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@PHDTHESIS{Niersbach:971116,
author = {Niersbach, Till Christian},
othercontributors = {Kargl, Florian and Bührig-Polaczek, Andreas},
title = {{A}ssessment of a universal nucleation and crystal growth
mechanism in undercooled alloys with orthorhombic
{C}r{B}-type structure},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2023-09478},
pages = {1 Online-Ressource : Illustrationen, Diagramme},
year = {2023},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2023},
abstract = {Electrostatic (ESL) and electromagnetic (EML) levitation
techniques were utilized to investigate the nucleation and
crystal growth in binary intermetallic melts, that solidify
in CrB-type structure. The high purity alloying and
subsequent contact free processing allowed the systems to
stay liquid at substantial under coolings below the
equilibrium melting temperature. The subsequent nucleation
event was observed insitu and the microstructure of the
solid sample was analyzed by optical and electron
microscopy. EBSD analysis was used to obtain a better
understanding through crystal-structure evaluation on the
mechanisms at play. Binary, intermetallic (A50B50) alloys
were chosen that are supposed to solidify inthe orthorhombic
CrB-type crystal structure. The present work bases on a
specialnucleation and growth mechanism, found by Kobold in
Ni50Zr50. Based on these observations, the
Hornfeck-Kobold-Kolbe growth model was developed, which
connects a quasi crystalline (QC) core structure with a
ten-fold twinned microstructure. According to the model,
other n-fold symmetries are possible in CrB-structures
alloys, based on their respective lattice parameters. This
work assesses a part of these CrB-structured alloys in order
to show the universal applicability of this model to
orthorhombic systems. Based on the prototype system of NiZr,
crucial features are stated, that are necessary for
nucleation and growth based on the model. Besides the
singular nucleation event and the icosahedral based nucleus
structure, especially the orthorhombic unit cell of the
CrB-structure (B33 phase) is of importance. It was shown,
that the lattice parameter ratio a/b is responsible for the
n-fold symmetry, since the twinning boundaries of the model,
creating ten differently orientated grains around a
pointsymmetric center, run along the diagonal of the unit
cell. In this work, systems were chosen, based on the
expected n-fold symmetry andit will focus on the
investigations of the ten-fold (Ni50Hf50, Ni50Zr25Hf25),
nine-fold(Ni50Gd50) and eight-fold (Ni50B50) systems. It
will give an analysis on whether their behavior can be
described by the proposed growth model and its crucial
features.Ni50Hf50 and Ni50Zr25Hf25 were chosen, since Hf and
Zr are very similar and almost completely miscible.
Additionally, a ten-fold growth was expected. Both systems
meet the expectations as most of the crucial features are
observed. These systems clearly show a singular nucleation
event, the growth front can be observed in situ and
connected with the front of the NiZr system. In both
systems, a single growth direction is present, with all
grains orientated around one common [001] direction. InNiHf
however, no singular growth structure could be identified in
the microstructureanalysis. Instead, several smaller
point-symmetric structures were found, that resemble the
symmetric core structure found in NiZr. A solid-solid
structural transition was found to disturb the optimal
growth. Several nucleation modes are proposed i this work,
and described, depending on the exact under cooling and
which phase is stable upon nucleation. Only one mode, with
the initial nucleation in the orthorhombicB33 phase, and two
subsequent solid-solid transitions, can be connected tothe
growth model. With the latent heat released during
nucleation, the system transitions into the high temperature
(HT) B2 phase. Upon cooling, all samples transition (back)
into the B33 phase. An initial nucleation in the cubic B2
phaseresults in different features, such as multiple growth
directions. Despite the twotransitions following the initial
B33 nucleation (B33→B2→B33) the microstructurecould
preserve the singular growth direction and some of the
symmetric structuresthat can be connected to tertiary
dendrites of the growth model. In Ni50Zr25Hf25, this effect
is not that heavily pronounced. The solid-solid transition
is present and itdisturbs the microstructure. However, a
singular underlying growth structure can still be identified
in the microstructure. Other crucial features are met,
according to the model, just as in NiHf.Ni50Gd50 was the
first system with a predicted uneven (nine-fold) symmetry.
As the model is based on an icosahedral core structure,
disturbances in a stable growth were anticipated. But,
despite only small undercoolings reached, the system
exhibitsa singular common growth direction with several
symmetric structures andthe expected grain boundary angle.
Only incomplete symmetric structures couldbe found, though.
Due to the low undercoolings, modifications had to be made,
to explain this microstructure in accordance to the proposed
model. A heterogeneous growth front propagates through the
melt and symmetric structures are formed regularly due to
stacking faults or impurities. These nuclei can act as
nucleation sites forthe model’s QC core structure. As
these structures have to compete in growth with the general
heterogeneous growth front, they take a cylindrical shape
and cannot grow through the whole of the sample. This can
also explain the deformations andmissing orientations found.
With Ni50B50, only small under coolings could be reached. It
was however possible,to connect this system to the proposed
model. A singular growth, throughout the whole sample, with
the expected eight-fold symmetry was observed. The twinning
boundaries exhibit the expected grain boundary angle, but
are highly distorted and shifted and it was shown, that
smaller symmetries shift the atomic arrangements, which, in
NiZr, extend almost perfectly along twinning boundaries and
impose therefore only a small energy barrier. Other systems
were investigated with expected symmetries from 8- to
11-fold. They are shortly described in this work and
evaluated. Some systems are promising andshould be
investigated further, others show no relation to the growth
model, e.g.when they solidify in the B2 structure. The
proposed nucleation and growth model, based on the NiZr
prototype system,was finally assessed, based on presence of
the defined crucial features in the investigated systems. It
is concluded, that the model can hold as a universal
characteristic in the liquid-solid phase transition of
CrB-structured alloys. Especially,the grain boundary angle
with the twinning boundary running through the diagonal of
the unit cell and the directly dependent n-fold symmetry are
highly preserved. The growth in a single direction is
another crucial feature, that is observed in basically every
case. Despite this, it is often difficult to observe the
perfect growth structure, as it was found in NiZr. It was
shown to be highly dependent on factors, such as solid-solid
transitions (Ni50Hf50), inner structural tensions
(Ni50B50)or disturbances during the formation of the core
structure (Ni50Gd50). Additionally, modifications were
implemented to adapt the model to a broad range of
nucleation events.},
cin = {523620 / 520000},
ddc = {620},
cid = {$I:(DE-82)523620_20140620$ / $I:(DE-82)520000_20140620$},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2023-09478},
url = {https://publications.rwth-aachen.de/record/971116},
}
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