A versatile class of composite materials composed of functional filler particles in polymer matrices are increasingly used in high-performance applications that leverage their adaptable mechanical, electrical, thermal, and other functional properties. The choice of constituent materials largely determines these properties: for example, long fibers redistribute stress efficiently, highly conductive filler particles impart their conductivity to the material, and combinations of ionic and electrically conductive fillers with energy storage materials increase the capacity of battery electrodes. The performance of these composites, however, is highly dependent on the internal arrangement of particles. Consequently, controlling this internal structure enables property improvements and novel functionalities. In particular, combining microstructure control with the design freedom of 3D printing presents opportunities to maximize the performance of functional components by spatially modulating properties and their directionality (e.g. reinforcing high-stress regions, prescribing electrical interconnects, or directing heat away from hot spots), as well as introducing novel combinations of properties.

To develop this structural control in printed materials, this work utilizes pressure fields to assemble filler particles into patterns within composites during 3D printing. Particle patterns are engineered to improve material properties by leveraging microscopic acoustic forces on particles, tailoring the particle arrangement for improvements in the functionality of the material. This acoustic patterning technique is integrated into 3D printing with attention to processing constraints (e.g. component and feature sizes, material properties, cost, throughput rate, etc.) required in existing and emerging technological applications. In particular, 3D printed composites with high electrical or thermal conductivity, which are simultaneously mechanically flexible or strain-tolerant, are targeted for advancing the emerging fields of flexible electronics and soft robotics, and for alleviating the bottleneck in cooling of electronic devices.

Acoustically patterned composites exhibit increased electrical and thermal conductivity (by many orders of magnitude under some conditions) compared to conventional, unpatterned composites via concentrated particle contact density and optimized transport pathways. Simultaneously, patterning enhances mechanical flexibility by reducing the required loading of filler particles and encapsulating conductive particle networks in thick sheathes of strain-absorbing polymer. The magnitude and directionality of electrical and thermal transport is configurable over a wide range via manipulation of the orientation and connectivity of filler particles, introducing processing routes for printing components with embedded electrical interconnects or directional cooling networks.

Computational modeling of acoustic forces probes the design space of possible particle structures, which are compared to detailed particle-level material characterization to understand and then optimize the patterning process for desired microstructures. These structural details inform the physics of electrical and thermal conduction between particles in the material as well as particle rearrangements during mechanical deformation. Finally, this work investigates methods for expanding control over composite microstructure, including patterning hierarchically structured particles and manipulating particles with multiple external fields.

Nanocrystalline metals exhibit exceptional mechanical properties, such as high strength and wear resistance, and are actively being investigated for use in high performance structural applications. The ubiquitous use of nanocrystalline alloys in engineering applications is currently limited by two intrinsic material instabilities: rampant, thermally induced grain growth; and catastrophic shear localization during plastic deformation. These instabilities, which originate in the high concentration of grain boundaries and corresponding structural disorder present in nanocrystalline metals, prevent the widespread use of these alloys, especially in mission- or safety-critical applications. However, outstanding questions remain regarding the precise role of grain boundary structure on these instabilities, presenting an exciting opportunity to enable their use.

Using both targeted processing and interface aware alloying strategies, the role of grain boundary structure and chemistry on these instabilities will be explored. Experiments suggest that ultrafast (fs) laser processing provides tunability of the mechanical behavior of nanocrystalline metals by selectively modifying grain boundaries. The degree of tunability is sensitive both to local chemistry and initial relaxation state, suggesting that laser processing increases the energy of the grain boundaries, akin to rejuvenation processing in amorphous metals.

A nanocrystalline aluminum alloy, doped with nickel and cerium, was developed to investigate the chemical and structural effects of multicomponent alloying. The Al-Ni-Ce alloy exhibits extremely desirable mechanical properties, including high hardness, tunable shear localization behavior, and strength retention at elevated temperature; as well as excellent microstructural stability. The origin of these desirable attributes is explored using ex- and in-situ diffraction experiments, which suggest the topological disordering of grain boundary regions during processing imparts thermal stability and strength at temperature. These results collectively underscore that grain boundary structure plays a deterministic role in the instabilities associated with nanocrystalline metals, providing novel strategies for alloy design and processing to circumvent these behaviors.

The impact of voids on structural metal performance has been investigated in the context of two case studies: nanovoids as strengtheners in irradiated materials, and void growth size effects during ductile failure.

Nanovoid Strengthening: When nanoscale in diameter, voids can act as obstacles to dislocation motion, resulting in strengthening reminiscent of precipitation hardening. This is especially relevant to irradiated metals in applications such as nuclear power plants. Local regions of very highly concentrated nanovoid defect clusters within irradiated metals are observed to cause material strengthening, meaning bulk yielding behavior is altered from what is expected in the original material design. However, it is not yet possible to reliably predict the consequence of nanovoids in metals with existing models, as they are not informed by the fundamental mechanisms behind this response. To address this, the meso-scale simulation technique Phase Field Dislocation Dynamics (PFDD) is applied to the problem in Chapter 2. This technique is an energetic framework within which rigorous discrete dislocation physics is applied, and which allows for larger simulation cell sizes than are commonly available to lower-length-scale methods.

Surprisingly, the critical stress is found to scale linearly with the ratio of the intrinsic to unstable stacking fault energies, and to scale directly with the linear void fraction. This novel empirical strengthening model is named the Linear Fraction (LF) Model, and is thought to be driven by the dynamic constriction and extension of the stacking fault width as the two dissociated partials of an fcc dislocation bow around a nanovoid obtacle. This deviates from the expected line-tension-approximation mechanism due to bowing around an obstacle, which has until now been used to inform analytical predictive models. Remarkably, this new strengthening trend holds for the vast majority of fcc metals and void size arrays tested, and in Chapter 3 it is discovered to fit compellingly with a number of MD studies as well as for nano-precipitates in PFDD. The boundaries of when the LF model will apply to nano-obstacle strengthening has only briefly been investigated, and the extent to which PFDD results will agree with other atomistic techniques such as MD is still an open question. With this in mind, a number of future work directions are discussed in Chapter 5, Sections 5.1 and 5.2.

Meso-Scale Void Growth: Voids are present in more than just irradiated structural metals, and evidence exists that nanovoids can also be found in large quantities during plastic deformation. While some of these may strengthen at the nanoscale, some may also grow during plastic deformation. It is still an open question which voids will grow and which will not, but growth into the hundreds of nanometers and beyond increase the likelihood that the presence of these voids will degrade the structural performance of the metal and possibly lead to part failure. Given the severity of the consequences, an understanding of this void growth behavior is of significant interest. A novel in-situ testing method is discussed next in Chapter 4, which was designed to target a specific gap in experimental void growth evidence at these lower, transitional length scales.

The design of this experiment was meant to be as versatile as possible while also providing as much control as possible over material, void size, void-void interactions, and void/grain size interactions. Two primary goals of this approach were to 1) directly observe the evolution of voids at length scales below what has been achieved in literature, and 2) to fabricate these intentional void defects within tensile specimen without the use of FIB which is known to damage film surfaces. For these goals moderate success was achieved, and this technique was deployed for the problem of void growth size effects at sub-micron void sizes. Existing literature on fundamental size effects are limited to computational theories for very simplified model systems. When applied to the two defected specimen for this work, the fundamental size effect theories proved elusive because of the dominance of a third size effect consideration: the ratio of the void size to the thin film grain size. As such, promising future work avenues to push the boundaries of what is currently known experimentally for void growth are provided in Chapter 5, Section 5.3.