Andrea M. Hodge

Nanoporous Au: A high yield strength material

The plastic deformation of nanoporous Au under compressive stress was studied by depth-sensing nanoindentation combined with scanning electron microscope characterization. The nanoporous Au investigated in the current study exhibits a relative density of 42%, and a spongelike morphology of interconnecting ligaments on a length scale of ∼100nm. The material is polycrystalline with a grain size on the order of 10–60nm. Microstructural characterization of residual indentation impressions reveals a localized densification via ductile (plastic) deformation under compressive stress and demonstrates the ductile behavior of Au ligaments. A mean hardness of 145(±11)MPa and a Young’s modulus of 11.1(±0.9)GPa was obtained from the analysis of the load-displacement curves. The hardness of investigated np‐Au is ∼10 times higher than the hardness predicted by scaling laws of open-cell foams thus potentially opening a door to a class of high yield strength—low-density materials.

Microscopic failure behavior of nanoporous gold

Nanoporous metals have recently attracted considerable interest fueled by potential sensor and actuator applications. One of the key issues in this context is the synthesis of high yield strength materials. Nanoporous Au (np-Au) has been suggested as a candidate due to its monolithic character. The material can be synthesized by dealloying Ag-Au alloys, and exhibits an open sponge-like morphology of interconnecting Au ligaments with a typical pore size distribution on the nanometer length scale. Unfortunately, very little is known about the mechanical properties of np-Au besides a length-scale dependent ductile-brittle transition. A key question in this context is: what causes the macroscopic brittleness of np-Au? Is the normal dislocation-mediated plastic deformation suppressed in nanoscale Au ligaments, or is the brittleness a consequence of the macroscopic morphology? Here, we report on the fracture behavior of nanoporous Au studied by scanning electron microscopy. Specifically, we demonstrate the microscopic ductility of nanometer-sized Au ligaments. The observed fracture behavior seems to be general for nanoporous metals, and can be understood in terms of simple fuse networks.

On the Microstructure of Nanoporous Gold: An X-ray Diffraction Study

The evolution of the grain structure, internal strain, and the lattice misorientations of nanoporous gold during dealloying of bulk (3D) Ag-Au alloy samples was studied by various in situ and ex situ X-ray diffraction techniques including powder and Laue diffraction. The experiments reveal that the dealloying process preserves the original crystallographic structure but leads to a small spread in orientations within individual grains. Initially, most grains develop in-plane tensile stresses, which are partly released during further dealloying. Simultaneously, the feature size of the developing nanoporous structure increases with increasing dealloying time. Finally, microdiffraction experiments on dealloyed micron-sized nanoporous pillars reveal significant surface damage introduced by focused ion beam milling.

Deformation Twinning During Nanoindentation of Nanocrystalline Ta

The deformation mechanism of body-centered cubic (bcc) nanocrystalline tantalum with grain sizes of 10–30 nm is investigated by nanoindentation, scanning electron microscopy and high-resolution transmission electron microscopy. In a deviation from molecular dynamics simulations and existing experimental observations on other bcc nanocrystalline metals, the plastic deformation of nanocrystalline Ta during nanoindentation is controlled by deformation twinning. The observation of multiple twin intersections suggests that the physical mechanism of deformation twinning in bcc nanocrystalline materials is different from that in face-centered cubic (fcc) nanocrystalline metals.

Synthesis of nickel–aluminide foams by pack-aluminization of nickel foams

Nickel–aluminide foams were synthesized from unalloyed nickel foams by using a two-step, high-activity pack-aluminizing process at 1273 K. After processing, the nickel–aluminide foams exhibited the original structure of the original nickel foams (open-cells with hollows struts and low density). Single-phase NiAl foams, with average composition within 1 wt.% of stoichiometry and with 92% open porosity, were produced by first selecting the appropriate aluminizing time, and then annealing to homogenize the structure. Foams of average Ni3Al composition were produced by the same method, but multiple intermetallic phases remained due to large variations in strut thickness and thus local composition. Nickel wires and tubes were also aluminized at 1273 K and homogenized for different times to further investigate the aluminizing kinetics and the creation of Kirkendall pores. For aluminization depths up to about 100 mm, Kirkendall pores can be avoided, leading to pore-free struts in the foam. # 2001 Elsevier Science Ltd. All rights reserved.

Deforming nanocrystalline nickel at ultrahigh strain rates

The deformation mechanism of nanocrystalline Ni (with grain sizes in the range of 30–100 nm) at ultrahigh strain rates (>107s−1) was investigated. A laser-driven compression process was applied to achieve high pressures (20–70 GPa) on nanosecond timescales and thus induce high-strain-rate deformation in the nanocrystalline Ni. Postmortem transmission electron microscopy examinations revealed that the nanocrystalline structures survive the shock deformation, and that dislocation activity is a prevalent deformation mechanism for the grain sizes studied. No deformation twinning was observed even at stresses more than twice the threshold for twin formation in micron-sized polycrystals. These results agree qualitatively with molecular dynamics simulations and suggest that twinning is a difficult event in nanocrystalline Ni under shock-loading conditions.

Elastic and viscoelastic characterization of agar

Agar is a biological polymer, frequently used in tissue engineering research; due to its consistency, controllable size, and concentration-based properties, it often serves as a representative material for actual biological tissues. In this study, nanoindentation was used to characterize both the time-independent and time-dependent response of agar samples having various concentrations (0.5%-5.0% by weight). Quasi-static indentation was performed at different loads and depths using both open- and closed-loop controls. Reduced modulus (Er) values change with agar concentration, ranging from ∼30 kPa for 0.5% samples to ∼700 kPa for 5.0% samples, which is the same modulus range as usually encountered in soft biological materials. Dynamic indentation was performed to assess the effects of load, dynamic frequency and amplitude. Storage modulus values ranged from approximately 30 to 2300 kPa depending on agar concentration. Loss modulus remained consistently less than 30 kPa at all conditions, indicating a diminished damping response in agar.

Incipient plasticity during nanoindentation at elevated temperatures

The onset of plastic deformation during nanoindentation is studied, focusing upon the effects of temperature variation. Indentations on pure (100)-oriented platinum at 20, 100, and 200°C reveal that the transition from elastic to plastic deformation occurs at progressively lower stress levels as temperature is increased. Additionally, it is shown that during plastic deformation, higher temperatures promote the discretization of plasticity into sharp bursts of activity. These results are in line with expectations for stress-biased, thermally activated deformation processes such as the nucleation of dislocations or the abrupt release of dislocation entanglements.

Monolithic nanocrystalline Au fabricated by the compaction of nanoscale foam

We describe a two-step dealloying/compaction process to produce nanocrystalline Au. First, nanocrystalline/nanoporous Au foam is synthesized by electrochemically-driven dealloying. The resulting Au foams exhibit porosities of 60 and 70% with pore sizes of {approx} 40 and 100 nm, respectively, and a typical grain size of <50 nm. Second, the nanoporous foams are fully compacted to produce nanocrystalline monolithic Au. The compacted Au was characterized by TEM and X-ray diffraction and tested by depth-sensing nanoindentation. The compacted nanocrystalline Au exhibits an average grain size of <50 nm and hardness values ranging from 1.4 to 2.0 GPa, which are up to 4.5 times higher than the hardness values obtained from polycrystalline Au.

Mechanical response of freestanding Au nanopillars under compression

We employ molecular dynamics simulations of defect-free nanopillars with realistic cylindrical geometries to obtain an atomic-level picture of their deformation behavior under compression. We find that dislocations are nucleated in the two outermost surface layers. Furthermore, plastic yield depends crucially on the particular arrangement of steps and facets at the surface of the nanopillars. We show that different facet orientations can differ dramatically in their response to external stresses. Freestanding nanopillars exhibit a highly nonuniform distribution of stresses along their height. This causes an elastic deformation that leads to a barrel-like shape attained by the nanopillars under compression. The stress concentration at the center of the pillars due to barreling causes dislocations to preferentially nucleate in this region.