Influence of Homogeneous Interfaces on the Strength of 500 nm Diameter Cu Nanopillars

Interfaces play an important role in crystalline plasticity as they affect strength and often serve as obstacles to dislocation motion. Here we investigate effects of grain and nanotwin boundaries on uniaxial strength of 500 nm diameter Cu nanopillars fabricated by e-beam lithography and electroplating. Uniaxial compression experiments reveal that strength is lowered by introducing grain boundaries and significantly rises when twin boundaries are present. Weakening is likely due to the activation of grain-boundary-mediated processes, while impeding dislocation glide can be responsible for strengthening by twin boundaries

Deformation mechanisms in nanotwinned metal nanopillars

Nanotwinned metals are attractive in many applications because they simultaneously demonstrate high strength and high ductility, characteristics that are usually thought to be mutually exclusive. However, most nanotwinned metals are produced in polycrystalline forms and therefore contain randomly oriented twin and grain boundaries making it difficult to determine the origins of their useful mechanical properties. Here, we report the fabrication of arrays of vertically aligned copper nanopillars that contain a very high density of periodic twin boundaries and no grain boundaries or other microstructural features. We use tension experiments, transmission electron microscopy and atomistic simulations to investigate the influence of diameter, twin-boundary spacing and twin-boundary orientation on the mechanical responses of individual nanopillars. We observe a brittle-to-ductile transition in samples with orthogonally oriented twin boundaries as the twin-boundary spacing decreases below a critical value (~3–4 nm for copper). We also find that nanopillars with slanted twin boundaries deform via shear offsets and significant detwinning. The ability to decouple nanotwins from other microstructural features should lead to an improved understanding of the mechanical properties of nanotwinned metals

Fracture behavior of brittle ceramics at the nanoscale

In spite of the excellent properties such as high hardness, low thermal expansion, enhanced resistance to chemical degradation and superior mechanical behavior at elevated temperature, ceramic materials usually suffer from the brittle fracture and catastrophic failure, which restrict them from being used for structural applications. While a number of researchers have strived to overcome this drawback of ceramic materials by constructing the microstructures that interfere with crack growth, recent theoretical and computational studies proposed another effective method to suppress the rapid crack propagation by reducing the specimen size down to the nanometer scale. In this study, we investigated the mechanical properties of brittle ceramics by changing sample sizes from bulk to nanoscale with particular focus on their fracture failure. For the ease of analysis, we chose the isotropic, homogeneous and purely brittle material, i.e., diamond-like carbon. In-situ fixed-ends bending experiments were conducted with different beam thicknesses and lengths, 1μm ~ 100nm and 3μm ~ 6μm, respectively. Additionally, in order to demonstrate the feasibility to intactly transfer the superior properties emergent only at the nanoscale to the macroscopically available form, we fabricated the large-area 3D hierarchical hollow ceramic nano-architectures using proximity nano-patterning technique

Catastrophic vs Gradual Collapse of Thin-Walled Nanocrystalline Ni Hollow Cylinders As Building Blocks of Microlattice Structures

Lightweight yet stiff and strong lattice structures are attractive for various engineering applications, such as cores of sandwich shells and components designed for impact mitigation. Recent breakthroughs in manufacturing enable efficient fabrication of hierarchically architected microlattices, with dimensional control spanning seven orders of magnitude in length scale. These materials have the potential to exploit desirable nanoscale-size effects in a macroscopic structure, as long as their mechanical behavior at each appropriate scale – nano, micro, and macro levels – is properly understood. In this letter, we report the nanomechanical response of individual microlattice members. We show that hollow nanocrystalline Ni cylinders differing only in wall thicknesses, 500 and 150 nm, exhibit strikingly different collapse modes: the 500 nm sample collapses in a brittle manner, via a single strain burst, while the 150 nm sample shows a gradual collapse, via a series of small and discrete strain bursts. Further, compressive strength in 150 nm sample is 99.2% lower than predicted by shell buckling theory, likely due to localized buckling and fracture events observed during in situ compression experiments. We attribute this difference to the size-induced transition in deformation behavior, unique to nanoscale, and discuss it in the framework of “size effects” in crystalline strength

Effect of grain size and grain-boundary structure on plasticity in nanocrystalline iron.

In the present study, plasticity of nanocrystalline Fe and its grain boundary structure have been studied systematically using nanoindentation and HREM, respectively. The effect of grain size of nanocrystalline Fe on plasticity was investigated. Samples with various grain sizes were synthesized by high-energy and low-energy ball milling at different milling times and milling amplitude (low-energy ball milling). Grain size and rms strain were determined by Warren-Averbach analysis of x-ray Bragg peak broadening. Hardness and strain-rate sensitivity were determined using nanoindentation. It is found that the hardness increases with decreasing grain size down to 18 nm (Hall-Petch relation), but decreases with decreasing grain size further below this value, behavior that has been termed inverse Hall-Petch relation. It is also found that the strain-rate sensitivity increases monotonically with decreasing grain size. Motivated by the fact that the strain-rate sensitivity of sintered nanocrystalline material is lower than that in the as-milled state for the same grain size, the effect of grain-boundary relaxation on plasticity of nanocrystalline Fe was investigated. To obtain samples with the same grain size, but different degree of grain-boundary relaxation, the as-milled nanocrystalline Fe samples were annealed at 80°C and 100°C for various times. Using nanoindentation, it was found that hardness variation with annealing time was slight, but strain-rate sensitivity changed significantly. The strain-rate sensitivity peaks as a function of time, suggesting two competing processes: one is responsible for the increase of the SRS and the other for the decrease. The process for the decrease of the strain-rate sensitivity is likely to be grain-boundary relaxation, but further study is required to understand what is responsible for the increase of SRS. Grain-boundary structure evolution during annealing was studied using HREM. It was found that disconnected lattice fringes at grain boundary of as-milled sample gradually changed to continuous lattice fringes with regularly spaced grain-boundary dislocations during annealing. The latter structure is suggested to be more relaxed.Ph.D.Applied SciencesMaterials scienceUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/125670/2/3208473.pd

Transition from a strong-yet-brittle to a stronger-and-ductile state by size reduction of metallic glasses

Amorphous metallic alloys, or metallic glasses, are lucrative engineering materials owing to their superior mechanical properties such as high strength and large elastic strain. However, their main drawback is their propensity for highly catastrophic failure through rapid shear banding, significantly undercutting their structural applications. Here, we show that when reduced to 100 nm, Zr-based metallic glass nanopillars attain ceramic-like strengths (2.25 GPa) and metal-like ductility (25%) simultaneously. We report separate and distinct critical sizes for maximum strength and for the brittle-to-ductile transition, thereby demonstrating that strength and ability to carry plasticity are decoupled at the nanoscale. A phenomenological model for size dependence and brittle-to-homogeneous deformation is provided