Publications

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List of publications:

Publication abstracts:

  • A Sharma, J Hickman, N Gazit, E Rabkin, Y Mishin, Nickel nanoparticles set a new record of strength, Nature Communications, volume 9, Article number: 4102 (2018)

    • Material objects with micrometer or nanometer dimensions can exhibit much higher strength than macroscopic objects, but this strength rarely approaches the maximum theoretical strength of the material. Here, we demonstrate that 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. Three factors are responsible for this record-high strength: the large Ni shear modulus, the smooth edges and corners of the nanoparticles, and the thin oxide layer on the particle surface. 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. The nucleation of a stable loop is preceded by multiple nucleation attempts accompanied by unusually large local atomic displacements caused by thermal fluctuations.

  • J. Hickman and Y. Mishin: Extra variable in grain boundary description, Phys. Rev. Materials 1, 010601(R) (2017).

    • Grain boundaries (GBs) in crystalline materials are traditionally described by five crystallographic angles, which are assumed to fully define the GB structure and energy. It has recently been realized that variations in the atomic density λ in the GB region can drastically alter the GB structure and cause transformations between different GB phases. Here we extend the previous studies of Cu 5 GBs by computing the structures and energies of a set of [001] symmetrical tilt GBs over the entire angular range by allowing arbitrary variations in λ. The results confirm the existence of stable and metastable phases in all GBs studied here. There are three types of structural units that can describe all GB structures obtained in this work. The work demonstrates that λ should be added to the description of GBs as an extra thermodynamic parameter that helps predict the GB phases and transformations among them.

  • Y. Mishin and J. Hickman: Energy spectrum of a Langevin oscillator , Phys. Rev. E 94, 062151. (2016).

    • We derive analytical solutions for the autocorrelation and cross-correlation functions of the kinetic, potential, and total energy of a Langevin oscillator. These functions are presented in both the time and frequency domains and validated by independent numerical simulations. The results are applied to address the long-standing issue of temperature fluctuations in canonical systems.

  • J. Hickman and Y. Mishin: Temperature fluctuations in canonical systems: Insights from molecular dynamics simulations, Phys. Rev. B 94, 184311. (2016)

    • Molecular dynamics simulations of a quasi harmonic solid are conducted to elucidate the meaning of temperature fluctuations in canonical systems and validate a well-known but frequently contested equation predicting the mean square of such fluctuations. The simulations implement two virtual and one physical (natural) thermostat and examine the kinetic, potential, and total energy correlation functions in the time and frequency domains. The results clearly demonstrate the existence of quasi equilibrium states in which the system can be characterized by a well-defined temperature that follows the mentioned fluctuation equation. The emergence of such states is due to the wide separation of time scales between thermal relaxation by phonon scattering and slow energy exchanges with the thermostat. The quasi equilibrium states exist between these two time scales when the system behaves as virtually isolated and equilibrium.

  • J. Hickman and Y. Mishin: Disjoining potential and grain boundary pre-melting in binary alloys, Phys. Rev. B 93, 224108 (2016).

    • Many grain boundaries (GBs) in crystalline materials develop highly disordered, liquid like structures at high temperatures. In alloys, this pre-melting effect can be fueled by solute segregation and can occur at lower temperatures than in single-component systems. A premelted GB can be modeled by a thin liquid layer located between two solid-liquid interfaces interacting by a disjoining potential. We propose a single analytical form of the disjoining potential describing repulsive, attractive, and intermediate interactions. The potential predicts a variety of pre-melting scenarios, including thin-to-thick phase transitions. The potential is verified by atomistic computer simulations of premelting in three different GBs in Cu-Ag alloys employing a Monte Carlo technique with an embedded atom potential. The disjoining potential has been extracted from the simulations by analyzing GB width fluctuations. The simulations confirm all shapes of the disjoining potential predicted by the analytical model. One of the GBs was found to switch back and forth between two (thin and thick) states, confirming the existence of thin-to-thick phase transformations in this system. The proposed disjoining potential also predicts the possibility of a cascade of thin-to-thick transitions caused by compositional oscillations (patterning) near solid-liquid interfaces.