Publications

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Comprehensive molecular dynamics tensile test simulations have been performed to study the delamination processes of seven different grain boundaries / cleavage planes (Σ1{111}, Σ3{111}, Σ5{100}, Σ7{111}, Σ9{411}, Σ11{311}, and R{100}/{411}) containing a helium bubble. Combinations of a variety of conditions are explored including different strain rates, system dimensions, bubble density, bubble radius, bubble pressure, and temperature. We found that in general, grain boundaries absorb less energies with decreasing strain rate but increasing bubble areal density, bubble pressure, bubble radius, and temperature. The propensity of grain boundary delamination is sensitive to grain boundary type: The random grain boundary R{100}/{411} is one of the most brittle boundaries whereas the Σ1{111} cleavage plane and the Σ3{111} twin boundary are two of the toughest boundaries. The sorted list of grain boundary fracture vulnerability obtained from our dynamic tensile test simulations differs from the one obtained from our decohesion energy calculations, confirming the important role of plastic deformation during fracture. Detailed mechanistic analyses are performed to interpret the simulated results.

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We explore the character angle dependence of dislocation-solute interactions in a face-centered cubic random Fe0.70Ni0.11Cr0.19 alloy through molecular dynamics (MD) simulations of dislocation mobility. Using the MD mobility data, we determine the phonon and thermally activated solute drag parameters which govern mobility for each dislocation character angle. The resulting parameter set indicates that, surprisingly, the solute energy barrier does not depend on character angle. Instead, only the zero-temperature flow stress—which is dictated by the activation area for thermal activation—is dependent on character angle. By analyzing the line roughness from MD simulations and the geometry of a bowing dislocation line undergoing thermal activation, we conclude that the character angle dependence of the activation area in this alloy is governed by the dislocation line tension, rather than the dislocation-solute interaction itself. Our findings motivate further investigation into the line geometry of dislocations in solid solutions.

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The fundamental interactions between an edge dislocation and a random solid solution are studied by analyzing dislocation line roughness profiles obtained from molecular dynamics simulations of Fe_0.70Ni_0.11Cr_0.19 over a range of stresses and temperatures. These roughness profiles reveal the hallmark features of a depinning transition. Namely, below a temperature-dependent critical stress, the dislocation line exhibits roughness in two different length scale regimes which are divided by a so-called correlation length. This correlation length increases with applied stress and at the critical stress (depinning transition or yield stress) formally goes to infinity. Above the critical stress, the line roughness profile converges to that of a random noise field. Motivated by these results, a physical model is developed based on the notion of coherent line bowing over all length scales below the correlation length. Above the correlation length, the solute field prohibits such coherent line bow outs. Using this model, we identify potential gaps in existing theories of solid solution strengthening and show that recent observations of length-dependent dislocation mobilities can be rationalized.

There is substantial demand for theoretical/computational tools that can produce correct predictions of the geometric structure and band gap to accelerate the design and screening of new materials with desirable electronic properties. DFT-based methods exist that reliably predict electronic structure given the correct geometry. Similarly, when good spectroscopic data are available, these same methods may, in principle, be used as input to the inverse problem of generating a good structural model. The same is generally true for gas-phase systems, for which the choice of method is different, but factors that guide its selection are known. Despite these successes, there are shortcomings associated with DFT for the prediction of materials' electronic structure. The present paper offers a perspective on these shortcomings. Fundamentally, the shortcomings associated with DFT stem from a lack of knowledge of the exact functional form of the exchange–correlation functional. Inaccuracies therefore arise from using an approximate functional. These inaccuracies can be reduced by judicious selection of the approximate functional. Other apparent shortcomings present due to misuse or improper application of the method. One of the most significant difficulties is the lack of a robust method for predicting electronic and geometric structure when only qualitative (connectivity) information is available about the system/material. Herein, some actual shortcomings of DFT are distinguished from merely common improper applications of the method. The role of the exchange functional in the predicted relationship between geometric structure and band gap is then explored, using fullerene, 2D polymorphs of elemental phosphorus and polyacetylene as case studies. The results suggest a potentially fruitful avenue of investigation by which some of the true shortcomings might be overcome, and serve as the basis for an appeal for high-accuracy experimental structure data to drive advances in theory.

TlBr can surpass CZT as the leading semiconductor for γ- A nd X-radiation detection. Unfortunately, the optimum properties of TlBr quickly decay when an operating electrical field is applied. Quantum mechanical studies indicated that if this property degradation comes from the conventional mechanism of ionic migration of vacancies, then an unrealistically high vacancy concentration is required to account for the rapid aging of TlBr seen in experiments. In this work, we have applied large scale molecular dynamics simulations to study the effects of dislocations on ionic migration of TlBr crystals under electrical fields. We found that electrical fields can drive the motion of edge dislocations in both slip and climb directions. These combined motions eject enormous vacancies in the dislocation trail. Both dislocation motion and a high vacancy concentration can account for the rapid aging of the TlBr detectors. These findings suggest that strengthening methods to pin dislocations should be explored to increase the lifetimes of TlBr crystals.

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In order to study the effects of Ni oxidation barriers on H diffusion in Zr, a Ni-Zr-H potential was developed based on an existing Ni-Zr potential. Using this and existing binary potentials H diffusion characteristics were calculated and some limited findings for the performance of Ni on Zr coatings are made.

It has been widely believed that crystalline TlBr can surpass CdZnTe to become the leading semiconductor for γ- and X- radiation detection. The major hurdle to this transition is the rapid aging of TlBr under the operating electrical field. Here, while ionic migration of vacancies has been traditionally the root cause for property degradation, quantum mechanical calculations indicated that the vacancy concentration needed to cause the observed aging must be orders of magnitude higher than the usual theoretical estimate. Recent molecular dynamics simulations and X-ray diffract ion experiments have shown that electrical fields can drive the motion of edge dislocations in both slip and climb directions. Furthermore, these combined motions eject a large number of vacancies. Both dislocation mot ion and vacancy ejection can account for the rapid aging of the TlBr detectors. Based on these new discoveries, the present work applies molecular dynamics simulations to “develop” aging-resistant TlBr crystals by inhibiting dislocation motions.

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Ionic liquids are a unique class of materials with several potential applications in electrochemical energy storage. When used in electrolytes, these highly coordinating solvents can influence device performance through their high viscosities and strong solvation behaviors. In this work, we explore the effects of pyrrolidinium cation structure and Li+ concentration on transport processes in ionic liquid electrolytes. We present correlated experimental measurements and molecular simulations of Li+ mobility and O2 diffusivity, and connect these results to dynamic molecular structural information and device performance. In the context of Li-O2/Li-air battery chemistries, we find that Li+ mobility is largely influenced by Li+-anion coordination, but that both Li+ and O2 diffusion may be affected by variations of the pyrrolidinium cation and Li+ concentration.

Austenitic stainless steels (Fe-Cr-Ni) are resistant to hydrogen embrittlement but have not been studied using molecular dynamics simulations due to the lack of an Fe-Cr-Ni-H interatomic potential. Herein we describe our recent progress towards molecular dynamics studies of hydrogen effects in Fe-Cr-Ni stainless steels. We first describe our Fe-Cr-Ni-H interatomic potential and demonstrate its characteristics relevant to mechanical properties. We then demonstrate that our potential can be used in molecular dynamics simulations to derive Arrhenius equation of hydrogen diffusion and to reveal twinning and phase transformation deformation mechanisms in stainless steels.

Fe‐Ni‐Cr stainless‐steels are important structural materials because of their superior strength and corrosion resistance. Atomistic studies of mechanical properties of stainless‐steels, however, have been limited by the lack of high‐fidelity interatomic potentials. Here using density functional theory as a guide, we have developed a new Fe‐Ni‐Cr embedded atom method potential. We demonstrate that our potential enables stable molecular dynamics simulations of stainless‐steel alloys at high temperatures, accurately reproduces the stacking fault energy—known to strongly influence the mode of plastic deformation (e.g., twinning vs. dislocation glide vs. cross‐slip)—of these alloys over a range of compositions, and gives reasonable elastic constants, energies, and volumes for various compositions. The latter are pertinent for determining short‐range order and solute strengthening effects. Our results suggest that our potential is suitable for studying mechanical properties of austenitic and ferritic stainless‐steels which have vast implementation in the scientific and industrial communities. Published 2018. This article is a U.S. Government work and is in the public domain in the USA.