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Effects of twinning and detwinning on the mechanical behavior of Mg and Mg-Gd alloy at the micron scale
Citation Link: https://doi.org/10.15480/882.5052
Publikationstyp
Doctoral Thesis
Date Issued
2023
Sprache
English
Author(s)
Advisor
Title Granting Institution
Technische Universität Hamburg
Place of Title Granting Institution
Hamburg
Examination Date
2022-10-28
TORE-DOI
Citation
Technische Universität Hamburg (2023)
In addition to dislocation slip, deformation twinning also plays an important role in
plasticity of magnesium (Mg). While there have been many numerical and experimental
studies of various aspects of twinning in polycrystalline Mg, little has been done to understand
the role of an individual twin, uncoupled from the effect of its grain neighborhood, on the
deformation of Mg. For this kind of analysis, micromechanical characterization has a great
advantage due its site-specific nature. Furthermore, twinning behavior can also be strongly
influenced by the presence of alloying elements. In this work, the scientific objective was
to understand how a single twin boundary can affect the concurrent slip activity during
twinning and detwinning in two material systems: (i) pure Mg and (ii) Mg-4wt.% Gd binary
alloy, where Gd solutes can segregate into twin boundaries upon thermal treatment. Such a
comparison of the cases (i) and (ii) could answer the question of how solutes influence the
twinning, detwinning and slip behavior at room temperature. To this end, microcompression
experiments were performed on differently oriented micropillars in the two materials: slipfavored
[0001] single crystals, twinning-favored [10-10] single crystals, and detwinning-favored
bicrystals including the both orientations.
Microcompression along with TEM observations confirmed that Mg [0001] single crystals,
as reference, deform by a typical stair-case c-axis hardening flow curve which is mainly
governed by (double) cross-slip of pyramidal <c + a> dislocations. The [10-10] single- and
[0001]/[10-10] bicrystalline micropillars underwent twinning and detwinning, respectively, leading
to a reorientation into the nominal [0001] orientation. Further compression of the (de)twinned
micropillars exhibited strong c-axis hardening with a considerably higher yield stress and
strain hardening rate than the reference samples. TEM investigations highlighted a unique
defect structure in the wake of a migrated twin boundary, comprised of a dense array of
basal stacking faults (SFs) along with numerous dislocations both of which are considered as
immobile and thus serve as a source for the observed (de)twinning-mediated hardening.
Mg-Gd micropillars with equal orientations as in the pure Mg were tested in the nonheat-
treated alloy as well as after heat-treatment at 300°C for 3.5 hours. Alloying with
Gd strengthened pyramidal slip and (de)twinning stress-strain characteristics. Such a solute
strengthening found to be the highest for the detwinning mechanism in the heat-treated
bicrystalline micropillars, where the pre-existing twin boundary was decorated with Gd after
heating, as confirmed by STEM-EDS analysis. The comparison of the normalized resolved
shear stresses at the onset of pyramidal slip and (de)twinning showed that the relative activity
of these deformation modes is modified in the Mg-Gd system. The experimental results in this
work suggest that the critical aspects regarding the twinning microstructure and the effects of
solutes on concurrent slip and twinning must be included in any continuum-based modelling
approach for understanding the deformation of Mg.
plasticity of magnesium (Mg). While there have been many numerical and experimental
studies of various aspects of twinning in polycrystalline Mg, little has been done to understand
the role of an individual twin, uncoupled from the effect of its grain neighborhood, on the
deformation of Mg. For this kind of analysis, micromechanical characterization has a great
advantage due its site-specific nature. Furthermore, twinning behavior can also be strongly
influenced by the presence of alloying elements. In this work, the scientific objective was
to understand how a single twin boundary can affect the concurrent slip activity during
twinning and detwinning in two material systems: (i) pure Mg and (ii) Mg-4wt.% Gd binary
alloy, where Gd solutes can segregate into twin boundaries upon thermal treatment. Such a
comparison of the cases (i) and (ii) could answer the question of how solutes influence the
twinning, detwinning and slip behavior at room temperature. To this end, microcompression
experiments were performed on differently oriented micropillars in the two materials: slipfavored
[0001] single crystals, twinning-favored [10-10] single crystals, and detwinning-favored
bicrystals including the both orientations.
Microcompression along with TEM observations confirmed that Mg [0001] single crystals,
as reference, deform by a typical stair-case c-axis hardening flow curve which is mainly
governed by (double) cross-slip of pyramidal <c + a> dislocations. The [10-10] single- and
[0001]/[10-10] bicrystalline micropillars underwent twinning and detwinning, respectively, leading
to a reorientation into the nominal [0001] orientation. Further compression of the (de)twinned
micropillars exhibited strong c-axis hardening with a considerably higher yield stress and
strain hardening rate than the reference samples. TEM investigations highlighted a unique
defect structure in the wake of a migrated twin boundary, comprised of a dense array of
basal stacking faults (SFs) along with numerous dislocations both of which are considered as
immobile and thus serve as a source for the observed (de)twinning-mediated hardening.
Mg-Gd micropillars with equal orientations as in the pure Mg were tested in the nonheat-
treated alloy as well as after heat-treatment at 300°C for 3.5 hours. Alloying with
Gd strengthened pyramidal slip and (de)twinning stress-strain characteristics. Such a solute
strengthening found to be the highest for the detwinning mechanism in the heat-treated
bicrystalline micropillars, where the pre-existing twin boundary was decorated with Gd after
heating, as confirmed by STEM-EDS analysis. The comparison of the normalized resolved
shear stresses at the onset of pyramidal slip and (de)twinning showed that the relative activity
of these deformation modes is modified in the Mg-Gd system. The experimental results in this
work suggest that the critical aspects regarding the twinning microstructure and the effects of
solutes on concurrent slip and twinning must be included in any continuum-based modelling
approach for understanding the deformation of Mg.
DDC Class
600: Technik
620: Ingenieurwissenschaften
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