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This dissertation proposes a multiscale Internal State Variable (ISV) inelasticity-corrosion damage model that is motivated by experimental microstructure-property relations of magnesium alloys. The corrosion damage framework was laid out based on observation of different corrosion mechanisms occurred on an extruded AM30 magnesium alloys. The extruded AM30 magnesium alloy was studied under two corrosion environments (cyclical salt spray and immersion) in order to observe the corrosion rates under different exposure environments. The coupons were examined at various times to determine the history effects of three corrosion mechanisms: (1) general corrosion; (2) pitting corrosion in terms of the nucleation rate, growth rate, and coalescence rate; and (3) intergranular corrosion. The multiscale ISV corrosion model was developed by bridging the macroscale corrosion damage to the mesoscale electrochemical kinetics, microscale material features, and nanoscale material activation energies. The corrosion testing results of Mg alloys (pure Mg, Mg-2% Al, and Mg-6% Al) were used to develop, calibrate, and validate the model, and good agreement was found between the model results and the corrosion testing data. Finally, the simultaneous effects of corrosion and cyclic loading were tested but not modelled for the extruded AM30 magnesium alloy by conducting fatigue experiments in a 3.5 wt.% NaCl solution environment. The corrosion fatigue life of the AM30 alloy was significantly reduced due to corrosion pit formation on specimen surface, hydrogen diffused into the material , and the fracture surface dissolved into the solution. The corrosion damage that arose on the fatigue specimens reduced the crack nucleation process and enhanced the crack propagation rate.

Methods designed for incorporation into multiscale modeling polycrystals are presented in this work in two tasks. This work contains mesoscale methods for capturing the effects of both the interactions of slip dislocations encountering twin grain boundaries and the simultaneous growth of multiple twin grain volume fractions on mechanical hardening and texture evolution. These are implemented in a crystal plasticity framework using the Los Alamos visco-plastic self consistent code, VPSC-7. Presented here, the effects of simultaneous growth in multiple twin variants on textural evolution is tracked using a Kalidindi-type twin volume transfer scheme. In Task 1, the implementation of this scheme in order to simulate the texture of Twinning Induced Plasticity steels (TWIP) subjected to Equal Channel Angular Pressing (ECAP) are summarized. In Task 2, the hardening effects of two types of interactions between slip dislocations and encountered twin grain boundaries, namely dislocation transmutation and dissociation, are captured by way of modifying the dislocation density based hardening model of [14]. Interactions of the first type are presented in a constitutive relation calculating the amount of dislocation density apportioned to a given slip system contained within the encountered twin volume fraction from each interacting slip system in the parent volume fraction. The amount transmuted from each interacting slip system described using the Correspondence Method, an onto mapping of slip systems in a parent grain to slip systems in considered twin grains. Interactions of the second type are then introduced into this constitutive relation as a disassociation parameter, the value of which is established by observations gleaned from the results of the molecular dynamics simulations of [11] and [36]. These methods are implanted to simulate the anisotropic hardening behavior of HCP magnesium under multiple load paths.

Magnesium alloys are excellent material candidate to reduce mass of automotive structures, and as such to meet the Department of Energy's targets in fuel economy and clean energy. However, magnesium alloys show poor ductility at room temperature, which is one of the most important impediments to achieving cost-effective manufacturing of wrought alloys and insuring good energy absorption in crash structures. This Master thesis aims to identify the mechanisms behind the low ductility of magnesium. Therefore, non-destructive EBSD analyses upon tension of both a strong and weak textured magnesium alloy were conducted with a focus on the role of twinning in fracture initiation. This study revealed five mechanisms responsible for early fracture, all of which relate to twinning activity. These mechanisms were involved directly in the shear incompatibility arising from interactions between twin-twin, twin-slip, twin-grain boundary, and double twinning. Backstress played a major role in twin-grain boundary and twin-twin boundary interactions.

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Sheet trimming is an important forming operation in stamping industry. However, trimming of automotive magnesium sheet materials is not well understood. The objective of present study was to investigate the trimming behavior of AZ31 and ZEK100 automotive magnesium sheet materials using a laboratory-based experimental set-up and complementary finite element (FE) simulations of the lab-based experiments. The effects of the trimming process parameters that included tool setup configuration, punch speed, clearance, sheet thickness and sheet orientation (rolling and transverse directions) on the quality of trimmed edge were analyzed. Experimental results indicated that the trimmed edge quality depended strongly on the trimming conditions. The optimal trimming parameters for AZ31 and ZEK100 sheets were experimentally obtained. Interrupted trimming experiments were conducted to examine crack initiation and development, the mechanism of fracture, and the generation of the fracture profile of the trimmed edges. The R-value as a measure of material anisotropy and fracture strain of both materials were measured using uniaxial tension and plane strain tests and incorporated in the FE model. General purpose Finite Element software ABAQUS/Explicit was employed to simulate the trimming process where five different fracture criteria and element deletion method were used to predict profile of trimmed edge and the fracture initiation and development during the trimming process. Good general agreement was observed between experiments and FE simulations. However, some discrepancies were also observed. These are presented and discussed in the thesis. / Thesis / Master of Applied Science (MASc)

3D printing concrete (3DPC) became one of the most investigated topics in cement and concrete research in the last decade. Research within this topic includes examining the role of admixtures on the fresh-state properties, specifically cement rheology, along with the mechanical performance of the printed materials and structures. 3DPC offers a promising platform for sustainable binders, optimized design, and economic and rapid construction that can help reduce the high CO2 impact of Portland cement. This thesis is aimed towards engineering nano-modified cement binders for 3DPC by characterizing the effects of dispersion on fresh, hardening and hardened properties, examining the potential of combining inorganic nanoparticles with organic admixtures to tailor select rheological properties, and developing printing performance measurements, and applying the findings in promising applications. Nanoclays (NC) are one of the most attractive rheological modifiers for 3DPC as they increase structuration and buildability with minimal increase to viscosity or pumping requirements. This work starts by studying the impact of different dispersion techniques (sonication, magnetic stirring, dry powder mixing) on the rheological properties of NC-modified cement paste. In addition, a novel dry dispersion technique that coats cement grains with nanomaterials was developed and compared to conventional dispersion methods. The results revealed that dry dispersion enhanced NC efficiency in increasing the static yield stress of cement paste, effectively reducing NC content requirements by 33% compared to solution dispersion to achieve the same print height. The observed changes in rheological properties at different NC contents and dispersion techniques implied that the origin of structuration in NC-modified cement pastes is mainly driven by the interactive forces between NC needles (NC-NC) more so than with cement (NC-cement). This incentivized exploring partial treatment of cement with dry dispersion at a relatively high NC content of 10 wt.% to maximize the aforementioned NC-NC interactions. The results confirmed that the mixture of 10 wt.% NC-coated cement with uncoated ones performed similar or better than mixtures where dry dispersion was applied to all cement grains while maintaining the same NC dosage. From the collective findings of this investigation, it was deduced that partial treatment of cement with NC using dry dispersion can maximize NC efficiency in increasing the static yield stress for 3DPC. Moreover, because NC-NC interactions were more influential than NC-cement interactions on the structuration of cement paste, NC are likely to be successful in increasing the static yield stress and buildability of alternative binders for 3DPC. The promising performance of NC-coated cement motivated further exploration of dry dispersion on other nanomaterial types. The impact of dry dispersed versus solution sonicated NC, silica and calcium nanoparticles (SNP and CCNP), and graphene nanoplatelets (GNP), on hydration kinetics and mechanical performance was investigated. Results of isothermal calorimetry and quantitative x-ray diffraction for nano-modified cement pastes showed the critical role of the method of dispersion on the progression of cement hydration, which in turn altered strength development. For example, SNP-coated cement paste exhibited a delay in ettringite formation by a few hours compared to solution sonication, which likely caused the delayed compressive strength development observed in SNP-coated cement mortars. Nevertheless, for mortars modified with NC, SNP, and CCNP, processing via dry dispersion and solution sonication showed comparable 28-day compressive strengths, implying the successful application of dry dispersion for all three nanomaterials. In addition, results showed an increase in electrical conductivity of GNP-coated cement pastes with dry dispersion whereas GNP were not dispersible with sonication without surfactants or functionalization. The collective results show the efficacy of dry dispersion as an alternative dispersion technique to sonication but one that offers ready-to-mix or just-add-water nano-engineered cement products. Therefore, nano-coated cements via dry dispersion could be very beneficial for remote 3DPC applications or commercialization of nano-engineered binders. One of the drawbacks of NC reported in literature and confirmed in this work is the significant increase in stiffness that causes filament breakage and tearing during extrusion. To remedy this, a new hybrid rheological modifier combining NC with methyl cellulose (MC) was introduced to tailor cement paste rheology and meet 3DPC requirements. The hybrid mixture of NC and MC proved to increase NC efficiency by up to 900 Pa/1 wt.% of MC in cement paste without jeopardizing its extrudability, essentially decreasing the NC content requirement, and associated costs, to achieve greater print heights. In addition, the hybrid admixture maintained similar or better mechanical performance compared to unmodified cement mortars whereas addition of NC or MC alone showed reduced 28-day mechanical strengths. To capture the effects of the new admixture on ink or filament properties, three recently proposed printer-based ink tests were applied – elastic buckling of thin walls, slug test and cable sag test. The results confirmed that despite the softening effect of the hybrid admixture on elastic modulus of cement paste compared to NC alone, the critical buckling height, which measures structural stability, was not similarly impacted. In addition, ink cohesion measured by both slug and cable sag tests improved with the hybrid admixture compared to NC or MC alone. The collective results suggest that the hybrid admixture can tailor cement rheology to meet 3DPC requirements by enhancing ink or filament properties while maintaining mechanical performance. The last investigation applied the previous findings to enable 3D printing and facilitate CO₂ mineralization for a new alternative binder. Magnesium oxide (MgO), similar to Portland cement, hardens through hydration but only develops mechanical strength through carbonation. However, atmospheric carbonation is a self-depreciating diffusion process where the carbonation of the exterior retards and limits further internal carbonation. Building upon the new understanding of the origin of structuration of NC in cement pastes and the high performance of the hybrid combination of NC and MC, the new admixture was used to enable 3D printing of MgO binders. The results confirmed that NC enhanced shape stability by increasing static yield stress while MC maintained ink cohesion, thereby effectively making MgO pastes printable. Compression tests of 3D printed and conventionally mold cast MgO paste cylinders showed that 3D printing can significantly increase strength by up to an order of magnitude. Examining the effects of different infill patterns (<100% and 100% infill density) and water-to binder ratios, results indicated that the increase in strength is attributed to 3D printing effects like the lack of protective formwork, which increased water evaporation and consequently increased carbon diffusion and intake. This study was the first to be published on tailoring the rheology of MgO binder and studying the effects of infill patterns on the compressive strength of 3D printed MgO pastes. The summary of results demonstrates that 3D printing can introduce significant benefits for carbon cured material systems, such as reactive MgO based systems, to potentially reach CO2 neutrality or negativity. Chapter 1 is the introduction, which describes how the main goal of this work is to explore the use of nanomaterials for 3DPC. Chapter 2 presents a literature review on 3DPC properties, cement rheology, and nanomaterials. Chapter 3 discusses the effects of NC dispersion, including the novel dry dispersion technique, on the structuration behavior of cement pastes. Chapter 4 dives deeper into the application of dry dispersion on other types of nanoparticles, i.e. SNP, CCNP and GNP. Chapter 5 revisits NC with the addition of MC to tailor cement rheology for 3DPC. Chapter 6 utilizes the results of Chapters 3 and 5 by examining 3D printed MgO paste and carbon intake. Chapter 7 summarizes this work and lists all the chapters’ conclusions. Additional discussions of the printer and extrusion head that were designed and built in Columbia CEEM Carleton Laboratory are also included in the appendix of this work, detailing gantry versus delta printers for 3DPC and the development of a low-cost concrete extrusion head with an open-to-atmosphere hopper that eliminates the need for a pumping system. Lastly, multiple in-situ printing properties and ink performance tests developed by the author, which utilize the printing system to characterize the fresh properties of inks on site, were expanded and detailed

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