This paper embraces several aspects related to dislocation self-organization in single slip. The formation of prismatic loops, seminal to the nucleation and growth of dislocation braids, is a dynamical process involving elastic dipolar interactions. Preliminary simulations of the concerted process of loop generation are presented. Elastic properties of dislocation dipoles are analysed as a function of dipole character. Equilibrium properties are investigated under the isotropic and anisotropic elastic approximations. The orientation dependence of the so-called dipole passing stress is examined, and its implication in the elementary mechanisms that take place under constrained deformation conditions, such as fatigue cycling, are discussed.

In the present study, hard TiSiN coating compounds with different Si content have been deposited by the magnetron sputtering technique. X-ray diffraction (XRD) and X-ray absorption spectroscopy (XAS) have been employed to investigate the effects of Si addition on the composition, structure and phase development of the cubic NaCl-type structure. To get some insight on these changes, X-ray absorption spectra have been measured at the Ti and Si K-edges in fluorescence yield mode. The collected data showed that the interaction between Ti and Si was weak and the formation of a-Si3N4 can be assumed. Hardness measurements were also consistent with the formation of hard nanocomposite films. A maximum hardness of about 36 GPa was obtained for the sample with 11 at. % Si.

This paper presents the results of an experimental study of the indentation size effects at the nano, sub-micron, and micron-scales. The size dependence of the indentation hardness in these regimes is shown to exhibit a bi-linear behavior when the data are fitted to Taylor Dislocation Hardening (TDH) model. The deformation in indent sizes corresponding to the onset of the transition from micro and deep nano indents to shallow nano indents represent dislocation substructures at the sub-micron and micron scales whereas the deformation at the nano-scale represents dislocation source-limited behavior.

Dislocation events are seen as excursions, or pop-in events, in the load-displacement trace of nanoindentation experiments. When indenting single crystal metals these events occur frequently during quasi-static and dynamic loading. A single crystal of Ni (111) has been indented quasi-statically using three different loading rates (10, 100, and 1000 μN/s) as well as with three different radii diamond indenter tips (1000 nm cone, 300 nm Berkovich, and 50 nm cube corner) to examine the occurrences of excursions. As expected, excursions at higher loads have larger displacements, and that initial loading follows Hertzian behavior up to the point of yield. Also, as the tip size is reduced the excursion loads are reduced. The excursion events depend mostly on the statistical distribution of surface sources and substructure dislocation arrangements.

Atomic defects, in the form of dislocation configurations, are identified following nanoindentation of Au(001) surfaces. Combined STM, dislocation theory and atomistic simulations fully characterise these defects and describe their motion. By comparing with AFM data the emission of those defects are correlated with the incipient stages of plasticity, the latter recognised from force vs penetration curves. The size distribution, the spatial pattern and the motion of these dislocations under stress have also been investigated and explained in terms of the theory of dislocations in a continuum.

Methods to obtain tensile stress-strain properties of materials from a practically non-destructive indentation test are of great industrial interest. However, to do this successfully, indentation size effects must be accounted for. Many indentation size effects, such as strain gradient plasticity and micro-pillar experiments [1], show a size dependence proportional to the inverse square root of a length scale, in common with Hall-Petch behavior. Recently, however, the indentation size effect from small radius spherical indenters has been shown, for a range of fcc metals, not to follow a Hall-Petch-like relationship but to be proportional to the inverse cube root of indenter radius [2]. Here, we investigate these differences further and present results for the indentation size effect with spherical indenters on Cu samples that have been engineered to have different grain sizes. The important experimental control parameter of the relative size of the indentation compared to the grain size is also explored since the cross over from grains significantly smaller than the contact radius to grains significantly larger than the contact radius occurs at different length scales in each sample. A thorough understanding of the various length-scale effects in the different test methods (e.g. the indentation size effect and grain size effect in indentation), is essential if a relationship, robust enough for industrial application, is to be defined to obtain tensile properties from an essentially non-destructive indentation test.

Conventional nanoindentation testing generally uses a peak penetration depth of less than 10 % of thin-film thickness in order to measure film-only mechanical properties, without considering the critical depth for a given thin film-substrate system. The uncertainties in this testing condition make hardness measurement more difficult. We propose a new way to determine the critical relative depth for general thin-film/substrate systems; an impression volume analyzed from the remnant indent image is used here as a new parameter. Nanoindents made on soft Cu and Au thin films with various indentation loads were observed by atomic force microscope. The impression volume calculated from 3D remnant image was normalized by the indenter penetration volume. This indent volume ratio varied only slightly in the shallow regime but decreased significantly when the indenter penetration depth exceeded the targeted critical relative depth. Thus, we determined the critical relative depth by empirically fitting the trend of the indent volume ratio and determining the inflection point. The critical relative depths for Cu and Au films were determined as 0.170 and 0.173, respectively, values smaller than 0.249 and 0.183 determined from the hardness variation of the two thin films. Hence the proposed indent volume ratio is highly sensitive to the substrate constraint, and stricter control of the penetration depth is needed to measure film-only mechanical properties.

Fracture tests have been performed for both macro- and micro-sized specimens prepared from an Fe-3%Si alloy single crystal, and the effects of length scale on fracture behavior have been investigated. Micro-sized cantilever beam specimens with dimensions of 10 × 10 × 50 μm3 were prepared by focused ion beam machining. Notches with a depth of 5 μm were introduced into the micro-sized specimens and fatigue pre-crack was also introduced ahead of the notch. Macro-sized three-point bending specimens with dimensions of 2 × 2 × 10 mm3 were also cut from the same Fe-3%Si alloy single crystal. Notches with a depth of 1mm were introduced into the macro-sized specimens, and fatigue pre-crack was also introduced. Notch plane was set to be (100), which is a cleavage plane of this material, and notch direction was set to be [010] for both size of specimens. For macro-sized specimens, cleavage fracture occurred during introducing fatigue pre-crack. In contrast, the micro-sized specimens were fractured by ductile manner. A plastic zone was clearly observed on the specimen surface near the crack tip and dimples were found on the fracture surface. The plastic zone size of this material is calculated to be 90 μm. This size is small enough to satisfy small scale yielding for macro-sized specimens, although this size corresponds to large scale yielding in micro-sized specimens. This may cause the size effect on the fracture behavior of this material.

An inverse Hall-Petch effect has been observed for nanocrystalline materials by a large number of researchers. This result implies that nanocrystalline materials get softer as grain size is reduced below a critical value. Postulated explanations for this behavior include dislocation based mechanisms and diffusion based mechanisms. In this paper, we report an explanation for the inverse Hall-Petch effect based on the statistical absorption of dislocations by grain boundaries, showing that the yield strength is both dependent on strain rate and temperature, and that it deviates from the Hall-Petch relationship at a critical grain size.

This study examines the deformation response of 20, 5, and 1 micron diameter samples fabricated by FIB-milling from bulk ultrapure LiF single crystals. The bulk crystals have a very low initial dislocation density as revealed by an etch-pit technique. Two types of <001> microsamples were deformed preferentially by single slip when compressed inside a nanoindentation system. Similar to previously studied FCC-derivative metals, LiF microsamples demonstrate dramatic strengthening achieving the engineering flow stress σ ≈ 650 MPa in 1-μm samples. The stress-diameter dependence obeys a power law, σ ∼ D−m , where m ≈ 0.8. Stochastic variation of flow stress, fast intermittent deformation events (“avalanches”) and highly localized slip bands after avalanches - all characteristic of size effects in metals, are also observed in LiF. Possible dislocation mechanisms of the observed size effects are discussed.

Deformation experiments at temperatures between 300 and 750 K have been performed in situ in the transmission electron microscope to investigate dislocation interactions and reactions with grain boundaries and other obstacles. Dislocations, both partial and perfect, as well as deformation twins have been observed being emitted from grain boundaries and, in some cases, even the same grain boundary. The ejection of dislocations from the grain boundary can result in its partial or total annihilation. In the latter case, the disintegration of the grain boundary was accompanied by grain growth and a change in misorientation.

Nanoporous gold (np-Au), produced by selectively removing silver from an AuAg alloy, has recently gained considerable attention from the scientific community. Biocompatibility, chemical inertness, increased surface area, relatively low elastic modulus, and ease of synthesis make np-Au an important candidate for biomedical, catalytic, and MEMS applications. Np-Au films also offer substantial ground for theoretical and empirical research, including mechanical characterization, fracture mechanics, and porosity evolution. Even though a significant effort has been directed towards exploring blanket np-Au films (i.e., foils, strips), to our knowledge no work has been done on fabricating or investigating freestanding np-Au structures (i.e., micro-beams, cantilevers). Recently we have developed techniques to create freestanding clamped np-Au beams with widths from 5 to 40 microns and lengths from 20 to 500 microns. The percentage yield was more than 97% for 2880 beams on a 2-inch wafer. A critical step in the fabrication process, necessary to prevent tensile failure of the beams during dealloying, is a thermal heat treatment prior to dealloying. The study of thermal treatment of beams at temperatures between 100°C and 600°C prior to dealloying revealed three distinct beam behavior regimes, namely quasi-elastic buckling, plastic buckling, and material interdiffusion. This paper will present the preliminary results from thermal treatment experiments particularly focusing on how beam dimensions affect percentage yield and beam fracture.

In this study, mechanical behavior of nanocrystalline Cu alloy thin films under constant tensile strain is studied, by taking advantage of the enhanced resistance to strain localization of a thin metal film on a thick elastomer with appropriate Young's modulus. Cu and Cu alloy thin films with a thickness of 100nm were deposited on heat-resistant polyimide substrates. On the top of the Cu alloy layer, Ta thin film was further deposited to suppress the surface diffusion of Cu alloy. A uniform uniaxial tensile strain was induced to the films by applying a constant radius of curvature to the polyimide substrate. Isothermal deformation and rupture modes of the films were studied by keeping the samples at various elevated temperatures. Microstructural observation was carried out using SEM, TEM, STEM, and confocal violet laser microscope at several stages of deformation. From the observation, the evolution of damage in the sub-critical strain level and the effect of alloying elements are discussed.

A majority of computational mechanical analyses of nanocrystalline materials or nanowires have been carried out using classical molecular dynamics (MD). Due to the fundamental reason that the MD simulations must resolve atomic level vibrations, they cannot be carried out at the timescale of the order of microseconds. Additionally, MD simulations have to be carried out at very high loading rates (∼108 s−1) rarely observed in experiments. In this investigation, a modified Hybrid Monte Carlo (HMC) method that can be used to analyze time-dependent (strain rate dependent) atomistic mechanical deformation of nanostructures at continuum timescales is established. In this method there is no restriction on the size of MD timestep except that it must be such that to ensure a reasonable acceptance rate between consecutive Monte-Carlo (MC) time-steps. For the purpose of comparison HMC analyses of Cu nanowires deformation at two different strain rates (108 s−1 and 109 s−1) (each with three different timesteps 2 fs, 4fs, and 8 fs) are compared with the analyses based on MD simulations at the same strain rates and a MD timestep of 2 fs. As expected, the defect formation is found to be strain rate dependent. In addition, HMC with timestep of 8 fs correctly reproduces defect formation and stress-strain response observed in the case of MD with 2 fs (for the interatomic potential used 2 fs is the highest MD timestep). Simulation time analyses show that by using HMC a saving of the order of 4 can be achieved bringing the atomistic analyses closer to the continuum timescales.

A unified dislocation density-based microstructural representation of f.c.c. crystalline materials, has been developed such that the microstructural behavior can be accurately predicted at different physical scales. This microstructural framework is based on coupling a multiple-slip crystal plasticity formulation to three distinct dislocation densities, which pertain to statistically stored dislocations (SSDs), geometrically necessary dislocations (GNDs), and grain boundary dislocations (GBDs). This interrelated dislocation-density formulation is then used with specialized finite-element modeling techniques to predict the evolving heterogeneous microstructure and the localized phenomena that can contribute to failure initiation as a function of inelastic deformation.

In nanoindentation, the plasticity size effect has been observed for several years, where a higher hardness is measured as indenter size decreases. In this paper, we report the size effect on the initiation of plasticity in ceramics by using spherical indenters. Here, we show a clear method that is able to determine the details of the onset of plasticity in nanoindentation. This enables us to measure the yield pressure with a high degree of accuracy and over a large range of indenter radii (hundreds of nanometers to several tens of micrometers). Our data shows clearly that there is a significant yield strength enhancement, which is inversely proportional to the cube root of the indenter radius. Also after normalization by the bulk yield strength, the increase in yield strength with decreasing indenter radius is shown to follow a single relation for all the ceramics studied in agreement with recent results for metals [1], and consistent with critical thickness theory for the initiation of yielding over a finite volume.