This is a copy of the slides presented at the meeting but not formally written up for the volume.

While material fracture is generally considered undesirable, recent experimental work has indicated that cracking can be utilized to create small scale nanowire structures with diameters of 100nm and smaller. Typical nanowire fabrication techniques include the use of a scanning tunneling microscope and electrochemical deposition. Compared to fracture fabricated nanowire structures current fabrication techniques are either extremely slow or require large amounts of post-processing. To become a viable nanowire fabrication technique the cracking of materials must be directed. We propose a computational model to predict propagation paths in large scale crack systems. We utilize the level set method to investigate the creation of nanoscale crack patterns. The level set method allows for large scale simulations of many crack tips while easily accommodating large scale deformations. Unlike traditional methods such as finite elements explicit modeling of the cracks is not needed. We show that the use of multiple materials and etched regions can effectively direct the cracking of a thin film. Using these patterns nanowire structures can be constructed which would be difficult to obtain otherwise.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

It is well-known that impurities affect the migration of intrinsic point defects in metals. For instance, carbon is a common impurity in Fe that significantly retards diffusion of vacancies. Under fusion irradiation conditions, high levels of He are produced by transmutation reactions. This element strongly interacts with vacancies produced during irradiation and agglomerate into stable He-vacancy clusters that can deteriorate the mechanical properties of the material. A physically-based model accounting for the interactions between He, point defects (interstitials and vacancies) and trapping impurities is therefore necessary to understand and predict damage evolution in Fe.We have used a multi-scale approach to predict the evolution of He in the presence of impurities in irradiated Fe. Density Functional Theory (DFT) calculations were performed to investigate the migration mechanisms and to determine the activation energies of the different atomistic processes. The influence of impurities - such as carbon - on the binding energies of small He-vacancy clusters was also studied. Using the information obtained by DFT a physically-based model was developed and implemented in a kinetic Monte Carlo (kMC) code to follow the evolution of He in Fe. In addition, a model based on the rate theory (RT) was developed in order to achieve larger simulation times and volumes. Results obtained with this model, which is based on a mean field approximation are compared to those obtained with kMC. Using this multiscale approach, the simulation results are used to interpret the different stages of thermal He desorption experiments and to determine the predominant migration mechanism. The influence of impurities which affect the diffusion of point defects or modify the binding energies of He-vacancy type clusters is also studied.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

A multiscale theoretical and computational methodology will be presented for describing liquid state, biomolecular, and nanoparticle systems across multiple length- and time-scales. The approach provides an interface between atomistic molecular simulations, mesoscale dynamics, and continuum mechanics. The underlying methodology couples atomistic-level simulations with mesoscale simulations which, in turn, can be bridged to continuum-level modeling where necessary. A new and systematic multiscale coarse-graining strategy for linking the atomistic-scale interactions to the mesoscale will be the primary focus of the presentation. Applications of the overall methodology will be given.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Understanding the basic mechanisms that determine microstructure changesin neutron irradiated steels is vital for a safe lifetime management of existing nuclear reactors and a safe design of future nuclear options. Low-alloyed ferritic steels containing Cu, Ni, Mn and Si as principal solute atoms are used as structural materials for current reactor vessels, while high-Cr ferritic-martensitic steels will be used in future nuclear options. The microstructural evolution under irradiation in alloys is decided by the interplay between defect formation and thermodynamic driving forces, together determining the appearance of phase transformations (precipitation, segregation, ...) and favouring or delaying the nucleation and growth of point-defect clusters, their diffusion and their mutual recombination or removal at sinks. A reliable description of the production, evolution and accumulation of radiation damage must therefore start from the atomic level and requires being able to describe multicomponent systems for timescales ranging from few picoseconds to years. This goal demands firstly the fabrication of interatomic potentials for alloys that must be both consistent with the thermodynamic properties of the system and capable of reproducing correctly the characteristic solute-point defect interactions, versus ab initio or experimental data. Secondly the performance of extensive molecular dynamics (MD) simulations, to grasp the main mechanisms of defect production, diffusion, mutual interaction, and interaction with solute atoms and impurities. Thirdly, the development of simulation tools capable of describing the microstructure evolution beyond the timeframe and lengthscale of MD, while reproducing as much as possible the atomic-level origin of the mechanisms governing the evolution of the system, including phase changes.In this presentation the results of recent efforts made in this direction in the case of Fe-Cu and Fe-Cr alloys, as basic model alloys for the description of steels of technological relevance, are highlighted. In particular, advanced techniques to fit interatomic potentials consistent with thermodynamics are proposed and the results of their application to the mentioned alloys are presented. The results of the use of advanced potentials to study the effect of high concentrations of solute atoms on self-interstitial cluster mobility and their correlation to changes in macroscopic properties, such as swelling, are summarised. And the development of advanced methods, based on the use of artificial intelligence, to improve both the physical reliability and the computational efficiency of kinetic Monte Carlo codes for the study of point-defect clustering and phase changes beyond the scale of MD, is reported. These recent progresses bear the promise of being able, in the near future, of producing reliable tools for the description of the microstructure evolution of realistic model alloys under irradiation.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Low temperature fracture behavior of multiphase metallic materials is controlled by the microcracks originated in brittle precipitates embedded metallic matrix. Fracture in these materials propagates by the extension of ‘critical microcrack’ situated in the plastic zone of macrocrack ahead of it. The crack-tip plasticity of both microcrack and macrocrack are simulated as dislocation arrays using as discrete dislocation simulation. The analysis reveals the factors that contribute to the exponential increase in fracture toughness with temperature at the brittle - ductile temperature (BDT). They are found to be: (a) the marginal increase in microscopic fracture stress, (b) the increase in crack-tip blunting with increase in plastic-flow with temperature, and (c) the increase in dislocation mobility with temperature. On applying the model to a set of microcrack distributions it has been found that (1) always it is one of the largest microcracks that lead to the fracture (2) the scatter in fracture toughness measurements is due the scatter in the size of the microcracks not their relative position to the macrocrack.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Current large scale atomistic simulations remain too computationally demanding to be generally applicable to industrial and bioengineering materials. It is desirable to develop multiscale modeling algorithms to perform efficient and informative mesoscopic simulations. Here we present a multipolar expansion method to construct coarse grained force fields (CGFF) for polymer nanostructures and nanocomposites. This model can effectively capture the stereochemical response to anisotropic long-range interactions and can be systematically improved upon adding higher order terms. The coarse-graining procedure forms the basis to perform a hierarchy of multiscale simulations starting with the quantum chemistry calculations to coarse grained molecular dynamics, hopefully toward continuum modeling. We have applied this procedure to molecular clusters such as alkane, benzene, and fullerene. For liquid alkane, molecular dynamics simulations using the CGFF can reproduce the pair distribution functions using atomistic force fields. Molecular mechanics simulations using the CGFF can well reproduce the energetics of benzene clusters from quantum chemistry electronic structure calculations. Subtle anisotropy in the interaction potentials of the fullerene dimer using the Brenner force field can also be well represented by the model. It is promising this procedure can be standardized and further extended.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

Micromechanics model incorporating with molecular dynamics (MD) simulation is developed to simulate the frictional behavior of carbon nanotube (CNT) arrays in ceramic nanocomposites. MD model is used to compute the interaction force and simulate failure mechanisms of individual nanotube at atomic length scale. The force and deformation calculated from MD simulation are passed to the continuum model to simulate the interaction between nanotube arrays and AFM tips. The coefficient of friction is determined at different load levels. The simulation shows that the low friction in the thick-wall CNT systems occurs because the stiffer CNTs are more resistant to collapse under the applied loads. The predictions for the coefficient of friction are consistent with nanoscale tests.

This is a copy of the slides presented at the meeting but not formally written up for the volume.

In order to understand the microstructural evolution in plasma sprayed coatings, the solidification process was modeled using a 2-D FEM model based on enthalpy formation. Studies of the surface of the coatings showed surface roughnesses across multiple length scales. The model was used to examine the effects of the substrate and splat temperatures and the surface roughness features on the onset of remelting of the underlying surface on which the splat solidifies. The surface roughness was found to promote remelting, indicating that it was an important parameter that determines splat solidification. The temperatures of the splat and substrate were consolidated into one non-dimensional parameter that captured the onset of remelting with a non-dimensional remelting point.A fully coupled thermo-mechanical finite element model was also run for a single splat case, to provide more insight stress buildup during solidification. An important result was that the relative size of the surface roughness features, as compared to the splat thickness, is very important. Very large wavelengths compared to splat thickness lead to smaller stresses, since the solidification and the interface are essentially 1-D. Very small wavelengths compared to splat thickness also leads to reduced stresses, since the solidification front quickly becomes 1-D. Only roughness features on the scale of splat thickness are important in providing locations of maximum stress concentration, which are locations of microcrack formation.