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Item type:Publication, Access status: without files , Effect of Gd solutes on the micromechanical response of twinning and detwinning in Mg(2023-10-01); ; ; ; An investigation of the effect of solutes on the migration of a single {101¯2} Gd decorated twin boundary in a Mg-4wt.% Gd binary alloy in terms of the corresponding stress-strain response and the defect structure in the wake of (de)twinning was undertaken in this work. The results were benchmarked against those of a pure Mg sample that was tested under same conditions. We established that the critical stresses for both twinning and detwinning increased by 29% and 10%, respectively, due to the addition of Gd. We showed that the latter is mediated by the pinning effect of Gd atoms that decorated the pre-existing twin boundaries in the alloy. Conversely, a 68% decrease in the detwinning stress was recorded in pure Mg. We attributed the decrease to the nucleation-free nature of detwinning along with the absence of solutes atoms at the pre-existing twin boundaries. Unlike previous investigations, the work does not just highlight how twinning and detwinning affects the concurrent slip activity in pure Mg and Mg alloys, it establishes a direct link between these activities and the magnitude of the observed mechanical response.Publicationtype: Journal Article18 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Effects of twinning and detwinning on the mechanical behavior of Mg and Mg-Gd alloy at the micron scaleIn 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.Publicationtype: Doctoral Thesis76 936 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , On the Influence of Twinning and Detwinning on the Deformation of Mg at the Micron ScaleThe influence of twinning and detwinning on the constitutive mechanical response of Mg was investigated via microcompression tests of single-crystalline pillars of nominal [0001] and [10-10] orientations, and bicrystalline pillars containing a single twin boundary. The [0001] pillars exhibit the highest initial yield strength while the [10-10] and bicrystalline pillars initially yield at significantly lower stress, at which twin nucleation and growth or migration commence. Depending on the extent of straining, the [10-10] and bicrystalline pillars exhibit a secondary yield point associated with the deformation of the newly formed [0001] oriented pillar. The mechanical results point to an anisotropy in the mechanical consequence of twin motion, as characterized through the comparison of the three micropillar orientations; a twin-mediated hardening is indicated by the relative stress-strain behavior.Publicationtype: Conference Paper115 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: without files , Detwinning-mediated hardening in Mg: A microcompression study of a single twin boundary(2020-01-20); ; We present the first experimental measurement of the hardening arising from the detwinning of a single twin boundary in Mg. Microcompression tests were performed on two sets of microcolumns: (i) single crystals having a “parent” grain orientation of nearly (0 0 0 1) along the compression axis, and (ii) bicrystals involving the parent grain and a single 101‾2 twin boundary. A comparison of the stress-strain data shows significant differences in the deformation characteristics of the deformed microcolumns. The bicrystalline microcolumns undergo detwinning, as indicated by a stress plateau, which leads to a nominally single crystalline microcolumn of the orientation of the parent grain. Microcompression beyond the plateau shows that the detwinned microcolumns exhibit a considerably higher yield stress and strain hardening rate than the single crystalline “parent” microcolumns. Electron back-scattered diffraction analyses on the cross-sections of the deformed microcolumns reveal higher misorientations in the detwinned region indicative of a high content of unpaired dislocations (so-called GNDs), compared to the deformed parent microcolumns. This discrepancy in misorientation distribution between the samples is consistent with a detwinning-mediated hardening response, as observed, and points to the creation of dislocation debris as a consequence of the detwinning process.Publicationtype: Journal Article166