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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