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Register a new userOrientation-dependent deformation mechanisms of bcc niobium nanoparticles
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Nanoparticles usually exhibit pronounced anisotropic properties, and a close insight into the atomic-scale deformation mechanisms is of great interest. In present study, atomic simulations are conducted to analyze the compression of bcc nanoparticles, and orientation-dependent features are addressed. It is revealed that surface morphology under indenter predominantly governs the initial elastic response. The loading curve follows the flat punch contact model in [110] compression, while it obeys the Hertzian contact model in [111] and [001] compressions. In plastic deformation regime, full dislocation gliding is dominated in [110] compression, while deformation twinning is prominent in [111] compression, and these two mechanisms coexist in [001] compression. Such deformation mechanisms are distinct from those in bulk crystals under nanoindentation and nanopillars under compression, and the major differences are also illuminated. Our results provide an atomic perspective on the mechanical behaviors of bcc nanoparticles and are helpful for the design of nanoparticle-based components and systems.
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Molecular dynamics simulations revealed significant difference in deformation behaviour of $<$100$>$ BCC Fe nanowires with and without twist boundary. The plastic deformation in perfect $<$100$>$ BCC Fe nanowire was dominated by twinning and reorientation to $<$110$>$ followed by further deformation by slip mode. On the contrary, $<$100$>$ BCC Fe nanowire with a twist boundary deformed by slip at low plastic strains followed by twinning at high strains and absence of full reorientation. The results suggest that the deformation in $<$100$>$ BCC Fe nanowire by dislocation slip is preferred over twinning in the presence of initial dislocations or dislocation networks. The results also explain the absence of extensive twinning in bulk materials, which inherently contains large number of dislocations.
The texture of high purity superconducting niobium sheets plays an important role in the physical and mechanical properties of high purity niobium sheet that are important for manufacturing of superconducting accelerator cavities. In a particular batch of sheet metal, orientation imaging microscopy showed an inhomogeneous texture from the surface to the mid-thickness of the sheet consisting a gamma fiber, {111}<uvw>), cube fiber, {100}<uvw>), and also some components on the alpha fiber, {hkl}<110>. With uniaxial deformation, peaks on the {uvw}<111> gamma fiber evolve differently depending on the in-plane direction of deformation, and the position in the sample (surface vs. center). Applying a different strain path such as balanced biaxial bulging, leads to development of rotated Goss, {110}<110> components in the texture of the deformed niobium. These results show that the texture of niobium is very sensitive to the deformation and strain path.
We model within the kinetic Monte Carlo method the initiation of neck formation and then later evolution of the resulting bridging regions for configurations involving small particles initially positioned fitted between large particles for situations typical for sintering of FCC nanocrystals, e.g., noble-metal nanoparticles. Neck initiation mechanisms by layering or clustering are identified. The stability of the resulting bridging configurations depends on several parameters, notably, on the relative small to large particle size ratio, and we explain recent experimental findings on improved sintering achieved for certain bimodal size distributions.
Atomistic simulations are performed to probe the anisotropic deformation in the compressions of face-centred-cubic metallic nanoparticles. In the elastic regime, the compressive load-depth behaviors can be characterized by the classical Hertzian model or flat punch model, depending on the surface configuration beneath indenter. On the onset of plasticity, atomic-scale surface steps serve as the source of heterogeneous dislocation in nanoparticle, which is distinct from indenting bulk materials. Under [111] compression, the gliding of jogged dislocation takes over the dominant plastic deformation. The plasticity is governed by nucleation and exhaustion of extended dislocation ribbons in [110] compression. Twin boundary migration mainly sustain the plastic deformation under [112] compression. This study is helpful to extract the mechanical properties of metallic nanoparticles and understand their anisotropic deformation behaviors.
The shape and alignment of silver nanoparticles embedded in a glass matrix is controlled using silicon ion irradiation. Symmetric silver nanoparticles are transformed into anisotropic particles whose larger axis is along the ion beam. Upon irradiation, the surface plasmon resonance of symmetric particles splits into two resonances whose separation depends on the fluence of the ion irradiation. Simulations of the optical absorbance show that the anisotropy is caused by the deformation and alignment of the nanoparticles, and that both properties are controlled with the irradiation fluence.
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