International Workshop on Interfaces at Bear Creek

September 28-October 1, 2015

Bear Creek Mountain Resort • 101 Doe Mountain Lane • Macungie, PA • 18062 • USA

1-866-754-2822 • www.skibearcreek.com

ORGANIZERS Martin P. Harmer Lehigh University, USA

Jian Luo University of California San Diego Shen Dillon University of Illinois Urbana Gregory Rohrer Carnegie Mellon University Andrea Harmer Kutztown University

SPONSORS • Office Of Naval Research • P.C. Rossin College of Engineering & Applied Science at Lehigh University • Center for Advanced Materials and Nanotechnology • Office of Vice President and Associate Provost for Research and Graduate Studies • Mechanical Engineering and Mechanics at Lehigh University • Department of Chemical and Biomolecular Engineering at Lehigh University • Materials Science and Engineering at Lehigh University • Department of Chemistry at Lehigh University • Electron Microscopy Laboratory at Lehigh University • Martindale Center at Lehigh University

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International Workshop on Interfaces at Bear Creek

September 28-October 1, 2015

Bear Creek Mountain Resort Macungie, PA USA

WORKSHOP PROGRAM

Monday September 28, 2015 3:00 p.m. – 6:00 p.m. Arrival at the Inn at Bear Creek 6:00 p.m. – 8:00 p.m. Welcome Reception and Dinner Session I – Opening Session 8:00 p.m. – 9:00 p.m. M. P. Harmer (Lehigh University)

Welcome and Perspectives Summary

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Tuesday, September 29, 2015 7:30 a.m. – 9:00 a.m. Breakfast 9:00 a.m. – 10:10 a.m. Session II – Grain Growth I

(Chair: Tony Rollett) 9:00 a.m. – 9:20 a.m. Carl Krill (Ulm University)

Probing the sociology of grain growth: Cyril Stanley Smith, German beer and microstructural mapping in 4D

9:20 a.m. – 9:40 a.m. Jeffrey Rickman (Lehigh University)

Parsing Abnormal Grain Growth 9:40 a.m. – 10:10 a.m. Discussion 10:10 a.m. – 11:30 p.m. Session II Continued – Grain Growth II

(Chair: Greg Rohrer) 10:10 a.m. – 10:30 a.m. Tony Rollett (Carnegie Mellon University)

How Grain Boundary Transitions Sustain Abnormal Growth 10:30 a.m. – 10:50 p.m. Michael Hoffmann (Karlsruhe Institute of Technology)

Grain growth transitions in perovskites: What do we know? What is important? What are we still missing? Where should we go to?

10:50 a.m. – 11:30 a.m. Discussion 11:30 a.m. – 12:10 p.m. Five Minute Poster Preview I (Chair: Ming Tang) 12:15 p.m. – 1:30 p.m. Lunch 1:30 p.m. – 3:15 p.m. Session III – Nanograin Stability (Chair: Reiner Kirchman) 1:30 p.m. – 1:50 p.m. Chris Schuh (MIT)

Thermodynamically stabilized nanostructures: nanoduplex alloys 1:50 p.m. – 2:10 p.m. Chris Marvel (Lehigh University)

The Influence of Contamination on the Thermal and Phase Stability of Nanocrystalline Ni-W Alloys

2:10 p.m. – 2:30 p.m. Kris Darling (Army Research Lab)

Microstructure and mechanical properties of bulk nanostructured Cu–Ta alloys consolidated by equal channel angular extrusion

2:30 p.m. – 3:00 p.m. Discussion 3:00 p.m. – 3:15 p.m. Coffee

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Tuesday, September 29, 2015 (continued) 3:15 p.m. – 4:45 p.m. Session IV – Interface Diffusion and Transformations

(Chair: Michael Hoffmann) 3:15 p.m. – 3:35 p.m. Patrick Cantwell (Rose-Hulman Institute of Technology)

Grain Boundary Complexion TTT Diagrams 3:35 p.m. – 3:55 p.m. Shen Dillon (University of Illinois Urbana)

The effect of grain boundary complexions on grain boundary transport 3:55 p.m. – 4:15 p.m. Jian Luo (University of California San Diego)

From Understanding Complexions in Binary Alloys to Stabilizing Nanoalloys Using “High-Entropy Grain Boundary Complexions”

4:15 p.m. – 4:45 p.m. Discussion 4:45 p.m. – 6:00 p.m. Poster Viewing Session I 6:00 p.m. – 8:00 p.m. Dinner

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Wednesday, September 30, 2015 7:30 a.m. – 9:00 a.m. Breakfast 9:00 a.m. – 11:00 a.m. Session V – Tailoring Interfaces for Electronic Applications

(Chair: Steve Garofalini) 9:00 a.m. – 9:20 a.m. Ming Tang (Rice University) Tailoring the Morphological Evolution of Interfaces in Lithium Battery

Electrode Materials 9:20 a.m. – 9:40 a.m. Ed Webb (Lehigh University)

Atomic Scale Exploration of Intercalation and Diffusion in Graphite Grain Boundaries

9:40 a.m. – 10:00 a.m. Steve McIntosh (Lehigh University)

Insights into Oxygen Anion Transport in Layered Oxides via In-Situ Powder Neutron Diffraction

10:00 a.m. – 10:20 a.m. Greg Ferguson (Lehigh University)

Studies of a Thermodynamically Unstable Oxide: Thin Films of Gold Oxide and Its Use in Selective Surface Chemistry

10:20 a.m. – 11:00 a.m. Discussion 11:00 a.m. – 11:15 a.m. Coffee 11:15 a.m. – 12:00 p.m. Five Minute Poster Preview Session II (Chair: Chris Shuh) 12:00 p.m. – 1:10 p.m. Lunch 1:10 p.m. – 2:45 p.m. Session VI – Solid/Liquid Surface Interactions (Chair: Ed Webb) 1:10 p.m. – 1:30 p.m. Jim Gilchrist (Lehigh University)

Large-area nanoparticle films by continuous automated Langmuir-Blodgett assembly and deposition

1:30 p.m. – 1:50 p.m. Joelle Frechette (John Hopkins University)

Large out-of-contact elastohydrodynamic deformation due to lubrication forces

1:50 p.m. – 2:10 p.m. Steve Garofalini (Rutgers University)

The Water/Glass Interface: Affect on Structure, Dissolution, and Proton Transport

2 :10 p.m. – 2:30 p.m. Discussion 2:30 p.m. -2:45 p.m. Coffee

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Wednesday, September 30, 2015 (continued) 2:45p.m. - 4:05 p.m. Session VII – Insights from First Principles Calculations

(Chair: Jeff Rickman) 2:45 p.m. – 3:05 p.m. Michael Widom (Carnegie Mellon University)

Bismuth and tungsten impurities in nickel 3:05 p.m. – 3:25 p.m. Heather Jaeger (Lehigh University)

Simulating electronic processes in molecular materials 3:25 p.m.-3:45 p.m. Andrew Rappe (University of Pennsylvania)

Interfaces between polar domains in oxides: Dynamics and Functionality 3:45 p.m. -4:05 p.m. Discussion 4:05 p.m. -4:30 p.m. Coffee 4:30 p.m. -6:00 p.m. Poster Viewing Session II 6:00 p.m. -7:00 p.m. Cocktail Hour 7:00 p.m. -10:00 p.m. Conference Dinner and Special Entertainment

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Thursday October 1, 2015 7:00 a.m. – 9:00 a.m. Breakfast 9:00 a.m. – 10:10 p.m. Session VIII – Corundum Conundrums

(Chair: Martin Harmer) 9:00 a.m. – 9:30 a.m. Arthur Heuer (Case Western Reserve University)

Al2O3 is a Wide-Band Gap Semiconductor: A Half Century of Myopia! 9:30 a.m. – 9:50 a.m. Helen Chan (Lehigh University)

Nanoscale Metal-Ceramic Matrix Composites by Reduction of Mixed Oxides 9:50 a.m. – 10:10 a.m. Discussion 10:10 a.m. – 12:10 p.m. Session IX – Mechanical Behavior

(Chair: Rick Vinci) 10:10 a.m. -10:30 a.m. Tim Rupert (University of California Irvine)

Using amorphous complexions to tailor the mechanical behavior of nanostructured metals

10:30 a.m. – 10:50 a.m. Rick Vinci (Lehigh University)

Fracture testing inside an SEM 10:50 a.m. – 11:10 a.m. Reiner Kirchheim (Institut für Materialphysik Georg-August-Universität

Göttingen) Chemomechanical effects on the separation of interfaces occurring during fracture

11:10 a.m. -11:30 a.m. Brandon Krick (Lehigh University)

Ultralow Wear Polytetrafluoroethylene and Alumina Composites: The Role of Tribochemistry and Nanomechanics

11:30 a.m. – 12:10 p.m. Discussion 12:10 p.m. – 1:20 p.m. Lunch 1:20 p.m. – 2:30 p.m. Session X – Architectured Materials

(Chair: Carl Krill) 1:20 p.m. – 1:50 p.m. Nicolas Argibay (Sandia National Labs)

Correlating surface microstructure evolution to regimes of friction in metals

1:50 p.m. -2:10 p.m. Natasha Vermaak (Lehigh University)

Exploiting interfaces in topology optimization for architectured materials 2:10 p.m. – 2:30 p.m. Discussion 2:30 p.m. – 3:00 p.m. Session XI – Conference Summary and General Discussion Greg Rohrer (Carnegie Mellon Uinversity) 3:00 p.m. Depart

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POSTER SESSION I

Ling Feng (University of Illinois at Urbana-Champaign)

Arvind Kalidindi (Massachusetts Institute of Technology)

Michael Kracum (Lehigh University)

Naixie Zhou (University of California San Diego )

Mingyan Wang (Ulm University)

Denise Yin (Lehigh University)

POSTER SESSION II

William Frazier (Carnegie Mellon University)

Nipun Goel (Lehigh University)

Philip Goins (Carnegie Mellon University)

Madeleine Kelly (Carnegie Mellon University)

Baiou Shi (Lehigh University)

Yan Wang (Lehigh University)

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International Workshop on Interfaces at Bear Creek

PRESENTATION SYNOPSES

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Probing the sociology of grain growth: Cyril Stanley Smith, German beer and microstructural mapping in 4D

Carl Krill

Institute of Micro and Nanomaterials, Ulm University, Albert-Einstein-Allee 47, 89081 Ulm, Germany Grain growth is one of those deceptively simple processes that have stymied materials scientists for decades. Seemingly tractable at the level of individual boundaries, grain growth is full of surprises when it comes to the collective behavior of densely packed crystallites. Actually, even the local level can pose significant challenges—what with phenomena like solute segregation, roughening/faceting transitions, defect-defect interactions and complexions all affecting the kinetics of grain boundary migration—but the macroscopic level is where the sheer complexity of grain growth really becomes apparent. Unafraid of this fact (or perhaps because of it?), the famed metallurgist, Cyril Stanley Smith, attacked the problem of grain growth precisely at the point it becomes “sociological” in nature, marked by a persistent struggle between neighboring grains to steal atoms from each other. Smith postulated that the topology of the resulting space-filling grain ensemble would provide important insight into the underlying growth mechanism(s), and he figured out how to acquire the desired morphological information long before 3D microstructural characterization methods had gained a firm foothold in materials science. In this short presentation, I aim to convince the audience that Smith’s approach to investigating grain growth inexorably leads to an up-close and personal encounter with German beer. But there’s no need for alarm! What might otherwise threaten to derail a scientific career is, in this particular instance, a key step in understanding the interaction that takes place between adjacent grains during coarsening and the ramifications of this interaction on the grain population as a whole. The endeavor that started with Cyril Stanley Smith’s pioneering experiments revealing the three-dimensional shapes and arrangements of grains in a polycrystal is coming to full fruition in time-resolved studies of microstructural evolution that are now, for the first time, being carried out in 4D (= 3D + time). The mapping techniques that are responsible for this breakthrough—3D x-ray diffraction microscopy (3DXRD) and diffraction-contrast tomography (DCT)—encompass both microscopic and macroscopic length scales; consequently, the methods can deliver sequences of “snapshots” showing the migration of individual grain boundaries as well as the evolution of the boundary network as a whole. But that’s not all: both 3DXRD and DCT also measure the crystallographic orientation of each grain involved in grain growth! Thus, the five degrees of freedom of individual grain boundaries are now non-destructively accessible in 4D, thereby overcoming the last major barrier to full-scale incorporation of local models for boundary migration into the computational simulation of grain ensembles. What do we want to know, and how can we learn it?

From the standpoint of applications, it would suffice in many circumstances to be able to predict the rate of grain growth and its general mode (normal vs. abnormal) as a function of processing conditions, like temperature, time and stress state. However, decades of research into the statistical properties of coarsening-induced microstructures (time dependence of the average grain size, shape of the normalized grain size distribution) indicate that predictive power will have been achieved only after validation of a comprehensive model for local boundary migration. How do the grain boundary energy and mobility depend on the crystallographic misorientation at that boundary and on its local inclination? To what extent can these dependencies account for the abnormally fast migration of certain grain boundaries (abnormal growth)? In other words, could abnormal grain growth (AGG) be an intrinsic feature of grain growth under certain conditions, or does AGG necessarily rely on the coupling of boundary migration to some other effect, like solute drag, particle pinning or inhomogeneous stress distributions? These questions appear to be particularly relevant to nanocrystalline metals, in which low-temperature coarsening can be extraordinarily abnormal, with some grains growing thousands of times larger than their neighbors and the outer edges of the abnormal grains manifesting fractal-like morphologies. In my estimation, the best way to test a (local) model for grain boundary migration is first to acquire time-resolved 3D mappings of polycrystalline specimens as they undergo grain growth. These data can then provide the initial state for a computational simulation of microstructural evolution, and the validity of that simulation (and, thereby, of its underlying model for boundary migration) can be assessed by comparison of the computational prediction to the subsequent growth that was actually measured. Optimization of the boundary migration model might be possible by fine-tuning its parameters to improve the agreement between simulation and experiment. The aforementioned techniques of 3DXRD and DCT are capable of spanning length scales from about 1 µm to 1 mm; for the study of nanocrystalline specimens, however, recourse must be taken to a higher-resolution method, like Orientation Mapping in a Transmission Electron Microscope (OMiTEM), which relies on contrast from dark-field images to determine the spatial extension of individual grains with nanometer accuracy.

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Parsing Abnormal Grain Growth

Jeffrey Rickman Department of Materials Science and Engineering, Lehigh University

Abnormal grain growth, the enlargement of a minority of grains in a polycrystal at the expense of the surrounding grains, occurs in both metallic and ceramic materials and can have a profound impact on their mechanical and electrical properties. Somewhat surprisingly then, there is little consensus as to which specific microstructural features provide a signature of abnormal growth. Indeed, some workers describe this phenomenon in terms of a bimodal grain size distribution, often without justification, while others focus on very few, elongated grains. Using specialty alumina (i.e., high-purity aluminum oxide with tailored impurity composition) as our prototype, we describe here a set of practical maps and metrics that are useful in quantifying various microstructural features that are associated with abnormal grain growth. These maps provide a visual "fingerprint" of abnormal growth, while the metrics permit the design of processing routes to obtain desired microstructures. We then present an application of correlation analysis that illustrates the efficacy of data analytics in quantifying which input (i.e., processing) variables exert the strongest influence on abnormal grain growth. Finally, we outline the use of this methodology to examine correlations among processing variables and the thermomechanical and kinetic properties of materials (e.g., strength, hardness, thermal conductivity, etc.).

How Grain Boundary Transitions Sustain Abnormal Growth

Anthony Rollet Department of Materials Science and Engineering, Carnegie Mellon University

The Potts Model of grain growth was adapted for the purpose of simulating abnormal grain growth (AGG) resulting from grain boundary complexion transitions. The transition in grain boundary structure between specific complexion types results in changes in properties. Where the transitions decrease energy and increase the mobility of boundaries, AGG occurs provided that such transitions predominantly occur via propagation to adjacent boundaries. Thus the model increases the mobility of selected boundaries on the basis of their adjacency to zero, one, or a multiplicity of boundaries that have already transitioned. The effect of transitions to a high mobility complexion was investigated separately from the effect of changes in energy. The influence of the frequency of complexion transitions with different adjacencies on the occurrence of AGG was explored. The simulations show how propagating complexion transitions can explain the AGG observed in certain ceramic systems. Further developments include a) including the effect of transitions induced by solute accumulation on boundaries and b) adding grain boundary anisotropy to the Potts model in order to reproduce the high aspect ratio abnormal grains observed in some systems.

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Network models for predicting rare microstructural events

Brian DeCost and Elizabeth A. Holm Department of Materials Science and Engineering, Carnegie Mellon University

One goal of mesoscale simulations is to understand rare events, such as failure initiation, hot spot formation, and abnormal grain nucleation. Because these phenomena typically arise from the localization of long-range interactions, identifying an incipient rare event is challenging. This talk will present a graph theory approach that analyzes extended grain neighborhoods to understand the nucleation of abnormal grain growth (AGG). Because AGG requires the persistent, concerted motion of most or all of the boundaries surrounding the abnormal grain, it is fundamentally a collective and long-range process within the grain boundary network. In order to predict AGG, we map the grain structure as a graph of connected grains (the transgranular network). We then apply graph theory algorithms, including current flow centrality and graph kernel methods, to characterize network properties over various length scales. Utilizing these results, machine-learning systems can be trained to classify potential abnormal growth events within the grain network. By operating beyond a nearest-neighbor or mean-field interaction distance, this approach has promise for characterizing a number of long-range microstructural phenomena.

Grain growth transitions in perovskites: What do we know? What is important? What are we still missing? Where should we go to?

Michael J. Hoffmann, Wolfgang Rheinheimer and Fabian Lemke

Karlsruhe Institute of Technology (KIT) Different perovskite ceramics show non-Arrhenius type grain growth; instead transition temperatures exists were the grain growth rate decreases or increases drastically. This behavior can be found e.g. in strontium titanate, barium titanate and lithium lanthanum titanate. In all cases the transitions are coupled to abnormal grain growth. On the microstructural level the phenomenon can be easily explained by the coexistence and transition of different grain boundary types. However, microscopically, the phenomenon is much more complicated. We are dealing with different microstructural effects (non-Arrhenius type grain growth, abnormal grain growth, growth stagnation), which have their origin on the atomic scale. Therefore, we need information on the grain boundary structure (faceting, wetting and complexions, chemistry and stoichiometry, space charge and defects). On the mesoscopic scale we have to think about anisotropy, faceting and kinetic/equilibrium shapes of interfaces. In general it is difficult to link these different scales to gain information on the microstructural behavior based on atomistic properties. This study summarizes all available information on grain growth in perovskites and highlights important parameters. It attempts to bridge the gap between different scales and specifies missing information for this task.

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Thermodynamically stabilized nanostructures: nanoduplex alloys

Christopher A. Schuh Massachusetts Institute of Technology

In some alloys, a minority solute addition can have a strong preference for grain boundary

segregation to the point where the segregated grain boundary state is the lowest free energy configuration available, even compared to bulk phases such as solid solutions or precipitates. Recently, our group has explored another stable nanocrystalline configuration—nano-duplex alloys—where grain boundary segregation and nano-precipitates occur simultaneously. For such a state to be thermodynamically stable, grain boundary segregation has to be entropically favored: segregated grain boundaries have a slightly higher enthalpy than precipitates, but allow the alloy to access higher entropies. We illustrate this concept through both lattice-based Monte Carlo simulations and through experiments in W-based alloys. We show that such nano-duplex alloys can also exhibit a special accelerated sintering mode, where precipitates form between powder particles and act as fast-diffusion pathways for rapid consolidation. This combination of structural stability and a viable processing route makes nano-duplex alloys technologically interesting bulk materials.

The Influence of Contamination on the Thermal and Phase Stability of Nanocrystalline Ni-W Alloys

Chris Marvel, Denise Yin, Patrick Cantwell*, Martin Harmer

Lehigh University, *Rose Hulman Most research efforts to stabilize nanocrystalline metals assume the alloys of interest are nominally pure and free of impurities. However, in many cases, contamination could play just as important of a role as the intentional solutes. This study directly investigated and revealed the role of impurities on electroplated Ni-W. The primary finding of this research was that several unexpected carbides and oxides precipitated. Most importantly, nanoscale oxides met the Zener criteria to stagnate grain growth. This evidence of particle pinning offers another mechanism to stabilize Ni-W, in addition to the well-known mechanism of W-segregation. Furthermore, the presence of the carbides highlighted potential flaws in the ASM binary phase diagram. The intermetallics NiW and NiW2 found on the diagram are in reality Ni6W6C and Ni2W4C. This research will hopefully guide future experiments to better understand the influence of impurities.

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Microstructure and mechanical properties of bulk nanostructured Cu–Ta alloys consolidated by equal channel angular extrusion

Kris Darling, Anthony Roberts, Mark Tschopp, and Laszlo Kecskes

Army Research Lab

Nanocrystalline materials have and continue to receive significant interest for their ability to enhance mechanical properties, specifically, strengthening via the Hall-Petch mechanism, compared to materials with a coarse-grained microstructure. While the nanometer-sized grains are clearly advantageous in imparting greater strength to the material, they do not contribute to or frequently are detrimental to the plasticity of the material. Furthermore, these nanometer-sized grains are highly susceptible to grain growth due to their thermal instability, which limits their use in practical applications. To address this limitation, considerable research has focused on enhancing the thermal stability of such systems to prevent/minimize the onset of grain growth. In the present study, Cu–Ta alloy powders, synthesized via high-energy cryogenic mechanical alloying, were consolidated into bulk nanostructured specimens using equal channel angular extrusion (ECAE) at high temperatures. Subsequent microstructure characterization indicated full consolidation, which resulted in an equiaxed grain structure for the Cu matrix along with the formation of fine Ta precipitates, the size distributions of which varied both with composition and processing temperature. Mechanical testing indicated, in some cases, an almost threefold increase in mechanical properties above that predicted by Hall–Petch estimates for pure nanocrystalline. Advanced characterization including TEM and Atom Probe Tomography in combination with molecular dynamics simulation are used to delineate the effect of Ta concentration on the retardation of grain growth and increase in mechanical strength of nano-crystalline Cu alloys.

“Grain Boundary Complexion TTT Diagrams”

Patrick R. Cantwell, Shuailei Ma, Stephanie A. Bojarski, Gregory S. Rohrer, Martin P. Harmer Lehigh University

Grain boundaries can exhibit phase-like behavior by transitioning from one equilibrium configuration to another as a function of temperature and bulk composition. Just as phase diagrams describe the equilibrium state of bulk matter, grain boundary complexion diagrams can be made to describe the equilibrium state of grain boundaries. Recent experimental data from grain growth studies of polycrystalline yttria and alumina shows that another analogue exists between phases and complexions: Grain boundary complexions exhibit time-temperature-transformation (TTT) behavior that parallels the TTT behavior of bulk materials. Grain boundary complexion TTT diagrams based on the recent grain growth data from polycrystalline alumina and yttria will be presented and discussed along with potential applications and future directions.

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The effect of grain boundary complexions on grain boundary transport

Shen J. Dillon Department of Materials Science and Engineering, University of Illinois at Urbana-Champaign

Anomalous diffusional grain boundary transport, such as activated sintering and abnormal grain growth, motivated much of the early work investigating grain boundary complexions. This presentation reviews key contributions in the literature that inform our understanding of the role of grain boundary complexions in affecting mass transport, describes recent results measuring the average grain boundary diffusivity in polycrystals containing different complexions, and highlights outstanding challenges and questions. The presentation will will focus on our recent work on diffusional transport along grain boundary complexions in Bi containing transition metals and aliovalent doped Al2O3.

From Understanding Complexions in Binary Alloys to Stabilizing Nanoalloys Using “High-Entropy Grain Boundary Complexions”

Jian Luo, jluo@ucsd.edu

University of California, San Diego Recent unpublished studies on the “systematics” of grain boundary adsorption transitions and construction of complexion diagrams for regular solutions as well as selected real systems such as Ni-Bi are briefly reviewed. The focus of this talk will be on the discussion of our most recent exploration to extend the theories and models to understand grain boundary adsorption in multicomponent alloys, particularly, a new idea of utilizing “high-entropy grain boundary complexions” to stabilize nanoalloys. Both new theories, modeling results of simplified multicomponent regular solutions, and experimental validations using Ni-based many-component nanoalloys will be presented and discussed. (See a poster presented by Naixie Zhou, entitled “High-Entropy Grain Boundary Complexions: A New Pathway to Stabilize Nanocrystalline Alloys,” for further details.)

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Tailoring the Morphological Evolution of Interfaces in Lithium Battery Electrode Materials

Ming Tang Department of Materials Science and NanoEngineering, Rice University

Various types of interfaces exist in electrode materials for electrochemical energy storage, including electrode/electrolyte interfaces, phase boundaries and grain boundaries, etc. During electrochemical cycling, these interfaces often undergo intricate morphological variation coupled with ion insertion/extraction, the understanding of which is not only of significant scientific interest but also provides useful insights on how to improve battery performance and reliability. This point is illustrated by our study on two electrode materials for lithium batteries, i.e. lithium iron phosphate olivine (LiFePO4) and silicon. In LiFePO4 cathodes, it is found that the morphology of the phase boundary between the lithiated and delithiated phases exhibits a strong overpotential dependence and is affected by the coherency strain and Li diffusivity. We show that interface evolution in LiFePO4 could be controlled by electrode particle geometry to achieve a more than10-fold increase of the rate capability. Different from LiFePO4 that accommodates transformation strain mainly through elastic deformation, silicon undergoes a >300% volume expansion upon Li insertion, which incurs significant plastic deformation and often results in extensive cracking. Using an efficient interface-tracking modeling technique, we simulate the concurrent processes of phase transition, plastic deformation and anisotropic shape evolution of crystalline Si electrodes upon lithiation. Based on simulation results, a novel strategy is proposed to mitigate lithiation-induced fracture in Si by using pristine crystalline Si structures with engineered anisometric morphologies. The effect of such morphological designs have been verified experimentally and could facilitate the practical application of micron-sized Si structures in Li batteries.

Atomic Scale Exploration of Intercalation and Diffusion in Graphite Grain Boundaries

Chris Shumeyko, Edmund B. Webb III Lehigh University

Although recent trends in energy storage technology research have moved away from traditional lithium-ion batteries, they remain highly germane due to their very wide deployment in existing electronics products. Furthermore, lithium-ion based batteries remain interesting given that there still exist many unknowns regarding their fundamental transport behavior. One example is the behavior of graphitic anodes during charging; experiments attempting to explore this have revealed a wide range of diffusion rates. This work will attempt to explore the origins of such varied observations via atomic scale simulations of Li intercalating into different graphite grain boundaries. Intercalation behavior for different boundaries provides compelling evidence that grain boundaries in graphite greatly affect the initial intercalation rates of low-mass intercalants such as lithium. These results will be discussed in terms of a possible explanation for the highly varied experimental diffusion rates of lithium in graphite. To better understand significant differences observed in grain boundary intercalation rate, structural features of different boundaries were compared. Though a number of structural metrics demonstrated essentially no difference between boundaries, it was found that carbon ring structure at the intersection of a given grain boundary with the free surface correlated well with intercalation rate. Ring structure is affected by local C atom hybridization and it was found that the degree of sp bonding at the surface correlated well with intercalation rate. In this talk, we will also present preliminary results from simulations of bulk grain boundary diffusion, with particular emphasis on the structure of intercalants at the boundary as well as observations of collective intercalant transport behavior.

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Insights into Oxygen Anion Transport in Layered Oxides via In-Situ Powder Neutron Diffraction

Alex C. Tomkiewicz1, Mazin A. Tamimi1, Ashfia Huq2, and Steven McIntosh1

1 Dept. of Chemical and Biomolecular Engineering, Lehigh University, Bethlehem, PA 18015, USA. 2 Neutron Sciences, Oak Ridge National Laboratory, Oak Ridge, TN, 37830, USA.

The promise of direct and efficient conversion of chemical to electrical energy makes fuel cell development an area of great technological interest. Solid Oxide Fuel Cells (SOFCs) are one of the most promising technologies to meet this goal. A significant barrier to progress is a lack of experimental techniques that can probe the properties of these materials under high temperate working conditions in both oxidizing and reducing gas environments. Neutron diffraction is one technique that can achieve this goal to reveal information relating to phase transition, order-disorder phenomenon, and the presence of anionic and cationic vacancies in crystalline oxides. Most powerfully, all of this information is collected in a single experiment over a wide variety of operating conditions. Analysis of these results enables visualization of transport pathways in ionic conductors, guiding future material development. This visualization is achieved through analysis of both occupancy (providing vacancy concentration) and the anisotropic atomic displacement (providing magnitude and direction of motion) of the oxygen sites in the structure. For layered oxides this reveals preferential vacancy localization and associated preferred transport pathways. This presentation will discuss results from a recently developed in-situ neutron diffraction cell developed for the POWGEN beam line at the Spallation Neutron Source, Oak Ridge National Laboratory. Results will be presented for a variety of materials to demonstrate the versatility of this technique.

Studies of a Thermodynamically Unstable Oxide: Thin Films of Gold Oxide and Its Use in Selective Surface Chemistry

Gregory S. Ferguson

Departments of Chemistry and Materials Science & Engineering, Lehigh University Anodization of a gold electrode in aqueous solution produces a ~1-nm film of the oxide on the surface. Although this material is unstable (thermodynamically) with respect to the elements, it is sufficiently inert (kinetically) to allow its study and use in specific applications. One such application is the selective formation of self-assembled monolayers (SAMs) on particular microelectrodes on a chip in the presence of others. This approach is conceptually related to one commonly used in synthetic organic chemistry: addition of a “protecting group” at one site to allow selective chemistry to occur at a second site. Subsequent removal of the protecting group (“deprotection”) then completes a scheme that provides spatially controlled reactivity. Thin films of oxide served as removable blocking layers in these studies.

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Large-area nanoparticle films by continuous automated Langmuir-Blodgett assembly and deposition

James Gilchrist

Department Chemical Engineering, Lehigh University We describe a general method to fabricate nanoparticle film by combining Langmuir-Blodgett deposition and continuous Landau-Levichthin film coating. Our approach is a scaled-up process for preparingnanoparticle films compared to regular batch method. In a continuously flowing trough, silica nanoparticles are injected onto a floatingwater surface and the lateral pressure of the circulating water helpsthe particles assemble into high density film. At the edge of the trough, a rolling system handles with dispensing spare substrate,transferring assembled nanoparticles film onto the substrate andcollecting the samples with nanoparticles film afterwards. An appropriate combination of particle mass flow rate and the web speed is a crucial factor in this approach so as to control the organization of particles in the film. The mechanism of this process was investigated both experimentally and theoretically.

Large out-of-contact elastohydrodynamic deformation due to lubrication forces

Joelle Frechette1,2 1Chemical and Biomolecular Engineering Department, and 2Hopkins Extreme Materials Institute,

Johns Hopkins University, Baltimore MD 21218.

Surface and interfacial phenomena in soft matter are profoundly affected by the mechanical compliance of the interacting materials and, as a result, lead to qualitatively different behaviors from those encountered in stiff materials. Hydrodynamic forces cause elastic deformation without physical contact (elastohydrodynamics or EHD). Elastohydrodynamic deformation during sliding of soft surfaces can cause lift and reduce friction. The deformation of an elastic boundary caused by drainage of fluid from a narrowing gap is analogous to the deformation of droplets or bubbles as they approach a rigid surface. However compliant solids can sustain higher pressures and do not break up or coalesce, leading to regimes absent in droplets or bubbles. A challenge in studying the coupling between elasticity and viscous forces has been with the simultaneous measurements of the hydrodynamic forces and of the surface profile, which is necessary to test theories, especially in for significant elastic deformation. Here we characterize the hydrodynamic forces and visualize the spatiotemporal evolution of the deformation in the drainage of fluid from a gap with an elastic boundary. We observe that elastic deformation prevents the surfaces from making contact via the formation of a dimple in the elastic film. The growth kinetics of the dimple follows a scaling derived for droplets and bubbles. We find excellent agreement between the experiments and lubrication theory combined with viscoelasticity, but also show systematic deviations that are attributed to the shear deformation associated with layering effects. Short Bio: Dr. Frechette received her PhD from Princeton University in chemical engineering and materials science in 2005 studying adhesion and double layer forces in electrochemical environment. After postdoctoral work at UC Berkeley where she investigated unwanted adhesion in microelectromechanical syststems, she joined the Hopkins Faculty in 2006. Joelle Frechette was awarded the NSF CAREER award in 2008 and the ONR Young Investigator Award in 2011. Her research interests are in the area of interfacial science, adhesion, wetting, and colloids.

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The Water/Glass Interface: Affect on Structure, Dissolution, and Proton Transport

Steve Garofalini Department of Materials Science and Engineering, Rutgers University

Recent experiments and computations have provided new insight about the water/glass interface. This interface affects the anomalous expansion of water in nanoconfinement, dissolution, and proton transport. Here we present reactive molecular dynamics simulations to determine the atomistic behavior at the interface between water and amorphous silica and its affect on these properties. Results are consistent with available experimental data, but more importantly provide specific atomistic mechanisms governing such results. For instance, the silica surface modifies the structure and hence properties of water at the interface, affecting expansion behavior; the local silica structure affects the activation barrier to dissolution; the electrochemically observed high proton conductivity can be explained by the role of the interface on proton transfer.

Bismuth and tungsten impurities in nickel

Qin Gao, Sanxi Yao and Michael Widom Carnegie Mellon University, Department of Physics

Atomic size and bonding characteristics lead to strongly contrasting solubilities of Bi and W in elemental Ni. The large size and weak bonding of semimetallic Bi cause it to segregate to Ni surfaces and grain boundaries, while the favorable size and transition metal character of W leads to a high temperature solid solution over a composition range of 20 atomic percent. Using first principles total energy calculations we identify several reconstructions of Bi on Ni surfaces and grain boundaries. That compare well with LEED and TEM investigations. Relative weakness of Bi-Bi bonding compared to Bi-Ni is identified as a contributing factor to embrittlement. Effective W-W interactions within the FCC solid solution of Ni and W lead to a pair of ordered FCC superlattice structures at low temperatures. We propose these phases form coherently at Ni surfaces and grain boundaries.

Simulating electronic processes in molecular materials

Heather Jaeger Department of Chemistry, Lehigh University

Electrical conductivities of molecular materials lie within a broad spectrum that ranges from ballistic transport to incoherent charge hopping. By simulating the time-dependent electron density of the systems explicitly, insight is gained into the effects of coupling to vibrations and the environment, the presence of defects, chemical modification, etc. Our quantum-classical approach places electronic transitions (electron transfer), electronic coherences, and polaron formation on the same level of the simulation, i.e., all contribute to the propagator of the quantum state. We have found that, especially in the case of inorganic-organic, hybrid coordination polymers, the charge distribution is rarely a mere consequence of standard chemical or physical rationale. In these cases, first-principle simulations are valuable assets to the ongoing research in conducting and semiconducting molecular materials.

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Interfaces between polar domains in oxides: Dynamics and Functionality

Andrew M. Rappe Department of Materials Science and Engineering, University of Pennsylvania

Ferroelectric materials form the core of many functional devices, including nonvolatile memory and piezoelectric transducers. These properties depend to a considerable extent on the interfaces between domains with different electric polarization orientation. In this talk I will present new insights into how these polar domain walls form, move, and function. (1) In relaxor ferroelectrics, polar domains of various sizes persist for various times in various locations, making experimental characterization challenging. (2) In ferroelectric thin films, the process of polarization reversal can occur via a single 180 degree flip or by successive 90 degree rotations. (3) The commonly seen creep and depinning modes of domain wall motion are not due to defects as is nearly universally assumed, but are intrinsic characteristics of the process.

Al2O3 is a Wide-Band Gap Semiconductor: A Half Century of Myopia!

A. H. Heuer Case Western Reserve University, Cleveland OH

Although the band gap in Al2O3 is very large, >8eV, the Fermi level cannot be ignored when considering defect energetics, as has traditionally been the case. Recent DFT calculations demonstrate that some long-held notions of defects arising from aliovalent solutes cannot be correct. These ideas will be discussed with regard to the corundum “conundrum”. Further, the electronic (band) structure of polycrystalline Al2O3, in particular the density of near-band edge grain boundary localized states, plays a significant role in a host of high temperature phenomena, including sintering, high temperature creep, oxygen permeability in dense “dry” Al2O3 ceramics, and Al2O3 scale formation on Al2O3 scale-forming alloys. All these phenomena involve creation or annihilation of charged point defects (vacancies and/or interstitials) at grain boundaries and interfaces, and must necessarily involve electrons and holes. Thus, the density of states (DOS) associated with grain boundaries in Al2O3 assume great importance, and have been calculated using DFT for both nominally un-doped and Y-doped 7 bi-crystal boundaries. These quantum mechanical calculations must be taken into account when considering why Y2O3 segregation to Al2O3 grain boundaries is so effective in enhancing high temperature creep resistance of polycrystalline Al2O3, and in understanding the reactive element effect in Al2O3 scale-forming alloys.

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Nanoscale Metal-Ceramic Matrix Composites by Reduction of Mixed Oxides

Helen Chan Department Chair Materials Science and Engineering, Lehigh University

Nanoscale metal-ceramic composites have the potential to offer improved properties over traditional composite materials. To date, due to processing constraints, the majority of work in this field has focussed on ceramic particles in a metal matrix. This talk will discuss the fabrication of novel Co-TiO3 and Cu-Al2O3 composites via the decomposition and partial reduction of mixed oxides. This approach can be used to achieve composite materials consisting of nanoscale metallic particles within a ceramic matrix. The influence of processing conditions on the microstructure and scale of the composite will be presented, as well as the corresponding mechanical properties as evaluated by nano-indentation.

Using amorphous complexions to tailor the mechanical behavior of nanostructured metals

Timothy J. Rupert Assistant Professor, Mechanical and Aerospace Engineering

Assistant Professor, Chemical Engineering and Materials Science University of California, Irvine

Email: trupert@uci.edu Doped interfaces can have unique structures and, in some cases, thermodynamically-stable interfacial complexions can form. In this talk, the potential of nanoscale amorphous intergranular films as structural features that can alter mechanical behavior will be discussed. Molecular dynamics simulations are used to show that amorphous grain boundaries can act as high-capacity sinks for dislocations, and to identify the processing conditions which promote the formation of such boundary structures. Experimental validation of this concept is provided by high resolution transmission electron microscopy in conjunction with energy dispersive x-ray spectroscopy, showing segregation of Zr to the boundaries of Cu-Zr alloys created with mechanically alloying and providing evidence for the formation of amorphous films. Microcompression and in-situ bending experiments are then used to quantify the effect of disordered complexions on mechanical behavior, showing that both strength and ductility can be controlled with segregation engineering, giving a potential path for producing structural metals with optimized mechanical properties.

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Fracture testing inside an SEM

Prof. Richard P. Vinci Department of Materials Science and Engineering, Lehigh University

Center for Advanced Materials and Nanotechnology The use of non-standard small-scale fracture test techniques is required when the specimen is available only in the form of a thin film or when a micrometer-sized feature such as a particular grain boundary is of interest. Using microcantilevers and microtensile specimens inside a scanning electron microscope (SEM) environment, we have measured the fracture behavior of specimens representing three such systems: nanocrystalline Ni-W films, Bi-doped Cu grain boundaries, and rare earth-doped spinel grain boundaries. All three systems show a significant correlation between boundary chemistry, structure, and fracture behavior. These examples will be used to demonstrate the value of performing quantitative fracture tests with micrometer-sized specimens inside an SEM. Even greater understanding can be gained when similar specimens are characterized using high resolution scanning transmission electron microscopy (HR-STEM). General conclusions will be drawn for the three material systems, illustrating how boundary segregation and fracture are tied together in each case.

Chemomechanical effects on the separation of interfaces occurring during fracture

Reiner Kirchheim Institut für Materialphysik

During fracture new surfaces are formed by a propagating crack. Depending on the chemical potential of the constituents of a material and their mobility the composition of the newly formed surfaces changes. Thus the surface energy as part of the ideal work to fracture will be affected. This will be treated by combining the work to fracture representing the mechanical aspect and the Gibbs Adsorption Isotherm covering the chemical aspect. Compared to previous studies the present one provides a more generalized but also a simpler insight into chemomechanical effects. In extreme cases separation of lattice planes or separation of two crystals with a common interface occurs without applied external forces. Closed solutions for the work of fracture are derived for brittle fracture and surface segregation of solutes in the limit of a mean field approach. Chemomechanical effects including plastic deformation by dislocation or vacancy generation are discussed qualitatively. Summary of missing knowledge: We should prefer to evaluate chemomechanical results in terms of chemical potentials of relevant solute atoms or molecules instead of using their concentrations, because the chemical potential allows both

(i) a direct access to excess solute at defects via Fermi-Dirac Statistics and (ii) a direct way of calculating changes of the defect formation energies via the extended

Gibbs Adsorption equation.

Chemical potentials should be measured or calculated as a function of solute concentration, temperature and defect density, although this is a difficult task.

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Ultralow Wear Polytetrafluoroethylene and Alumina Composites: The Role of Tribochemistry

and Nanomechanics

Authors: Brandon A. Kricka, Mark A. Sidebottoma, Angela A. Pitenisb, Kathryn Harrisb, John F. Currya, Fiona Cuia, Richard P. Vincia, Christopher P. Junkc, Heidi E. Burchc, Gregory S. Blackmanc

and W. Gregory Sawyerb a Lehigh University

b University of Florida c DuPont Central Research and Development

Polytetrafluoroethylene has the lowest coefficient of friction among bulk polymers, however it has one of the highest wear rates (K~ 4 x 10-4 mm3/Nm) among engineering polymers. Remarkably, the wear rate of PTFE and other fluoropolymers can be reduced by three to four orders of magnitude (K as low as 4 x 10-8 mm3/Nm) with the inclusion of less than 5 wt. % (< 2.5 vol. %) of certain alumina fillers. Not all varieties of alumina fillers can achieve this dramatic reduction in wear; in fact, there is a three order of magnitude variation in wear with changes in the alumina filler. The true mechanisms responsible for this extraordinary wear reduction in these materials has remained elusive, largely due to their multiple, synergistic size, length, and force scale origins. New studies reveal how material, mechanical, physical and chemical effects come together to form tribofilms that are structurally, morphologically, chemically and mechanically different from the bulk. The prevailing discoveries are 1) a tribochemically formed transfer film promotes adhesion to a countersample and 2) multi-scale mechanics of the filler dominate the variation in wear rate with filler type.

Correlating surface microstructure evolution to regimes of friction in metals Nicolas Argibay1, Michael Chandross1, Shengfeng Cheng2, Paul G. Kotula1 and Joseph R. Michael1 1 Materials Science and Engineering Center, Sandia National Labs, Albuquerque NM USA 2 Department of Physics and Macromolecules and Interfaces Institute, Virginia Polytechnic Institute

and State University, Blacksburg, VA 24061 Engineering design of sliding metallic contacts is generally guided by the rule of thumb that increased hardness leads to lower friction. Recent molecular dynamics simulations performed by our group have demonstrated that this relationship is more accurately described as correlation rather than causation. Our work shows that both these macroscopic observations have their roots in the same atomistic mechanisms, namely accommodation of stress through grain boundary sliding as opposed to dislocation mediated plasticity. This result has been confirmed by a series of experiments, and has allowed us to develop a general, predictive model of the friction regimes observed in bare and boundary lubricated metallic contacts. The model is based on material properties, microstructural stability and its evolution. We will present details of the proposed framework, including physics-based predictions of applied load thresholds, and the resulting tribological response. Experimental results that confirm the model predictions will also be presented. Sandia National Laboratories is a multi-program laboratory managed and operated by Sandia Corporation, a wholly owned subsidiary of Lockheed Martin Corporation, for the U.S. Department of Energy's National Nuclear Security Administration under contract DE-AC04-94AL85000

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Exploiting interfaces in topology optimization for architectured materials

Natasha Vermaak Department of Mechanical Engineering and Mechanics, Lehigh University

Architectured or hybrid materials have attributes not offered by any one material alone. In designing these materials, one feature that is critical is determining how combinations of materials or materials and space are arranged, their connectivity, their geometry, their configuration. Topology optimization offers a systematic methodology for arranging multiple materials in space to meet loading and design requirements. Recent advances in the mathematical formulations for topology optimization have enhanced the treatment of a key feature introduced and typically ignored in multi-material optimization models: interfaces. Material interfaces play a pivotal role in the actual performance of a composite material, often dictating lifetime and failure characteristics, tolerances and processing choices. Several elastic and thermoelastic engineering problems ranging from structural components for Big Area Additive Manufacturing (BAAM) to the design of cellular microstructures are used to demonstrate significant performance changes that are linked to local interface properties.

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International Workshop on Interfaces At Bear Creek

POSTER SESSION I

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Dopant Effect on Cr3+ Lattice Diffusion and Grain Boundary Diffusion in α-Al2O3

Authors: Lin Feng and Shen Dillon

University of Illinois at Urbana-Champaign

Cr3+ lattice and grain boundary diffusion in α-Al2O3 were measured by energy dispersive spectroscopy and secondary ion mass spectrometry at temperature range of 1100℃to 1400℃. The dopant effect on cation lattice and grain boundary diffusion are investigated. It is found that Si4+ dopant enhances Cr3+ lattice diffusion relative to undoped α-Al2O3 by about a factor of 2, , while Mg2+ has no effect on Cr lattice diffusion. For grain boundary diffusion, both aliovalent cation dopants have negligible effect. These grain boundaries display different complexions, which also appear to have negligible effect on the measured diffusivity. keywords: Lattice Diffusion; Grain Boundary Diffusion; Dopant effect; Polycrystalline Alumina

Stability of Nanocrystalline Alloys against Grain Growth and Ordered Compound Formation

Arvind R. Kalidindi, Christopher A. Schuh Massachusetts Institute of Technology

Alloying can stabilize the nanocrystalline state against grain growth if the solute species energetically prefer to segregate to the grain boundary over forming bulk states, and in some cases it appears possible to stabilize such a structure against phase separation as well. Here, we describe our recent work extending the concept of nanocrystalline stabilization to systems that tend to form ordered compounds in equilibrium. We developed a lattice-based Monte Carlo simulation that is tailored to study the thermodynamic competition between nanostructured states and bulk states, including the opportunity to form intermetallics. The results reveal nanocrystalline states with solute segregated grain boundaries as well as nano-duplex states that simultaneously exhibit ordered compounds and solute segregated grain boundaries at equilibrium, which are presented in a thermodynamic stability map for selecting alloying systems with stable nanocrystalline states.

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Ti Microstructure development of a copper-alumina composite from decomposition of mixed oxide CuAlO2

Authors: M. Kracum, Z. Yu, M. P. Harmer, H. M. Chan

Lehigh University

The copper aluminum oxide, CuAlO2, has been a topic of interest for its electrical properties, as well as its potential as an oxygen storage material for fuel cells. The authors have previously shown that under reducing conditions, the CuAlO2 will decompose into an Al2O3 polymorph and metallic copper with a novel composite microstructure related to the original CuAlO2 microstructure. By adjusting the heat treatment temperature and oxygen partial pressure, both the reaction and diffusion kinetics can be adjusted and will produce microstructures with different features and scales. The expressed goal is to develop a suitable heat treatment schedule which will allow the production of alumina/copper ceramic-matrix composites with selected microstructural features (e.g. copper in the form of nanoparticles or as microscale lamella). Coupled with the microstructural examination, aberration-corrected STEM will be used to examine the decomposition at the reaction front. This should elucidate some details of the transformation itself.

High-Entropy Grain Boundary Complexions: A New Pathway to Stabilize Nanocrystalline Alloys

Naixie Zhou (presenter), Tao Hu, Jiajia Huang, Jian Luo

Department of NanoEngineering, University of California San Diego, La Jolla, CA 92122

Nanocrystalline metals and alloys may exhibit many desirable mechanical properties. However, normal and abnormal grain growth often prevents nanoalloys to be used at high or even moderate temperatures. Both theoretical models and experimental results of binary (and selected ternary) alloys showed that the impurity segregation (adsorption) at the grain boundaries (GBs) could suppress grain growth. However, the stabilization effects are still somewhat unsatisfactory in most cases. In this work, we further explore the feasibility of utilizing “high-entropy GB complexions” to stabilize nanoalloys both thermodynamically and kinetically. In the modeling thrust, we extend a Wynblatt-Chatain type lattice model to understand GB adsorption (segregation) in multicomponent alloys. We have conducted theoretical analyses as well as systematic numerical experiments to determine the conditions for thermodynamic stabilization of nanoalloys via forming high-entropy GB complexions. Guided by the theories and modeling results, we have designed several Ni-based multicomponent nanoalloys with exceptional high-temperature stabilities and subsequently validated the predictions via experiments.

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Simulation of fractal abnormal grain growth in nanocrystalline materials

Mingyan Wang,1 Jules Dake,1 Rainer Birringer2 and Carl Krill1 Institute of Micro and Nanomaterials, Ulm University, Germany FR 7.3 Technical Physics, University of the Saarland, Germany

Despite an implicit rarity in its name, abnormal grain growth (AGG) appears to be a common mode of coarsening in nanocrystalline materials regardless of the specimen’s composition or synthesis route. During AGG, a subpopulation of grains manifests rapid growth, leading to grain volumes that not only are much larger than those of their neighbors, but also are sometimes highly irregular in shape. The nature of this irregularity can be described by the geometric concept of fractals. This fractal morphology suggests that, in certain cases, AGG might proceed by some kind of percolation process along the “grid” defined by the initial ensemble of grains. We have investigated this possibility by extending a phase field algorithm for simulating grain growth to include selection rules for percolation. For properly chosen parameter values, the abnormal grains generated by simulation can be strikingly similar in shape to their experimental counterparts. Quantifying the comparison between simulation and experiment by fractal dimensionality, we hope to gain insight into at least one of the underlying physical mechanisms behind AGG in nanocrystalline materials.

Effect of Microstructure and Impurity Contamination on the Fracture Behavior of Nanocrystalline Ni-W Alloys.

Denise Yin, Wanjun Cao, Chris Marvel, Yuanyao Zhang, Patrick Cantwell, Martin Harmer, Jian

Luo, Richard Vinci Lehigh University

Nanocrystalline Ni - 23 at.% W films were produced by electrodeposition in an aqueous sodium citrate bath using the reverse pulse plating technique. Samples were annealed in an Ar - 5% H2 atmosphere at 300°C and 700°C for four hours. The samples were ion-beam machined and subjected to in-situ micro-scale cantilever beam testing as well as nanoindentation hardness testing. The fracture surfaces were inspected with SEM, and the microstructures were characterized with aberration-corrected STEM - HAADF imaging. The as-deposited and 300°C samples were amorphous in nature and contained oxide streaks that were located on the interfaces of columnar domains. When tested, fracture took on the path of the domain interfaces, and it is expected to have been influenced by the oxide streaks. The 700°C sample was fully crystalline and produced an increase in fracture toughness, which can be attributed to the development of the crystalline structure. The varied response in fracture demonstrate that mesoscale features and impurities have an effect on the mechanical behavior.

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International Workshop on Interfaces at Bear Creek

POSTER SESSION II

29

Modeling Abnormal Grain Growth with Grain Boundary Complexion Transitions"

William Frazier Carnegie Mellon University

The Potts Model of grain growth was adapted for the purpose of simulating abnormal grain growth (AGG) resulting from grain boundary complexion transitions. The transition in grain boundary structure between specific complexion types results in changes in properties. Where the transitions decrease energy and increase the mobility of boundaries, AGG occurs provided that such transitions predominantly occur via propagation to adjacent boundaries. AGG was also found to occur provided that transitions occur after a preset amount of grain boundary motion. The effect of transitions to a high mobility complexion was investigated separately from the effect of changes in energy. The influence of the processes causing these complexion transitions on the occurrence of AGG was explored. The simulations show how the AGG observed in certain ceramic systems can occur. Possible experiments to discriminate the processes causing complexion transitions in such materials are discussed.

Dopant Effects on Thermal Grain Boundary Resistance in SiC: MD Simulations

Nipun Goel (presenter), Edmund B. Webb III, Alparslan Oztekin, Jeffrey Rickman, Sudhakar Neti Lehigh University

Silicon Carbide is a candidate material for high temperature microelectronic applications; as such, it is attractive to consider its use in thermoelectric devices for harvesting waste. However, for SiC to be a viable thermo-electric material for high temperature applications, its thermoelectric figure of merit ZT = S2σT/κ must be improved significantly. It is customary to include absolute temperature T on both sides of the figure of merit expression; S is Seebeck coefficient, σ is electrical conductivity, and κ is thermal conductivity. A strategy that has been extensively explored for improving ZT in materials is to reduce κ while not affecting σ by introducing phonon scattering features in the material structure that have minimal effect on electron transport. Thermal boundary, or Kapitza, resistance at microstructure grain boundaries is one mechanism advanced as being able to hamper phonon transport but not electron transport, partly because the mean free path of phonons in many materials is significantly larger than that of electrons. However, it has been advanced that the Kapitza resistance revealed by, e.g., Molecular Dynamics (MD) simulations in different materials is too low to account for certain experimental observations of thermal conductivity dependence on grain size. Thus, grain boundary impurities have been described as possibly being able to increase Kapitza resistance. This phenomenon is explored here via non-equilibrium MD simulations of β-Silicon Carbide (3C-SiC) to investigate the effect of doping at the grain boundaries on the thermal transport across the grain boundaries at 1000K. Symmetric tilt grain boundaries with a tilt about the [111] plane are explored and the effect of tilt angle (i.e. the degree of misorientation) is explored. For each GB modeled, the role of dopant concentration at the boundary, dopant mass, and dopant/matrix interaction strength are explored. Generally, Kapitza resistance is observed to increase with increasing dopant concentration. At higher dopant/matrix interaction strength, there exists a tendency for dopant atoms to form more layered structures, resulting in less significant increases in Kapitza resistance. At low interaction strength, clustered dopant structures form and, for the highest concentration clustered structures, Kapitza resistance is observed to increase by more than a factor of 50. Our talk will conclude with an outline discussion of our efforts to incorporate atomics scale results into a microstructure informed continuum thermal transport model capable of capturing the effects of varying grain size distribution and varying Kapitza resistance on cumulative thermal conductivity.

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The Material Points Monte Carlo Method

Authors: Philip E. Goins, Elizabeth A. Holm Carnegie Mellon University

In recent years, the Monte Carlo Potts Model for grain growth has been falling out of favor due to lattice artifacts that occur from the symmetry of the underlying simulation grid. A new Monte Carlo simulation for microstructural evolution is proposed, which eliminates the impact of these artifacts. Such a method involves use of a non-repeating, time-evolving set of simulation points. This method has been shown to overcome the lattice artifacts found in the conventional Potts model. The Material Point Monte Carlo method is shown to yield the correct curvature-driven growth found in interfacial networks, and is more extensible with new material physics. For instance, simulations of systems with volumetric energy, anisotropic boundary properties, system deformations, particle pinning, and solute-grain boundary interaction are all possible with the MPMC in three dimensions.

Temperature dependence of the relative grain boundary energy in yttria-doped alumina

Madeleine Kelly Carnegie Mellon University

The ratio of grain boundary energy to surface energy of 100ppm and 500ppm yittria-doped alumina was measured from 1350°C-1650°C using the grain boundary thermal grooving technique. A complexion transition had been previously identified in this temperature range. The 100ppm yttria-doped sample exhibited an increase in energy with temperature from 1350°C-1450°C. In the 1450°C-1550°C region, AFM imaging showed the presence of abnormal grains and the energy decreased. The maximum energy observed was when the decreasing trend began, suggesting the transition from a metastable state to equilibrium. As temperature was increased further, increasing energy resumed. The 500ppm yttria-doped sample exhibited an overall decreasing energy from 1350°C-1550°C. Between 1550°C-1650°C an increase in energy occurred with increasing temperature. The changes in relative grain boundary energy as a function of temperature are discussed in relation to the temperature dependence of segregation, grain boundary complexion transitions, and the precipitation of excess yttria as yttrium aluminum garnet.

31

Molecular Dynamics Simulations of Self-pinning in Nanosuspension Droplets

Authors: Baiou Shi (poster presenter), Edmund B. Webb III Lehigh University

The behavior of nano-fluids, or fluid suspensions containing nanoparticles, has garnered tremendous attention recently for applications in advanced manufacturing. Contact line pinning by the particles, or self-pinning, has been extensively considered during contact line retreat due to solvent evaporation. Here we will present results from MD simulations of self-pinning for an advancing contact line. For a wetting system of identical liquid, solid and particle chemistry yet significant difference in advancing contact angles 𝜃𝜃𝑎𝑎𝑎𝑎𝑎𝑎, self-pinning is observed for low 𝜃𝜃𝑎𝑎𝑎𝑎𝑎𝑎 whereas it is not for high 𝜃𝜃𝑎𝑎𝑎𝑎𝑎𝑎. The role of contact angle in determining likelihood for self-pinning is investigated on fundamental time and length scales. Relations between contact line velocity, advancing contact angle, and time dependent forces experienced by particles entrained to the contact line will be discussed from atomic scale computation results. For a pinned case, a precursor film continues to advance across the surface even after pinning halts contact line advance. However a single layer of liquid on the outer facet of the particle surfaces is observed, which manifests a rate-limiting step for the precursor film advance.

Grain Boundary Complexion Transition Affecting Oxidation Behavior in Hafnia doped Alumina

Yan Wang, Zhiyang Yu, Qian Wu, Helen M. Chan, Jeffrey M. Rickman, Martin P. Harmer

Department of Materials Science and Engineering, Lehigh University It is well known that Hf additions enhance the oxidation resistance of high temperature alloys that are alumina formers. This work investigates the temperature dependence of oxygen grain boundary transport kinetics in Hf doped polycrystalline alumina by using Ni marker particles. At temperatures above 1250oC, 500ppm Hf doping retards the oxygen diffusion kinetics by a factor of ~ 8, whereas the factor increases to ~ 40 below 1150oC. In the temperature between 1150oC and 1250oC, we identify a discontinuity in the Arrhenius plot of the rate constant (k), strongly indicating the presence of a grain boundary complexion transition. The existence of the complexion transition is confirmed by atomic resolution imaging of the grain boundary structures of the corresponding high and low temperature samples.

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Thank you for attending the

International Workshop on Interfaces at Bear Creek.

We look forward to seeing you again.