DEFORMATION MECHANISMS IN NANOCRYSTALLINE ALUMINUM THIN FILMS: AN EXPERIMENTAL INVESTIGATION

Materials consisting of grains or crystallites with sizes below a hundred nanometers have exhibited unprecedented physical and mechanical properties in comparison to their coarse-grained counterparts. As a result, nanocrystalline materials have garnered considerable interest and a quest to uncover the new deformation mechanisms that give rise to this superior response has revealed that nanoscale behavior is quite different from that described by continuum plasticity. While the production of nanocrystalline materials with reasonable sizes for structural applications remains a challenge, thin metallic films used in next-generation MEMS and NEMS devices can be nanostructured by virtue of their limited dimensions. Ultimately, the reliability and lifetime prediction of these devices will hinge on the accurate modeling of their mechanical response. This dissertation describes efforts to elucidate the deformation mechanisms operating in nanocrystalline aluminum freestanding submicron thin films. Results obtained from these films demonstrate unique mechanical behavior, where discontinuous grain growth results in a fundamental change in the way in which the material deforms. In contrast to the low tensile ductility generally associated with nanocrystalline metals, these nanocrystalline films demonstrate extended tensile ductility. In situ X-ray diffraction and post-mortem transmission electron microscopy point to the importance of stress-assisted room temperature grain growth in transforming the underlying processes that govern the ii mechanical response of the films; nanoscale deformation mechanisms give way to microscale plasticity. The findings highlighted in this work emphasize that the microstructure and the attendant properties are dynamic; they evolve as the nanocrystalline material is being deformed. Experiments designed to address the role of impurities in stabilizing the microstructure against an applied stress are used to demonstrate that a critical concentration of impurities can effectively pin the grain boundaries from any motion. A detailed comparison of the characteristics of grain growth with traditional driving forces for grain boundary migration reveals the need for an alternative description. Measurements of surface topography evolution indicate that shear stresses directly couple to grain boundaries, induce motion, and result in grain growth that dramatically changes the mechanical behavior of these films. Finally, comparison with recently published theoretical formulations and molecular dynamics simulations is shown

Robust Scaling of Strength and Elastic Constants and Universal Cooperativity in Disordered Colloidal Micropillars

We study the uniaxial compressive behavior of disordered colloidal free-standing micropillars composed of a bidisperse mixture of 3- and 6-μm polystyrene particles. Mechanical annealing of confined pillars enables variation of the packing fraction across the phase space of colloidal glasses. The measured normalized strengths and elastic moduli of the annealed freestanding micropillars span almost three orders of magnitude despite similar plastic morphology governed by shear banding. We measure a robust correlation between ultimate strengths and elastic constants that is invariant to relative humidity, implying a critical strain of ∼0.01 that is strikingly similar to that observed in metallic glasses (MGs) [Johnson WL, Samwer K (2005) Phys Rev Lett 95:195501] and suggestive of a universal mode of cooperative plastic deformation. We estimate the characteristic strain of the underlying cooperative plastic event by considering the energy necessary to create an Eshelby-like ellipsoidal inclusion in an elastic matrix. We find that the characteristic strain is similar to that found in experiments and simulations of other disordered solids with distinct bonding and particle sizes, suggesting a universal criterion for the elastic to plastic transition in glassy materials with the capacity for finite plastic flow

Extremely Low Drift of Resistance and Threshold Voltage in Amorphous Phase Change Nanowire Devices

Time-dependent drift of resistance and threshold voltage in phase change memory (PCM) devices is of concern as it leads to data loss. Electrical drift in amorphous chalcogenides has been argued to be either due to electronic or stress relaxation mechanisms. Here we show that drift in amorphized Ge2Sb2Te5 nanowires with exposed surfaces is extremely low in comparison to thin-film devices. However, drift in stressed nanowires embedded under dielectric films is comparable to thin-films. Our results shows that drift in PCM is due to stress relaxation and will help in understanding and controlling drift in PCM devices

Linear complexions directly modify dislocation motion in face-centered cubic alloys

Linear complexions are defect phases that form in the presence of dislocations and thus are promising for the direct control of plasticity. In this study, atomistic simulations are used to model the effect of linear complexions on dislocation-based mechanisms for plasticity, demonstrating unique behaviors that differ from classical dislocation glide mechanisms. Linear complexions impart higher resistance to the initiation and continuation of dislocation motion when compared to solid solution strengthening in all of the face-centered cubic alloys investigated here, with the exact strengthening level determined by the linear complexion type. Stacking fault linear complexions impart the most pronounced strengthening effect, as the dislocation core is delocalized, and initiation of plastic flow requires a dislocation nucleation event. The nanoparticle and platelet array linear complexions impart strengthening by acting as pinning sites for the dislocations, where the dislocations unpin one at a time through bowing mechanisms. For the nanoparticle arrays, this event occurs even though the obstacles do not cross the slip plane and instead only interact through modification of the dislocation's stress field. The bowing modes observed in the current work appear similar to traditional Orowan bowing around classical precipitates but differ in a number of important ways depending on the complexion type. As a whole, this study demonstrates that linear complexions are a unique tool for microstructure engineering that can allow for the creation of alloys with new plastic deformation mechanisms and extreme strength

The Role of Confinement on Stress-Driven Grain Boundary Motion in Nanocrystalline Aluminum Thin Films

3D molecular dynamics simulations are performed to investigate the role of microstructural confinement on room temperature stress-driven grain boundary (GB) motion for a general population of GBs in nanocrystalline Al thin films. Detailed analysis and comparison with experimental results reveal how coupled GB migration and GB sliding are manifested in realistic nanoscale networks of GBs. The proximity of free surfaces to GBs plays a significant role in their mobility and results in unique surface topography evolution. We highlight the effects of microstructural features, such as triple junctions, as constraints to otherwise uninhibited GB motion. We also study the pinning effects of impurities segregated to GBs that hinder their motion. Finally, the implications of GB motion as a deformation mechanism governing the mechanical behavior of nanocrystalline materials are discussed

Binary nanocrystalline alloys with strong glass forming interfacial regions: Complexion stability, segregation competition, and diffusion pathways

Stabilization of grain structure is important for nanocrystalline alloys, and grain boundary segregation is a common approach to restrict coarsening. Doping can alter grain boundary structure, with high temperature states such as amorphous complexions being particularly promising for stabilization. Dopant enrichment at grain boundaries may also result in precipitate formation, giving rise to dopant partitioning between these two types of features. The present study elucidates the effect of dopant choice on the retention of amorphous complexions and the stabilization of grain size due to various forms of interfacial segregation in three binary nanocrystalline Al-rich systems, Al-Mg, Al-Ni, and Al-Y as investigated in detail using transmission electron microscopy. Amorphous complexions were retained in Al-Y even for very slow cooling conditions, suggesting that Y is the most efficient complexion stabilizer. Moreover, this system exhibited the highest number density of nanorod precipitates, reinforcing a recently observed correlation between amorphous complexions and grain boundary precipitation events. The dopant concentration at the grain boundaries in Al-Y is lower than in the other two systems, although enrichment compared to the matrix is similar, while secondary segregation to nanorod precipitate edges is much stronger in Al-Y than in Al-Mg and Al-Ni. Y is generally observed to be an efficient doping additive, as it stabilizes amorphous features and nanorod precipitates, and leaves very few atoms trapped in the matrix. As a result, all grains in Al-Y remained nanosized whereas abnormal grain growth occurred in the Al-Mg and Al-Ni alloys. The present study demonstrates nanocrystalline stability via simple alloy formulations and fewer dopant elements, which further encourage the usage of bulk nanostructured materials

Lattice Anharmonicity in Defect-Free Pd Nanowhiskers

We have investigated anharmonic behavior of Pd by applying systematic nanoscale tensile testing to near defect-free nanowhiskers offering a large range of elastic strain. We measured size-dependent deviations from bulk elastic behavior in nanowhiskers with diameters as small as ∼30  nm. In addition to size-dependent variations in Young’s modulus in the small strain limit, we measured nonlinear elasticity at strains above ∼1%. Both phenomena are attributed to higher-order elasticity in the bulklike core upon being biased from its equilibrium configuration due to the role of surface stresses in small volumes. Quantification of the size-dependent second- and third-order elastic moduli allows for calculation of intrinsic material nonlinearity parameters, e.g., δ. Comparison of the size-independent values of δ in our nanowhiskers with studies on bulk fcc metals lends further insight into the role of length scales on both elastic and plastic mechanical behavior

Quantifying the commonalities in structure and plastic deformation in disordered materials

The nonequilibrium nature of kinetically frozen solids such as metallic glasses (MGs) is at once responsible for their unusual properties, complex and cooperative deformation mechanisms, and their ability to explore various metastable states in the rugged potential energy landscape. These features coupled with the presence of a glass transition temperature, above which the solid flows like a supercooled liquid, open the door to thermoplastic forming operations at low thermal budget as well as thermomechanical treatments that can either age (structurally relax) or rejuvenate the glass. Thus, glasses can exist in various structural states depending on their synthesis method and thermomechanical history. Nanocrystalline (NC) metals, also considered to be far-from-equilibrium materials owing to the large fraction of atoms residing near grain boundaries (GBs), share many commonalities with MGs both in terms of plastic deformation and its dependence on processing history. Despite these similarities, the disorder intrinsic to both classes of materials has precluded the development of structure-property relationships that can capture the multiplicity of energetic states that glasses and GBs may possess. Here, we report on experimental studies of MG and NC materials and novel synthesis and processing routes for controlling the structural state – and as a consequence, the mechanical properties. A particular focus will be on strategies for rejuvenation of disorder with the goal of suppressing shear localization and endowing damage tolerance. We also describe a microscopic structural quantity designed by machine learning to be maximally predictive of plastic rearrangements and further demonstrate a causal link between this measure and both the size of rearrangements and the macroscopic yield strain. We find remarkable commonality in all of these quantities in disordered materials with vastly different inter-particle interactions and spanning a large range of elastic modulus and particle size

Intermetallic particle heterogeneity controls shear localization in high-strength nanostructured Al alloys

The mechanical behavior of two nanocrystalline Al alloys, Al-Mg-Y and Al-Fe-Y, is investigated with in-situ micropillar compression testing. Both alloys were strengthened by a hierarchical microstructure including grain boundary segregation, nanometer-thick amorphous complexions, carbide nanorod precipitates with sizes of a few nanometers, and submicron-scale intermetallic particles. The maximum yield strength of the Al-Mg-Y system is measured to be 950 MPa, exceeding that of the Al-Fe-Y system (680 MPa), primarily due to a combination of more carbide nanorods and more amorphous complexions. Both alloys exhibited yield strengths much higher than those of commercial Al alloys, and therefore have great potential for structural applications. However, some micropillar specimens were observed to plastically soften through shear banding. Post-mortem investigation revealed that intermetallic-free deformation pathways of a few micrometers in length were responsible for this failure. Further characterization showed significant grain growth within the shear band. The coarsened grains maintained the same orientation with each other, pointing to grain boundary mechanisms for plastic flow, specifically grain rotation and/or grain boundary migration. The presence of intermetallic particles makes it difficult for both matrix and intermetallic grains to rotate into the same orientation due to the different lattice parameters and slip systems. Therefore, we are able to conclude that a uniform distribution of intermetallic particles with an average spacing less than the percolation length of shear localization can effectively prevent the maturation of shear bands, offering a design strategy for high-strength nanocrystalline Al alloys with both high strength and stable plastic flow