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.

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.

Surface Mechanical Attrition Treatment (SMAT) is a process used to coat metallic alloy surfaces with a nanocrystallized layer via mechanical abrasion. SMAT has garnered a significant amount of interest from the scientific community as a surface treatment technique due to the ability of this fine grain top layer to provide several benefits to its constituent alloy, namely significantly higher hardness, fatigue strength, and most pertinently, greatly improved corrosion resistance. Emerging research suggests that SMAT can also be used to apply powder coatings onto target substrates. A given substrate can be installed in a ball mill, where stainless steel balls coated with pure elemental powder deliver sustained impact onto the substrate, embedding the powders onto its surface. This paper will explore the process of coating aluminum 7075 coating with chromium powder via SMAT, and the effects doing so will have on the corrosion resistance properties of the aluminum 7075. Traditionally, high-strength alloys have been treated with chromium via the process of electroplating, where the alloys are subjected to a hexavalent chromium plating procedure that is known to risk releasing toxic carcinogens into the environment. Coating these alloys with SMAT could minimize such negative externalities, while yielding benefits unique to the SMAT coating process itself. Baseline corrosion testing reveals that the corrosion resistance properties of the aluminum 7075 improved marginally when exposed to SMAT without the addition of any chromium powder. A literature review conducted in this paper of select studies on SMAT coating also demonstrates that material properties intrinsic to aluminum 7075 and pure chromium powder, as well as interaction effects occurring between aluminum and chromium when subjected to mechanical alloying, could enable the SMAT coating of aluminum 7075 with chromium to result in greatly enhanced corrosion resistance properties. While this was not accomplished within the duration of the Honors Project due to logistical difficulties brought forth by the COVID-19 epidemic, the baseline corrosion testing performed, as well as the literature review of studies directly relevant to the matter, should hopefully provide some information of value in any future exploration of the topic.

With the advancements in technology, it is now possible to synthesize new materials with specific microstructures, and enhanced mechanical and physical properties. One of the new class of materials are nanoscale metallic multilayers, often referred to as nanolaminates. Nanolaminates are composed of alternating, nanometer-thick layers of multiple materials (typically metals or ceramics), and exhibit very high strength, wear resistance and radiation tolerance. This thesis is focused on the fabrication and mechanical characterization of nanolaminates composed of Copper and Cobalt, two metals which are nearly immiscible across the entire composition range. The synthesis of these Cu-Co nanolaminates is performed using sputtering, a well-known and technologically relevant physical vapor deposition process. X-ray diffraction is used to characterize the microstructure of the nanolaminates. Cu-Co nanolaminates with different layer thicknesses are tested using microelectromechanical systems (MEMS) based tensile testing devices fabricated using photolithography and etching processes. The stress-strain behavior of nanolaminates with varying layer thicknesses are analysed and correlated to their microstructure.

The exceptional mechanical properties of polymers with heterogeneous structure, such as the high toughness of polyethylene and the excellent blast-protection capability of polyurea, are strongly related to their morphology and nanoscale structure. Different polymer microstructures, such as semicrystalline morphology and segregated nanophases, lead to coordinated molecular motions during deformation in order to preserve compatibility between the different material phases. To study molecular relaxation in polyethylene, a coarse-grained model of polyethylene was calibrated to match the local structural variable distributions sampled from supercooled atomistic melts. The coarse-grained model accurately reproduces structural properties, e.g., the local structure of both the amorphous and crystalline phases, and thermal properties, e.g., glass transition and melt temperatures, and dynamic properties: including the vastly different relaxation time scales of the amorphous and crystalline phases. A hybrid Monte Carlo routine was developed to generate realistic semicrystalline configurations of polyethylene. The generated systems accurately predict the activation energy of the alpha relaxation process within the crystalline phase. Furthermore, the models show that connectivity to long chain segments in the amorphous phase increases the energy barrier for chain slip within crystalline phase. This prediction can guide the development of tougher semicrystalline polymers by providing a fundamental understanding of how nanoscale morphology contributes to chain mobility. In a different study, the macroscopic shock response of polyurea, a phase segregated copolymer, was analyzed using density functional theory (DFT) molecular dynamics (MD) simulations and classical MD simulations. The two models predict the shock response consistently up to shock pressures of 15 GPa, beyond which the DFT-based simulations predict a softer response. From the DFT simulations, an analysis of bond scission was performed as a first step in developing a more fundamental understanding of how shock induced material transformations effect the shock response and pressure dependent strength of polyurea subjected to extreme shocks.

Aluminum alloys are ubiquitously used in almost all structural applications due to their high strength-to-weight ratio. Their superior mechanical performance can be attributed to complex dispersions of nanoscale intermetallic particles that precipitate out from the alloy’s solid solution and offer resistance to deformation. Although they have been extensively investigated in the last century, the traditional approaches employed in the past haven’t rendered an authoritative microstructural understanding in such materials. The effect of the precipitates’ inherent complex morphology and their three-dimensional (3D) spatial distribution on evolution and deformation behavior have often been precluded. In this study, for the first time, synchrotron-based hard X-ray nano-tomography has been implemented in Al-Cu alloys to measure growth kinetics of different nanoscale phases in 3D and reveal mechanistic insights behind some of the observed novel phase transformation reactions occurring at high temperatures. The experimental results were reconciled with coarsening models from the LSW theory to an unprecedented extent, thereby establishing a new paradigm for thermodynamic analysis of precipitate assemblies. By using a unique correlative approach, a non-destructive means of estimating precipitation-strengthening in such alloys has been introduced. Limitations of using existing mechanical strengthening models in such alloys have been discussed and a means to quantify individual contributions from different strengthening mechanisms has been established.

The current rapid pace of technological progress necessitates the demand for more resilient and high-performance alloys. To achieve this, a thorough understanding of the relationships between material properties and its structure is indispensable. To establish this correlation and achieve desired properties from structural alloys, microstructural response to mechanical stimuli needs to be understood in three-dimensions (3D). To that effect, in situ tests were conducted at the synchrotron (Advanced Photon Source) using Transmission X-Ray Microscopy as well as in a scanning electron microscope (SEM) to study real-time damage evolution in such alloys. Findings of precipitate size-dependent transition in deformation behavior from these tests have inspired a novel resilient aluminum alloy design.

The goal of this research is to compare the mechanical properties of CP-Ti and Ti-O and to understand the relationship between a material's microstructure and its response to fatigue. Titanium has been selected due to its desirable properties and applicability in several engineering fields. Both samples are polished and etched in order to visualize and characterize the microstructure and its features. The samples then undergo strain-controlled fatigue tests for several thousand cycles. Throughout testing, images of the samples are taken at zero and maximum load for DIC analysis. The DIC results can be used to study the local strains of the samples. The DIC analysis performed on the CP-Ti sample and presented in this study will be used to understand how the addition of oxygen in the Ti-O impacts fatigue response. The outcome of this research can be used to develop long-lasting, high strength materials.

In real world applications, materials undergo a simultaneous combination of tension, compression, and torsion as a result of high velocity impact. The split Hopkinson pressure bar (SHPB) is an effective tool for analyzing stress-strain response of materials at high strain rates but currently little can be done to produce a synchronized combination of these varying impacts. This research focuses on fabricating a flange which will be mounted on the incident bar of a SHPB and struck perpendicularly by a pneumatically driven striker thus allowing for torsion without interfering with the simultaneous compression or tension. Analytical calculations are done to determine size specifications of the flange to protect against yielding or failure. Based on these results and other design considerations, the flange and a complementary incident bar are created. Timing can then be established such that the waves impact the specimen at the same time causing simultaneous loading of a specimen. This thesis allows research at Arizona State University to individually incorporate all uniaxial deformation modes (tension, compression, and torsion) at high strain rates as well as combining either of the first two modes with torsion. Introduction of torsion will expand the testing capabilities of the SHPB at ASU and allow for more in depth analysis of the mechanical behavior of materials under impact loading. Combining torsion with tension or compression will promote analysis of a material's adherence to the Von Mises failure criterion. This greater understanding of material behavior can be implemented into models and simulations thereby improving the accuracy with which engineers can design new structures.

The main field of study research through this project is to study the effect of history of deformation in materials subjected to complex loading, useful for producing lightweight alloys and composites optimized for absorbing shock and impact. This is accomplished by creating a digital model of a system in which the material undergoes tension and compression through colliding bars. The results show that the system generated is accurate when compared to real tests, so the program used to create the model can be used in the future for simulated tests using different materials or applied loads.