Local Structure of Al-based Amorphous Alloys after Microalloying by Element Substitution and after

125 Pages·2012·4.54 MB·English

SCHRIFTENREIHE DES HZB · EXAMENSARBEITEN

Local Structure of Al-based
Amorphous Alloys after
Microalloying by Element
Substitution and after Ball
Milling

Yao Liu
Dissertation

Institut für Angewandte Materialforschung F-I1
Oktober 2011
HZB–B 25
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ISSN 1868-5781
doi: http://dx.doi.org/10.5442/d0019

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Local Structure of Al-based Amorphous Alloys
after Microalloying by Element Substitution
and after Ball Milling

vorgelegt von
Yao Liu

von der Fakultät III - Prozesswissenschaften
der Technischen Universität Berlin
zur Erlangung des akademischen Grades

Doktor der Ingenieurwissenschaften
-Dr.-Ing.-

genehmigte Dissertation

angefertigt am Helmholtz-Zentrum Berlin für Materialien und Energie
Institut für Angewandte Materialforschung

Promotionsausschuss:

Vorsitzender: Prof. Dr. rer. nat. Helmut Schubert
Berichter: Prof. Dr. rer. nat. John Banhart
Berichter: Prof. Dr. rer. nat. Gerhard Wilde

Tag der wissenschaftlichen Aussprache: 08. Dezember 2011

Berlin, Oktober 2011
D83

Abstract

Structural stability of glassy Al Ni La as-atomized powder was investigated after ball mill-
85 10 5
ing using scanning electron microscopy (SEM), differential scanning calorimetry (DSC), X-
ray diffractometry (XRD), transmission electron microscopy (TEM), and X-ray absorption
spectroscopy (XAS). Fcc-Al nanocrystals were found after ball milling. The crystallization
behaviour upon thermal treatment depends on the deformation level. Because the solute atoms
Ni and La diffuse out of the Al crystal and create a solute rich area around the crystal, the
composition of the residual amorphous phase changes towards higher contents of Ni and La.
Furthermore, crystallization under ball milling depends on the ball milling temperature indi-
cating a combined role of temperature and plastic deformation for the crystallization behavior
of Al Ni La powder during ball milling. The coordination number N of Ni obtained by
85 10 5
XAS measurements decreases exponentially as a function of ball milling time. The exponen-
tial decrease in N(Ni) during plastic deformation was described in the frame of a shear band
model, in which N(Ni) is 6 outside the shear bands and 5 within the shear bands.

The local structures around nickel and cobalt atoms in Al Ni Y and Al Ni Co Y La
86 8 6 86 6 2 4.5 1.5
bulk amorphous alloys were measured by XAS. The same local structure around Ni and Co
atoms was found which is ascribed to a site substitution of Ni by Co atoms in the amorphous
structure. The configurational entropy is estimated to be the main thermodynamic driving
force for the increase in glass forming ability when substituting Ni by Co and Y by La. The
cluster-line model is not supported by our results.

The local structure after minor substitution of Ni by Co, Ge, Pd, Ag and Zr in Al-Ni-Y and
Al-Ni-La systems was also investigated by XAS. Three groups of elements which are “small”,
“medium” and “large” atoms were identified and discussed in terms of coordination number
as a function of the interatomic distance. The most striking result is the bond-shortening be-
tween the absorbing atoms and Al resulting in large changes of the effective radii compared to
their metallic radii and a concomitant reduction of the number of nearest neighbours. This
effect is most pronounced for the small atoms, especially for Ag atoms, allowing the Ag at-
oms to have the same local environment as Ni and Co.

I
II
Acknowledgements

I have been very fortunate with my supervisor, Prof. John Banhart, who offered me the oppor-
tunity to do my PhD at the Institute of Applied Materials of the Helmholtz-Zentrum Berlin für
Materialien und Energie (HZB). I would like to thank the second examiner Prof. Gerhard
Wilde, who is ready to examine my thesis. I would like to thank Prof. Helmut Schubert for
being the chairman during the defence of my PhD. I would like to express my greatest grati-
tude to PD Dr. Gerhard Schumacher, who guided me through the science of metallic glasses
during my PhD. With his enthusiasm, his inspiration, and his great efforts to explain things
clearly and simply, he helped to make scientific fun for me. Throughout my thesis-writing
period, he provided encouragement, sound advice, good teaching, good company, and lots of
good ideas. I would have been lost without him.

I would like to thank Dr. Ivo Zizak and Dr. Heinrich Riesemeier from Berlin synchrotron ra-
diation facility (BESSY) for their interest in my topic and their support in performing the
XAS experiments. Many thanks to my colleagues: Dr. Tobias Scherb, Dr. Stefan Mechler,
Jatinkumar Rana and Jessica Kohnke for the kind assistance on the XAS experiments at the
various beam times. I am especially grateful to Dr. Tobias Scherb, who gave me thousand
times of helps. He is a great scientist and of course my role model.

For this research, data were essential. Thanks therefore to Dr. Markus Wollgarten and Dr.
Heinz-Eberhard Mahnke who let me their EXAFS raw data measured at HASYLAB and give
me a chance to reanalyse them to compare them with my own measurements. Markus Woll-
garten also helped me to use TEM. Thank you.

I would like to thank Mrs Christiane Ciceron, who is a great secretary in our department and
gave me a lot of help for documents management. Special thanks to Soeren Zimmerman, who
was kind of my student, whose work is really valuable for my thesis.

There were many people who helped me to prepare sample and assisted me to do many in-
house experiments, for which I would like to thank them: Claudia Leistner, Christiane Foer-
ster, Holger Kropf and Harald Stapel.
I would like to thank my parents and my fiancée, who are the toughest and nicest shield to
support me to go through the hardest time. I love you.

Finally, I would like to thank my country and the Chinese Scholarship Council for giving me
the financial support for research.

IV
Contents
Abstract.....................................................................................................................................I
Acknowledgements................................................................................................................III
1 Introduction........................................................................................................................1
2 Literature Survey...............................................................................................................3
2.1 Metallic glasses..............................................................................................................3
2.1.1 Glass Forming Ability...........................................................................................4
2.1.2 Al-based Bulk Metallic Glasses............................................................................6
2.2 Topological Models for the Atomic Structure................................................................8
2.2.1 Free Volume..........................................................................................................8
2.2.2 Dense Random Packing and Bernal’s Model........................................................9
2.2.3 Miracle’s Model..................................................................................................11
2.2.4 Atomic Configuration in Al-based Metallic Glasses...........................................12
2.3 Stability of Metallic Glasses.........................................................................................13
2.3.1 Alloy Stability during Thermal Treatment..........................................................14
2.3.2 Stability during Plastic Deformation...................................................................15
3 Experimental.....................................................................................................................19
3.1 Materials.......................................................................................................................20
3.2 Splat Quenching...........................................................................................................21
3.3 Ball Milling..................................................................................................................21
3.4 Optical Microscope......................................................................................................23
3.5 Scanning Electron Microscope.....................................................................................23
3.6 X-ray Diffractometry....................................................................................................24
3.7 Differential Scanning Calorimetry...............................................................................26
3.8 Transmission Electron Microscopy..............................................................................27
3.9 X-ray Absorption Spectroscopy...................................................................................28
4 Microstructure of Al85Ni10La5 Metallic Glass after Ball milling...............................31
4.1 Results..........................................................................................................................31
4.1.1 Microstructure of As-atomized and Ball-milled Powders...................................31
V
VI CONTENTS

4.1.2 Crystallization during Continuous Heating Characterised by DSC....................33
4.1.3 α-Al Precipitation during Ball milling................................................................36
4.1.4 Heat Treatment on As-atomized Powder............................................................39
4.1.5 Heat Treatment of Ball-milled Powder...............................................................41
4.1.6 Temperature Effects during Mechanical Alloying..............................................42
4.1.7 Change of Local Structure after Ball milling......................................................44
4.1.7.1 Local Structure around Ni Atoms in Ball-milled Powder.......................44
4.1.7.2 Local Structure around La Atoms in Ball-milled Powder......................48
4.2 Discussion....................................................................................................................51
4.2.1 Thermodynamical Model of the Crystallization of Al Ni La .........................51
85 10 5
4.2.2 Size Limitation of Fcc-Al Nanocrystals..............................................................52
4.2.3 Deformation Mechanism.....................................................................................54
4.2.4 A Schematic Model of Crystallization by Ball milling and Subsequent Heat
Treatment.....................................................................................................................55
4.2.5 Local Structure of Al Ni La As-atomized Powder after Ball milling............56
85 10 5
4.2.6 Shear Bands Induced by Ball milling.................................................................60
5 Local Structure in Microalloyed Al-based Metallic Glasses........................................65
5.1 Results..........................................................................................................................65
5.1.1 Ni and Co edge on Al TM RE .........................................................................65
86 8 6
5.1.2 Substitution of Ni by Elements with Various Atomic Radii in an Al-based
Amorphous Alloy.........................................................................................................69
5.1.2.1 Local Structure of Ge Al Ni Y ............................................................71
1 86 7 6
5.1.2.2 Local Structure of Pd Al Ni Y .............................................................74
1 86 7 6
5.1.2.3 Local Structure of Ag Al Ni Y and Ag Al Ni La .............................76
1 86 7 6 1 87 5 7
5.1.2.4 Local Structure of Zr Al Ni Y and Zr Al Ni La ...............................80
1 86 7 6 1 87 5 7
5.2 Discussion....................................................................................................................84
5.2.1 Site Substitution of Ni by Co in Amorphous Al Ni Co Y La Alloy...........84
85 6 2 4.5 1.5
5.2.2 Glass-Forming-Ability Improvement by Increasing the Mixing Entropy..........86
5.2.3 The Cluster Line Model......................................................................................89
5.2.4 Dense Random Packing of Hard Spheres...........................................................90
6 Summary and Conclusions..............................................................................................97