................................... 3
1.4 Neutron scattering studies ................................................................................................ 5
1.5 Bragg-Williams Model ..................................................................................................... 6
1.6 Monte Carlo Simulations ................................................................................................. 8
1.7 Microscopy Studies .......................................................................................................... 9
References ................................................................................................................................. 12
Appendix 1A: Chapter 1 Figures .............................................................................................. 15
Chapter 2: Deviation from high-entropy configurations in the atomic distributions of a multi-
principal-element alloy ................................................................................................................. 21
Abstract ..................................................................................................................................... 23
2.1 Introduction .................................................................................................................... 24
2.2 Results ............................................................................................................................ 27
Room-temperature microstructures ....................................................................................... 27
Complementary neutron and X-ray scattering studies .......................................................... 30
High-temperature neutron-scattering studies ......................................................................... 32
Ab-initio molecular-dynamics simulations ........................................................................... 33
Combined results revealing the structural evolution during cooling ..................................... 34
2.3 Discussion ...................................................................................................................... 37
Methods ..................................................................................................................................... 39
Sample preparation ................................................................................................................ 39
Microscopy and atom probe tomography .............................................................................. 39
Ab-initio molecular-dynamics (AIMD) simulations ............................................................. 39
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Applying ordering parameters to a multi-component B2-ordered structure ......................... 40
Neutron scattering .................................................................................................................. 43
Synchrotron X-ray powder diffraction .................................................................................. 44
References ................................................................................................................................. 45
Appendix 2A: Chapter 2 Tables ................................................................................................ 49
Appendix 2B: Chapter 2 Figures ............................................................................................... 51
Appendix 2C: Chapter 2 Supplementary Table and Figures..................................................... 60
Chapter 3: High-entropy alloys provide a new perspective on order-disorder transitions ........... 67
3.1 Introduction .................................................................................................................... 70
3.2 Order-disorder transitions involving multiple elements................................................. 70
3.3 Experimental observations ............................................................................................. 72
3.4 Simulated order-disorder transitions in HEAs ............................................................... 73
3.5 Decomposition of the partially-ordered phase ............................................................... 74
3.6 Factors affecting HEA microstructure ........................................................................... 75
3.7 Conclusion ...................................................................................................................... 76
3.8 Methods .......................................................................................................................... 77
Neutron scattering .................................................................................................................. 77
Bragg-Williams Model .......................................................................................................... 78
References ................................................................................................................................. 81
Appendix 3A: Chapter 3 Table ................................................................................................. 84
Appendix 3B: Chapter 3 Figures ............................................................................................... 85
Appendix 3C: MatLab Code ..................................................................................................... 91
Chapter 4: Conclusion................................................................................................................. 100
Vita .............................................................................................................................................. 103

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List of Tables

Table 2.1 Phase compositions ....................................................................................................... 49
Table 2.2 Microstructure evolution with temperature .................................................................. 50
Table S2.1: Simulated diffusion constants.................................................................................... 60
Table 3.1 Nearest-neighbor interaction energies .......................................................................... 84

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