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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 944 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Electro-chemo-mechanical coupling of nanoporous gold at the microscale(2022); Its tunable strengthening, stiffening and actuation behavior, induced by a change of surface state controlled via electrochemical potential, make nanoporous gold (NPG) a promising candidate for many applications. Due to the complex network structure, a fundamental understanding of the functional behavior of NPG requires accurate measurements of the electro-chemo-mechanical coupling. While many experiments at the macro-scale were published, only few experiments have been carried out at the micro-scale. In this work, the micromechanical testing with and without electrochemical control has been carried out, using a modified nanoindentation system with which the effects of deformation length-scale and microstructural length-scale are investigated. Based on the results of nanoindentation and microcompression without electrochemical environment, the elastic modulus, strength and hardness show a decrease with increasing ligament size. By varying ligament size (L) and sample diameter (D) of NPG micropillars, a critical ratio (α=D/L=20) was found, above which the test structure can be considered a representative volume element (RVE) resulting in reproducible stress-strain behavior and uniform deformation. Below this critical ratio, both flow stress and elastic modulus decrease with decreasing pillar diameter, as evidenced for a fixed ligament size of L=350 nm. Stochastic behavior along with non-uniform deformation for α<10, indicate that the size of the load-bearing unit is close to 10 times the corresponding ligament size. In the case of in situ microcompression, a novel loading profile was developed to decouple the contribution of displacement due to the actuation from the compression-induced deformation. The flow stress of pillars under potential jumps exhibited the same trend as the corresponding macroscopic results; strength is enhanced significantly due to the surface adsorption, and this response is reversible. The amount of previous deformation of pillars with a clean surface has no impact on the subsequent flow stress coupled with adsorption. The elastic modulus was found not to depend on the potential, in contrast to what was found with dynamic mechanical analysis at the macro-scale. The stress-strain curves of pillars with varying α indicate that the relative change in strength induced by adsorption (∆σ/σ_off) decreases with increasing ligament size, i.e. the absolute change in strength and the strength itself scale differently with the ligament size. A change in pillar size does not impact the ∆σ/σ_off for a fixed ligament size, even though α is below the ratio of the RVE for the mechanical behavior.Publicationtype: Doctoral Thesis285 942 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , On the factors impacting the mechanical response of nanoporous gold-polymer compositesNanoporous gold (NPG) is a bicontinuous network of nanoscale gold ligaments and pores, the dimensions of which can be adjusted to exploit strengthening at small ligament sizes. Nevertheless, its mechanics are limited by the lack of ductility under tensile loading. Recently, the impregnation of the open porous structure with an epoxy phase created a metal-polymer composite that combined strengthening at small structural length-scales with enhanced tensile ductility. The present work aims to clarify the effect of factors influencing the mechanical behaviour of composite materials, particularly the properties of the epoxy phase and interface – properties that are still unexplored in NPG-epoxy composites. To this end, a series of novel approaches were developed to investigate the independent contribution of each factor, focusing on the development of representative model samples, micromechanical testing, and finite element simulation. A novel method to understand the effect of the composite microstructure in the epoxy was made possible by isolating the matrix phase. Taking inspiration from mining techniques, an approach based on gold etching in iodine-iodide solutions successfully leached out the Au from NPG-polymer composites. The remaining bi-continuous open porous polymer (NPP) was investigated using nanoindentation to explore possible size effects as those found in nanoporous gold. The interface was investigated using a model system where a single planar boundary between gold and epoxy was achieved. Using both nanoindentation approaches and novel microshear experiments, the interphase regime and interfacial strength were measured. Moreover, the experimental results inspired a modelling approach of the interfaces for finite element simulations, which was employed to study the interfacial behaviour as a function of the balance between the properties of constituent phases and interfaces. The interfacial model was also employed to simulate the effect of the interfacial strength on the mechanical response and delamination evolution of NPG-epoxy composites. The findings from the investigations on the isolated factors improved the interpretation of the composite mechanical response. No evidence of a structural length-scale effect in the polymer was found, and the response of the isolated polymer phase could be well predicted by an isostrain law applied to the properties measured on the bulk polymer. Furthermore, no evidence supporting the presence of an interlayer was found between the gold and epoxy phases. The measurement of the interfacial strength showed that the interface in NPG-epoxy composite is the weakest link in the composite's deformation, which was also observed in the simulations. The isolated effect of the interfacial properties was attenuated in the composite by the interconnected structure, which in compression allows the load transfer even after interface failure.Publicationtype: Doctoral Thesis218 1483 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Micromechanical and three-dimensional microstructural characterization of nanoporous gold-epoxy composites(2017); Nanoporous gold (npg), a bicontinuous network of nanoscale gold ligaments and pores, displays tunable mechanical behavior through the variation of internal length-scales. However, it is severely limited by its lack of ductility in tension. By infiltrating the porous structure with epoxy, a composite material with enhanced tensile ductility and ow stress is achieved. The present work aims at a three-fold investigation of micromechanical behavior, mechanisms of deformation and failure of the npg-epoxy composite, focusing on 3D microstructural characterization, finite element simulation and micromechanical testing. In order to understand the composite microstructure, high resolution 3D reconstructions of the npg ligament network were achieved with focused ion beam (FIB) based tomography, taking advantage of epoxy infiltration on FIB machining of porous media. It is assumed that the infiltration of epoxy does not significantly alter the geometry of the npg network. Samples of varying structural length-scales were used, with mean ligament diameters in the range of tens to hundreds of nanometers, as achieved through isothermal annealing of npg. Quantitative analyses of the 3D reconstructions were carried out in terms of metric properties (e.g., relative density, ligament diameter distribution and specific surface area), topological properties (e.g., connectivity density), and directional properties (e.g., directional tortuosity). Importantly, representative volumes (RVs) were identified, which reflects the global structural properties of the material. It was found that npg coarsens in a nearly self-similar manner. This allows the identification of structural parameters, which can be used to describe the mechanical behavior of the global npg structure over varying length scales. FEM simulations applied to meshed RVs of the 3D reconstructions strongly suggest that the effective relative density of the load bearing ligament structure is the critical structural parameter in determining the mechanical behavior of npg structural geometry, rather than the solid relative density alone, as is often assumed. After infiltration with epoxy, the densification during deformation of npg-epoxy composites is strongly suppressed. This leads to a strongly enhanced strength compared to pure npg. However, the dependence of the yield strength on the mean ligament size is much weaker in the composite structure as compared to pure npg. The size effects in pure npg have been commonly attributed to dislocation activities within the gold ligaments, whereas in the composite material the influence of the interface on the motion of the dislocations and the epoxy chains must be considered. The interfacial behavior leads to a strengthening effect in the composites but weakens the size effects. The comparisons of the elastic moduli from the analytical models predictions, the experiments and the FEM simulations demonstrated the influences of the connectivity, the effective relative density of the load bearing ligament structure, the directional tortuosity of the ligaments and the interface on the elastic behavior of the npg-epoxy composites. Interfacial failure was experimentally observed under compression and tension. The tension-compression asymmetry investigation revealed that the npg-epoxy composite is stronger in compression than in tension. These various observations point to the important roles that the hard phase, i.e., the interconnected ligament network, the soft phase, i.e., the continuous epoxy, and the interface between them play on the mechanical behavior of npg-epoxy composites.Publicationtype: Doctoral Thesis753 615 - Some of the metrics are blocked by yourconsent settings
Item type:Publication, Access status: with files , Micromechanisms governing plastic instability in Al–Li based alloys(2015); The investigation of the underlying microscopic mechanisms that govern plastic in- stability in solution strengthened and precipitation strengthened Al alloys has been the subject of several studies. These studies are largely motivated by the need to devise strategies to mitigate the undesirable effects, such as reduction in ductility and formation of surface striations, which occur in alloys that exhibit this phenomenon. While the microscopic origin of plastic instability in solution strengthened alloys Al alloys is fairly well established, there is yet no convincing model that is consistent with experimental observations and gives a clear mechanistic description of the origin of the phenomenon in precipitation strengthened Al alloys. In this work, detailed experimental investigations of several tempers of a multi– component Al–Li based alloy, AA2198, have been carried out. Both mechanical and mi- crostructural characterization techniques were employed in order to correlate microstruc- tural characteristics to global and local mechanical behaviour. Specifically, high resolution nanoindentation and micro–tensile testing were used for mechanical testing, while trans- mission electron microscopy based methods – including in situ TEM tensile straining, along with high energy x–ray diffraction and atom probe tomography were used to inves- tigate the relevant microstructural characteristics. The experimental results clearly showed that dynamic strain aging of temporarily trapped mobile dislocations by Li atoms, widely accepted as the underlying mechanism for plastic instability in Al–Li based alloys, cannot sufficiently account for the occurrence of plastic instability in AA2198. Moreover, theoretical analyses of strengthening mecha- nisms in the investigated tempers showed that only the overaged temper, which is also the only temper that displayed plastic instability, is governed by order hardening. In light of the wealth of experimental results, a mechanistic model describing the microscopic mechanisms underlying plastic instability in precipitation strengthened Al–Li based alloy systems was developed. It is proposed that the phenomenon is governed by an altogether different mechanism than what has so far been considered, namely a diffusion controlled pseudo–locking mechanism that accompanies order hardening at low strain rates. The applicability of the model to other Al–Li alloy based systems was also examined. It was demonstrated, by critical examination of the instability behaviour of a number of binary and multi–component Al–Li based alloy systems reported in literature, that plastic instability only occurs in these alloy systems when strength is governed by order hardening.Publicationtype: Doctoral Thesis711 726