The present collection of articles focuses on the mechanical strength properties at micro- and nanoscale dimensions of body-centered cubic, face-centered cubic and hexagonal close-packed crystal structures. The advent of micro-pillar test specimens is shown to provide a new dimensional scale for the investigation of crystal deformation properties. The ultra-small dimensional scale at which these properties are measured is shown to approach the atomic-scale level at which model dislocation mechanics descriptions of crystal slip and deformation twinning behaviors are proposed to be operative, including the achievement of atomic force microscopic measurements of dislocation pile-up interactions with crystal grain boundaries or with hard surface coatings. A special advantage of engineering designs made at such small crystal and polycrystalline dimensions is the achievement of an approximate order-of-magnitude increase in mechanical strength levels. Reasonable extrapolation of macro-scale continuum mechanics descriptions of crystal strength properties at micro- to nano-indentation hardness measurements are demonstrated, in addition to reports on persistent slip band observations and fatigue cracking behaviors. High-entropy alloy, superalloy and energetic crystal properties are reported along with descriptions of deformation rate sensitivities, grain boundary structures, nano-cutting, void nucleation/growth micromechanics and micro-composite electrical properties.

Single crystalline, μm-sized cantilevers are fabricated out of epitaxially grown Ag thin films by a lithography-based procedure and are deflected by a nanoindenter system. The microstructure of the plastically deformed cantile-vers is investigated using transmission Kikuchi diffraction (TKD) on the cantilever cross section. 3D discrete dislocation dynamics simulations (DDD) are performed for further analysis. A mechanism to explain the formation of dislocation networks upon loading is suggested.

Several basic aspects concerning the effects of grain boundaries in niobium bicrystals were explored. The investigation led to the following findings: (1) A new method of growing oriented niobium bicrystals (the Y shaped seed technique) was developed. Large cylindrical bicrystals, 0.63 cm in diameter and over 10 cm in length, have been grown from the melt using this method. (2) Niobium bicrystals exhibit excess hardening at the grain boundary, as shown by microhardness measurements. The degree of boundary hardening increased as the angle of misorientation increases. In addition, hardening is greater in bicrystals with tilt boundaries than with twist boundaries. (3) In tensile deformation of niobium bicrystals, Stage I hardening is absent. In Stage II, extra slip traces are activated from the boundary. In some cases, the primary slip system of the bicrystal differs from that of the single crystal of similar orientation. (4) In conjunction with the experimental program, a refined analysis was made to examine the dislocation-boundary interaction. It was found that in non-symmetrical bicrystals an edge dislocation wall has a long-range stress field which contrasts with the classical result in a single crystal. (Author).

This thesis investigates the interaction between gamma' precipitates and grain boundaries in a Ni-Al system during deformation. This interaction is investigated using molecular dynamics, and gamma'/boundary configurations were built to investigate how the orientation, size, and interaction of gamma' change the deformation behavior of the grain boundary. The gamma' aided in nucleating defects (i.e., dislocations) that contributed to the boundary sliding mechanism. By increasing the size of precipitates that bisect the boundary, the boundary becomes stronger, whereas increasing the size of precipitates adjacent to the boundary makes the boundary weaker. Additionally, the interaction of multiple gamma' plays a role in grain boundary sliding behavior. Low concentrations of gamma' produce sliding dominated by atomic shuffling, whereas high concentrations of gamma' produce sliding dominated by dislocation emission. More work is needed to investigate the effects of temperature, initial defects, and different grain boundary configurations on sliding behavior.

Abstract : Grain boundary segregation is well known to cause significant embrittlement of alloys. But in certain cases, it has also been observed to increase mechanical strength. This project attempts to assess local mechanical behavior of specific grain boundaries with and without segregation in order to understand association between grain boundary chemistry and deformation mechanism utilizing instrumented nanoindentation technique. It is hypothesized that solute segregation strongly affects the grain boundary energy which in turn affects the deformation mechanism processes. This project also utilizes a unique ability provided by the instrumented indentation technique to interrogate local grain boundary strengthening mechanisms proposed by Hall-Petch and Taylor-Ashby using two different indentation geometries. Grain boundary mechanical properties have typically been interpolated from macroscopic mechanical testing on polycrystalline materials, or alternatively, mechanical test procedures carried out on bulk bicrystals. The disadvantages to these types of studies relate to the difficulty in extracting the local response of a particular grain boundary (in the case of polycrystalline materials) or the grain boundary region (in the case of a bicrystal material) from the overall response of the complex interaction between the presence of the grain boundary and the deformation behavior far from the grain boundary. That is, the grain boundary causes a non-local response to the mechanical behavior. This non-local response is particularly evident in bicrystal deformation, where the macroscopic plastic displacement is inconsistent with that observed for single crystal deformation. Moreover, local hardness testing of grain boundary regions in macroscopically deformed materials show that the deformation in the grain boundary region is leads to greater local dislocation density than found in the grain center. This project is designed to use nanoindentation to isolate the mechanical response of the grain boundary as the dependent variable, where indentation geometry, indentation rate, grain boundary misorientation and sample chemistry are the independent experimental variables. It is proposed that this approach can provide insight into long standing hypotheses regarding grain boundary strengthening mechanisms, including the Hall-Petch pile-up theory, grain boundary source theory, grain boundary layer theory and the Ashby-Taylor strain incompatibility theory.

A dislocation-density grain boundary interaction scheme (DDGBI) has been developed to account for complex interrelated dislocation-density interactions of emission, absorption and transmission in grain-boundary (GB) regions for bicrystals and polycrystals with different random and coincident site lattice (CSL) GB arrangements. This scheme is coupled to a dislocation-density crystalline plasticity formulation and specialized finite-element scheme at different physical scales. The DDGBI scheme is based on slip-system compatibility, local resolved shear stresses, and immobile and mobile dislocation-density activities at GBs. A conservation law for dislocation-densities is used to balance dislocation-density absorption, transmission and emission in GB regions. It is shown that dislocation-density absorptions and pile-ups will increase immobile dislocation-densities in high angle CSL boundaries, such as Σ17b. Lower angle CSLs, such as Σ1, are characterized by high transmission rates and insignificant GB dislocation-density accumulations. The identification of how different material mechanisms dominate underscores that GB activities, such as dislocation-density absorption, transmission and emission are interrelated interactions. These GB processes can be potentially controlled for desired material behavior. This methodology, together with grain boundary sliding (GBS) scheme and a misorientation dependence on initial GB dislocation-densities, was extended to account for grain size effects on strength. The behavior of polycrystalline aggregates with random low angle and random high angle GBs was also investigated with different crack lengths. For aggregates with random low angle GBs, dislocation-density transmission dominates at the GBs, which indicates that the low angle GB will not significantly change crack growth orientations. For aggregates with random high angle GBs, extensive dislocation-density absorption and pile-ups occur. The high stresses along the GB regions.