A. Caro

Plastic behavior of nanophase Ni: A molecular dynamics computer simulation

We report molecular dynamics computer simulations of low temperature-high load plastic deformation of Ni nanophase samples with several mean grain sizes in the range of 3–5 nm. The samples are polycrystals nucleated from different seeds, with random location and orientation. Among the mechanisms responsible for the deformation, grain boundary sliding and motion, as well as grain rotation are identified. No dislocation activity is detected, in contrast to the behavior of coarse grain metals. Interpreting the results in terms of grain boundary viscosity, a linear dependence of strain rate with the inverse of the grain size is obtained.

Influence of chemical disorder on energy dissipation and defect evolution in concentrated solid solution alloys

A grand challenge in materials research is to understand complex electronic correlation and non-equilibrium atomic interactions, and how such intrinsic properties and dynamic processes affect energy transfer and defect evolution in irradiated materials. Here we report that chemical disorder, with an increasing number of principal elements and/or altered concentrations of specific elements, in single-phase concentrated solid solution alloys can lead to substantial reduction in electron mean free path and orders of magnitude decrease in electrical and thermal conductivity. The subsequently slow energy dissipation affects defect dynamics at the early stages, and consequentially may result in less deleterious defects. Suppressed damage accumulation with increasing chemical disorder from pure nickel to binary and to more complex quaternary solid solutions is observed. Understanding and controlling energy dissipation and defect dynamics by altering alloy complexity may pave the way for new design principles of radiation-tolerant structural alloys for energy applications.

A molecular dynamics study of polycrystalline fcc metals at the nanoscale: grain boundary structure and its influence on plastic deformation

Molecular dynamics computer simulation of nanocrystalline Ni and Cu show that grain boundaries in nanocrystalline metals have the short range structure of most grain boundaries found in conventional polycrystalline materials. The simulations also indicate the presence of a critical grain size below which all plastic deformation is accommodated in the grain boundary and no intra-grain deformation is observed.

Are nanoporous materials radiation resistant

The key to perfect radiation endurance is perfect recovery. Since surfaces are perfect sinks for defects, a porous material with a high surface to volume ratio has the potential to be extremely radiation tolerant, provided it is morphologically stable in a radiation environment. Experiments and computer simulations on nanoscale gold foams reported here show the existence of a window in the parameter space where foams are radiation tolerant. We analyze these results in terms of a model for the irradiation response that quantitatively locates such window that appears to be the consequence of the combined effect of two length scales dependent on the irradiation conditions: (i) foams with ligament diameters below a minimum value display ligament melting and breaking, together with compaction increasing with dose (this value is typically ∼5 nm for primary knock on atoms (PKA) of ∼15 keV in Au), while (ii) foams with ligament diameters above a maximum value show bulk behavior, that is, damage accumulation (few hundred nanometers for the PKAs energy and dose rate used in this study). In between these dimensions, (i.e., ∼100 nm in Au), defect migration to the ligament surface happens faster than the time between cascades, ensuring radiation resistance for a given dose-rate. We conclude that foams can be tailored to become radiation tolerant.

Structural, elastic, and electronic properties of Fe3C from first principles

Using first-principles calculations within the generalized gradient approximation, we predicted the lattice parameters, elastic constants, vibrational properties, and electronic structure of cementite (Fe3C). Its nine single-crystal elastic constants were obtained by computing total energies or stresses as a function of applied strain. Furthermore, six of them were determined from the initial slopes of the calculated longitudinal and transverse acoustic phonon branches along the [100], [010] and [001] directions. The three methods agree well with each other, the calculated polycrystalline elastic moduli are also in good overall agreement with experiments. Our calculations indicate that Fe3C is mechanically stable. The experimentally observed high elastic anisotropy of Fe3C is also confirmed by our study. Based on electronic density of states and charge density distribution, the chemical bonding in Fe3C was analyzed and was found to exhibit a complex mixture of metallic, covalent, and ionic characters.

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.

Scalable parallel Monte Carlo algorithm for atomistic simulations of precipitation in alloys

We present an extension of the semi-grand-canonical (SGC) ensemble that we refer to as the variance-constrained semi-grand-canonical (VC-SGC) ensemble. It allows for transmutation Monte Carlo simulations of multicomponent systems in multiphase regions of the phase diagram and lends itself to scalable simulations on massively parallel platforms. By combining transmutation moves with molecular dynamics steps, structural relaxations and thermal vibrations in realistic alloys can be taken into account. In this way, we construct a robust and efficient simulation technique that is ideally suited for large-scale simulations of precipitation in multicomponent systems in the presence of structural disorder. To illustrate the algorithm introduced in this work, we study the precipitation of Cu in nanocrystalline Fe.

Molecular dynamics computer simulation of nanophase Ni: structure and mechanical properties

Abstract Molecular dynamics computer simulations of creep experiments on large Ni nanophase samples with different mean grain size are performed on a massively parallel platform (Cray-T3D). The samples have been constructed by filling up an assigned volume with a polycrystal nucleated from different seeds with random location and orientation. After relaxation to a minimum enthalpy, the samples have been submitted to a constant uniaxial stress. Among the deformation mechanism responsible for the accomodation of the applied stress, grain boundary sliding, grain rotation and grain boundary motion are identified.

Surface effects on the radiation response of nanoporous Au foams

We report on an experimental and simulation campaign aimed at exploring the radiation response of nanoporous Au (np-Au) foams. We find different defect accumulation behavior by varying radiation dose-rate in ion-irradiated np-Au foams. Stacking fault tetrahedra are formed when np-Au foams are irradiated at high dose-rate, but they do not seem to be formed in np-Au at low dose-rate irradiation. A model is proposed to explain the dose-rate dependent defect accumulation based on these results.

The role of grain size and the presence of low and high angle grain boundaries in the deformation mechanism of nanophase Ni: A molecular dynamics computer simulation

Abstract The mechanisms of plastic deformation of computer simulated nanophase Ni are studied for samples with mainly high angle (HA) grain boundaries and with mean grain sizes ranging from 3 to 12 nm, and for samples with mainly low angle (LA) grain boundaries with mean grain size of 5.2 nm. The influence of the grain size and grain boundary type on the deformation mechanism is discussed on the atomic level.