Composition, structure and magneto-mechanical properties of Ni-Mn-Ga magnetic shape-memory alloys [Elektronische Ressource] / vorgelegt von Markus Chmielus

Markus Chmielus

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• Publié par: technische_universitat_berlin
• Publié le: 01 janvier 2010
• Nombre de lectures: 58
• Poids de l'ouvrage: 9 Mo

Extrait

Composition,StructureandMagneto-
MechanicalPropertiesofNi-Mn-Ga
MagneticShape-MemoryAlloys
MarkusChmielus
λογος

COMPOSITION, STRUCTURE AND MAGNETO-
MECHANICAL PROPERTIES OF NI-MN-GA
MAGNETIC SHAPE-MEMORY ALLOYS

vorgelegt von
Diplom-Ingenieur und Master of Science
Markus Chmielus


Von der Fakultät III – Prozesswissenschaften
der Technischen Universität Berlin
zur Erlangung des akademischen Grades

Doktor der Ingenieurswissenschaften
Dr.-Ing.

genehmigte Dissertation





Promotionsausschuss:

Vorsitzender: Prof. Dr. rer. nat. H. Schubert
Berichter: Prof. Dr. rer. nat. W. Reimers
Berichter: Prof. Dr. sc. techn. P. Müllner

Tag der wissenschaftlichen Aussprache: 14.06.2010


Berlin 2010
D83

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INTERNET: http://www.logos-verlag.de Acknowledgement
ACKNOWLEDGEMENT
I would like to thank my advisors Prof. P. Müllner, Prof. W. Reimers and Dr. R.
Schneider, for their support, ideas, guidance, and organizational help during the last three
years. I also want to thank Prof. Schubert for taking the chair of the dissertation defense
committee. During the course of this delocalized Ph.D. program, I had the honor to work
with numerous colleagues and collaborators in Berlin at the Helmholtz Centre for Materials
and Energy (HZB), at Boise State University, at Northwestern University and at the Technical
University of Braunschweig. I want to thank especially my colleague Katharina Rolfs for her
support, sample preparation and very fruitful discussions, as well as Dr. R. Wimpory, Mirko
Boin, Dr. J.-U. Hoffmann and Dr. I. Glavatskyy of the HZB and Arno Mecklenburg of MSM
Krystall our collaborators at the Technical University of Braunschweig Jan Guldbakke, and
Prof. A. Raatz. At Boise State University, I want to thank my colleagues Adrian Rothenbühler,
Brittany Muntifering, Brittany Siewert, Cassie Witherspoon, Dave Carpenter, Dave Schenker,
Doug Kellis, Kimo Wilson, Matt Reinhold, Mike Hagler, Dr. Paul Lindquist, and Nikki Kucza for
their great help, ideas, and enthusiasm. The design and fabrication of upgrades and new
experiments would not have been possible without the help of Phil Boysen (BSU). I want
further thank Prof. D. Dunand (Northwestern University, NU), Prof. X. Zhang (Harbin Institut
of Technology, China), and Peiqi Zheng (NU) for their collaboration on MSMA foams as well
as Dr. S. Vogel and Dr. D. Brown of the Los Alamos Neutron Scattering Facility for their
support during beamtime and data evaluation. Without samples of Prof. Kostorz (ETH Zürich)
and Prof. Chernenko (Universidad del País Vasco, Bilbao) comparisons with samples of other
research groups would have not been possible. I also want to express my gratitude to Prof.
D. Butt (BSU), Prof. M. Frary (BSU), Prof. B. Knowlton (BSU) and their students for their help
with sample preparation and characterization, as well as Dr. N. Kardilov, A. Hilger and A.
Paulke for their support with x-ray tomography experiments and reconstruction work. I also
want to thank Dr. K. Ullakko for his interest and inspiration, and Prof. R.C. Pond, and Prof. B.
Schönfeld for their discussions regarding neutron diffraction experiments and analysis. I also
want to express my gratitude to the German Research Foundation priority program SPP1239
which partially funded this project, the generous support of the HZB institutes G-I1 and M-I1
and their heads Dr. K.Habicht and Prof. Dr. A. Tennant as well as funding from the U.S.
National Science Foundation and the Department of Energy, Office of Basic Energy Studies.
Finally, I want to thank my friends, my family, parents and siblings and especially my
wife Jennifer for their continuous support and confidence.

Boise, ID, May 2010 Markus Chmielus
3
Abstract
ABSTRACT
Magnetic shape-memory alloys (MSMAs) are smart materials which show in single
crystalline form a magnetic field induced plastic and recoverable deformation of up to 10%.
Ni-Mn-Ga is the as most prominent representative. The shape change of MSMAs is based on
the motion of twin boundaries driven by a magneto-stress due to an applied magnetic field.
The plastic deformation takes place in the martensite phase and does not require a phase
change as needed in shape-memory alloys (SMAs). The combination of high strain of SMAs
and high actuation frequencies positions MSMAs as attractive smart actuator materials. A
challenge of MSMAs is that magneto-mechanical properties are still very difficult to predict
and to reproduce. The production of Ni-Mn-Ga single crystals is rather difficult and time
consuming. Chemical segregation leads to a continuous variation of the Mn concentration in
single crystals in growth direction. The composition change has a strong influence on all
properties of MSMA. Furthermore, sample preparation influences magneto-mechanical
properties.
This dissertation is therefore divided in three parts: first, the characterization of
composition, structure, transformation temperatures, magnetic and mechanical properties as
a study on position within an ingot. Second, the influence of surface polishing and surface
deformation on the twinning stress. Third, the influence of training and constraints on
magneto-mechanical properties.
This study demonstrates that MSMA properties depend on the position within a
single crystal ingot. With increasing Mn content the martensite structure changes from 10M
over 14M to nonmodulated martensites. The decrease of surface roughness leads to a
decrease of twinning stress. On the other hand, polished samples have only very few twin
boundaries moving rapidly which results in serrated stress-strain curves. Furthermore,
surface deformations pin twin boundaries and lead to dense twin microstructures. MSMAs
with twinning stresses of above 1 MPa and few twin boundaries moving through the sample
only show a magnetic field-induced strain when tilting of the sample is not restricted by
constraints. In soft MSMAs, tilting is not necessary since multiple twin boundaries move in
different orientations. Therefore, soft samples can adapt to constraints much better than
harder MSMAs and show large magnetic field-induced strain.


4
Zusammenfassung
ZUSAMMENFASSUNG
Magnetische Formgedächtnislegierungen (MSMA) mit Ni-Mn-Ga als
prominenstensten Vertreter sind faszinierende Materialien, die eine magnetfeldinduzierte
plastische Verformung von bis zu 10% in einkristalliner Form aufweisen. Die Verformung der
MSMA basiert dabei auf der Bewegung von Zwillingsgrenzen, die durch
magnetfeldinduzierte interne Spannungen angetrieben werden. Die Verformung benötigt
also nicht wie bei traditionellen Formgedächtnis-legierungen (SMA) eine
Phasenumwandlung sondern findet im der Martensit Phase statt. Aus diesem Grund vereinen
MSMA die hohe plastische Verformung von SMA mit den schnellen Verformungsfrequenzen
und bilden somit eine attraktive Alternative zu etablierten aktiven Materialien. Die magneto-
mechanischen Eigenschaften dieser Materialien sind immernoch nicht klar zu reproduzieren.
Die Herstellung von Ni-Mn-Ga Einkristallen ist schwierig und zeitaufwändig. Wegen
chemischer Segregation verändert sich die Zusammensetzung von Ni-Mn-Ga Einkristallen
kontinuierlich in Wachstumsrichtung und mit ihr alle MSMA Eigenschaften. Ausserdem
haben Probenbearbeitung und Training der Proben einen entscheidenen Einfluss auf die
magneto-mechanischen Eigenschaften von MSMA.
In dieser Doktorarbeit werden aufeinander aufbauende Themen bearbeitet. Im
ersten Teil werden Zusammensetzung, Struktur, Phasenumwandlungstempera-turen,
magnetische und mechanische Eigenschaften von Proben aus drei Einkristallen in Bezug auf
deren Position im Kristall ausgewertet. Im zweiten Teil wird der Einfluss von
Oberflächenrauhigkeiten und –verformungen auf die Zwillingsspannung von MSMA
untersucht. Im dritten Teil wird der Einfluss von Training und Einspannungen auf das
magneto-mechanische Verhalten analysiert werden. Die Untersuchungen dieser Dissertation
haben den Zusammenhang von MSMA Eigenschaften mit Bezug auf die Position und damit
auf diese chemische Zusammensetzung herausgestellt. Als besonders wichtiges Resultat ist
hier die Veränderung der Martensitstruktur von 10M über 14M zu nichtmoduliertem
Martensit