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Abstract: In the present work we investigate the mechanical properties of multiple slip oriented single crystal Cu(100) compression samples to shed light on size-dependent yield and hardening behavior at small-scales. Samples with diameters ranging from 90nm to 1700nm were fabricated using focused ion beam milling and tested in situ in a transmission electron microscope. The results demonstrate a dislocation source-limited size-dependent yield strength, as evidenced by size-dependent changes in the deformation morphology. Moreover, we report size dependency and strain dependency in the hardening behavior at these dimensions, where higher hardening is observed for smaller samples and at lower strains. This is explained by the source-limited nature of plasticity in small dimensions, which we demonstrate affects not just yield but also the hardening behavior in the nanopillars. [ABSTRACT FROM AUTHOR]

Abstract: Micro-compression testing is a promising technique for determining mechanical properties at small length scales since it has several benefits over nanoindentation. However, as for all new techniques, experimental constraints influencing the results of such a micro-mechanical test must be considered. Here we investigate constraints imposed by the sample geometry, the pile-up of dislocations at the sample top and base, and the lateral stiffness of the testing setup. Using a focused ion beam milling setup, single crystal Cu specimens with different geometries and crystal orientations were fabricated. Tapered samples served to investigate the influence of strain gradients, while stiff sample top coatings and undeformable substrates depict the influence of dislocation pile-ups at these interfaces. The lateral system stiffness was reduced by placing specimens on top of needles. Samples were loaded using an in situ indenter in a scanning electron microscope in load controlled or displacement controlled mode. The observed differences in the mechanical response with respect to the experimental imposed constraints are discussed and lead to the conclusion that controlling the lateral system stiffness is the most important point. [Copyright &y& Elsevier]

Recent investigations have reported significantly higher flow stresses for microcompression compared to microtensile testing. To clarify this point a load reversal test was performed on a microtensile specimen and equal flow stresses in tension and compression were observed. In contrast, if the lateral sample compliance constrains the deformation, microcompression testing overestimates the flow stresses. Additional contributions to the measured flow stresses stem from the aspect ratio of the sample and dislocation pile-ups. [Copyright &y& Elsevier]

This article discusses crystal rotations in micro-compressed single copper columns that are dislocation-induced. Although flow stress of such single crystal compression samples has been found to be size dependent through the use of a focused ion beam, no model has been proposed to explain this unusual behavior. Experiments with dislocation starvation, surface image stresses, and consideration of stochastic single-ended dislocation sources have also failed to develop a model. This study applies an in situ set-up for the loading experiments and, subsequently, local crystal orientation of the deformed samples is analyzed by electron backscatter diffraction to examine the microstructural development.

Abstract: A method for in situ testing of miniaturized tension specimen was developed. The size effects of the plastic deformation behavior of copper single crystals loaded along the 〈−234〉 direction were investigated. The diameter was varied between 0.5μm and 8μm, and the aspect ratio, gauge length to side length, between 1:1 and 13.5:1. At high aspect ratios hardening was negligible. However, an increase of the flow stress with decreasing diameter was observed. This increase was small for diameters above 2μm, and somewhat larger below 2μm. These findings are explained by individual dislocation sources which govern the plastic deformation. For low aspect ratios the behavior is significantly different. A pronounced hardening and a very strong size effect was observed. Both are a result of dislocation pile-ups due to the constrained glide of the dislocations caused by the sample geometry. [Copyright &y& Elsevier]

Abstract: Cu specimens were exposed to Ga+ ion bombardment for varying conditions of ion energy, ion dose, and incident angle in a focussed ion beam workstation. Conventional transmission electron microscopy investigations were employed to analyze the Ga+ ion induced damage. The extent of visible damage was minimized by reducing the ion energy and furthermore by using grazing incident ions. Concentration depth profiles of the implanted Ga were measured by Auger electron spectroscopy. Concentrations of up to 20at.% Ga were found several nanometers below the surface. Ga contents of more than 2at.% were detected within a depth of up to ∼50nm. Mechanical consequences in terms of possible hardening mechanisms are discussed, taking into account the experimental findings along with Monte Carlo simulations. A non-negligible influence of the ion damage is predicted for submicron-sized samples. [Copyright &y& Elsevier]

Abstract: In order to characterize the deformed volume beneath microindents in tungsten and copper, several cross-sections through the indents were machined using a focused ion beam workstation. Electron backscattering diffraction scans were performed on these cross-sections to determine local orientations and changes of orientation due to plastic deformation with respect to the undeformed crystal. It was found that the deformation is highly localized beneath the tip and flanks of the indent. For small indentations a constant ratio between the maximum misorientation beneath the indent flanks and the diameter of the imprint was found. This conflicts with the idea of self-similar deformation. [Copyright &y& Elsevier]

Service materials are often designed for strength, ductility, or toughness, but neglect the effects of cyclic time-variable loads ultimately leading to macroscopic mechanical failure. Fatigue originates as local plasticity that can first only be observed on the micro scale at defects serving as stress concentrators such as inclusions or grain boundaries. Thus, a recently developed technique to perform in-situ observation of micro scale bending fatigue experiments was applied. Micro-beams fabricated from copper, single grained and ultrafine grained (ufg), and an oxide dispersion strengthened (ODS) steel were subject to cyclic deformation and subsequent damage. The elastic stiffness, yield strength, dissipated energy, and maximum stress were measured as a function of cycle number and plastic strain amplitude. From these properties, cyclic stress-strain curves were developed. Initial pronounced monotonic hardening and an increasing Bauschinger effect were observed in all samples with increasing strain amplitude. Cyclic stability was maintained until plastic strain amplitudes reached a critical value. At this point, dramatic cyclic softening and microcracking occurred. The critical strain amplitude was found to be approximately 5.4×10 −3 for the copper with a refined grain structure and 1.2×10 −2 for the steel specimen. Grain rotation and noticeable changes in sub-grain structure were evident in the ufg copper after a critical strain amplitude of ε a =8.3×10 −3 . In-situ micro fatigue bending couples the cyclic evolution of bulk mechanical properties measurements with real-time electron microscopy analysis techniques of damage and failure mechanisms, which renders it a powerful method for developing novel fatigue resistant materials. [ABSTRACT FROM AUTHOR]

Advanced die application materials, utilizing pressure-less sintered copper, show great prospects regarding cost effectiveness, power density, withstanding high switching speeds and temperature loading for novel eco-friendly and high efficiency semiconductors. In general, to preserve high reliability in combination with electrical functionality the design of elastic as well as electrical material parameters is of great importance. Here, we present a multi-method characterization approach to understand the impact of the morphology on the elastic as well as electrical behavior, which facilitates a strategy to design the relevant material parameters by tuning the morphology. Nano-SEM/FIB tomography and SEM/EBSD are applied to probe the morphology of three representative copper films. Nanoindentation and 4-point probe are used to extract the elastic modulus and specific electrical resistivity, respectively. The evaluated material parameters are compared with modeling results using the analyzed image data as an input. For the crucial image analysis, we develop a validated objective image analysis workflow. We obtain a quantified insight about the effect of the heterogeneous morphologies on the elastic modulus and specific electrical resistivity, thereby delivering important information about the necessary homogeneous copper morphology- and nano-scale pore-design. The strategy shall provide design guidelines to ensure reliable and high-performance die attachments. Unlabelled Image • We present a multi-method characterization approach to understand the morphology-property relationship of sintered copper. • We facilitate a strategy to design relevant material parameters for sintered copper films by tuning the morphology. • We obtain a quantified insight about the effect of the heterogeneous morphologies on the investigated material properties. [ABSTRACT FROM AUTHOR]