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@PHDTHESIS{Mouhib:979734,
author = {Mouhib, Fatim-Zahra},
othercontributors = {Korte-Kerzel, Sandra and Sinclair, Chad},
title = {{S}ynergistic effects in magnesium-rare-earth-zinc alloys -
a gateway to informed alloy design},
school = {Rheinisch-Westfälische Technische Hochschule Aachen},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2024-01797},
pages = {1 Online-Ressource : Illustrationen},
year = {2024},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, Rheinisch-Westfälische Technische
Hochschule Aachen, 2024, Kumulative Dissertation},
abstract = {In dilute magnesium (Mg) alloys, solute effects were proven
crucial for microstructure and texture development rendering
a deep understanding of solute-solute and solute-defect
interactions indispensable for property predictions and
alloy design. This dissertation aimed at clarifying the
underlying factors contributing to the formation of texture
components deviating from the common basal texture in
ternary magnesium-rare earth (RE) -zinc (Zn) alloys. The
main focus was on understanding the role of solute defect
interactions and resulting synergistic solute effects. A
multi-scale characterization involving classical electron
backscatter diffraction (EBSD) analysis, mechanical testing,
and atom probe tomography (APT) was adopted to establish
microstructure correlations concerning atomic scale
segregation phenomena, slip activation and arising material
properties. Binary and ternary dilute Mg-Gd-(Zn), Mg-Er-(Zn)
and Mg-Ca-(Zn) alloy sheets were investigated subsequent to
rolling deformation and various heat treatments. The results
confirmed the formation of a unique ±40° transverse
direction (TD) recrystallization texture in ternary Mg-RE-Zn
alloys. A texture transition from an RD to an TD dominated
texture was found to take place during the early stages of
recrystallization. To understand texture selection during
recrystallization, further investigations on nucleation and
early nucleus growth during static recrystallization were
carried out. An as-cast Mg-Gd-Zn sample was subjected to
uniaxial compression at 200°C up to $40\%$ strain and
subsequently cut into two mirroring sample halves of which
one was annealed for 60 min. Classical and quasi-in-situ
EBSD analysis proved the importance of specific nucleation
sites, especially compression-tension double twins, to the
final texture. Quasi-insitu EBSD investigations revealed
selective growth of off-basal texture components, which
would ultimately dominate the resulting recrystallization
texture. Combined solute effects and their dependence on the
solute ratio, were explored by comparing solute segregation
and texture formation of three Mg-Gd-Zn alloys, with varying
Gd:Zn ratios of 2:1, 1;1 and 1:2. A low Gd:Zn ratio led to
higher solute segregation and a more pronounced TD texture.
Solute clusters with a ratio of approximately 0.33 were
found in all alloys indicating that the formation of
effective solute clusters may require a high Zn
concentration. These results prove that alterations of the
solute ratio rather than the absolute solute concentration
govern segregation and texture formation. A possible
replaceability of RE solutes by Ca, was addressed by EBSD
investigations, atom probe tomography and ab initio
calculations of binary and ternary Mg-Gd-(Zn) and Mg-Ca-(Zn)
alloys. Texture investigations showed similar off-basal
texture trends in Mg-Ca alloys compared to RE containing
alloys. However, significant combined solute effects were
observed in ternary RE containing alloys, e.g. Mg-Gd-Zn,
rather than the counterpart Ca alloy. Ab initio calculations
of binding energies revealed preferential Ca-Zn binding
compared to Ca-Zn. Additionally, solute-vacancy binding was
investigated to estimate the behaviour of solutes in vacancy
rich regions, e.g. grain boundaries, and revealed that
solute-vacancy binding is preferential for Ca rather than
Gd, which may explain the off-basal texture trends emerging
in binary Mg-Ca. In terms of mechanical properties, Mg-Er-Zn
and Mg-Gd-Zn both exhibited an enhancement in the yield
strength, strain hardening capability, and failure ductility
compared to binary alloy systems. EBSD assisted slip trace
analysis at $5\%$ strain showed promoted non-basal slip in
ternary alloys compared to the binary counterpart alloy.
Alterations of the Gd:Zn ratio lead to further enhancements
of non basal slip activation for lower Gd:Zn ratios.
Additionally, a RD/TD yield stress anisotropy, equally
scaling with the Gd:Zn ratio, was observed and attributed to
an increased activation of twinning under strain in TD. To
explore alternative deformation modes in Mg-Ca alloys
synchroshear in Mg2Ca Laves phases was investigated by
classical atomistic simulation and ab initio calculations.
It was proven that synchro-shear is energetically favourable
and therefore the main mechanism for dislocation motion on
the basal plane.},
cin = {523110 / 520000},
ddc = {620},
cid = {$I:(DE-82)523110_20140620$ / $I:(DE-82)520000_20140620$},
pnm = {DFG project 394480829 - Zur Korngrenzsegregation in
Magnesiumlegierungen und deren Einfluss auf die Einstellung
der Mikrostruktur und der mechanischen Eigenschaften
(394480829)},
pid = {G:(GEPRIS)394480829},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2024-01797},
url = {https://publications.rwth-aachen.de/record/979734},
}
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