Table Of ContentMICROSTRUCTURAL CHARACTERIZATION AND SHAPE MEMORY
RESPONSE OF Ni-RICH NiTiHf AND NiTiZr HIGH TEMPERATURE SHAPE
MEMORY ALLOYS
A Dissertation
by
ALPER EVIRGEN
Submitted to the Office of Graduate and Professional Studies of
Texas A&M University
in partial fulfillment of the requirements for the degree of
DOCTOR OF PHILOSOPHY
Chair of Committee, Ibrahim Karaman
Committee Members, Dimitris C. Lagoudas
K. Ted Hartwig
Ronald D. Noebe
Xinghang Zhang
Head of Department, Ibrahim Karaman
August 2014
Major Subject: Materials Science and Engineering
Copyright 2014 Alper Evirgen
ABSTRACT
NiTiHf and NiTiZr high temperature shape memory alloys (HTSMAs) have
drawn a great deal of attention as cheaper alternatives to Pt, Pd and Au alloyed NiTi-
based HTSMAs while NiTiZr alloys also providing at least 20% weight reduction then
its NiTiHf counterparts with the same stoichiometry. (Ti + Hf/Zr)-rich compositions
were already reported to have high thermal hysteresis, poor dimensional and thermal
stability due to their low matrix strength hampering their practical applications.
However, Ni-rich compositions of NiTiHf alloys were shown to have very promising
shape memory responses recently due to generation of fine Ni-rich particles after proper
heat treatments not only strengthening the matrix but also leading to relatively high
transformation temperatures. Comparable studies have not been performed on Ni-rich
NiTiZr compositions. Furthermore, very few published work are present on these new
Ni-rich NiTiHf and NiTiZr systems. Hence many critical characteristics still remains
unknown and further investigation is necessary to reveal the effect of precipitation on the
microstructures and its subsequent effect on the transformation characteristics and shape
memory responses.
The present study focuses on the extensive microstructural and thermo-
mechanical property characterizations of the Ni-rich NiTiHf and NiTiZr HTSMAs in
order to develop the fundamental knowledge necessary for the optimization and
development of reliable, cheap, lightweight HTSMAs operating up to 300 °C with
improved thermal and dimensional stability. Several different compositions of Ni-rich
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NiTiHf and NiTiZr HTSMAs are systematically precipitation heat treated for the
microstructural control and then subjected to multi-scale microstructural and thermo-
mechanical characterizations to achieve this goal. Differential scanning calorimetry
measurements are conducted on the aged samples to reveal the transformation
characteristics and furthermore generate the time-temperature-transformation
temperature (TTT) diagrams of the individual alloy systems. The shape memory
response and characteristics of the alloys are investigated through load-biased thermal
cycling and superelasticity tests. The microstructures of the aged samples are extensively
characterized using transmission electron microscopy (TEM) to build up microstructure-
property relationships as well as providing deeper understanding of precipitate crystal
structure, composition and morphology.
Such an experimental approach is crucial for the development of new ternary
alloy compositions and for the careful control of the microstructure to obtain desired
properties. The outcomes of the present study is expected to help to reveal the potential
of these alloys to be utilized in a wide range of applications at elevated temperatures in
aerospace, automotive and oil-gas industries.
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To my family: my mother Zeynep Evirgen, my father Baki Evirgen and my brother
Caner Evirgen for their never ending support and trust
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ACKNOWLEDGEMENTS
It has been four tough years since I began my PhD studies in College Station. For
four tough years I followed my career objective, the same one I’ve had ever since my
undergraduate career. Despite all of the grueling work, these four years have been
intellectually illuminating due to my scientific research and discoveries. These
endeavors would not have been successful without the wonderful people I have met,
YOU.
Prof. Ibrahim Karaman, I am not only grateful that I had the opportunity to have
you as my advisor but more importantly I have developed a deep respect for you as a
person. I would like to thank you for all your trust and the unconditional support you
gave me during my PhD studies. I am very lucky to have had the chance to work under
your supervision. In addition, Prof. Dimitris C. Lagoudas, Prof. K. Ted Hartwig and
Prof. Xinghang Zhang, I would like to express my gratitude to you for serving on my
committee and giving me advice on my studies and scientific work.
Dr. Ronal D. Noebe from NASA Glenn Research Center, Structural Materials
Division, without your help this study would not have been successful and complete. I
am especially thankful to you for all your guidance and valuable comments on our joint
publications along with my dissertation and also serving as a member on my committee.
Prof. Jaume Pons and Dr. Ruben Santamarta, there was no other way that this
study would have had successfull microscopy investigations without your help, your
experience and your deep insights provided the framework for numerous scientific
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discoveries. I am very glad to have met you and I am very thankful that I had the chance
to work in your group at “La Isla Bonita” Mallorca, Spain. Furthermore, I would also
like to thank you for all your suggestions and contributions on our joint publications.
Dr. Aaron Stebner from Colorado School of Mines, I would like to thank you for
letting me use your MATLAB code in order to calculate the maximum strains of the
alloys in the present study. I am also thankful to Dr. Raymundo Arroyave and Anjana
Talapatra from Texas A&M University for carrying out DFT calculations to calculate
elastic constants of the alloys in the present study.
Finally, the people of “MESAM Research Group”, I spent quite a significant
time, not only of my PhD years but also of my life in room 102 of Doherty building. It
was neither the laboratory nor the devices that made my stay here memorable and
tolerable but it was the people. James A. Monroe, Nevin Ozdemir, Sonia M. Razavi,
Ruixian Zhu, Nick Bruno, Nick Barta, Li Wei Tseng, Ankush Kothalkar, Liangfa Hu,
Hande Ozcan, Taymaz Jo and Omer Karakoc, thank you all for being there when I
needed help and for the birthdays and fun we had in laboratory. Ji Ma, thank you for
your patience during my training and also your indepth explanations on my never ending
questions. Brian Franco, dude, thanks for your help on fixing almost everything that no
one else is capable of to do so and also frequent discussions on the “precipitate story” in
shape memory alloys. Special thanks go out to Brian for the shooting trip and watching
“Game of Thrones” on a huge wall projector. Pinar Karpuz, Ceylan Hayrettin and
Ebubekir Dogan, guys we talked about politics, life, and almost everything whenever we
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wanted a break. Thank you for all your support and help on my studies and also for the
coffee and tea sessions. I wish you all the best with the rest of your careers.
Samet Mutlu, Mehtap Deniz Unlu, Tumer Yildirim and Ugur Cem Ozder, you
guys are more like brothers and sisters to me. We have been far apart for years, even
living in different continents, however you have always been beside me. Thank you for
making me feel at home with your never ending support and belief in my endeavors.
And the beautiful people I met in College Station, Eda Aydogan, Gokhan
Gungor, Nese and Emrah Yilmaz, Sema and Volkan Firat, Duygu and Ceylan Hayrettin,
Kivanc and Meltem Ustun, Sema Asik, Tugba Oner, Sevil Sarikurt, Murat Cobanoglu,
Tuba Aydin, Zeynep Dincer, Gokhan Ileri, Andreas Larsson, Amanda Cain, Aishwarya
Sooresh, Youxing Chen, Miao Song and Shamik Basu, thank you for all the good time
and the friendship in College Station. I hope to see you somewhere in future. Mehmet
Tatli, the good guy of Antakya and my roommate, thanks for everything and being a
brother to me.
I would like to thank the students I mentored during their studies, Fabian Basner,
Paul Joan and Flavien Hatton, thank you for carrying out experiments and your
contribution in several parts of the present work.
In addition, I would like to thank the people that made my stay in Spain and the
Netherlands memorable, Esteban, Anca, Marysia, Karol, Sebastian, David, Corrado,
Mario, Vincent, Sarah, Eleonora, Neera, Jennifer, Aquilina, Maude, Lucie and Anjana; a
big thank you.
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And finally I would like to thank my family, Baki Evirgen, Zeynep Evirgen and
Caner Evirgen who deserve my biggest gratitude for their infinite support, help and
advice during all my life making me the person that I am now.
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NOMENCLATURE
A Austenite finish
f
A Austenite start
s
ATAT Alloy theoretic automated toolkit
BSE Back-scattered electron
CC Clasius-Clapeyron
CSS Critical shear stress
CVP Corresponding variant pair
DFT Density functional theory
DSC Differential scanning calorimetry
EDM Electrical discharge machining
EDS Energy-dispersive spectroscopy
EDX Energy-dispersive X-ray
EM Electron microprobe
ε Irrecoverable strain
irr
ε Recovered transformation strain
rec
ε Total strain
total
FC Furnace cooling
FFT Fast fourier transform
HRTEM High resolution transmission electron microscopy
HTSMA High temperature shape memory alloy
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HV Vickers hardness
ICP-AES Inductively coupled plasma – atomic emission spectrometry
LIS Lattice invariant shear
M Martensite finish
f
M Martensite peak
p
M Martensite start
s
MT Martensitic transformation
SADP Selected area diffraction pattern
SE Superelasticity
SEM Scanning electron microscopy
SHT Solution heat treated
SIMT Stress induced martensitic transformation
SMA Shape memory alloy
SME Shape memory effect
σ Critical stress to induce martensite
SIM
TEM Transmission electron microscopy
TRIP Transformation induced plasticity
TTT Time-temperature-transformation temperature
XRD X-ray diffraction
WDS Wavelength-dispersive spectroscopy
WQ Water quenched
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Description:alloy compositions and for the careful control of the microstructure to obtain Gungor, Nese and Emrah Yilmaz, Sema and Volkan Firat, Duygu and
