The increasing demand for structural materials with superior mechanical properties has provided a strong impetus to the discovery of novel materials, and innovations in processing techniques to improve the properties of existing materials. Methods like severe plastic deformation (SPD) and surface mechanical attrition treatment (SMAT) have led to significant enhancement in the strength of traditional structural materials like Al and Fe based alloys via microstructural refinement. However, the nanocrystalline materials produced using these techniques exhibit poor ductility due to the lack of effective strain hardening mechanisms, and as a result the well-known strength-ductility trade-off persists. To overcome this trade-off, researchers have proposed the concept of heterostructured materials, which are composed of domains ranging in size from a few nanometers to several micrometers. Over the last two decades, there has been intense research on the development of new methods to synthesize heterostructured materials. However, none of these methods is capable of providing precise control over key microstructural parameters such as average grain size, grain morphology, and volume fraction and connectivity of coarse and fine grains. Due to the lack of microstructural control, the relationship between these parameters and the deformation behavior of heterostructured materials cannot be investigated systematically, and hence designing heterostructured materials with optimized properties is currently infeasible. This work aims to address this scientific and technological challenge and is composed of two distinct but interrelated parts. The first part concerns the development of a broadly applicable synthesis method to produce heterostructured metallic films with precisely defined architectures. This method exploits two forms of film growth (epitaxial and Volmer-Weber) to generate heterostructured metallic films. The second part investigates the effect of different microstructural parameters on the deformation behavior of heterostructured metallic films with the aim of elucidating their structure-property relationships. Towards this end, freestanding heterostructured Fe films with different architectures were fabricated and uniaxially deformed using MEMS stages. The results from these experiments are presented and their implications for the mechanical properties of heterostructured materials is discussed.

Nanocrystalline (NC) materials are of great interest to researchers due to their multitude of properties such as exceptional strength and radiation resistance owing to their high fraction of grain boundaries that act as defect sinks for radiation-induced defects, provided they are microstructurally stable. In this dissertation, radiation effects in microstructurally stable bulk NC copper (Cu)- tantalum (Ta) alloys engineered with uniformly dispersed Ta nano-precipitates are systematically probed. Towards this, both ex-situ and in-situ irradiations using heavy (self) ion, helium ion, and concurrent dual ion beams (He+Au) followed by isochronal annealing inside TEM were utilized to understand radiation tolerance and underlying mechanisms of microstructure evolution in stable NC alloys. With systematic self-ion irradiation, the high density of tantalum nanoclusters in Cu-10at.%Ta were observed to act as stable sinks in suppressing radiation hardening, in addition to stabilizing the grain boundaries; while the large incoherent precipitates experienced ballistic mixing and dissolution at high doses. Interestingly, the alloy exhibited a microstructure self-healing mechanism, where with a moderate thermal input, this dissolved tantalum eventually re-precipitated, thus replenishing the sink density. The high stability of these tantalum nanoclusters is attributed to the high positive enthalpy of mixing of tantalum in copper which also acted as a critical driving force against atomic mixing to facilitate re-precipitation of tantalum nanoclusters. Furthermore, these nanoclusters proved to be effective trapping sites for helium, thus sequestering helium into isolated small bubbles and aid in increasing the overall swelling threshold of the alloy. The alloy was then compositionally optimized to reduce the density of large incoherent precipitates without compromising on the grain size and nanocluster density (Cu-3at.%Ta) which resulted in a consistent and more promising response to high dose self-ion irradiation. In-situ helium and dual beam irradiation coupled with isochronal annealing till 723 K, also revealed a comparable microstructural stability and enhanced ability of Cu-3Ta in controlling bubble growth and suppressing swelling compared to Cu-10Ta indicating a promising improvement in radiation tolerance in the optimized composition. Overall, this work helps advancing the current understanding of radiation tolerance in stable nanocrystalline alloys and aid developing design strategies for engineering radiation tolerant materials with stable interfaces.

Fatigue damage accumulation under multiaxial loading conditions is an important practical problem for which there is a need to collect additional experimental data to calibrate and validate models. In this work, a sample with a special geometry capable of producing biaxial stresses while undergoing uniaxial load was fabricated and tested successfully and used, along with standard dogbone samples, to monitor the evolution of surface roughness development under cyclic loading using optical microscopy. In addition, a Michelson interferometer was successfully designed, built and tested that can be used to monitor surface roughness for lower levels of load than those used in this work. Results of testing and characterization in 2024-T3 samples tested at a maximum stress slightly below their yield strength and load ratio ~ 0.1 indicate that most of the surface roughness development under cyclic loads occurs on the second half of the fatigue, with the bulk of it close to failure. However, samples with load axes perpendicular to the rolling direction showed earlier development of roughness, which correlated with shorter fatigue lives and the expected anisotropy of strength in the material.

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.

Polyurea is a highly versatile material used in coatings and armor systems to protect against extreme conditions such as ballistic impact, cavitation erosion, and blast loading. However, the relationships between microstructurally-dependent deformation mechanisms and the mechanical properties of polyurea are not yet fully understood, especially under extreme conditions. In this work, multi-scale coarse-grained models are developed to probe molecular dynamics across the wide range of time and length scales that these fundamental deformation mechanisms operate. In the first of these models, a high-resolution coarse-grained model of polyurea is developed, where similar to united-atom models, hydrogen atoms are modeled implicitly. This model was trained using a modified iterative Boltzmann inversion method that dramatically reduces the number of iterations required. Coarse-grained simulations using this model demonstrate that multiblock systems evolve to form a more interconnected hard phase, compared to the more interrupted hard phase composed of distinct ribbon-shaped domains found in diblock systems. Next, a reactive coarse-grained model is developed to simulate the influence of the difference in time scales for step-growth polymerization and phase segregation in polyurea. Analysis of the simulated cured polyurea systems reveals that more rapid reaction rates produce a smaller diameter ligaments in the gyroidal hard phase as well as increased covalent bonding connecting the hard domain ligaments as evidenced by a larger fraction of bridging segments and larger mean radius of gyration of the copolymer chains. The effect that these processing-induced structural variations have on the mechanical properties of the polymer was tested by simulating uniaxial compression, which revealed that the higher degree of hard domain connectivity leads to a 20% increase in the flow stress. A hierarchical multiresolution framework is proposed to fully link coarse-grained molecular simulations across a broader range of time scales, in which a family of coarse-grained models are developed. The models are connected using an incremental reverse–mapping scheme allowing for long time scale dynamics simulated at a highly coarsened resolution to be passed all the way to an atomistic representation.

Shock loading produces a compressive stress pulse with steep gradients in density, temperature, and pressure that are also often modeled as discontinuities. When a material is subject to these dynamic (shock) loading conditions, fracture and deformation patterns due to spall damage can arise. Spallation is a dynamic material failure that is caused by the nucleation, growth, and coalescence of voids, with possible ejection of the surface of the material. Intrinsic defects, such as grain boundaries are the preferred initiation sites of spall damage in high purity materials. The focus of this research is to study the phenomena that cause void nucleation and growth at a particular grain boundary (GB), chosen to maximize spall damage localization.

Bicrystal samples were shock loaded using flyer-plates via light gas gun and direct laser ablation. Stress, pulse duration, and crystal orientation along the shock direction were varied for a fixed boundary misorientation to determine thresholds for void nucleation and coalescence as functions of these parameters. Pressures for gas gun experiments ranged from 2 to 5 GPa, while pressures for laser ablation experiments varied from 17 to 25 GPa. Samples were soft recovered to perform damage characterization using electron backscattering diffraction (EBSD) and Scanning Electron Microscopy (SEM). Results showed a 14% difference in the thresholds for void nucleation and coalescence between samples with different orientations along the shock direction, which were affected by pulse duration and stress level. Fractography on boundaries with strong damage localization showed many small voids, indicating they experience rapid nucleation, causing early coalescence. Composition analysis was also performed to determine the effect of impurities on damage evolution. Results showed that higher levels of impurities led to more damage. ABAQUS/Explicit models were developed to simulate flyer-plate impact and void growth with the same crystal orientations and experimental conditions. Results are able to match the damage seen in each grain of the target experimentally. The Taylor Factor mismatch at the boundary can also be observed in the model with the higher Taylor Factor grain exhibiting more damage.

The stability of nanocrystalline microstructural features allows structural materials to be synthesized and tested in ways that have heretofore been pursued only on a limited basis, especially under dynamic loading combined with temperature effects. Thus, a recently developed, stable nanocrystalline alloy is analyzed here for quasi-static (<100 s-1) and dynamic loading (103 to 104 s-1) under uniaxial compression and tension at multiple temperatures ranging from 298-1073 K. After mechanical tests, microstructures are analyzed and possible deformation mechanisms are proposed. Following this, strain and strain rate history effects on mechanical behavior are analyzed using a combination of quasi-static and dynamic strain rate Bauschinger testing. The stable nanocrystalline material is found to exhibit limited flow stress increase with increasing strain rate as compared to that of both pure, coarse grained and nanocrystalline Cu. Further, the material microstructural features, which includes Ta nano-dispersions, is seen to pin dislocation at quasi-static strain rates, but the deformation becomes dominated by twin nucleation at high strain rates. These twins are pinned from further growth past nucleation by the Ta nano-dispersions. Testing of thermal and load history effects on the mechanical behavior reveals that when thermal energy is increased beyond 200 °C, an upturn in flow stress is present at strain rates below 104 s-1. However, in this study, this simple assumption, established 50-years ago, is shown to break-down when the average grain size and microstructural length-scale is decreased and stabilized below 100nm. This divergent strain-rate behavior is attributed to a unique microstructure that alters slip-processes and their interactions with phonons; thus enabling materials response with a constant flow-stress even at extreme conditions. Hence, the present study provides a pathway for designing and synthesizing a new-level of tough and high-energy absorbing materials.

Understanding damage evolution, particularly as it relates to local nucleation and growth kinetics of spall failure in metallic materials subjected to shock loading, is critical to national security. This work uses computational modeling to elucidate what characteristics have the highest impact on damage localization at the microstructural level in metallic materials, since knowledge of these characteristics is critical to improve these materials. The numerical framework consists of a user-defined material model implemented in a user subroutine run in ABAQUS/Explicit that takes into account crystal plasticity, grain boundary effects, void nucleation and initial growth, and both isotropic and kinematic hardening to model incipient spall. Finite element simulations were performed on copper bicrystal models to isolate the boundary effects between two grains. Two types of simulations were performed in this work: experimentally verified cases in order to validate the constitutive model as well as idealized cases in an attempt to determine the microstructural characteristic that define weakest links in terms of spall damage. Grain boundary effects on damage localization were studied by varying grain boundary orientation in respect to the shock direction and the crystallographic properties of each grain in the bicrystal. Varying these parameters resulted in a mismatch in Taylor factor across the grain boundary and along the shock direction. The experimentally verified cases are models of specific damage sites found from flyer plate impact tests on copper multicrystals in which the Taylor factor mismatch across the grain boundary and along the shock direction are both high or both low. For the idealized cases, grain boundary orientation and crystallography of the grains are chosen such that the Taylor factor mismatch in the grain boundary normal and along the shock direction are maximized or minimized. A perpendicular grain boundary orientation in respect to the shock direction maximizes Taylor factor mismatch, while a parallel grain boundary minimizes the mismatch. Furthermore, it is known that <1 1 1> crystals have the highest Taylor factor, while <0 0 1> has nearly the lowest Taylor factor. The permutation of these extremes for mismatch in the grain boundary normal and along the shock direction results in four idealized cases that were studied for this work. Results of the simulations demonstrate that the material model is capable of predicting damage localization, as it has been able to reproduce damage sites found experimentally. However, these results are qualitative since further calibration is still required to produce quantitatively accurate results. Moreover, comparisons of results for void nucleation rate and void growth rate suggests that void nucleation is more influential in the total void volume fraction for bicrystals with high property mismatch across the interface, suggesting that nucleation is the dominant characteristic in the propagation of damage in the material. Further work in recalibrating the simulation parameters and modeling different bicrystal orientations must be done to verify these results.

Measuring the dynamic strength of a material based on stress and strain data is challenging due to the diculty in recording strain and stress under the short times and large loads typical of dynamic events, such as impact and shock loading. The research involved in this study aims to perform nite element simulations for a new experimental method that can provide information on material dynamic strength, which is crucial for many engineering applications. In this method, a shock wave is applied to a metallic sample with a perturbed surface, i.e, one with periodic ripples machined or etched on the surface. The speed and magnitude of the change of am- plitude of the ripples are recorded. It is known that these parameters are functions of both geometry and material strength. The experimental data are compared with the simulation results produced. The dynamic yield strength of a material is taken to be the same as the strength used in simulations when a close match is found. The simulations have produced results that closely matched the experimental data and predicted the dynamic yield strength of metallic samples and have led to the discov- ery of a new experimental technique to lower the impact velocity required to induce amplitude changes in surface perturbations under shock loading. Thus, shock experi- ments to measure strength using surface perturbations will become easier to conduct and span a wider range of conditions. However, the existing simulation models are not adequate to examine the relations among hardening behavior and the change of amplitude and velocity on the sample surface. Thus, the models should be further modied to study dierent material hardening behaviors under dynamic loadings.