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@PHDTHESIS{Evertz:808513,
author = {Evertz, Simon},
othercontributors = {Schneider, Jochen M. and Dehm, Gerhard},
title = {{Q}uantum mechanically guided design of mechanical
properties and topology of metallic glasses},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2020-12118},
pages = {1 Online-Ressource : Illustrationen, Diagramme},
year = {2020},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University 2021; Dissertation, Rheinisch-Westfälische
Technische Hochschule Aachen, 2020, Kumulative Dissertation},
abstract = {Metallic glasses are promising structural materials due to
their unique property combinations such as high fracture
toughness and high strength. For structural applications and
processing, the coefficient of thermal expansion is an
important design parameter. Here, it is demonstrated that
predictions of the coefficient of thermal expansion for
metallic glasses by density functional theory based ab
initio calculations are efficient both with respect to time
and resources. The coefficient of thermal expansion is
predicted by an ab initio based method utilising the
Debye-Grüneisen model for a Pd-based metallic glass, which
exhibits a pronounced medium range order. The predicted
coefficient of thermal expansion of 3.4∙10−5 K−1 at
room temperature iscritically appraised by in situ
synchrotron X-ray diffraction and excellent agreement is
observed. Through this combined theoretical and experimental
research strategy, the feasibility to predict the
coefficient of thermal expansion from the ground state
structure of a metallic glass until the onset of structural
changes is shown. This strategy provides a method to
efficiently probe a potentially vast number of metallic
glass alloying combinations regardingthermal expansion. For
the application of metallic glasses as structural materials,
high fracture toughness is crucialto avoid catastrophic
failure of the material in a brittle manner. One fingerprint
for fracture toughness in metallic glasses is the fraction
of hybridized bonds, which is affected by
alloyingPd57.4Al23.5Y7.8M11.3 with M = Fe, Ni, Co, Cu, Os,
Ir, Pt, and Au. It is shown that experimental fracture
toughness data is correlated to the fraction of hybridized
bonds which scale with the localized bonds at the Fermi
level. Thus, the localized bonds at the Fermi level are
utilized quantitatively as a measure for fracture toughness.
Based on ab initio calculations, the minimum fraction of
hybridized bonds was identified for Pd57.4Al23.5Y7.8Ni11.3.
According to the ansatz that the crystal orbital overlap
population at the Fermi level scales with fracture
toughness, for Pd57.4Al23.5Y7.8Ni11.3 a value of around 95
± 20 MPa·m0.5 is predicted quantitatively for the first
time. Consistent with this prediction, in micro-mechanical
beam bending experiments Pd57.4Al23.5Y7.8Ni11.3 thin films
show pronounced plasticity and absence of crack growth. As
the properties of metallic glasses depend on the electronic
structure, which in turn is definedby chemical composition,
the influence of metalloids such as B on glass transition,
topology, magnetism, and bonding is investigated
systematically for B concentrations x = 2 to 92 $at.\%$
inthe (Co6.8±3.9Ta)100-xBx system. From an electronic
structure and coordination point of view, theB concentration
range is divided into three regions: Below 39 ± 5 $at.\%$
B, the material is a metallic glass due to the dominance of
metallic bonds. Above 69 ± 6 $at.\%,$ the presence of an
icosahedra-like B network is observed. As the B
concentration is increased above 39 ± 5 $at.\%,the$ B
network evolves while the metallic coordination of the
material decreases until the Bconcentration of 69 ± 9
$at.\%$ is reached. Hence, a composite is formed. It is
evident that, based on the B concentration, the ratio of
metallic bonding to icosahedral bonding in the compositecan
be controlled. It is proposed that, by tuning the
coordination in the composite region, glassy materials with
defined plasticity and processability can be designed. While
it is accepted that the plastic behaviour of metallic
glasses is affected by their free volume content, the effect
thereof on chemical bonding has not been investigated
systematically. According to electronic structure analysis,
the overall bond strength is not significantly affected by
the free volume content. However, with increasing free
volume content, the average coordination number decreases.
Furthermore, the volume fraction of regions containing atoms
with lower coordination number increases. As the local
bonding character changes from bonding to anti-bonding with
decreasing coordination number, bonding is weakened in the
volume fraction of lower coordination number. During
deformation, the number of strong, short-distance bonds
decreases more for free volume containing samples than for
samples without free volume, resulting in additional bond
weakening. Thus, it is shown that the introduction of free
volume causes the formation of volume fractions oflower
coordination number resulting in weaker bonding and proposed
that this is the electronic structure origin of the enhanced
plastic behaviour reported for glasses containing free
volume.},
cin = {521110 / 520000},
ddc = {620},
cid = {$I:(DE-82)521110_20140620$ / $I:(DE-82)520000_20140620$},
pnm = {SPP 1594 - Topological Engineering of Ultrastrong Glasses
(198574154)},
pid = {G:(GEPRIS)198574154},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2020-12118},
url = {https://publications.rwth-aachen.de/record/808513},
}
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