In-situ tensile tests have been performed in a dual beam focused ion beam and scanning electron microscope on as-grown and prestrained single-crystal molybdenum-alloy (Mo-alloy) fibers. The fibers had approximately square cross sections with submicron edge lengths and gauge lengths in the range of 9–41 μm. In contrast to previously observed yield strengths near the theoretical strength of 10 GPa in compression tests of ∼1–3-μm long pillars made from similar Mo-alloy single crystals, a wide scatter of yield strengths between 1 and 10 GPa was observed in the as-grown fibers tested in tension. Deformation was dominated by inhomogeneous plastic events, sometimes including the formation of Lüders bands. In contrast, highly prestrained fibers exhibited stable plastic flow, significantly lower yield strengths of ∼1 GPa, and stress–strain behavior very similar to that in compression. A simple, statistical model incorporating the measured dislocation densities is developed to explain why the tension and compression results for the as-grown fibers are different.

Nanopillars and nanocoils fabricated by chemical vapor deposition using a focused ion beam were used to estimate bending and torsional rigidities under infinitesimal deformation and to investigate nonlinear large deformation behaviors. For the pillars, we performed bending tests using a unique double-cantilever specimen, which was made by joining two pillars together using focused electron beam deposition in a scanning electron microscope. The reproducible load–deflection curves, which were not severely disturbed by the ambiguous chuck condition of the specimens, indicated that the pillar deformation resistance decreased after the linear response (called softening), and it was dependent on the pillar diameter and the ratio of diameter to length. However, all pillars became extremely hardened at large deformation. At diameters of less than 300 nm, and at diameter/length ratios of over 10−2, this nanopillar size effect (characterized as softening) was consistently observed.

The ultimate properties of a fibrous composite system depend highly on the transverse mechanical properties of the fibers. Here, we report the size dependency of transverse elastic modulus in cellulose nanocrystals (CNCs). In addition, the mechanical properties of CNCs prepared from wood and cotton resources were investigated. Nanoindentation in an atomic force microscope (AFM) was used in combination with analytical contact mechanics modeling (Hertz model) and finite element analysis (FEA) to estimate the transverse elastic moduli (Et) of CNCs. FEA modeling estimated the results more accurately than the Hertz model. Based on the AFM–FEA calculations, wood CNCs had higher transverse elastic moduli in comparison to the cotton CNCs. Additionally, Et was shown to increase with a reduction in the CNCs’ diameter. This size-scale effect was related to the Iα/Iβ ratio and crystalline structure of CNCs.

Nanofiber yarns with controlled twist levels were prepared by twisting a narrow fibrous strip cut directly from electrospun nanofiber mats. The effects of fiber morphology, diameter and orientation, as well as the yarn twist level on the yarn tensile properties were examined. For the yarns made from randomly oriented fine uniform nanofibers (e.g., diameter 359 nm) and beaded nanofibers, the tensile strength increased with increasing the yarn twist level. Higher fiber diameter (e.g., 634 nm) led to the tensile strength having an initial increase and then decrease trend. The modulus increased with the twist level for all the yarns studied. However, the elongation at break increased initially with the twist level and subsequently decreased. The orientation of aligned fibers within the fiber strip greatly influenced the yarn tensile properties. When the fibers were oriented along the fiber length direction, both tensile strength and modulus were the largest.

Crystalline pentagonal nano- and microrods (PRs) and pentagonal nano- and microparticles (PPs) with 5-fold symmetry are studied. Structure of PRs and PPs and their elastic distortions are characterized in the framework of the disclination approach. Relaxation of mechanical stresses due to disclinations causes structural transformations in PRs and PPs. Experimental evidence of such transformations, namely, the appearance of internal cavities and pores, and growth of whiskers in copper PRs and PPs grown in the process of electrodeposition is demonstrated. A brief review of existing models of stress relaxation in PRs and PPs is presented. We discuss a new model of nanowhisker growth based on the nucleation of two dislocation loops of opposite signs near the surface of the crystal with disclination. As a result, vacancy-type dislocation loop remains in the material and serves as a nucleus for cavity, while the interstitial loop comes to the free surface and contributes to whisker growth.

To understand the brittleness transition in low-toughness materials, the nucleation and kinetics of dislocations must be measured and modeled. One aspect overlooked is that the apparent activation energy for plasticity is modified at very high stresses. Coupled with state of stress and length scale effects on plasticity, the lowering of the brittle-to-ductile transition (BDT) in such materials can be partially understood. Experimental evidence in silicon single crystals in the length scale regime of 40 nm to 1 mm is presented. It is shown that high stress affects both length scale and temperature-dependent properties of activation volume and activation energy for dislocation nucleation and/or mobility. Nanoparticles and nanopillars of single-crystal silicon demonstrate unexpectedly high fracture toughness at low temperatures under compression. A thermal activation approach can model the three decades of size associated with the factor of three absolute temperature shift in the BDT.

In this work, the ultimate bending strengths of as-grown Si and fully oxidized Si nanowires (NWs) were investigated by using a new atomic force microscopy (AFM) bending method. NWs dispersed on Si substrates were bent into hook and loop configurations by AFM manipulation. The adhesion between NWs and the substrate provided sufficient restraint to retain NWs in imposed bent states and allowed subsequent AFM imaging. The stress and friction force distributions along the bent NWs were calculated based on the in-plane configurations of the NWs in the AFM images. As revealed from the last-achieved bending state, before fracture, fracture strengths close to the ideal strength of materials were attained in these measurements: 17.3 GPa for Si NWs and 6.2 GPa for fully oxidized Si NWs.

Nanomechanical testing of silicon is primarily motivated toward characterizing scale effects on the mechanical behavior. “Defect-free” nanoscale silicon additionally offers a road to large deformation permitting the investigation of transport characteristics and surface instabilities of a significantly perturbed atomic arrangement. The need for developing simple and generic characterization tools to deform free-standing silicon beams down to the nanometer scale, sufficiently equipped to investigate both the mechanical properties and the carrier transport under large strains, has been met in this research through the design of a versatile lab-on-chip. The original on-chip characterization technique has been applied to monocrystalline Si beams produced from Silicon-on-Insulator wafers. The Young’s modulus was observed to decrease from 160 GPa down to 108 GPa when varying the thickness from 200 down to 50 nm. The fracture strain increases when decreasing the volume of the test specimen to reach 5% in the smallest samples. Additionally, atomic force microscope-based characterizations reveal that the surface roughness decreases by a factor of 5 when deforming by 2% the Si specimen. Proof of concept transport measurements were also performed under deformation up till 3.5% on 40-nm-thick lightly p-doped silicon beams.

Friction forces for a nanowire (NW) elastically bent on flat substrate were investigated both theoretically and experimentally. Models based on elastic beam theory were proposed considering balance of external, frictional, and elastic forces along the NW. The distributed friction force was determined for two cases: (i) the NW was uniformly dragged at its midpoint and bent by kinetic friction forces and (ii) the NW was held in a bent state by static friction forces. The first case considers a uniform distribution of kinetic friction along the NW and enables the measurement of the friction force from the elastically deformed NW profile. The second case exploits the interplay between static friction and elastic forces inside the NW to find the distributed friction force. An original method for the measurement of frictional forces in both cases while maintaining total force and momentum equilibrium was introduced and demonstrated for ZnO NWs on a Si wafer. Averaged kinetic and static friction forces were compared for the same individual NW.

Nanowires have attracted tremendous research interests due to their potential applications. Their mechanical properties are critical for the reliability and durability of the nanowire-based devices. Compared to many other characterization techniques, the lateral probing of a nanowire using nanoindentation has the advantage of relatively simple sample preparation. However, the data analysis is difficult due to the complex contact mechanics. In all previous studies, some questionable approximations have been made to proceed with data analysis. In this study, a quantitative physical picture of nanowire lateral probing is proposed, which we believe is the first time in the literature. Three-dimensional finite element analysis (FEA) is performed and compared to a double-contact analytical model in which the two contacts, namely contact 1 (indenter/nanowire) and contact 2 (nanowire/substrate), are considered. Both the FEA and analytical models are for a specific case: an elastic spherical indention of a GaN nanowire on a Si substrate. We find that contact 1 cannot be well approximated by a Hertzian elliptical contact as assumed in many studies. We also find a large contact deformation at contact 2, which has been ignored in almost all previous studies. Finally, the adhesion condition and nanowire-receding at contact 2 are found to have insignificant effects on the data analysis.

Microcompression tests were performed on the Al/Nb multilayers of incoherent interfaces with the layer thicknesses of 5 nm Al/5 nm Nb and 50 nm Al/50 nm Nb. The Al-Nb multilayers showed increase in strength as the layer thickness was reduced; the average flow stresses at 5% plastic strain from the 5 nm Al/5 nm Nb and 50 nm Al/50 nm Nb layer thickness specimens were determined to be 2.1 GPa and 1.4 GPa respectively. The results from this Al-Nb microcompression study were compared with those of the previous report on Cu-Nb multilayer microcompression results that indicated that the flow stresses of the Al-Nb multilayer are lower than those of Cu-Nb with the same bilayer spacing. The observed difference in strength was attributed to a potential difference in the interfacial strength of the two incoherent multilayer systems.

There is much interest in the recent years in the nanoscale metallic multilayered composite materials due to their unusual mechanical properties, such as very high flow strength and stable plastic flow to large strains. These unique mechanical properties have been proposed to result from the interface-dominated plasticity mechanisms in nanoscale composite materials. Studying how the dislocation configurations and densities evolve during deformation will be crucial in understanding the yield, work hardening, and recovery mechanisms in the nanolayered materials. In an effort to shed light on these topics, uniaxial compression experiments on nanoscale Cu/Nb single-crystal multilayer pillars using ex situ synchrotron-based Laue x-ray microdiffraction technique were conducted. Using this approach, we studied the nanoscale Cu/Nb multilayer pillars before and after uniaxial compression to about 14% of plastic strain and found significant Laue peak broadening in the Cu phase, which indicates storage of statistically stored dislocations, while no significant Laue peak broadening was observed in the Nb phase in the nanoscale multilayers. These observations suggest that at 14% plastic strain of the nanolayered pillars, the deformation was dominated by plasticity in the Cu nanolayers and elasticity or possibly a zero net plasticity (due to the possibility of annihilation of interface dislocations) in the Nb nanolayers.

Constitutive models that describe crystal microplasticity in a continuum framework can be envisaged as average representations of the dynamics of dislocation systems. Thus, their performance needs to be assessed not only by their ability to correctly represent stress–strain characteristics on the specimen scale but also by their ability to correctly represent the evolution of internal stress and strain patterns. Three-dimensional discrete dislocation dynamics (3D DDD) simulations provide complete knowledge of this evolution, and averages over ensembles of statistically equivalent simulations can therefore be used to assess the performance of continuum models. In this study, we consider the bending of a free-standing thin film. From a continuum mechanics point of view, this is a one-dimensional (1D) problem as stress and strain fields vary only in one dimension. From a dislocation plasticity point of view, on the other hand, the spatial degrees of freedom associated with the bending and piling up of dislocations are essential. We compare the results of 3D DDD simulations with those obtained from a simple 1D gradient plasticity model and a more complex dislocation-based continuum model. Both models correctly reproduce the nontrivial strain patterns predicted by 3D DDD for the microbending problem.

When thin nanomaterials spontaneously deform into nonflat geometries (e.g., nanorods into nanohelices, thin sheets into ruffled forms), their properties may change by orders of magnitude. We discuss this phenomenon in terms of a formal mathematical concept: codimension c = D − d, the difference between the dimensionality of space D, and that of the object d. We use several independent examples such as the edge stress of graphene nanoribbons, the elastic moduli of nanowires, and the thermal expansion of a modified bead-chain model to demonstrate how this framework can be used to generically understand some nanomaterial properties and how these properties can be engineered by using mechanical constraints to manipulate the codimension of the corresponding structure.