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Nickel Nanoparticles Set a New Record of Strength

Penelitian - Researchers demonstrate faceted single-crystalline nickel (Ni) nanoparticles exhibit an ultrahigh compressive strength up to 34 GPa unprecedented for metallic materials. This strength matches the available estimates of Ni theoretical strength.

This finding is supported by molecular dynamics simulations that closely mimic the experimental conditions, which show that the mechanical failure of the strongest particles is triggered by homogeneous nucleation of dislocation loops inside the particle.

Penelitian Nickel Nanoparticles Set a New Record of Strength

Mechanical properties of small crystalline objects of sub-micrometer dimensions have been the subject of intensive research during the past two decades. It is well established that deformation mechanisms change once the sample size is reduced in at least one dimension into the micrometer range.

The Smaller is Stronger paradigm is now universally accepted. A natural question to ask is whether further downscaling can produce objects reaching the theoretical strength of the material. In ductile metals and alloys, the latter is defined as the resolved shear stress in the primary glide plane that causes homogeneous sliding of two neighboring atomic planes past each other.

An alternate definition relates the theoretical strength to the shear stress required for homogeneous, barrier-free nucleation of a dislocation loop in a perfect crystal. According to different estimates, the theoretical shear strength varies from G/30 to G/8, where G is the shear modulus of the material.

In nanoindentation tests on well-annealed metallic single crystals, the abrupt displacement bursts are usually explained by homogeneous dislocation nucleation when the local shear stress reaches the theoretical strength.

This nucleation process is usually discussed in terms of the Hertz elastic contact theory, which predicts that the maximum shear stress is reached at a point some distance away from the contact.

In a well-annealed single crystal, chances are high that this point is located in a defect-free region and thus a dislocation can only nucleate homogeneously. This argument also applies to spherical metallic nanoparticles tested with a hard flat punch.

The situation is different in micro-compression or tension tests, which essentially represent a downscaled version of the classical mechanical tests. For a cylindrical or prismatic sample, the material is homogeneously exposed to the applied stress.

Thus, any dislocation or dislocation source present in the sample due to its processing history can be activated. Even in defect-free samples, dislocation half-loops or quarter-loops can nucleate at multiple locations at the surface at stresses much smaller than those required for homogeneous nucleation.

As a result, only a fraction of the theoretical strength can be usually achieved. In faceted nanoparticles compressed by a flat punch, the facet edges at the indenter–particle interface similarly act as dislocation nucleation sites.

So far, a strength close to theoretical was only demonstrated for Cu and Pd defect-free nanowhiskers and in Au microparticles. However, since the shear moduli of these metals are relatively low (e.g., 27 to 48 GPa for Au and Cu, respectively), the absolute value of their strength is only a few GPa.

The team of the George Mason University in Fairfax and Technion-Israel Institute of Technology in Haifa showed faceted single-crystalline Ni nanoparticles obtained by solid-state dewetting exhibit an ultrahigh compressive strength unprecedented for metallic materials.

The three factors chiefly responsible for this strength record are the large shear modulus of Ni (78.7 GPa for {111} slip in the ⟨110⟩ direction), the smooth edges and corners of the nanoparticles that reduce the stress concentration, and the thin oxide layer on the particle surface that softens the contacts with the substrate and indenter.

The experimental findings supported by molecular dynamics (MD) simulations that closely reproduce the experimental conditions and provide additional insights into the dislocation mechanisms increasing the particle strength.

Journal : A. Sharma et al. Nickel nanoparticles set a new record of strength, Nature Communications, 05 October 2018, DOI:10.1038/s41467-018-06575-6



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