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@PHDTHESIS{Zubair:839718,
author = {Zubair, Muhammad},
othercontributors = {Korte-Kerzel, Sandra and Tasan, Cemal Cem},
title = {{C}o-deformation of metallic and intermetallic phases in
{M}g-{A}l-{C}a alloys},
school = {RWTH Aachen University},
type = {Dissertation},
address = {Aachen},
publisher = {RWTH Aachen University},
reportid = {RWTH-2022-00793},
pages = {1 Online-Ressource : Illustrationen, Diagramme, Karten},
year = {2022},
note = {Veröffentlicht auf dem Publikationsserver der RWTH Aachen
University; Dissertation, RWTH Aachen University, 2022,
Kumulative Dissertation},
abstract = {Mg-Al-Ca alloys have a dual phase microstructure comprising
a soft α-Mg phase reinforced with a hard intermetallic
interconnected Laves phase skeleton. The excellent creep
properties of these alloys are attributed to the presence of
Laves phases in the microstructure. However, it is not
entirely clear how the amount, type, and morphology of the
Laves phases can affect the elevated temperature tensile and
creep properties of these alloys. Furthermore, the two
mechanically and crystallographically different phases
(α-Mg and Laves phases) provide an opportunity to study the
co-deformation behaviour of such heterogeneous materials.
This thesis, therefore, focuses on the two main aspects: i)
effect of Laves phases on the mechanical properties of
Mg-Al-Ca alloys and ii) co-deformation of metallic and
intermetallic phases. The Ca/Al ratio can be used to
manipulate the amount, type, and morphology of Laves phases.
Therefore, three different Mg-Al-Ca alloys with varying
Ca/Al ratio (Ca/Al: 0.32, 0.62 and 1.03) were produced. The
alloys were microscopically and mechanically investigated
using SEM, EDS, EBSD, micro-hardness, tensile and creep
testing. The results show that an increase in Ca/Al ratio
from 0.32 to 1.03 results in a higher volume fraction of
Laves phase in the as-cast microstructure, higher yield
strength, UTS and better creep properties at a temperature
of 170 °C. However, the alloy with the highest Ca/Al ratio
exhibits lowest ductility. The co-deformation mechanisms of
the same Mg-Al-Ca alloys were studied using DIC, quasi
in-situ tensile deformation in SEM (at 170 °C), EBSD, and
TEM. The strain maps obtained from DIC experiments showed
that the strain is highly heterogeneous at the
microstructural level and tends to concentrate along slip
lines and twins in the α-Mg phase and along the α-Mg/Laves
phase interfaces. Moreover, it was found that cracks
preferentially nucleate in the Laves phase at the
intersections of slip in the α-Mg and Mg-Laves phase
interfaces as well as at the intersections of twins in the
matrix and Mg-Laves phase interfaces. Consequently, cracks
in the Laves phase were mainly observed in microstructural
regions that underwent significant basal slip and tensile
twinning. Euler number analysis also confirmed that the
interconnectivity of the Laves phase decreases with
deformation because of cracking. In addition to cracks in
the Laves phase, slip transfer was also observed in the
(Mg,Al)2Ca phase at strain concentration points. Atomistic
simulations of the Mg/Mg2Ca system confirmed that
dislocation slip in the Mg2Ca phase was triggered by the
interaction of basal dislocation of the Mg matrix with the
interface. However, the slip transfer mechanisms across the
Mg/Mg2Ca interface were affected by temperature and
orientation relationship between both phases. In line with
atomistic simulations, basal slip lines in the (Mg,Al)2Ca
phase were also observed at strain concentration points in
deformed alloys investigated using SEM and TEM. The
co-deformation mechanism based on experimental results is
proposed within this thesis. Finally, nanoindentation with
constant and variable strain rate was conducted to determine
the mechanical properties and thermally activated
deformation mechanisms of the individual phases in Mg-Al-Ca
alloys. It was observed that the hardness of the α-Mg phase
decreases with temperature while that of the Laves phases
stays constant until 200 °C. The strain rate sensitivity,
m, was nearly the same for α-Mg and α-Mg/Laves interfaces
while the activation volume was lower for indents made
across interfaces. Nanoindentation creep tests indicated
that the creep resistance of the Mg2Ca phase is higher than
that of the α-Mg phase. The findings of this thesis provide
valuable insights for the design of creep resistant Mg-Al-Ca
alloys by manipulating the Ca/Al ratio. Additionally, the
methods involved in this thesis are generally applicable to
study the co-deformation of multiphase alloys with
mechanically heterogeneous microstructural components. They
may, therefore, be of interest to researchers working on
other multi-phase alloys such as dual-phase steels or
titanium alloys.},
cin = {523110 / 520000},
ddc = {620},
cid = {$I:(DE-82)523110_20140620$ / $I:(DE-82)520000_20140620$},
pnm = {SFB 1394 C02 - Ko-Verformung von (inter)metallischen
Kompositen (C02) (437514011) / DFG project 409476157 - SFB
1394: Strukturelle und chemische atomare Komplexität
$\u2013$ Von Defekt-Phasendiagrammen zu
Materialeigenschaften (409476157)},
pid = {G:(GEPRIS)437514011 / G:(GEPRIS)409476157},
typ = {PUB:(DE-HGF)11},
doi = {10.18154/RWTH-2022-00793},
url = {https://publications.rwth-aachen.de/record/839718},
}
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