14 research outputs found
Extraordinary strain hardening from dislocation loops in defect-free Al nanocubes
The interaction of crystalline defects leads to strain hardening in bulk
metals. Metals with high stacking fault energy (SFE), such as aluminum, tend to
have low strain hardening rates due to an inability to form stacking faults and
deformation twins. Here, we use in situ SEM mechanical compressions to find
that colloidally synthesized defect-free 114 nm Al nanocubes combine a high
linear strain hardening rate of 4.1 GPa with a high strength of 1.1 GPa. These
nanocubes have a 3 nm self-passivating oxide layer that has a large influence
on mechanical behavior and the accumulation of dislocation structures.
Post-compression TEM imaging reveals stable prismatic dislocation loops and the
absence of stacking faults. MD simulations relate the formation of dislocation
loops and strain hardening to the surface oxide. These results indicate that
slight modifications to surface and interfacial properties can induce enormous
changes to mechanical properties in high SFE metals.Comment: 10 pages, 7 figure
Stress Induced Structural Transformations in Au Nanocrystals
Nanocrystals can exist in multiply twinned structures like the icosahedron,
or single crystalline structures like the cuboctahedron or Wulff-polyhedron.
Structural transformation between these polymorphic structures can proceed
through diffusion or displacive motion. Experimental studies on nanocrystal
structural transformations have focused on high temperature diffusion mediated
processes. Thus, there is limited experimental evidence of displacive motion
mediated structural transformations. Here, we report the high-pressure
structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure
in a diamond anvil cell that is driven by displacive motion. In-situ X-ray
diffraction and transmission electron microscopy were used to detect the
transformation of multiply twinned nanocrystals into single crystalline
nanocrystals. High-pressure single crystalline nanocrystals were recovered
after unloading, however, the nanocrystals quickly reverted back to multiply
twinned state after redispersion in toluene solvent. The dynamics of recovery
was captured using transmission electron microscopy which showed that the
recovery was governed by surface recrystallization and rapid twin boundary
motion. We show that this transformation is energetically favorable by
calculating the pressure-induced change in strain energy. Molecular dynamics
simulations showed that defects nucleated from a region of high stress region
in the interior of the nanocrystal, which make twin boundaries unstable.
Deviatoric stress driven Mackay transformation and dislocation/disclination
mediated detwinning are hypothesized as possible mechanisms of high-pressure
structural transformation.Comment: 32 pages, 14 figures, and 1 movie (please open pdf with Adobe Acrobat
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High pressure induced precipitation in Al7075 alloy
Precipitate-matrix interactions govern the mechanical behavior of precipitate
strengthened Al-based alloys. These alloys find a wide range of applications
ranging from aerospace to automobile and naval industries due to their low cost
and high strength to weight ratio. Structures made from Al-based alloys undergo
complex loading conditions such as high strain rate impact, which involves high
pressures. Here we use diamond anvil cells to study the behavior of Al-based
Al7075 alloy under quasi-hydrostatic and non-hydrostatic pressure up to ~53
GPa. In situ X-ray diffraction (XRD) and pre- and post-compression transmission
electron microscopy (TEM) imaging are used to analyze microstructural changes
and estimate high pressure strength. We find a bulk modulus of 75.2 +- 1.9 GPa
using quasi-hydrostatic pressure XRD measurements. XRD showed that
non-hydrostatic pressure leads to a significant increase in defect density and
peak broadening with pressure cycling. XRD mapping under non-hydrostatic
pressure revealed that the region with the highest local pressure had the
greatest increase in defect nucleation, whereas the region with the largest
local pressure gradient underwent texturing and had larger grains. TEM analysis
showed that pressure cycling led to the nucleation and growth of many
precipitates. The significant increase in defect and precipitate density leads
to an increase in strength for Al7075 alloy at high pressures.Comment: 15 pages, 5 figure
Nucleation of Dislocations in 3.9 nm Nanocrystals at High Pressure
As circuitry approaches single nanometer length scales, it is important to
predict the stability of metals at these scales. The behavior of metals at
larger scales can be predicted based on the behavior of dislocations, but it is
unclear if dislocations can form and be sustained at single nanometer
dimensions. Here, we report the formation of dislocations within individual 3.9
nm Au nanocrystals under nonhydrostatic pressure in a diamond anvil cell. We
used a combination of x-ray diffraction, optical absorbance spectroscopy, and
molecular dynamics simulation to characterize the defects that are formed,
which were found to be surface-nucleated partial dislocations. These results
indicate that dislocations are still active at single nanometer length scales
and can lead to permanent plasticity.Comment: 33 pages, 12 figure
Nanoscale Thin Films of Niobium Oxide on Platinum Surfaces: Creating a Platform for Optimizing Material Composition and Electrochemical Stability
A nanoscale thin film of niobium oxide on a platinum substrate was evaluated for its influence on the electronic and chemical properties of the underlying platinum towards the oxygen reduction reaction with applications to proton exchange membrane fuel cells. The nanoscale thin film of niobium oxide was deposited using atomic layer deposition onto the platinum substrate. A film of niobium oxide is a chemically stable and electronically insulating material that can be used to prevent corrosion and electrochemical degradation when layers are several nanometers thick. These layers can be insulating if sufficiently thick and may not be sufficient to protect the platinum from corrosion if too thin. An ∼3 nm thin film of niobium oxide was fabricated on the platinum surface to determine its influence on the electronic and chemical properties at the interface of these materials. The atomic layer deposition process enabled a precise control over the material composition, structure, and layer thickness. The niobium oxide film was evaluated using cyclic voltammetry and electrochemical impedance spectroscopy to evaluate whether a balance could be found between the inhibition of platinum degradation and electronic insulation of the platinum for use in proton exchange membrane fuel cells. The 3 nm thin niobium oxide film was found to be sufficiently thin to permit electronic conductivity while reducing the incidence of platinum dissolution
Synthesis of Multifunctional Amorphous Metallic Shell on Crystalline Metallic Nanoparticles
Colloidal nanoparticles can be coated with a conformal shell to form multifunctional nanoparticles. For instance, plasmonic, magnetic, and catalytic properties, chemical stability and biocompatibility can be mixed and matched. Here, a facile synthesis for depositing metal boride amorphous coatings on colloidal metallic nanocrystals is introduced. The synthesis is independent of core size, shape, capping ligand, and composition. Shell thickness can be as thin as 3 nm with no apparent pinholes. High pressure studies show that the coatings are highly resistant to crystallization and are strongly bonded to the crystalline core. By choosing either CoB or NiB for the coating, the composite nanoparticles can be either ferromagnetic or paramagnetic at room temperature, respectively
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Stress-Induced Structural Transformations in Au Nanocrystals.
Nanocrystals can exist in multiply twinned structures like icosahedron or single crystalline structures like cuboctahedron. Transformations between these structures can proceed through diffusion or displacive motion. Experimental studies on nanocrystal structural transformations have focused on high-temperature diffusion-mediated processes. Limited experimental evidence of displacive motion exists. We report structural transformation of 6 nm Au nanocrystals under nonhydrostatic pressure of 7.7 GPa in a diamond anvil cell that is driven by displacive motion. X-ray diffraction and transmission electron microscopy were used to detect the structural transformation from multiply twinned to single crystalline. Single crystalline nanocrystals were recovered after unloading, then quickly reverted to the multiply twinned state after dispersion in toluene. The dynamics of recovery was captured using TEM which showed surface recrystallization and rapid twin boundary motion. Molecular dynamics simulations showed that twin boundaries are unstable due to defects nucleated from the interior of the nanocrystal