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- All Subjects: Materials Science
- Creators: Rajagopalan, Jagannathan
Description
Traditionally nanoporous gold is created by selective dissolution of silver or copper from a binary silver-gold or copper-gold alloy. These alloys serve as prototypical model systems for a phenomenon referred to as stress-corrosion cracking. Stress-corrosion cracking is the brittle failure of a normally ductile material occurring in a corrosive environment under a tensile stress. Silver-gold can experience this type of brittle fracture for a range of compositions. The corrosion process in this alloy results in a bicontinuous nanoscale morphology composed of gold-rich ligaments and voids often referred to as nanoporous gold. Experiments have shown that monolithic nanoporous gold can sustain high speed cracks which can then be injected into parent-phase alloy. This work compares nanoporous gold created from ordered and disordered copper-gold using digital image analysis and electron backscatter diffraction. Nanoporous gold from both disordered copper-gold and silver-gold, and ordered copper-gold show that grain orientation and shape remain largely unchanged by the dealloying process. Comparing the morphology of the nanoporous gold from ordered and disordered copper-gold with digital image analysis, minimal differences are found between the two and it is concluded that they are not statistically significant. This reveals the robust nature of nanoporous gold morphology against small variations in surface diffusion and parent-phase crystal structure.
Then the corrosion penetration down the grain boundary is compared to the depth of crack injections in polycrystal silver-gold. Based on statistical comparison, the crack-injections penetrate into the parent-phase grain boundary beyond the corrosion-induced porosity. To compare crack injections to stress-corrosion cracking, single crystal silver-gold samples are employed. Due to the cleavage-like nature of the fracture surfaces, electron backscatter diffraction is possible and employed to compare the crystallography of stress-corrosion crack surfaces and crack-injection surfaces. From the crystallographic similarities of these fracture surfaces, it is concluded that stress-corrosion can occur via a series of crack-injection events. This relationship between crack injections and stress corrosion cracking is further examined using electrochemical data from polycrystal silver-gold samples during stress-corrosion cracking. The results support the idea that crack injection is a mechanism for stress-corrosion cracking.
Then the corrosion penetration down the grain boundary is compared to the depth of crack injections in polycrystal silver-gold. Based on statistical comparison, the crack-injections penetrate into the parent-phase grain boundary beyond the corrosion-induced porosity. To compare crack injections to stress-corrosion cracking, single crystal silver-gold samples are employed. Due to the cleavage-like nature of the fracture surfaces, electron backscatter diffraction is possible and employed to compare the crystallography of stress-corrosion crack surfaces and crack-injection surfaces. From the crystallographic similarities of these fracture surfaces, it is concluded that stress-corrosion can occur via a series of crack-injection events. This relationship between crack injections and stress corrosion cracking is further examined using electrochemical data from polycrystal silver-gold samples during stress-corrosion cracking. The results support the idea that crack injection is a mechanism for stress-corrosion cracking.
ContributorsKarasz, Erin (Author) / Sieradzki, Karl (Thesis advisor) / Chawla, Nikhilesh (Committee member) / Peralta, Pedro (Committee member) / Rajagopalan, Jagannathan (Committee member) / Arizona State University (Publisher)
Created2020
Description
Physical vapor deposition (PVD) of phase-separating multicomponent alloy films generates a rich variety of unique self-organized nanoscale morphologies. However, an understanding of how the different material and process parameters influence the formation of these nanostructures is limited. My dissertation aims to bridge this gap by developing phase-field models that can predict an entire spectrum of nanostructures as a function of processing conditions and composition in multicomponent alloys under a set of material-specific constraints. Firstly, I developed a numerical model to simulate nanoscale phase separation in codeposited immiscible binary alloy films. An investigation on the influence of deposition rates, phase-fraction, and temperature, on the evolution of self-assembled nanostructures yielded many characteristic patterns, including well-known morphologies such as the lateral and vertical concentration modulations, as well as some previously undocumented variants. I also simulated phase-separation in ternary alloyed PVD films, and studied the influence of deposition rate and composition on the evolution of self-assembled nanostructures, and recorded many novel nanoscale morphologies. I then sought to understand the role of material properties such as elastic misfit due to lattice mismatch between phases, grain boundaries formed in polycrystalline films, and the interplay of interphase and surface boundaries at the film surface. To this end, I developed phase-field models of binary PVD film deposition that incorporated these constraints and studied their role in altering the temporal and spatial characteristics of the evolving morphologies. I also investigated the formation of surface hillocks and the role of surface and interfacial energies in their evolution. By studying the change in total free energy across the different deposition models, I established that, in addition to influencing the temporal and spatial characteristics of nanoscale structures in the films, this quantity is also responsible for driving morphological transitions as the rate of deposition is increased. Insights gained from this computational study will demonstrate the viability of these models in predicting experimentally observed morphologies and form a basis for understanding the various factors involved in driving phase-separation and morphological transitions. In addition, morphology maps will serve as templates for developing new pathways for morphology control in the manufacturing of PVD alloy films.
ContributorsRaghavan, Rahul (Author) / Ankit, Kumar (Thesis advisor) / Rajagopalan, Jagannathan (Committee member) / Mushongera, Leslie T (Committee member) / Arizona State University (Publisher)
Created2021
Description
There is a high demand for customized designs of various types of cement-based materials in order to address specific purposes in the construction field. These demands stem from the need to optimize the cementitious matrix properties and reinforcement choices, especially in high reliability, durability, and performance applications that include infrastructure, energy production, commercial buildings, and may ultimately be extended to low risk/high volume applications such as residential applications. The typical tools required to guide practicing engineers should be based on optimization algorithms that require highly efficient capacity and design alternatives and optimal computational tools. The general case of flexural design of members is an important aspect of design of structural members which can be extended to a variety of applications that include various cross-sections such as rectangular, W-sections, channels, angles, and T sections. The model utilized the simplified linear constitutive response of cement-based composite in compression and tension and extends into a two-segment elastic-plastic, strain softening, hardening, tension-stiffening, and a multi-segment system. The generalized parametric model proposed uses a dimensionless system in the stress-strain materials diagram to formulate piecewise equations for an equilibrium of internal stresses and obtains strain distributions for the closed-form solution of neutral axis location. This would allow for the computation of piecewise moment-curvature response. The number of linear residual stress implemented is flexible to a user to maintain a robust response. In the present approach bilinear, trilinear, and quad-linear models are addressed and a procedure for incorporating additional segments is presented. Moreover, a closed-form solution of moment-curvature can be solved and employed in calculating load-deflection response. The model is adaptable for various types of fiber-reinforced and textile reinforced concrete (FRC, TRC, UHPC, AAC, and Reinforced Concrete). The extensions to cover continuous fiber reinforcement such as textile reinforced concrete (TRC, FRCM) strengthening and repair are addressed. The theoretical model is extended to incorporate the hybrid design (HRC) with continuous rebar with FRC to increase the ductility and ultimate moment capacity. HRC extends the performance of the fiber system to incorporate residual capacity into a serviceability-based design that reduced the reliance on the design based on the limit state. The design chart for HRC and as well as conventional RC has been generated for practicing engineering applications. Results are compared to a large array of data from experimental results conducted at the ASU structural lab facilities and other published literature.
Contributorspleesudjai, chidchanok (Author) / Mobasher, Barzin (Thesis advisor) / Neithalath, Narayanan (Committee member) / Rajan, Subramaniam (Committee member) / Arizona State University (Publisher)
Created2021
Description
Layer-wise extrusion of soft-solid like cement pastes and mortars is commonly used in 3D printing of concrete. Rheological and mechanical characterization of the printable binder for on-demand flow and subsequent structuration is a critical challenge. This research is an effort to understand the mechanics of cementitious binders as soft solids in the fresh state, towards establishing material-process relationships to enhance print quality. This study introduces 3D printable binders developed based on rotational and capillary rheology test parameters, and establish the direct influence of packing coefficients, geometric ratio, slip velocities, and critical print velocities on the extrudate quality. The ratio of packing fraction to the square of average particle diameter (0.01-0.02), and equivalent microstructural index (5-20) were suitable for printing, and were directly related to the cohesion and extrusional yield stress of the material. In fact, steady state pressure for printing (30-40 kPa) is proportional to the extrusional yield stress, and increases with the geometric ratio (0-60) and print velocity (5-50 mm/s). Higher print velocities results in higher wall shear stresses and was exponentially related to the slip layer thickness (estimated between 1-5μ), while the addition of superplasticizers improve the slip layer thickness and the extrudate flow. However, the steady state pressure and printer capacity limits the maximum print velocity while the deadzone length limits the minimum velocity allowable (critical velocity regime) for printing. The evolution of buildability with time for the fresh state mortars was characterized with digital image correlation using compressive strain and strain rate in printed layers. The fresh state characteristics (interlayer and interfilamentous) and process parameters (layer height and fiber dimensions) influence the hardened mechanical properties. A lower layer height generally improves the mechanical properties and slight addition of fiber (up to 0.3% by volume) results in a 15-30% increase in the mechanical properties. 3D scanning and point-cloud analysis was also used to assess the geometric tolerance of a print based on mean error distances, print accuracy index, and layer-wise percent overlap. The research output will contribute to a synergistic material-process design and development of test methods for printability in the context of 3D printing of concrete.
ContributorsAmbadi Omanakuttan Nair, Sooraj Kumar (Author) / Neithalath, Narayanan (Thesis advisor) / Rajan, Subramaniam (Committee member) / Mobasher, Barzin (Committee member) / Hoover, Christian (Committee member) / Chawla, Nikhilesh (Committee member) / Arizona State University (Publisher)
Created2021
Description
Thin films are widely used for a variety of applications such as electrical interconnects, sensors, as well as optical, mechanical, and decorative coatings. Thin films made of NiTi, commonly referred to as nitinol, have generated recent interest as they are highly suitable for high frequency thermal actuation in microelectromechanical devices because of their small thermal mass and large surface-to-volume ratio. The functional properties of NiTi arise from a diffusionless phase transformation between two of its primary phases: austenite and martensite. This transformation leads to either the shape memory or pseudoelastic effect, where inelastic deformation is recovered with and without the application of heat, respectively. It is well known that the mechanical properties of NiTi are highly dependent on the microstructure, but few studies have been performed to examine the mechanical behavior of thin NiTi films (thickness below 200 nm), which are expected to have grain sizes in a similar range. The primary intent of this work is the synthesis of NiTi thin films with controlled microstructures, followed by characterization of their microstructure and its relationship to the mechanical properties. Microstructural control was achieved by utilizing a novel synthesis technique in which amorphous precursor films are seeded with nanocrystals, which serve as nucleation sites during subsequent crystallization via thermal annealing. This technique enables control of grain size, dispersion, and phase composition of thin films by varying the parameters of seed deposition as well as annealing conditions. The microstructures and composition of the NiTi thin films were characterized using X-ray Diffraction, Electron Microprobe Analysis, High-resolution Transmission Electron Microscopy, Secondary Ion Mass Spectroscopy, Differential Scanning Calorimetry, as well as other complementary techniques. Mechanical properties of the films were investigated using uniaxial tensile testing performed using a custom microfabricated tensile testing stage. The NiTi thin films exhibit mechanical behavior that is distinct from bulk NiTi, which is also highly sensitive to small changes in microstructure and phase composition. These findings are rationalized in terms of the changes in deformation mechanisms that occur at small grain sizes and sample dimensions.
ContributorsRASMUSSEN, Paul (Author) / Rajagopalan, Jagannathan (Thesis advisor) / Solanki, Kiran (Committee member) / Sieradzki, Karl (Committee member) / Arizona State University (Publisher)
Created2021
Description
This paper investigates Surface Mechanical Attrition Treatment (SMAT) and the influence of treatment temperature and initial sample surface finish on the corrosion resistance of 7075-T651 aluminum alloy. Ambient SMAT was performed on AA7075 samples polished to 80-grit initial surface roughness. Potentiodynamic polarization and electrochemical impedance spectroscopy (EIS) tests were used to characterize the corrosion behavior of samples before and after SMAT. Electrochemical tests indicated an improved corrosion resistance after application of SMAT process. The observed improvements in corrosion properties are potentially due to microstructural changes in the material surface induced by SMAT which encouraged the formation of a passive oxide layer. Further testing and research are required to understand the corrosion related effects of cryogenic SMAT and initial-surface finish as the COVID-19 pandemic inhibited experimentation plans.
ContributorsDeorio, Jordan Anthony (Author) / Solanki, Kiran (Thesis director) / Rajagopalan, Jagannathan (Committee member) / Mechanical and Aerospace Engineering Program (Contributor) / Barrett, The Honors College (Contributor)
Created2020-05
Description
Alkali-activated aluminosilicates, commonly known as "geopolymers", are being increasingly studied as a potential replacement for Portland cement. These binders use an alkaline activator, typically alkali silicates, alkali hydroxides or a combination of both along with a silica-and-alumina rich material, such as fly ash or slag, to form a final product with properties comparable to or better than those of ordinary Portland cement. The kinetics of alkali activation is highly dependent on the chemical composition of the binder material and the activator concentration. The influence of binder composition (slag, fly ash or both), different levels of alkalinity, expressed using the ratios of Na2O-to-binders (n) and activator SiO2-to-Na2O ratios (Ms), on the early age behavior in sodium silicate solution (waterglass) activated fly ash-slag blended systems is discussed in this thesis. Optimal binder composition and the n values are selected based on the setting times. Higher activator alkalinity (n value) is required when the amount of slag in the fly ash-slag blended mixtures is reduced. Isothermal calorimetry is performed to evaluate the early age hydration process and to understand the reaction kinetics of the alkali activated systems. The differences in the calorimetric signatures between waterglass activated slag and fly ash-slag blends facilitate an understanding of the impact of the binder composition on the reaction rates. Kinetic modeling is used to quantify the differences in reaction kinetics using the Exponential as well as the Knudsen method. The influence of temperature on the reaction kinetics of activated slag and fly ash-slag blends based on the hydration parameters are discussed. Very high compressive strengths can be obtained both at early ages as well as later ages (more than 70 MPa) with waterglass activated slag mortars. Compressive strength decreases with the increase in the fly ash content. A qualitative evidence of leaching is presented through the electrical conductivity changes in the saturating solution. The impact of leaching and the strength loss is found to be generally higher for the mixtures made using a higher activator Ms and a higher n value. Attenuated Total Reflectance-Fourier Transform Infrared Spectroscopy (ATR-FTIR) is used to obtain information about the reaction products.
ContributorsChithiraputhiran, Sundara Raman (Author) / Neithalath, Narayanan (Thesis advisor) / Rajan, Subramaniyam D (Committee member) / Mobasher, Barzin (Committee member) / Arizona State University (Publisher)
Created2012
Description
This thesis presents a study of Boron Nitride (BN) and Copper (Cu)/BN multilayer thin films in terms of synthesis, chemical, structural, morphological, and mechanical properties characterization.
In this study, the influence of Ar/N₂ flow rate in synthesizing stoichiometric BN thin films via magnetron sputtering was investigated initially. Post magnetron sputtering, the crystalline nature and B:N stoichiometric ratio of deposited thin films were investigated by X-ray diffraction (XRD) and X-ray Photoelectron Spectroscopy (XPS) respectively. Thicknesses revealed by ellipsometry analysis for nearly stoichiometric B:N thin films and their corresponding deposition times were used for estimating BN interlayer deposition times during the deposition of Cu/BN multilayer thin films. To characterize the microstructure of the synthesized Cu/BN multilayer thin films, XRD and scanning electron microscopy (SEM) have been used. Finally, a comparison of nanoindentation measurements on pure Cu and Cu/BN multilayer thin films having different number of BN interlayers were used for studying the influence of BN interlayers on improving mechanical properties such as hardness and elastic modulus.
The results show that the stoichiometry of BN thin films is dependent on the Ar/N₂ flow rate during magnetron sputtering. An optimal Ar/N₂ flow rate of 13:5 during deposition was required to achieve an approximately 1:1 B:N stoichiometry. Grazing incidence and powder XRD analysis on these stoichiometric BN thin films deposited at room temperature did not reveal a phase match when compared to hexagonal boron nitride (h-BN) and cubic boron nitride (c-BN) reference XRD patterns. For a BN thin film deposition time of 5 hours, a thickness of approximately 40 nm was achieved, as revealed by ellipsometry. XRD and microstructure analysis using scanning electron microscopy (SEM) on pure Cu and Cu/BN thin films showed that the Cu grain size in Cu/BN thin films is much finer than pure Cu thin films. Interestingly, nanoindentation measurements on pure Cu and Cu/BN thin films having a similar overall thickness demonstrated that hardness and Young’s modulus of the films were improved significantly when BN interlayers are present.
ContributorsCaner, Sumeyye (Author) / Rajagopalan, Jagannathan (Thesis advisor) / Oswald, Jay (Committee member) / Solanki, Kiran (Committee member) / Arizona State University (Publisher)
Created2023