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Understanding Atomic Scale Structure in Four Dimensions to Design and Control Corrosion Resistant Alloys

Location, University of Virginia, Department of Materials Science and Engineering Thursday, June 30, 2016, 8:00 AM to 5:15 PM (EDT) Mechanical Engineering Building, Room 205 Friday, July 1, 2016, 8:00 AM to 12:20 PM (EDT) Mechanical Engineering Building, Room 205 PROGRAM Thursday, June 30th, Mechanical Engineering Building, Room 205 8:00 Registration 8:45 Overview of MURI Program, L. D. Marks, Northwestern University 9:15 Oxidation resistant Mo-Si-B and Ni-Cr-Al Alloys-Phase Structures and Evolution, J. Perepezko, University of Wisconsin-Madison 10:00 Exploring the Electrochemical Passivation of Ni-Cr Compared to Ni-Cr-Mo Alloys: Interfaces between Corrosion Science, Surface Science and First Principles Based Modelling, J. Scully, University of Virginia 10:45 Coffee 11:00 Towards an Integrated Multiscale Model of Oxidation, P. W. Voorhees, Northwestern University 11:30 Imaging Early Transient Oxidation L. D. Marks, Northwestern University 12:00 Lunch in Wilsdorf café 1:00 Accurate Thermodynamics and Electrochemistry from First Principles, J. M. Rondinelli, Northwestern University 1:40 Nanoscale Modeling of Ni-Cr-Al and Mo-Si Oxidation and Interfacial Properties, H. Heinz, University of Colorado, Boulder

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2:20 Investigating the Early Stages of Corrosion with Scanning Probe Microscopy and Spectroscopy, P. Reinke, University of Virginia 3:00 Coffee & Poster Session in Wilsdorf Hall atrium & café 5:00 Adjourn 6:00 Dinner at Vivace Restaurant and Lounge, 2244 Ivy Road Friday, July 1st, Mechanical Engineering Building, Room 205 8:00 Coffee and Breakfast 8:30 Transient Oxidation, J. Perepezko, University of Wisconsin-Madison 9:10 3D Characterization of Oxidized Mo-Si-B, Ni-Cr and Ni-Cr-Mo Systems with Elemental Specificity, J. Miao, UCLA 9:50 Homogeneity and Inhomogeneity in Early Transient Oxidation, L. D. Marks, Northwestern University 10:30 Coffee Break 10:45 Mesoscale Modeling of Oxide Growth, P. W. Voorhees, Northwestern University 11:25 Understanding the Electrochemical Passivation of Ni-Cr and Ni-Cr-Mo Alloys – Opportunities Facilitated by Operando Experimental Characterizations Combined with Multi-Scale Computational Modeling , J. Scully, University of Virginia 12:05 Summary, L. D. Marks, Northwestern University 12:20 Adjourn Posters The Surface Kinetics of the Initial Stages of Cu Oxidation Judith C. Yang University of Pittsburgh Oxidation of MoSi2 observed at the Nanoscale C. Volders and P. Reinke University of Virginia

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Atomic and Mesoscale Insights into the Oxidation of NiCr and NiCrMo Alloys – A Scanning Tunneling Microscopy & Spectroscopy Study G. Ramalingam, E. Monazami and P. Reinke University of Virginia Predicting and Controlling the Role of Minor Elements in the Passivation and Local Dissolution of Ni-Cr-based Alloys K. Lutton, K. Gusieva and J. R. Scully University of Virginia Connection between Atomic Scale Characterization and Electrochemical Behavior during Passivation of Single Crystals on Ni-Cr and Ni-Cr-Mo Alloys K. Gusieva, G. Ramalingam, K. Lutton, J. R. Scully University of Virginia Experimental Investigation of the Nickel Pourbaix Diagram through Electrochemical and Spectroscopic Techniques M. Hutchison, R. J. Santucci and J. R. Scully University of Virginia Revisiting the Effect of Crystallographic Orientation on the Corrosion of Commercially Pure Mg L. G. Bland, K. Gusieva and J. R. Scully University of Virginia A Phase Field Model for Thin Film Oxide Growth Q. Sherman, P. Reinke, L.D. Marks, J.M. Rondinelli and P.W. Voorhees Northwestern University and University of Virginia The Growth of Oxide Islands during Oxidation R. Ramanathan, G. Ramalingam, P. Reinke, J.H. Perepezko, and P.W. Voorhees

Northwestern University, University of Wisconsin-Madison and University of Virginia Accurate and Efficient First-Principles Calculations of the Thermodynamics and Electrochemistry of Solids L-F Huang and J. M. Rondinelli Northwestern University Energetics of Ni-Al Intermetallic Alloys Point Defects and Strained NiO Vacancies E, Tennessen and J. M. Rondinelli Northwestern University Incommensurate Structures in A15 Mo3Si

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A. Gulec, X. Yu, M. Taylor, J. Perepezko and L. D. Marks Northwestern University and University of Wisconsin-Madison Cabrera-Mott Oxidation: Interfaces, Chemistry and More X. Yu, L. Huang, J. M. Rondinelli and L. D. Marks Northwestern University Sample Design for Early Stage Oxidation Experiments A. Gulec, X. Yu and L. D. Marks Northwestern University Evolution of Oxides with Time and Thickness in MoSiB A. Gulec, X. Yu, M. Taylor, J. Perepezko and L. D. Marks Northwestern University and University of Wisconsin-Madison Imaging the Development of Aqueous Corrosion X. Yu, A. Gulec, J. Scully and L. D. Marks Northwestern University and University of Virginia Pulsed Oxidation Chamber – A Novel Instrument Enabling Short Time High Temperature Oxidation Experiments M. Taylor, E. Zeitchick, J. Perepezko, University of Wisconsin

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Oxidation/Reduction Behavior of the Metal/Metal-oxide observed using Environmental TEM Aram Yoon, Jian-Min Zuo1,2 1Dept of Materials Science and Engineering, University of Illinois, Urbana-Champaign, IL 61801 2Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana-Champaign, IL 61801 Tribocorrosion of 2507 Super Duplex Stainless Steel J. Michael Shockley, Derek Horton, and Kathy Wahl Chemistry Division, Naval Research Laboratory 3D Tomographic Imaging of the Oxidized A15 Phase in the Mo-Si-B System with Elemental Specificity J. Zhou, G. Melinte, M. Taylor, P. Ercius, J. Perepezko, and J. Miao UCLA, University of Wisconsin-Madison, Lawrence Berkeley National Laboratory Atom Probe Tomography Research at NRL K. Knipling Naval Research Laboratory Reactive Modeling of Mo3Si Oxidation and Resulting Silica Morphology C. Dharmawardhana, J. Perepezko, J. Miao, and H. Heinz

University of Colorado, University of Wisconsin-Madison, UCLA MoSi2 Oxidation: Mechanism and Silica Growth C. Dharmawardhana, H. Heinz and P. Reinke University of Colorado and University of Virginia Surface Diffusion of Oxygen Species on Ni, Al, and NiCr Alloy Surfaces K. Kanhaiya, N. Mehio, H. Heinz, R. Ramanathan, Q. Sherman and P. W. Voorhees

University of Colorado and Northwestern University Electronically Refined Force Fields for BCC and HCP Metals C. Dharmawardhana, S. Deshmukh and H. Heinz University of Colorado Electronegativity Concepts and Atomic Charge Differences in Alloys to Understand Alloy Formation and Defect Energies N. Saikia, E. Tennessen, J. M. Rondinelli and H. Heinz

University of Colorado and Northwestern University

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Presentation Abstracts

Overview of MURI Program L. D. Marks, Northwestern University The critical importance of corrosion has been known for centuries as have ways to try and reduce it. However, much of our current knowledge is the phenomena from the mesoscale down to several nanometers. In other areas there has been an explosion of tools to image materials at the atomic scale, as well as accurately calculate their behavior. For instance, not only can single atoms be imaged, their chemical state can be measured. Modern ab-initio methods such as density functional theory are now starting to be able to handle materials such as transition metal oxides, where older functionals can go catastrophically wrong. The focus of this MURI project is to bring the full power of these new tools to bear on corrosion at the atomic scale to unravel how the nanoscale structure influences the properties in 4D. This talk will provide a general introduction to the program, with more details provided in the later presentations. Oxidation resistant Mo-Si-B and Ni-Cr-Al Alloys-Phase Structures and Evolution, J. Perepezko, University of Wisconsin-Madison The oxidation of Ni-Al-Cr alloys that are the basis of many superalloys is dependent upon composition (i.e Al/Cr ratio) and often yields a multilayer oxide microstructure with alumina, chromia and spinel phases. The evolution of the oxide structure is controlled by competitive nucleation and growth kinetics since the thermodynamically most stable oxide is alumina. To elucidate the initial stage of the kinetic competition an oxide nucleation map has been developed based upon the relative driving free energies of the competing oxide phases for comparison to the final steady state oxide structures. A kinetic model analysis for the oxide nucleation and growth is available. In companion experimental examinations oxide nucleation is being measured with a pulse oxidation technique for comparison to the calculated oxide nucleation map. With STM studies the evolution of surface oxide development has been examined on a Ni14Cr alloy from step edges to terraces to full layers. In addition, TEM studies on a representative Ni67.5Cr22.5Al10 alloy reveals that at low Po2 exposure NiO is favored initially at low temperature, but competes with Cr2O3 + spinel at higher temperatures before alumina formation. This reaction sequence is opposite to that expected based upon thermodynamic driving free energies and demonstrates the importance of kinetics in the phase selection. The challenges of a high temperature environment impose severe material performance constraints in terms of melting point, oxidation resistance and structural functionality. In metallic systems the multiphase microstructures that can be developed in the Mo-Si-B system based upon the coexistence of the high melting temperature (>2100°C) ternary intermetallic Mo5SiB2 (T2) phase with Mo and the Mo3Si phases offer useful options for high temperature applications.. The early stage of oxidation behavior of Mo-based alloys has been studied at the microstructure scale where a transient period was identified involving the Mo3Si phase before the onset of steady state oxidation. The evolution of the oxide structure is being quantified by multidimensional electron microscopy and MD

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simulations. The development of oxide phases on other silicides such as Mo5SiB2 and MoSi2 has also been established by TEM and STM/STS/XPS studies. Exploring the Electrochemical Passivation of Ni-Cr Compared to Ni-Cr-Mo Alloys: Interfaces between Corrosion Science, Surface Science and First Principles Based Modelling J. Scully, University of Virginia The roles of Cr and Mo in the passivation of Ni-based alloys are not yet understood at an atomistic scale. It has been established that the addition of Mo to the alloys increases the pitting resistance of the material and it has additionally been theorized that this occurs due to either Mo enabling repassivation once break down occurs or improved resistance to processes in the oxide that lead to breakdown such as cation vacancy transport rates and formation of cation vacancy clusters. The role of Mo in the passive film has been largely neglected despite early proposals on its role by luminaries such as by Macdonald, Clayton, Marcus and others. However, the proposed mechanisms of Mo and Mo-Cr synergy were often made at a length scale that defied confirmation. Therefore, this aspect of passivation has been stuck until the advance in techniques has enabled further progress. The talk begins with this background and review of the high impact of minor elements on corrosion in the Ni-Cr solid solution system. A more accurate and atom-based explanation for these behaviors requires a multi-faceted and operando approach that combines corrosion science, surface science, and first principles based modeling. The grouping of these techniques allows the tracking of the evolution of metal, oxide, and solution interfaces and the fate of alloying elements controlling the corrosion with spatial resolution over multiple length scales. Additionally, the controlling unit processes can be identified in the growth of passive films and their eventual breakdown in order to link nanoscale information with the macroscale corrosion properties. This approach is enabled by the MURI team. The surfaces of electrochemically passivated NiCr, NiCrMo, and NiCrMoW solid solution alloys have been studied using in-situ or operando ex-situ AFM, STM, XPS, APT and TEM techniques with the MURI team generating one of the first corrosion studies well connected to the nano-scale using multiple probes to study corrosion in “4-D.” The information obtained will enable a better understanding of the role of major and minor alloying elements and ultimately guide scientific evolution of alloy optimization replacing trial and error approaches lacking fundamental insights. Towards an Integrated Multiscale Model of Oxidation P. W. Voorhees, Northwestern University Computational and theoretical efforts in understanding corrosion processes at length scales from the atomic to the mesoscale will be explored. The talk will highlight recent results using multiscale approaches, including electronic structure calculations based on density functional theory (DFT), molecular dynamics simulations, and phase field simulations as well as the essential integration with experiment to both identify new

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phenomena and to verify the simulations. More specifically, we shall discuss: DFT predictions of Pourbaix diagrams, an accurate methodology to compute the thermodynamic properties of oxides, the formation of atomically thin films of NiO, MD simulations of oxygen diffusion and adsorption on alloy surfaces as a function of composition, surface orientation and temperature, reactive MD simulations of oxidation reactions, a mesoscale model of oxide island growth, and a phase field method for oxide growth for films with thicknesses less than the Debye length. By combining these results and methods, along with insights and measurements from experiments, accurate models of oxide growth at the earliest stages of growth will be developed. Imaging Early Transient Oxidation L. D. Marks, Northwestern University A core component of the MURI project is to exploit a range of different experimental tools to better understand the early stages of oxidation and aqueous corrosion. In this talk I will over view some of our progress with these, ranging from aberration-corrected microscopies for structural details through three dimensional imaging via APT or electron tomography to new strategies under development for aqueous corrosion using methods such as SERS or in-situ electrochemical tests within an electron microscope. Accurate Thermodynamics and Electrochemistry from First Principles J. M. Rondinelli, Northwestern University Accurate thermodynamic quantities and electrochemical Pourbaix diagrams are critical to understanding oxidation and corrosion processes. Building on our previously implemented computational workflow to construct Pourbaix diagrams [Phys. Rev. B 92, 245126 (2015)], I discuss our recent progress in obtaining an accurate Ni electrochemical phase diagram. Remarkably, a reliable Pourbaix diagram is still lacking, despite Ni (and Ni-based materials) being of considerable importance in modern society. I show that results obtained from density functionals that include exact-exchange agree with recent experiments performed by the Scully group at UVA and improve on those published diagrams obtained using inaccurate experimental formation free energies of Ni compounds derived from high-temperature heats of combustion or low-temperature solubilities. I also demonstrate how probability profiles may be used as an analytical tool to understand phase competition and coexistence in aqueous environments. Next, I describe our adaptation of the workflow to enable high-throughput calculations, showcasing the simulation of 3d transition metal Pourbaix diagrams which facilitates the study of binary alloys, e.g., Ni-Cr alloys and relevant metal oxides and hydroxides appearing under aqueous conditions. Finally, I conclude by describing our plans to incorporate defective structures into phase equilibria diagrams with the aim of improving our understanding and prediction of oxidation and corrosion at the atomic scale, including (i) substitutional molybdenum in Ni-Cr, (ii) point defects in NiO and Cr2O3, and (iii) boron-doping in Mo-Si alloys.

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Nanoscale Modeling of Ni-Cr-Al and Mo-Si Oxidation and Interfacial Properties H. Heinz, University of Colorado, Boulder We show how charge separation in alloys due to electronegativity differences between the constituting metals is essential to understand alloy formation energies, defects, and the reactivity in oxidation and aqueous corrosion reactions. Initial results for defect formation in NiAl and Ni3Al alloys, consistent with DFT, reveals a new paradigm for understanding alloy formation and defect energies on the nanoscale. This new chemical concept is implemented in atomistic models to evaluate the course of oxidation and corrosion reactions up to the 100 nm scale in comparison to measurements. We studied oxygen diffusion on Ni, Cr, and alloy surfaces and report first modeling results for the oxidation of Ni, Cr, and NiCr alloys in comparison to measurements. We also carried out reactive molecular dynamics to study the oxidation of Mo3Si and MoSi2 alloys and obtained nanoscale morphologies of silica, including an analysis of pore volume, length, and connectivity. Results are consistent with experimental observations on thin films. Accurate models for alloys that include refinements of electronic structure features (d electrons), ions in aqueous solution, and deposition of salts have been carried out that are now initially being applied to study interfaces between alloy, oxide, and aqueous solution during corrosion at different pH and ionic strength. Investigating the Early Stages of Corrosion with Scanning Probe Microscopy and Spectroscopy P. Reinke, University of Virginia The initial stages of alloy oxidation are critical to the development of the protective oxide layer. Remarkably our current understanding of the early steps in oxidation which play out at the transition from clean alloy surface to the Mott-Cabrera regime, is limited. Our work focuses on unraveling the fundamental reaction steps in the oxidation by using scanning probe microscopy and spectroscopy, which affords a nanoscale view of the surface. These are highly controlled experiments, which interface directly with modeling and simulations. The current focus is on two materials systems: NiCr and NiCr(Mo) alloys, and Mo-silicides. The oxidation of NiCr/NiCr(Mo) alloys depends strongly on the Cr content and we target the critical regime between 5-35at%Cr. The bottleneck in chromia nucleation and growth is not surface diffusions or O2 dissociation but the rate of supplying Cr from the bulk to the surface nucleus. We will present a process-model, which integrates experimental and modeling/simulation results, to describe the nucleation, and growth of oxide islands as a function of Cr concentration and temperature. This work also points to a pathway for the promotion of chromia growth and suppression of NiO formation. In addition, we will discuss data on the role of Mo as a minor alloying element in the oxide nucleation and growth. Specifically, we aim to unravel the controversy whether Mo impacts the initial oxidation process, or rather contributes only later on to the "repair" of an oxide scale. The second materials system, Mo-silicides, is well established as a technical alloy, but the formation of voids in the protective SiO2 layer, which are attributed to the sublimation

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of MoO3, remains a challenge. The surfaces and surface reactions of Mo-silicides have not been studies in any depth at the nanoscale, which allowed us to break new ground. We will discuss the synthesis of MoSi2 crystallites in-situ and the discovery of a highly coveted silicene-like surface reconstructions. Silicene is a graphene equivalent where C atoms are substituted by Si atoms. The oxidation studies for MoSi2 illustrate the extreme temperature dependence of the process, albeit the surface reactions do not proceed by a nucleation-growth process as seen for the metal alloys but seem to progress much more rapidly once started. This is somewhat reminiscent of a self-catalyzing process - once oxidation has started it progresses very rapidly and the kinetic limitations play a significant role. Recent results also strongly suggest that the Mo-oxides participate in a complex surface chemistry, which is not described by a simple sublimation process. Transient Oxidation J. Perepezko, University of Wisconsin-Madison To elucidate the transient period mechanism a nanometer scale investigation of the initial oxidation was conducted based upon direct observation by means of in-situ environmental TEM. A nanometer size porous SiO2 develops at very early stage of the oxidation of Mo3Si between 600-800°C and a Po2 = 2x10-3 Pa as a competition between loss of volatile MoO3 and the growth of SiO2. For longer exposures the porous silica evolves into a continuous layer covering a lamellar Mo/SiO2 structure. With Mo5SiB2 the initial oxide structure develops as a continuous silica layer (with B) covering an intermediate Mo oxide layer. In order to examine the early nucleation stage a pulse oxidation facility has been constructed that allows for accurate and reproducible control of oxidation exposures. As a test of the pulse oxidation facility the initial efforts are focused on the oxidation of Ni. Sample preparation procedures have been optimized to achieve large grain sizes that allow for the examination of the orientation dependence of oxide nucleation and clean surfaces. An image analysis protocol s been implemented to permit the quantification of the oxide island size distribution. By monitoring the evolution of the oxide island size distribution with exposure time we have developed an analytical kinetics analysis that will yield nucleation rates and elucidate the growth and coarsening behavior. 3D Characterization of Oxidized Mo-Si-B, Ni-Cr and Ni-Cr-Mo Systems with Elemental Specificity J. Miao, UCLA

Atomic electron tomography (AET) has been applied to characterize the 3D structure of oxidized A15 (Mo3Si) phase at various experimental conditions (1,2). By acquiring tomographic tilt series from oxidized A15 samples prepared by Dr. Perepezko’s group, we achieved 3D reconstructions of the sample as a function of the time and temperature, which allow us to quantitatively study the effects of time and temperature on resultant pore size. We have also used electron dispersive spectroscopy (EDS) to measure the Mo, Si, and O distribution inside oxidized A15 samples. The combination of high-resolution electron tomography and EDS enabled us to observe the formation of MoO2

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islands in porous silica during the initial stage of oxidation of the A15 phase. Furthermore, we have also performed structure study of the interface between porous silica and A15 phase with elemental specificity. We believe that our results may open a door to understand the mechanism of the pore formation in oxidized A15 phase, which would ultimately help to achieve robust oxidation resistance.

We have also conducted preliminary AET experiments for 3D structure determination of oxidized Ni-Cr and Ni-Cr-Mo alloys at the atomic scale. Oxidized Ni-Cr and Ni-Cr-Mo alloys were prepared by Dr. Scully’s group and then fabricated to needle-shaped specimens by Dr. Marks’ group, allowing the acquisition of tomographic tilt series. The AET experiment was performed on the TEAM I microscope at the Lawrence Berkeley National Laboratory. In this talk, I will present some preliminary results on AET studies of oxidized Ni-Cr and Ni-Cr-Mo alloys. Our goal is to determine the 3D positions of individual atoms in Ni-Cr and Ni-Cr-Mo alloys and to correlate the measured atomic positions with density functional theory calculations.

1. J. Miao, P. Ercius and S. J. L. Billinge, “Atomic electron tomography: 3D structures without crystals”, Science, in press (2016).

2. R. Xu, C.-C. Chen, L. Wu, M. C. Scott, W. Theis, C. Ophus, M. Bartels, Y. Yang, H. Ramezani-Dakhel, M. R. Sawaya, H. Heinz, L. D. Marks, P. Ercius and J. Miao, “Three-Dimensional Coordinates of Individual Atoms in Materials Revealed by Electron Tomography”, Nature Mater. 14, 1099-1103 (2015).

Homogeneity and Inhomogeneity in Early Transient Oxidation L. D. Marks, Northwestern University One of the targets of the MURI program is to understand how the selvedge layer of the metal, that is the region just below the surface, can be used to control corrosion. A picture of the key components is starting to emerge based upon combining very careful sample preparation with aberration-corrected microscopies and theoretical work within the team. This talk will focus on some aspects of this. One of the key enabling technologies has been a way to make samples which can then be used for pulsed oxidation or aqueous corrosion studies with both advanced electron microscopy methods as well as atom probe tomography. Using this as well as theoretical models we have found a number of phenomena, including the role of dipoles at the buried interface, how differential oxidation can lead to a competition of phases and a working hypothesis for the role of molybdenum in reducing aqueous corrosion. Mesoscale Modeling of Oxide Growth P. W. Voorhees, Northwestern University Two mesoscale simulations of oxide formation and growth are being developed. The first is a model for the growth of 2D or 3D oxide islands on a surface. The objective is a predictive model for the nucleation and growth of oxide particles, and the evolution of the oxide island size distribution. The challenge is to determine the thermophysical properties needed to make the simulations predictive. We are therefore using information from both

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experiment and atomic scale simulation. The model employs the diffusion coefficients and desorption rates determined from MD simulations. Other parameters, however, are difficult to calculate, such as the oxide interfacial mobility, or require parameters with high degree of accuracy such as interfacial energies. So, we employ experiments to provide these parameters. We find that we can reproduce the growth rate of the average oxide island size seen experimentally, but the simulated island size distribution is different than that measured experimentally. This indicates that there is an important contribution of island coalescence during growth, which is straightforward to include in the simulation in the future. The second approach is a phase field model for the growth of oxides below the Debye length, where the films are no longer charge neutral. We employ Galvani potentials from DFT calculations. The model is based upon a free-electron gas model for the electrons in the metal and anion vacancy diffusion in the oxide (cation diffusion is a simple modification of the code). We find classical Thomas-Fermi screening in the metal at the metal-oxide interface for small oxygen concentrations in the metal. A double layer forms at the oxide-metal interface that gives rise to a dependence of the oxide-metal interfacial energy on the Galvani potential. The phase field model has been benchmarked against results for the growth of oxides in the Wagner limit and the solutions to Poisson’s equation for the potential in certain limits, and the correct equilibrium state of an oxide film on a metal is recovered. We find that the growth of oxides below the Debye thickness does not display the classical parabolic kinetics, and that the growth kinetics depends on the potential, the oxygen reaction rate on the surface, and the mobility of the oxide-metal interface. Understanding the Electrochemical Passivation of Ni-Cr and Ni-Cr-Mo Alloys – Opportunities Facilitated by Operando Experimental Characterizations Combined with Multi-Scale Computational Modeling J. R. Scully, University of Virginia The thermodynamic and kinetics of passivation of Ni-Cr and Ni-Cr-Mo alloys was investigated using operando spectro-electrochemistry experiments coupled with ex-situ characterization methods and computational modeling. Pourbaix E-pH diagrams based on first principles computational modeling were explored with the MURI Team to predict the thermodynamically probable stable species at a given potential and pH. These products from Rondinelli’s group at Northwestern were verified using macroscale electrochemistry coupled with in-situ Raman spectroscopy to identify the chemical species present in different Ni alloys across the E-pH ranges for Ni and in the future, NiCr and NiCrMo, NiCrMoW alloys. Regarding passivation kinetics in the transient stages and steady state regime, spectro-electrochemical techniques such as electrochemical impedance spectroscopy (EIS) have been used to study macro-scale corrosion properties, including the oxide thickness, oxide resistance, and passive dissolution rate among others. Prior work has theorized the role of Cr and Mo in preventing and resisting pitting corrosion at either the oxide passivation and breakdown stage or breakdown/repair stage but the large length scale of typical corrosion techniques prevents an atomistic study from being possible. Therefore the

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proposals for the role of Mo at the atomistic scale in the oxide are unexplored. The revolution in nano-scale techniques, however, can now be used in combination with operando spectro-electrochemistry to examine the nanoscale growth of passive oxide films and the time-dependent impact of the minor alloying elements like Mo as well as the effects of Mo and Cr. Electrochemically measured oxide thicknesses was validated and connected to atom scale surface science using nano-scale sensitive techniques such as APT AFM, STM, STEM, XPS, at every decade of time (e.g. 1-10,000 seconds) following potentiostatic growth in the passive range in order to characterize nanoscale properties such as the bandgap, topography, oxide identity and physical characteristics, chemical species dissolved, and distribution of elements across the metal, oxide, and at their interface. Such data can also be analyzed in the context of established models for aqueous passive film growth to identify a controlling mechanism. Macdonald’s Point Defect Model and the Cabrera-Mott Model and their expected trends for potentiostatic and galvanostatic experiments were used to test applicability to our alloy system. The discovery of a controlling method for the oxide growth for either or both NiCr and NiCrMo would assist a nano-scale analysis of the role of the minor alloying elements on the growth and breakdown of the local passive film. This understanding is furthered by operando surface analysis throughout the passivation period and analysis of the time-dependent concentration of oxide species in the film. Pioneering connections have been achieved between meso-scale spectro-electrochemical and surface science measurements and computational modeling has begun to examine possible role of minor elements in unit processes in the passive film.

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Poster Abstracts The Surface Kinetics of the Initial Stages of Cu Oxidation Judith C. Yang University of Pittsburgh The transient stages of oxidation from the nucleation of the metal oxide to the formation of the thermodynamically stable oxide represent a scientifically challenging and technologically important terra incognito. As engineered materials approach the nanometer regime, control of their environmental stability at this scale becomes crucial. As environmental stability is an essential property of most engineered materials, many oxidation theories exist to explain its mechanisms. However, most classical oxidation theories assume a uniform growing film, where structural changes are not considered due to the lack of traditional experimental procedure to visualize this non-uniform growth under conditions that allow highly controlled surfaces and impurities. Yet, in situ transmission electron microscopy studies reveal that the initial stages of Cu oxidation are due to surface diffusion of oxygen followed by nucleation and growth of oxide islands, and thereby challenge the common assumption of a uniform oxide formation. Understanding this initial oxidation of the metal surface, from the atomic to mesoscale, using in situ electron microscopy is the focus of this work Oxidation of MoSi2 observed at the Nanoscale C. Volders and P. Reinke University of Virginia We use a surface science approach to study the early stages of oxidation for molybdenum silicides. The molybdenum-silicon system has garnered much attention due to the various intermetallic phases as well as composites which exhibit high temperature oxidation resistance. The current approach has attacked this complex system with the use of Scanning Tunneling Microscopy (STM) and Scanning Tunneling Spectroscopy (STS) to probe the surface morphology and electronic structure. MoSi2 crystallites and thin films are synthesized via annealing of Mo thin films which were deposited on Si (001) (2x1) surfaces. These are subsequently exposed to increasing amounts of oxygen at temperatures between 500 and 800ºC, which is the critical regime for the onset of MoO3 sublimation. The reaction of the MoSi2 surfaces accelerates significantly once the temperature exceeds 650ºC and leads to the formation of nanosized SiO2 nuclei. In contrast to the oxidation of alloy surfaces such as NiCr the oxidation progresses very rapidly, and does not appear to follow the conventional island growth and coalescence process. The Si-terminated h-MoSi2 surface shows the highest resistance to oxidation compared to all other facets. It presents a silicene-like reconstruction, which is of interest in the context of new synthesis pathways for 2D materials. Smaller crystallites appear to

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react much faster than larger ones and show significant shrinkage during the oxidation, which is commensurate with expectations of higher vapor pressures from thermodynamic considerations although we have not yet unambiguously confirmed this observation. The sublimation of MoO3 is likely accompanied by localized re-deposition, which is interesting observation that affects the overall oxidation of the surface structures. Atomic and Mesoscale Insights into the Oxidation of NiCr and NiCrMo Alloys – A Scanning Tunneling Microscopy & Spectroscopy Study G. Ramalingam, E. Monazami and P. Reinke University of Virginia We have used scanning tunneling microscopy and spectroscopy to unravel the initial reaction steps of oxidation as a function of Cr and Mo composition progressing from a clean Ni surface to a Ni-base alloy with 33 wt.% Cr including alloys with 10 wt.% Cr and 6 wt.% Mo. We demonstrate the use of spectroscopy maps to obtain spatially resolved electronic structure information and the distribution of oxides on the surface. We observe that the amount of oxygen required for complete oxide coverage strongly depends on the Cr content: after 30 L oxidation at 300 °C, 45% of the surface is covered with oxide in Ni-14Cr alloy while only 28% of the surface is oxidized in Ni-5Cr alloy, and >75% of a Ni-33Cr surface is oxidized after only 11.5 L while a continuous oxide is not observed on Ni even after 180 L oxidation. Select images show atomic resolution of the alloy and oxide surfaces. Complementary STS maps capture the evolution of the electronic structure and reflect the bandgap evolution in ultrathin layer. The detailed island distributions (height, area, volume, bandgap) directly interface with phase field, and molecular dynamics simulations. This work provides insights into the early stages of oxidation of NiCr and NiCrMo alloys at the nano- and meso-scale and also serves as input for theoretical modeling to capture the temporal-scale evolution of alloy oxidation. Note: [L] = [Langmuir] is equal to exposure for 1s at 10-6 torr of gas. If all atoms stick to the surface this corresponds approximately to a single layer of atoms. Predicting and Controlling the Role of Minor Elements in the Passivation and Local Dissolution of Ni-Cr-based Alloys K. Lutton, K. Gusieva and J. R. Scully University of Virginia In Fe-Cr based steels, the effects of low concentrations of alloying elements such as N, Mo, and W are somewhat understood with respect to corrosion and passivation. Such alloying often achieves significant improvements in corrosion resistance [1]. However, the roles of alloying elements in a Ni-Cr system are much less well understood. Upon adding small to intermediate concentrations of Mo to Ni-Cr, the corrosion resistance improves dramatically [2]. The Pitting Resistance Equivalent Number can forecast corrosion resistance based on major and minor alloying element concentrations but is empirical and does not provide predictive capabilities nor scientific insights [4]. The goal of this study is to identify the mechanism(s) by which alloying elements improve aqueous passivation in

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the Ni-Cr-Mo system at the atomic scale and to investigate the oxide properties of the protective passive film. The alloys under study are Ni-11Cr, Ni-11Cr-6Mo, Ni-22Cr, Ni-22Cr-6Mo, and Ni-22Cr-6Mo-3W, wt%, polycrystalline solid solutions. Previous studies have postulated the influence of different elements on passivity in acidic and high [Cl-] environments [3,5]. Cr has been shown to provide passivity during repassivation and enhance resistance to corrosion in oxidizing acids while Mo enables repassivation during oxide film breakdown and increases the resistance to reducing acids [3]. Electrochemical impedance spectroscopy (EIS) studies have been used to analyze the passive film for a Ni-22Cr-13Mo-3W-3Fe-2.5Co, wt%, alloy in varying environments [4]. This presentation explores DC and AC electrochemical techniques for exploring controlled growth of passive oxide films on Ni-Cr alloys. Potentiostatic and galvanostatic holds were used in aqueous chloride and chloride-free solutions of increasing acidity. The data was analyzed using the Cabrera-Mott (C-M) and Point Defect (PDM) models in order to investigate their applicability to the case of Ni-Cr oxide growth with varying concentrations of minor alloying elements [6,7]. A major challenge to the application of either C-M or PDM is that the film thickening rate must be assessed independently from the total oxidation rate. Inductively Coupled Plasma-Optical Emission Spectroscopy indicates that a significant portion of the applied anodic charge during oxidation is associated with direct cation ejection and/or dissolution of the oxide into solution rather than thickening of the film. Therefore, the oxide growth rate and charge must be extracted from the total current for C-M and PDM analysis. EIS techniques were used to characterize and model the electrochemically grown oxides, represented as a constant phase element, to enable the calculation of oxide thickness. The impedance data following potentiostatic oxide growth was compared to the data produced from single frequency EIS where the impedance was continuously recorded in order to extract the variation of oxide charge and thickness over time. The oxide thicknesses calculated using these methods was related to the charge that goes toward oxide film growth. By comparing these values to the potentiostatic or galvanostatic total charge, the portion involved in dissolution was removed for C-M and PDM analysis. Subsequently, diagnostics were used to test the applicability of C-M and PDM models. This approach was systematically used over various alloys, pH levels and Cl- vs. sulfate solutions. Findings are reported herein. Ex situ Raman and X-ray Photoelectron Spectroscopy were utilized to characterize the chemical and molecular identities of the oxide film, accurately calculate the thickness, and test the accuracy of the EIS-dependent oxide calculations. These ex situ methods corroborated the determination of the potentiostatic and galvanostatic charges associated with the growth of the oxide. References [1] Sedriks, A.J. (1996). Corrosion of stainless steels (2nd ed.). New York: Wiley

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[2] Bocher, F., Huang, R., & Scully, J. R. (2010). Prediction of critical crevice potentials for Ni-Cr-Mo alloys in simulated crevice solutions as a function of molybdenum content. Corrosion, 66(5). [3] Lloyd, A. C.; Noel, J. J., McIntyre, S.; & Shoesmith, D. W. (2004). Cr, Mo, and W alloying additions in Ni and their effect on passivity. Electrochimica Acta, 49, 3015-3027. [4] Combrade, P. (2001). Corrosion mechanisms in theory and practice (2nd ed.). CRC Press: Marcus. 349-397. [5] Jakupi, P.; Zagidulin, D.; Noel, J.J.; & Shoesmith, D.W. (2011). The impedance properties of the oxide film on the Ni-Cr-Mo Alloy-22 in neutral concentrated sodium chloride solution. Electrochimica Acta, 56, 6251-6259. [6] Davenport, A.J. & Lee, B. K. (2012). Passive film growth kinetics for iron and stainless steel, Electrochemical Society Proceedings, 13, 187-197. [7] Marshall, P. I. & Burstein, G. T. (1984). Effects of alloyed molybdenum on the kinetics of repassivation on austenitic stainless steels, Corrosion Science, 24(5), 463-478. Connection between Atomic Scale Characterization and Electrochemical Behavior during Passivation of Single Crystals on Ni-Cr and Ni-Cr-Mo Alloys K. Gusieva, G. Ramalingam, K. Lutton, J. R. Scully University of Virginia Ni-Cr and Ni-Cr-Mo alloys owe their outstanding corrosion resistance to the surface enrichment of passivating Cr(III) oxides and synergistic effect of Cr and Mo in case of Mo-containing alloys [1-3]. However, the specific roles of minor elements are not well understood especially with respect to precise location of Mo relative to the oxide/metal interface, nor the atomistic processes responsible for protective oxide layer growth and breakdown. The composition, structure and thickness of the passivating oxide films are challenging to characterize considering their nanoscale dimensions and the high electric field imposed during growth in solution. Key processes that take place within the oxide and regulate passivation are controlled by defect interactions that are atomic, ionic, and electronic in nature and currently poorly understood, often needed to be studied at the resolution and detection limits of experimental methods [4]. Common electrochemical methods such as AC and DC electrochemistry are rarely connected with atomic scale ex situ techniques such as scanning tunneling (STM), atomic force (AFM) and transmission electron (TEM) microscopies to yield crucial information on structure, molecular and electronic properties of the oxides [5]. The main goal of this work is to integrate single crystal electrochemical measurements with nanoscale characterization to advance fundamental understanding and eventually predict the roles of Cr and Mo on surface stability in corrosive environments. Such a connection between global electrochemistry and atomic studies is infrequently reported [6]. Polycrystalline alloys contain many grain orientations that may affect passivation behavior [7]. This work focuses on examining the oxide growth kinetics on single crystals. It is expected that grain surface facet evolution and oxide thickness will vary according to crystallographic orientation [8, 9]. Model alloys Ni-22Cr and Ni-22Cr-6Mo, wt%, were arc-melted, cold-rolled to achieve ~12% deformation, and then heat treated to allow for re-crystallization and growth of sufficiently large grains to enable electrochemical

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measurements. A ~4 mm2 area of a sample was first scanned with Electron Backscattered Diffraction (EBSD) to obtain a “catalog” of grain orientations, and then each grain within the EBSD map was probed regarding its electrochemical behavior. The kinetics of the passive state of single grains were determined potentiostatically at +0.2 V SCE in solution, where a previously formed on air oxide was partially reduced at -1.3 VSCE. Electrochemical impedance spectroscopy (EIS) was then collected after various stages of oxide growth and the data was fit assuming a constant phase element to enable calculation of thickness [10]. Oxide charge formation, current density, and thickness for each alloy and grain orientation were studied using Mott-Cabrera and Point Defect models. Ex situ Raman and XPS conducted on polycrystalline samples aided in determining molecular identity and the efficiency of oxide growth versus dissolution. AFM topography images and profiles across single grain surfaces at intermediate stages of potentiostatically controlled oxide growth were obtained to give insight on the nucleation and morphology of oxides formed on various grains. Furthermore, the structural and electrochemical properties of oxides grown potentiostatically on specific crystallographic orientations were correlated with the TEM analysis performed on the oxides grown chemically under equivalent conditions using potassium persulfate or hydrogen peroxide oxidants. It was found the EIS, TEM and XPS oxide thicknesses were in agreement with each other. The broader relevancy of this presentation, besides connecting electrochemistry and nano- to mesoscale characterization, is its contribution to concepts beneficial towards development of predictive correlations associated with the passivity of Ni alloys during aqueous exposure. References [1] F. Bocher, R. Huang, J.R. Scully, Corrosion 66 (2010). [2] A.C. Lloyd, J.J. Noël, S. McIntyre, D.W. Shoesmith, Electrochimica Acta, 49 (2004) 3015-3027. [3] A.C. Lloyd, J.J. Noël, N.S. McIntyre, D.W. Shoesmith, JOM, (2005) 31-35. [4] P. Jakupi, D. Zagidulin, J.J. Noël, D.W. Shoesmith, Electrochimica Acta, 56 (2011) 6251-6259. [5] V. Maurice, H. Peng, L.H. Klein, A. Seyeux, S. Zanna, P. Marcus, Faraday Discuss, 180 (2015) 151-170. [6] J.R. Scully, Faraday Discuss, 180 (2015) 577-593. [7] P. Marcus, V. Maurice, H.H. Strehblow, Corrosion Science, 50 (2008) 2698-2704. [8] J.J. Gray, B.S. El Dasher, C.A. Orme, Surface Science, 600 (2006) 2488-2494. [9] D.J. Horton, A.W. Zhu, J.R. Scully, M. Neurock, MRS Communications, (2014) 1-7. [10] B. Hirschorn, M.E. Orazem, B. Tribollet, V. Vivier, I. Frateur, M. Musiani, Journal of The Electrochemical Society, 157 (2010) C458.

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Experimental Investigation of the Nickel Pourbaix Diagram through Electrochemical and Spectroscopic Techniques M. Hutchison, R. J. Santucci and J. R. Scully University of Virginia The electrochemical stability of pure nickel at various potentials and pH values were evaluated with a combination of in situ and ex situ electrochemical and spectro-electrochemical methods. This experimental investigation sought to test a modern Density Function Theory (DFT) model of the nickel-water system against the same system experimentally modeled by Marcel Pourbaix in his famous “Atlas of Electrochemical Equilibria”. The nickel Pourbaix diagram was revisited using first principle calculations of the free energy of formation for all possible species instead of experimentally determined free energies used by Pourbaix. The result was an aqueous nickel system prediction that exhibited a much wider range of stable oxide/hydroxide/oxyhydroxide. Correct determination and verification of the nickel Pourbaix diagram is crucial for the implementation of nickel-based engineering materials. The techniques used here for pure nickel have been developed for application to higher ordered corrosion resistant nickel-based super alloys. Future studies will focus on the electrochemical stability of these super alloys as a function of potential and pH. Pure nickel was subjected to a variety of potentials and pH values. A combination of spectroscopic techniques were used to observe how the nickel interface responded to these various regions of the E-pH diagram. Conditions were potentiostatically and potentiodynamically controlled to fix electrochemical potentials at the nickel surface, pH buffers were utilized where appropriate to fix the pH at the surface, and [Ni2+] ion buffers to fix the activity of [Ni2+] at the interface when required. Electrochemical Impedance Spectroscopy (EIS) was used to assess the electrochemical kinetics of the nickel interface at cathodic reduction potentials and at oxidizing anodic potentials during the progress of the experiment. EIS correctly monitored vigorous hydrogen evolution during the cathodic potential holds. At the oxidizing anodic potentials, EIS measured a notable difference in the impedance response of the nickel surface ranging from freely corroding to highly passivating behavior as a function of pH. Results show support for extended nickel oxide/hydroxide/oxyhydroxide stability range beyond what is predicted by the traditional Pourbaix diagram as suggested by the proposed DFT model. Additionally, Raman spectroscopy was used to determine the molecular identity of any Ni-based oxidation product on the surface exposed to testing after the anodic potential hold was completed. Raman signal was enhanced by the presence of silver nanoparticles on the nickel surface under testing. Raman spectroscopy identified Ni oxidation products for air-formed films and also for films formed during the electrochemical testing procedures. The relative intensities of characteristic Ni-O stretching modes scaled with the presence of passivating oxide/hydroxide/oxyhydroxide as recorded by EIS. Furthermore, for tests conducted at these passivating pH’s, additional Raman modes beyond those noted for the air-formed film were measured, indicating an oxidation product formed during the potential hold and not solely from natively occurring air-oxidation. In all, this investigation found evidence for an extended stability of Ni-based oxide/hydroxide/oxyhydroxide for the aqueous Ni Pourbaix diagram as indicated by the proposed DFT model.

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Revisiting the Effect of Crystallographic Orientation on the Corrosion of Commercially Pure Mg L. G. Bland, K. Gusieva and J. R. Scully University of Virginia For many metals and alloys, the corrosion rate has a strong dependence on crystallographic orientation [1-5]. In order to fully understand the effect of crystallographic orientation on corrosion rates, an analysis of the effects of crystal structure on atomistic bonding, the number of dangling bonds, surface energy, film thickness, film/substrate epitaxy and other properties are useful [2, 6, 7] as well as understanding metal-oxide-solution interface during both passivation and active corrosion [3, 8]. Assessing anisotropic corrosion properties by using single crystal samples limits the number of facets tested [3, 4, 7, 8]. While such studies give an indication of the corrosion properties of specified orientations, they are limited to a few crystal planes. This falls short of providing an overarching understanding of the corrosion rate as a function of orientation since corrosion properties can vary with only a few degrees difference in crystallographic orientation [1]. Orientation studies are also difficult due to highly crystallographically textured Mg alloy sheet or plate formed upon mechanical processing of the samples [9-12]. For these reasons, conflicting results have been reported regarding the orientation dependence of Mg corrosion [13, 14]. Electron Backscatter Diffraction (EBSD) correlated with surface topography measurements, such as confocal laser scanning microscopy (CLSM) enabled understanding of the dissolution behavior of a large number of crystal planes [1]. For Mg, depending on the exposure environment, the oxide film thickness will vary dramatically. A crystalline MgO layer is formed during exposure to air. After longer periods of time, a hydroxyl-oxide layer (Mg(OH)2) will form which is amorphous [15]. Therefore, the surface film formed on Mg consists of a thin, nano-crystalline MgO inner layer and an amorphous Mg(OH)2 platelet layer [15-17]. The thickness of the oxide layer on Mg has been determined through several techniques, including focused ion beam (FIB) cross-section [17]. The outer, Mg(OH)2 layer is typically on the order of ~500 nm while the MgO layer is only on the order of 50-90 nm. The electrochemical dissolution of Mg shows strong crystallographic dependence in chloride-containing, alkaline environments.[18-20] However, the trend does not match surface energy. For non-chloride containing, neutral pH environments which do not support film growth, such as Tris(hydroxymethyl)aminomethane (TRIS) and Ethylenediaminetetraacetic (EDTA), there is limited to no crystallographic orientation dependence on the corrosion rate. For the environments exhibiting crystallographic dependence, the origins of the corrosion rate trend is better understood utilizing electrochemical impedance spectroscopy (EIS) [21]. However, correct assessment of

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corrosion rate requires measurement of EIS to low frequency (1 MHz). EIS constant phase elements may also be exploited to determine oxide thicknesses. In order to describe the index of each plane, {hkil}, the variables from the inverse pole figure (IPF) were used. The {hkil} is defined in terms of the interplanar angles, βi. β is the angle measured from a primary low index hexagonal plane normal to the {hkil} of interest and αi, is the angle between βi and either the angular distance from the {0 0 0 1} towards �0 1 1� 0� or {1 �2 1 �0}. β and α describe the exact irrational orientation of a corroding crystal plane in the stereographical triangle. In unbuffered 0.6 M NaCl, the corrosion rate varied with the crystallographic orientation, with the fastest corrosion rate occurring along the basal plane and the slowest corrosion rate along the low index, prismatic and pyramidal planes. The corrosion rate correlated with oxide thickness as a function of the β{0001} angle (with β{0001}=0° corresponding to the basal plane and β{0001}=90° corresponding to the prismatic and pyramidal orientations). In the work herein it is shown that much of the variation in the corrosion kinetics with crystallographic orientation correlates with film thickness suggested to be related to epitaxial stress. In particular, the variation in the MgO growth kinetics along various crystallographic orientations will potentially alter the corrosion kinetics. References [1] D. Horton, A. Zhu, J. Scully, M. Neurock, MRS Communications, 4 (2014) 113-119. [2] K. Fushimi, K. Miyamoto, H. Konno, Electrochim Acta, 55 (2010) 7322-7327. [3] B.W. Davis, P.J. Moran, P.M. Natishan, Corros Sci, 42 (2000) 2187-2192. [4] M. Yasuda, F. Weinberg, D. Tromans, J Electrochem Soc, 137 (1990) 3708-3715. [5] H.G. Kim, T.H. Kim, Y.H. Jeong, Journal of Nuclear Materials, 306 (2002) 44-53. [6] B. Holme, N. Ljones, A. Bakken, O. Lunder, J.E. Lein, L. Vines, T. Hauge, O. Bauger, K. Nisancioglu, J Electrochem Soc, 157 (2010) C424-C427. [7] R.S. Lillard, G.F. Wang, M.I. Baskes, J Electrochem Soc, 153 (2006) B358-B364. [8] G.M. Treacy, C.B. Breslin, Electrochim Acta, 43 (1998) 1715-1720. [9] U. Konig, B. Davepon, Electrochim Acta, 47 (2001) 149-160. [10] E.V. Koroleva, G.E. Thompson, P. Skeldon, B. Noble, P Roy Soc a-Math Phy, 463 (2007) 1729-1748. [11] A. Schreiber, J.W. Schultze, M.M. Lohrengel, F. Karman, E. Kalman, Electrochim Acta, 51 (2006) 2625-2630. [12] A. Shahryari, J.A. Szpunar, S. Orrianovic, Corros Sci, 51 (2009) 677-682. [13] G.-L. Song, Z. Xu, Corros Sci, 63 (2012) 100-112. [14] M. Liu, D. Qiu, M.-C. Zhao, G. Song, A. Atrens, Scripta Mater, 58 (2008) 421-424. [15] J.H. Nordlien, S. Ono, N. Masuko, J Electrochem Soc, 142 (1995) 3320-3322. [16] Y. Zhu, G. Wu, Y.H. Zhang, Q. Zhao, Appl Surf Sci, 257 (2011) 6129-6137. [17] M. Taheri, R.C. Phillips, J.R. Kish, G.A. Botton, Corros Sci, 59 (2012) 222-228. [18] L. Yang, X. Zhou, M. Curioni, S. Pawar, H. Liu, Z. Fan, G. Scamans, G. Thompson, J Electrochem Soc, 162 (2015) C362-C368.

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[19] P. Schmutz, V. Guillaumin, R.S. Lillard, J.A. Lillard, G.S. Frankel, J Electrochem Soc, 150 (2003) B99-B110. [20] M.A.H. C.R. McCall, R.S. Lillard, Corros Eng Sci Techn, 40 (2005) 337-343. [21] L.G. Bland, A.D. King, N. Birbilis, J.R. Scully, Corrosion Journal, 71 (2014) 128-145. A Phase Field Model for Thin Film Oxide Growth Q. Sherman, P. Reinke, L.D. Marks, J.M. Rondinelli and P.W. Voorhees Northwestern University and University of Virginia Modeling the growth of oxide scales below the Wagner limit remains a difficult problem due to the coupling of reaction kinetics, electrostatics, and thermodynamics. To address this problem, we have developed an electrochemical phase-field model of oxidation that resolves the ionic diffusion across an oxide layer, and the resulting oxidation rate, without the assumptions of coupled-currents and diffusion-limited growth that Wagner used to derive the classic parabolic rate law. The model captures the formation of a charged double layer at the oxide-metal interface and resolves the electrostatic potential difference across the growing film. Electric-field screening occurs over the Debye length in the oxide and the Thomas-Fermi length in the metal. Therefore, as the oxide scale becomes large compared to the Debye length, the phase-field model approaches the Wagner limit, where the electrostatic effects are localized at the interfaces. We compare the phase-field model to the experimental growth rate of NiO using parameters from both atomistic theory and experiment. Particularly, we use defect formation energies and work functions provided by DFT and experimental values for diffusion coefficients and thermodynamic driving forces to model the growth of NiO. We find good agreement with experimental growth rate in the Wagner regime, and can predict how the growth rate evolves below the Wagner limit due to the interface mobility, Galvani potential, and reaction rate coefficients. The Growth of Oxide Islands during Oxidation R. Ramanathan, G. Ramalingam, P. Reinke, J.H. Perepezko, and P.W. Voorhees

Northwestern University, University of Wisconsin-Madison and University of Virginia The Cabrera-Mott model for the formation of an early oxide scale on a metal assumes that the scale grows layer by layer on the surface of the metal. This is in contrast the behavior observed for oxide formation on Cu, Ni, Mn, and other metals and alloys, where nucleation and growth of distinct oxide islands on the surface leads to the formation of a continuous oxide layer that then thickness. In this work, we present a model of oxide nucleation and growth that allows for the evolution of the size distribution of oxide islands to be computed. This model combines parameters computed through MD simulations as well as those estimated from experimental observations of oxide growth on Ni and Ni-Cr alloys. We performed simulations using size distributions from experiments as initial conditions and evolving the size distribution over time. Comparisons of the simulated size distribution to that observed in the experiments reveals that coalescence of oxide islands on the surface greatly affects the shape of the observed island size distributions. We also

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show that our model recovers the experimentally observed capture areas of oxide islands within a factor of 2. Accurate and Efficient First-Principles Calculations of the Thermodynamics and Electrochemistry of Solids L-F Huang and J. M. Rondinelli Northwestern University Thermodynamics and electrochemistry are two important and mutually dependent properties of materials. In this poster, we present our related progress in both methodological developments and property simulation. First, we introduce our new density-functional theory (DFT) method to efficiently and accurately calculate various thermodynamic properties of solids, as well as describe a novel negative thermal expansion mechanism uncovered in layered perovskites discovered by using this method. The calculated formation energies of Ni-Cr alloys are also shown, based on which some useful conclusions are made on phase stability. Next, accurate Ni Pourbaix diagrams are constructed using the high-level DFT methods, which we show are consistent with results obtained from the Scully group at Virginia University using electrochemical impedance spectroscopy and surface-enhanced Raman spectroscopy. Last, we propose a high-throughput first-principles method to accurately simulate the Pourbaix diagrams of magnetic transition metals (Cr, Mn, Fe, Co, and Ni), where the efficiency and accuracy of different hierarchies of DFT methods are exploited for the large-scale structural screening, accurate electronic energy calculation, and vibrational free energy calculation. Energetics of Ni-Al Intermetallic Alloys Point Defects and Strained NiO Vacancies E, Tennessen and J. M. Rondinelli Northwestern University Point defects play a crucial role in the mass transport characteristic of the process of corrosion in metals and alloys. Specifically, the point defect energetics of Ni-Al intermetallic alloys are investigated using density functional calculations. The energetic stability of vacancies and anti-sites in the Ni-Al alloys can be primarily attributed to reconstructions in the electronic structure of the local surrounding environment. Point defects, especially vacancies, are just as important during the development of oxide layers, such as with NiO, which may grow epitaxially on the metal support. The vacancy formation energies of bulk NiO are investigated with respect to the degree of biaxial compressive strain expected at the beginning of oxidation. We hypothesize that the large degree of biaxial compression will cause a significant change in the formation energy of vacancies as well as modify the barriers of diffusion, affecting the process of diffusion and the development of oxide layers. The compression should also change the relative stability of magnetic configurations in the presence of vacancies, which should have further effects on diffusion pathways.

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Incommensurate Structures in A15 Mo3Si A. Gulec, X. Yu, M. Taylor, J. Perepezko and L. D. Marks Northwestern University and University of Wisconsin-Madison We utilized Z contrast imaging, electron diffraction, atom probe tomography (APT) and density functional theory (DFT) calculations to study the crystal structure of Mo3Si phase which was previously reported to have an A15 crystal structure. Our results showed that Mo3Si has an incommensurate crystal structure with a non-cubic unit cell. The small off-stoichiometry in composition of the sample which revealed by APT and atomic resolution Z contrast imaging suggested that site substitution caused the development of split atomic positions, disorder and vacancies. Cabrera-Mott Oxidation: Interfaces, Chemistry and More X. Yu, L. Huang, J. M. Rondinelli and L. D. Marks Northwestern University The classic model for self-limited ultrathin oxide film growth on metal surfaces is due to Cabrera and Mott (CM): the driving force for rapid initial growth of the oxide is the electric field that accrues across it by adsorption of oxide ions at the surface and subsequent mass transport from the metal support. However, the role of the metal-oxide interface on surface oxidation and how it influences the oxide surface, especially for complex multivalent oxides, remains to be explored. We decipher the role of the buried interface through density functional calculations performed on the model system comprised of nickel oxide on aluminum NiO(100)/Al(100) with varying oxide film thicknesses and interfacial oxygen concentrations. We find that the ionizing charges transferred to the adsorbed oxygen mainly originate from the NiO surface, whereas the electric dipole at the buried NiO/Al interface alters the band alignment (work function of the oxide surface) and the adsorption energies. Furthermore, the diffusion barrier of negative charged Ni vacancy decreases substantially in ultra-thin NiO film compared to that in the bulk, and it is even lower after oxygen adsorption. Our findings support the CM model but show that it should be extended to include other established interfacial effects, and importantly, with these additional interactions provides a route to control surface oxidation by metal-oxide interface engineering. Sample Design for Early Stage Oxidation Experiments A. Gulec, X. Yu and L. D. Marks Northwestern University The information which is provided by the correlated use of transmission electron microscopy (TEM) and atom-probe tomography (APT) is unique and unobtainable by means of any other tool. As TEM carries geometric and elemental information on a relatively large scale compared to APT, APT offers a unique combination of high spatial resolution and single-atom analytical sensitivity. Although correlated TEM/APT studies

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are state of the art in material science, they have not yet been used for surface applications like early stage of oxidation in high temperature alloys. Most common correlative methods require FIB milling which is not suitable for surface application due to the large Ga damage on the surface. In this work, we demonstrate the novel approach to correlative TEM/ APT by utilizing electro-polished Tip mounted on TEM grid. Here, we show preliminary data from the first correlated TEM and APT study of oxidized Mo3Si. Evolution of Oxides with Time and Thickness in MoSiB A. Gulec, X. Yu, M. Taylor, J. Perepezko and L. D. Marks Northwestern University and University of Wisconsin-Madison Mo-Si based superalloys are the candidate of new high temperature material by having the balanced mixture of Mo5SiB2 (T2 phase) and Mo3Si (A15 phase) in Mo solid state solution as this composition provides oxidation resistance, creep resistance, and strength at a wide range of temperatures. In this work, we utilized high resolution transmission electron microscopy (HRTEM) and high angle annular dark field (HAADF) imaging, combining with controlled high vacuum furnace oxidation and pulse oxidation system in order to analyze the very early stage of oxidation of each phases at various high temperatures between 500oC and 900oC. Our results showed that porous silica formation on top of A15 phase starts at temperature as below as 550oC as porosity of silica formed over T2 substrate at similar temperature is smaller. Also 10 second of oxygen exposure at 900oC on T2 phase creates continues coverage of Mo oxide between the metal substrate and thin layer of silica as the thickness of silica and Mo oxide varies on different crystallographic surface orientation. While solid Mo oxide is observed at oxidation of both phases, continues coverage is achieved only at 900oC. Imaging the Development of Aqueous Corrosion X. Yu, A. Gulec, J. Scully and L. D. Marks Northwestern University and University of Virginia The characterization on aqueous corrosion of NiCr and NiCrMo were performed by the combination of electron microscopy and atom probe tomography. The observations show that cube-cube epitaxy of the NiO firstly initiates in short time (~1 hour) in the corrosive media. And the Cr oxide grows out at longer corrosion time (~3 hours). Mo is prone to diffuse out to form oxide in NiCrMo corroded sample and consume locally. APT results show the Mo depletion gap between the oxide and the bulk. The extra protective Mo oxide layer could be the reason of the difference of passive behavior of NiCrMo compared to NiCr samples. Pulsed Oxidation Chamber – A Novel Instrument Enabling Short Time High Temperature Oxidation Experiments M. Taylor, E. Zeitchick, J. Perepezko, University of Wisconsin

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A novel instrument enabling short-time, high temperature oxidation exposures for the purpose of oxide nucleation analysis is presented. The chamber permits tight, independent control of both sample temperature as well as gas pressure and composition, allowing samples to be heated in an inert atmosphere before being exposed to oxygen at the target temperature. This allows isothermal, isobaric oxidation exposures to be carried out, which is impossible in a traditional furnace exposure experiment. This is the ideal instrumentation to run an oxide nucleation analysis on bulk samples. Furthermore, this device serves as an effective method for performing controlled oxidations of TEM specimens which has been mechanically thinned prior to exposure.

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Oxidation/Reduction Behavior of the Metal/Metal-oxide observed using Environmental TEM Aram Yoon, Jian-Min Zuo1,2 1Dept of Materials Science and Engineering, University of Illinois, Urbana-Champaign, IL 61801 2Frederick Seitz Materials Research Laboratory, University of Illinois, Urbana-Champaign, IL 61801 In-situ study of the redox reaction of metal/metal oxide using environmental TEM (ETEM) is promising for solving the puzzle of oxidation of different materials under different conditions. With the high resolving power, the direct observation of reduction/oxidation inside an ETEM is expected to resolve whether intermittent processes are present during the reaction, as well as the presence of transient phases at the initial stage of oxidation and the role of alloying elements in the increase in corrosion resistance. However, in-situ ETEM experiment is complex due to the simultaneous interplay between several environmental parameters: the temperature, pressure, electron beam and the sample itself. Therefore, interpretation of ETEM results requires a comparison of different sets of experiments performed under varying conditions of reduction/oxidation and a measurement of the effect of each parameter on the reaction, which then could be used to build a thermodynamic model and to predict the conditions under which oxidation/reduction occurs. In this study, the redox reaction of three different materials are illustrated: the oxidation of iron whiskers, the reduction of NiO/YSZ nanoparticles, and the oxidation of zirconium alloy (zircaloy-2). The redox reactions were categorized into three based on the observed phenomena: shell-void formation, nanoparticle formation, and amorphization/recrystallization. The shell-void formation was observed at the redox of the single phase material such as iron or nickel followed by the volume change (Figure 1). The oxidation of the zircaloy-2 was more complex and showed different oxidation behaviors at the different temperatures. Oxygen induced the transient nanoparticle formation (Figure 2b, c) which soon disappeared (Figure 2d) during the low temperature oxidation at 150 °C, while introducing oxygen triggered recrystallization of the amorphous layer which was formed during the annealing at high temperature near 700 °C (Figure 3). A Hitachi H9500 80-300kV TEM with a LaB6 emitter is used. The ETEM is equipped with of a gas handling and mixing system and a high temperature sample heating holder up to 1500 °C. The gas pressure was increased from TEM vacuum (~5x10-5 Pa) to 2x10-3 Pa in the experiments.

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Figure 1 (a) Iron whisker before oxidation. (b) A shell-void shape of the iron oxide was formed on the iron whisker after oxidation with O2 of 2x10-3Pa at 350 °C (c) Nickel oxide nanoparticle before reduction. (d) A shell of the Nickel was formed on the nickel oxide during the reduction with H2 of 2x10-3Pa at 200 °C.

Figure 2 The process of the zircaloy-2 oxidation by the time: (a) The grain coarsening after heating for 70 minutes at 150 °C in vacuum (5x10-5Pa), (b) The particle started to form as soon as the oxygen gas (2x10-3Pa) was introduced (20 minutes after (a)), (c) and the particle gets bigger 7 minutes after introducing the oxygen gas in. (d) The nano particles starts to disappear 13 min after introducing the oxygen gas in (still oxygen is flowing) (e) 18 min after, The grains gets bigger to (f) After 30 minutes, the phase was separated into two.

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Figure 3 Amorphization and recrystallization of the Zircaloy-2. Amorphous layer was formed on the zircaloy-2 metal surface during the annealing (700 °C) before introducing oxygen gas. The amorphous layer recrystallize consuming outer amorphous layer after introducing oxygen gas (2x10-3 Pa) at 700 °C.

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Tribocorrosion of 2507 Super Duplex Stainless Steel J. Michael Shockley, Derek Horton, and Kathy Wahl Chemistry Division, Naval Research Laboratory Grade 2507 duplex stainless steel achieves a good balance of mechanical properties and corrosion resistance, in particular its resistance to pitting corrosion. Under ideal conditions the alloy consists of a mix of α-ferrite and γ-austenite grains, but in certain temperature ranges, σ-phase can develop and considerably worsen the corrosion resistance. In the present study, we used heat treatments to express varying quantities of σ-phase in a 2507 alloy. These alloys were subjected to sliding wear while submerged in a chloride-containing salt solution under controlled electrochemical conditions. By performing friction and electrochemical measurements at a high sampling rate, evidence for depassivation and wear were resolved in time and space during testing. Complementary analysis is performed ex situ to examine the relationship of microstructural features, in particular the size and location of σ-phase grains, to the observed friction, electrochemical, and wear behavior. 3D Tomographic Imaging of the Oxidized A15 Phase in the Mo-Si-B System with Elemental Specificity J. Zhou, G. Melinte, M. Taylor, P. Ercius, J. Perepezko, and J. Miao UCLA, University of Wisconsin-Madison, Lawrence Berkeley National Laboratory The high temperature behavior of intermetallic Mo-Si alloys has attracted broad interests due to the alloy properties of high melting temperatures and high strengths at elevated temperatures. Our main goal of this work is to understand the mechanism of oxidation of A15 phase Mo3Si in Mo-Si-B system. We studied the oxidized phases formed at varying temperatures and oxidation times and probe the kinetics. We used the advanced electron microscopes at the National Center for Electron Microscopy at Lawrence Berkeley National Lab to acquire many tomographic tilt series of images and EDS spectra from oxidized A15 samples as a function of varying temperatures and oxidation times. By combining 3D electron tomography and electron dispersive spectroscopy (EDS), we got the information about pore structures in oxidized A15 phase with elemental specificity. Atom probe tomography research at the Naval Research Laboratory Keith E Knipling U.S. Naval Research Laboratory, Code 6356 Atom-probe tomography (APT) enables true 3D atomic-scale reconstructions of material volumes with sub-nanometer resolution, and with chemical sensitivity approaching 10 at.ppm. This poster presents examples of current research at the U.S. Naval Research Laboratory where APT is used to study various nanoscale microstructures in advanced structural alloys.

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Al-Sc alloys, strengthened by nanoscale Al₃Sc precipitates, exhibit coarsening and creep resistance to 300 °C. By alloying with faster-diffusing Er and slower-diffusing Zr and V additions, we have demonstrated that complex core/triple-shell precipitates are formed, with precipitates consisting of Er- and Sc-enriched cores surrounded by Zr- and V-enriched shells. These complex nanoscale Al-Er-Sc-Zr precipitates are stable against long-term coarsening at 400°C, because of the slow Zr and V outer shells, while exhibiting significant creep threshold stresses due to increased lattice parameter mismatch from Er additions. Oxide dispersion-strengthened (ODS) steels are used in a variety of nuclear and other high-temperature power applications. Traditional welding processes are unsuitable for joining these alloys, because the oxide particles agglomerate during melting. Friction stir welding (FSW) is a solid-state joining process thought to prevent these deleterious effects, but we present experimental evidence of yttrium oxide particle agglomeration, coarsening, and phase transformation after FSW of a MA956 ODS steel. High-entropy alloys (HEAs) typically consist of five or more elements in nearly equiatomic proportions, where the large configurational entropy often favors the formation of simple crystalline solid solutions. In reality, HEAs can undergo phase transformations such as spinodal decomposition, ordering, or precipitation, and these nanoscale microstructural heterogeneities have a profound influence on the alloy's properties. Using APT, we investigate nanoscale phase separation occurring in a AlCoCrFeNiCu HEA. Reactive Modeling of Mo3Si Oxidation and Resulting Silica Morphology C. Dharmawardhana, J. Perepezko, J. Miao, and H. Heinz

University of Colorado, University of Wisconsin-Madison, UCLA Corrosion of high temperature resistant alloy Mo-Si-B alloys leads to the evaporation of MoOx and formation of a Mo-containing borosilicate layer. Early stage oxidation of Mo3Si (A15 phase) results predominantly in a silica layer, whereby the mechanisms of formation and morphology of this silica layer are not well understood and affect further oxidation of the alloy. We use reactive molecular dynamics simulation to study the formation and morphology of silica upon oxidation of Mo3Si using extensions of the INTERFACE force filed. Silica structural features are represented using non-bonded parameters, nd the oxidation reaction modeled through layer-by-layer elimination of Mo and oxidation of remaining Si. First results of the silica morphology are presented as the oxidation reaction progresses through layers of 1, 5, 20, 40, and 100 nm thickness. Irregular size and shape distribution of pores is observed with consistent small size of 1 to 2 nm in size and contact lengths of less than 1 nm. These results agree with experiment for thin films (few nm), yet highly irregular shapes of larger pores (1 to 50 nm size) are found in the laboratory upon oxidation of larger (e.g. 100 nm) samples. It appears that larger pores could be the result of fast evaporation of MoO3 gas, which was neglected in the first models. Follow-on

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studies will test this hypothesis and evaluate ways to computationally predict realistic oxide morphologies upon A15 oxidation. MoSi2 Oxidation: Mechanism and Silica Growth C. Dharmawardhana, H. Heinz and P. Reinke University of Colorado and University of Virginia The surface science experiment of formation of molybdenum silicides on silicon substrate and subsequent oxidation is a complex problem and it is challenging to fully characterize the products. Experiments show that mainly MoSi2 (tetragonal phase) is formed. Here we used reactive molecular dynamics simulation to study the silica formation from the MoSi2 nanocrystals in the early stage oxidation. We observe the formation of silica islands when freestanding nanocrystals on the substrate are oxidized, and the formation of thin silica films locally on the surface of the nanocrystals. The simulation morphologies are compared to STM and STS experiments, including the structure of the silica layer and nanocrystals bounded by different (h k l) facets, e.g., (001) and (103). Surface Diffusion of Oxygen Species on Ni, Al, and NiCr Alloy Surfaces K. Kanhaiya, N. Mehio, H. Heinz, R. Ramanathan, Q. Sherman and P. W. Voorhees

University of Colorado and Northwestern University Adsorption of molecular oxygen to metal and alloy surfaces as well as its subsequent diffusion plays a role in the initial steps of corrosion and plays a role in mesoscale models. Molecular dynamics simulations indicate the motion of molecular oxygen from gas towards the surface, surface hopping, associated energy barriers, as well as diffusion coefficients for a series of metals and facets, including (100), (110), and (111) facets of Al and Ni. Adsorption energies were calculated as the difference in the energy when an oxygen molecule is far from the surface and when oxygen molecule is at its minimum energy location above the surface. Diffusion coefficients were calculated from the mean square displacement of oxygen molecules for long time scales up to 100 ns. The surface potential energy map (Figure A) could then be calculated from the potential energy of a probe oxygen molecule across different locations on the surface, which further allowed to quantify the activation energy for diffusion using the difference in potential energy at the minima versus saddle points of the hypersurface. The results show that diffusion was is isotropic on (100) surfaces and anisotropic on (110) and (111) surfaces. The reason are the same diffusion barriers in all directions on the (100) surfaces versus the observation of low diffusion barrier in one and high diffusion barrier in other direction on (110) and (111) surfaces. Adsorption energies of molecular oxygen were similar for both Al and Ni, whereby diffusion was faster on Al surfaces. The surface also have a low corrugation ratio (diffusion barrier/adsorption energy), which means that oxygen molecules favor surface diffusion over desorption. The parameters will be compared to and used in phase field models for morphology prediction. Work in progress focuses on remaining alloys, metals, and oxides (Cr, Mo, Ni-Al, Ni-Cr, Ni-Al-Cr, Al2O3, Cr2O3, NiO) to point out the exact role of each component in adsorption and diffusion.

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Figure A. Potential energy map of an oxygen molecule atop an Al (100) surface in kcal/mol. The approximate diffusion barrier is 0.65-0.05=0.60 kcal/mol.

Electronically Refined Force Fields for BCC and HCP Metals C. Dharmawardhana, S. Deshmukh and H. Heinz University of Colorado Chemically realistic, accurate, and broadly compatible force fields for metals and alloys are of great necessity for large scale simulation of multiphase materials, such as metal-oxide interfaces, metal-oxide-aqueous interfaces, as well as metal/oxide-organic-aqueous interfaces in corrosion-protected materials. Existing force fields (EAM models) are not element specific and cannot be applied by multicomponent alloy systems or multiphase systems easily. We are extending developments of electronically refined force fields that can be generalized to multicomponent alloys as well as interfaces and used up to tens of millions of atoms. We achieve this by representing the metal atoms with a positively charged core and appropriate number of negatively charged dummy atoms guided by the electronic structure (e.g. number of d electrons). This method, based on Coulomb and Lennard-Jones (LJ) potentials for each metal atom, allows to obtain realistic crystal structures, densities, surface energies, and element-specific mechanical properties. This approach is particularly necessary for BCC and HCP metals, for which the crystal structures and mechanical properties could not at all be reproduced by simple LJ potentials. The new potential, besides broad applicability to interfaces and higher accuracy than EAM/MEAM/ADP-type many-body potentials, are also computationally less expensive and quantum mechanically better justified by using only two-body

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interactions. So far, we have optimized parameters for the BCC metals W, Cr and Mo and the HCP metal Ti as example and developed general guidelines to obtain parameters for further BCC and HCP metals. Electronegativity Concepts and Atomic Charge Differences in Alloys to Understand Alloy Formation and Defect Energies N. Saikia, E. Tennessen, J. M. Rondinelli and H. Heinz

University of Colorado and Northwestern University

Charge separation in alloys due to electronegativity differences between the constituting metals explains alloy formation energies for a wide range of alloys, and can be implemented in atomistic models. We quantify these differences for NiAl, Ni3Al, and NiCr alloys and further test the associated defect generation energies using both atomistic molecular dynamics and DFT calculations, as well as previous results from EAM/MEAM many-body models. Thereby, the charge redistribution upon defect generation was found to be large restricted to the first neighbor shell. We demonstrated broad agreement among the data and conclude that atomic charge distributions in alloys are a helpful quantitative measure for internal polarity, which is fully consistent with chemical theory but has not found inroads into alloy simulations to-date. The charge differences are expected not only to contribute to and explain experimentally observed cohesion versus phase separation in alloys, they are also expected to show clear impacts on NiCrAl and NiCrMo alloy oxidation which is currently further examined by simulation in comparison with imaging and spectroscopy. The models now used by our team are first to include electronegativity differences between metals quantitatively. They are also applied to explain observations of oxide growth and interfacial capacitance in aqueous corrosion of NiCrAl/NiCrMo and guide in suggestions of materials formulations once the mechanisms of corrosion are better understood.