Computational Thermodynamics and Kinetics: Poster Session
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Computational Materials Science and Engineering Committee
Program Organizers: Nana Ofori-Opoku, Canadian Nuclear Laboratories; Jorge Munoz, University of Texas at El Paso; Sara Kadkhodaei, University Of Illinois Chicago; Vahid Attari, Texas A&M University; James Morris, Ames Laboratory
Tuesday 5:30 PM
February 25, 2020
Room: Sails Pavilion
Location: San Diego Convention Ctr
Session Chair: Zachary Morgan, Oak Ridge National Laboratory; Mohsen Asle Zaeem, Colorado School of mines; Pascal Bellon, University of Illinois; Fadi Abdeljawad, Clemson University; Anton Van der Ven, UC Santa Barbara; Zi-Kui Liu, Penn State University
O-1 (Invited): Computational Simulation of Precipitation during Continuous Cooling of 6xxx Al-alloys: Bernhard Miesenberger1; Ernst Kozeschnik2; Benjamin Milkereit3; Erwin Povoden-Karadeniz1; 1Christian Doppler Laboratory for Interfaces and Precipitation Engineering CDL-IPE; 2Institute of Materials Science and Technology; 3Mechanical Engineering and Marine Technology, University of Rostock
Precipitation during cooling of alloys from an annealing temperature is strongly influenced by the cooling rate and directly affects the hardening response of the material.The aim of the present study is the predictive understanding of cooling-rate dependence of precipitate evolution by physics-based thermokinetic simulation of heat flow curves based on Calphad thermodynamics, diffusion mobilities, nucleation theory and growth kinetics of precipitates, using the software package MatCalc. Al-Mg-Si alloys are studied, where main precipitates during cooling are β-Mg2Si and B’-Al4Mg9Si7. Experimental determination of their sizes and nucleation sites was done by scanning and transmission electron microscopy. Whereas the β-phase nucleates at grain boundaries, the B’-phase nucleation is associated with AlFeSi dispersoids. By combining these experimental findings with previous differential scanning calorimetry data, a suitable dataset for the validation of thermokinetic simulation is developed. Our simulation results go along with the experimental findings for various cooling rates.
O-2: A Model Fusion Approach to Modeling Microstructure Development during Heat Treatment: Richard Couperthwaite1; Raymundo Arróyave1; Ankit Srivastava1; Douglas Allaire1; Ibrahim Karaman1; 1Texas A&M University
Advanced high strength steels are essential to the automotive industry and efforts to increase the strength to density ratio of these materials relies on the tailoring of composition and heat treatment procedures. Utilizing the processing-structure-property design paradigm it is necessary to be able to predict the microstructure of materials from a given processing procedure. In most cases, doing this prediction accurately requires highly expensive computations or experiments. However, there are low cost, empirical models that are capable of predicting some of the trends in the results. Therefore, using a model fusion technique known as Reification, it is possible to fuse these multi-fidelity models together in a process that allows for the design of relatively complex heat treatment procedures for steel materials while dramatically reducing the use of both costly computation and experimental methods.
O-3: Atomistic Modeling of Austenite-ferrite Transformation Interface: Olha Nakonechna1; Helena Zapolsky1; Frederic Danoix1; Mohamed Gouné2; Didier Huin3; Nicolas Charbonnier3; 1University of Rouen Normandy; 2ICMCB, UPR CNRS 9048, University of Bordeaux; 3ArcelorMittal research SA, voie romaine, 57 280 Maizières Les Metz
The austenite to ferrite phase transformation plays a key role in the processing of a modern steels. One of the crucial parameters that control the final microstructure of steels is the mobility of austenite/ferrite transformation interface. It’s well established that its mobility depends on a complex interaction of the alloying elements with a moving interface. In this paper, using atomistic modeling based on the quasiparticle approach, we investigate the energetics and dynamics of the phase transformation in an Fe bicrystal containing a bcc/fcc interface. Three interfaces, close to the Bain, Pitch and Kurdjumov-Sachs orientation relations (OR), have been examined. It is shown that the OR between the fcc and bcc phase may affect the mechanism of nucleation and growth of the newly formed bcc phase. The influence of C segregation on the interface mobility is also under discussion.
O-5: Computational and Experimental Thermodynamics of Fe-Cr-Al-O Quaternary System: Can Agca1; Jake McMurray2; Joerg Neuefeind2; Alexandra Navrotsky1; 1University of California, Davis; 2Oak Ridge National Laboratory
FeCrAl alloys are of importance in as accident tolerant nuclear fuel cladding materials. During a loss of coolant accident, these materials form (Fe,Cr,Al)3O4 spinel solid solutions. It is necessary to understand the thermodynamics of these spinels for modeling accident scenarios. We investigated the melting behavior of these materials using ultra-high temperature (>1500°C) differential thermal analysis and cooling traces with aerodynamic levitation under controlled atmosphere. Atmosphere control is crucial for fixing the Fe2+/Fe3+ ratio affecting the prevalent phase and melting temperature of the material. Solution calorimetry measurements provided enthalpies of formation. The structure of these spinel solid solutions at different temperatures using in-situ synchrotron and neutron diffraction experiments were also probed. Using the CALPHAD method, the Fe-Cr-Al-O quaternary solution phases were represented with Compound Energy Formalism and Ionic Liquid Models. The findings from experimental calorimetry, diffraction and melting experiments were used to optimize the adjustable parameters of these high temperature phases.
O-6: Computational Study of the Structure and Thermodynamics of GP Zones in Mg Alloys: Kang Wang1; Du Cheng1; Bi-Cheng Zhou1; 1University of Virginia
Recent experimental studies have found that GP zones (nanometer-sized solute clusters) formed during early stage aging process in Mg alloys can significantly enhance their strength while retaining high ductility. However, the structure and thermodynamics of GP zones in Mg alloys were poorly studied, which hinders a coherent alloy design strategy. In the present study, we used first-principles calculations, cluster expansion, and Monte Carlo simulations to study the metastable orderings on the hcp lattice and structures and stability of GP zones in Mg-Sn and Mg-Zn alloys, which are the only known binary, rare-earth-free GP-zone forming systems in Mg alloys. Numerous potential GP zone structures in Mg-Sn and Mg-Zn alloys are predicted and metastable phase diagrams are constructed. Our calculations also suggest new metastable precipitation sequences in Mg-Sn and Mg-Zn systems.
O-8: Effect of Free Surface on Grain Growth by Monte Carlo Potts Model: Sokyun Hong1; Kyung Jong Lee1; 1Hanyang University
It is generally accepted that grain growth in thin films is quite different from that in bulk materials. Normal grain growth slows down or even stops when average grain size is comparable to thickness and with subsequent proper conditions, abnormal grain growth occurs. The mechanism for stagnation of grain growth in thin films is not yet clear. The anisotropy of grain boundary and grain boundary groove drag near free surface are suggested to be main causes of stagnation. Recently, Phase field and Monte Carlo simulation of grain growth in thin films are studied but results are not enough to explain the experimental results. In this study, the improved logic to apply free surface effect on grain growth by Monte Carlo Potts model is suggested. Results of grain size and topology distribution of new model are analyzed, and it shows good agreement with the experimental results.
Cancelled
O-9: Engineering Improved Electron-emitting Materials: Examining the Desorption and Diffusion Behavior of Scandate Cathodes at Operating Temperature: Mujan Seif1; Sydney Kolnsberg1; Kerry Baker1; Thomas Balk1; Matthew Beck1; 1University of Kentucky
Scandate cathodes are poised to become the state-of-the-art in thermionic cathodes, which are critical electron-emitting components in a wide range of electronic systems, chiefly RADAR and micro- and millimeter-wave devices. Currently, widespread integration is hindered by outstanding issues all stemming from an incomplete understanding of scandate cathodes' fundamental operating mechanisms. A prominent theory explaining the cathodes' superior emission behavior is the presence of Ba-O surface dipoles atop the Sc-doped W nanoparticles comprising the scandate cathode body. Here, we examine Ba desorption and surface diffusion, phenomena thought to be directly linked to cathode lifetime. Phonon density of states and binding energy calculations yield insight into the proclivity of Ba to desorb off or diffuse across prominent cathode surfaces at operating temperature (950 C). XPS and RGA measurements are included to complement the calculations. These results are a new insight into scandate cathodes and will be instrumental in engineering improved electron-emitting surfaces.
O-10: First-principles Study of Substitutional Solute Diffusion along a Screw Dislocation in fcc Ni: Luke Wirth1; Amir Farajian1; Christopher Woodward2; 1Wright State University; 2Air Force Research Laboratory
Mass transport through high-diffusivity pathways along dislocation cores is commonly a rate-limiting process for climb and creep mechanisms in fcc metals. Diffusion of solutes through the γ matrix of Ni-base superalloys can create conditions leading to dissolution of the γ' intermetallic precipitates that lend the materials their characteristic strength. Here, the solutes Co, Cr, Al, Ti, and Mo are considered due to their frequent presence in these superalloys. Calculations are based on nudged elastic band-derived migration energies and a kinetic Monte Carlo diffusion model. The results of how solute type affects diffusion rates in an isolated a/2<1 -1 0> screw dislocation partial core (and how those rates differ from those in fcc regions) provide insight on how atoms of each species may inhibit or expedite creep. This behavior in the purely Ni environment will inform expectations for diffusion in the more complex geometry of the γ matrix.
Cancelled
O-11: Experimental and Modeling Studies Using Fe2O3 Doped Coke: Ziming Wang1; Kejiang Li1; Jianliang Zhang1; Minmin Sun1; Chunhe Jiang1; Hongtao Li1; 1University of Science and Technology Beijing
The gasification process of metallurgical coke with 0, 0.5, 1, and1.5 wt pct Fe2O3 was investigated through thermogravimetric method from ambient temperature to 1573 K in purified CO2 atmosphere. Characteristic temperature Ti Tf and Tm decreases gradually with increasing Fe2O3, it indicates that the coke gasification process can be catalyzed by the Fe2O3 addition. Two nth-order representative gas-solid models: volume reaction model (VM) and unreaction core model (URCM) were used to interpret the carbon conversion data.The kinetic analysis showed that the VM described the coke gasification with CO2 better than that of URCM. Using the kinetic model to calculate the coke samples, the activation energy lied in the range of 193.7-246.2 kJ/mol, which were in the same decreasing trend with the temperature parameters analyzed by the thermogravimetric method.
O-13: Kinetic Monte Carlo Simulations of Effect of Grain Boundary Variability on Forming Times of RRAM Conductive Filaments: Yang Hao Lau1; Zhun Yong Ong1; Hiroyo Kawai1; Liling Zhang1; Gang Wu1; Bharathi Madurai Srinivasan1; David Wu1; 1Institute Of High Performance Computing
Due to its robust performance, resistive random access memory (RRAM) has garnered interest as a potential replacement for non-volatile memory (NVM), with potential applications in neuromorphic computing for AI. However, its deployment has been hindered by the large variability of properties, including the forming times of conductive filaments in RRAM devices. Empirically, this forming time has been shown to be correlated with the microstructure of the RRAM. Since the microstructure can be modified via processing conditions, such as anneal time and temperature, there is potential to reduce undesirable heterogeneity in forming times through process control. As a first step towards this goal, Raghavan has studied the effect of grain boundaries having a different defect formation rate from the bulk on the forming times in a percolation model. To capture the potential effect of different grain boundaries, we extend Raghavan’s model by varying the defect generation rates between grain boundaries.
Cancelled
O-14: Kinetics of Scrap Melting in Iron-carbon Bath: Mengke Liu1; Guojun Ma1; Xiang Zhang1; 1Key Laboratory for Ferrous Metallurgy and Resources Utilization of Ministry of Education, Wuhan University of Science and Technology; Hubei Provincial Key Laboratory of New Processes of Ironmaking and Steelmaking, Wuhan University of Science and Technology; State Key Laboratory of Refractories and Metallurgy, Wuhan University of Science and Technology, Hubei Wuhan, China
The melting kinetics of scarp is a limiting factor in controlling the temperature trajectory and scrap ratio of BOF as well as the energy consumption and productivity of EAF. In this paper, the interface between melt and scrap in melting process is analyzed, and the moving boundary layer concept is further optimized. Through Fick's law and Fourier equation state, combined with heat and mass balance of moving interface, the theoretical analysis model of scrap melting is established and programmed. The computation results show that the formation time of solidified layer and its maximum thickness decreases with an increase of bath temperature, bath carbon content and scrap initial temperature, while it increases with an increase of scrap characteristic length. The final stable melting rate increases with an increase of bath temperature and carbon content. However, it remains almost constant as characteristic length of scrap decreases and initial temperature of scrap increases.
Cancelled
O-15: Mesoscopic Model of Free Surface Behavior in the Continuous Casting Mold: Peng Zhao1; Bin Yang2; Liang Li2; Yinhe Lin3; 1Kunming University of Science and Technology; Panzhihua University; 2Kunming University of Science and Technology; 3Yangtze Normal University
Free surface fluctuations in the continuous casting mold were simulated using the three-dimensional Lattice Boltzmann Method (LBM) coupled with Free Surface Scheme. The LBM models are verified by the good agreement between the calculated results and the measurements. The results showed that free surface fluctuations at both sides of the mold were asymmetrical. Operation parameters of continuous caster played an essential role in the fluctuations of free surface. The distributions of free surface fluctuations were also quantified. The peak and trough fluctuations on both sides of the mold alternated thereon, accompanied by the stochastic formation of the vortices at the top mold. The simulation results suggest that the LBM models offer a promising way to study free surface behaviour and related phenomena in a continuous casting mold.
O-16: Multi-phase-field Modeling and Simulations of Polycrystalline Microstructure with Grain Boundary Phase: Aoshi Nakai1; Eisuke Miyoshi1; Tomohiro Takaki1; 1Kyoto Institute of Technology
High accurate control of polycrystalline microstructure is key to produce high-performance Nd-Fe-B based permanent magnets [Scripta Mater., 67 (2012) 530-535]. This has been clarified by micromagnetic simulations considering microstructures [Mater. Trans., 57 (2016) 1221-1229.]. However, simulations for microstructure formation processes are very limited [J. Japan Inst. Met. Mater., 81 (2017) 43-48]. In this study, in order to develop a prototype model for microstructure prediction of Nd-Fe-B magnets, we investigate a multi-phase-field model to express the formation process of polycrystalline microstructure with high anisotropy and with grain boundary phase. Moreover, we show some simulation examples of grain growth for a binary alloy.
O-17: Multi-phase-field Modeling for Interaction of Moving Dendritic Particles: Namito Yamanaka1; Tomohiro Takaki1; Shinji Sakane1; Yasushi Shibuta2; Munekazu Ohno3; 1Kyoto Institute of Technology; 2The University of Tokyo; 3Hokkaido University
Simulation of motion of multiple growing dendrites is a challenging theme in alloy solidification. In our previous study, we enabled the simulation of growth, motion, collision, and coalescence of multiple equiaxed dendrites and grain growth after coalescence by employing a phase-field lattice Boltzmann model [Comput. Mater. Sci., 147 (2018) 124–131]. However, in the model we assumed a perfect inelastic collision, where two equiaxed dendrites are immediately coalesced into one solid at the collision. To improve the accuracy of the model, we introduce an elastic collision among the dendritic particles in this study. We also introduce a double-obstacle potential, instead of a double-well obstacle, to effectively treat the massive number of grains. We show some simulation examples of multiple dendrite particles with motion.
O-19: Phase-field Simulation Study of Crystallization of Polymer Droplets on Surfaces: Yang Xia1; Jason Liu1; Rodney Priestley1; Craig Arnold1; Mikko Haataja1; 1Princeton University
Crystallization of thin polymer droplets on a substrate is a complicated process often controlled by substrate-polymer interactions and lattice mismatch. In this work, a diffuse-interface approach is employed to study the crystallization kinetics and morphology of thin polymer droplets on substrates under epitaxial conditions. Experimentally observed spherulites and highly anisotropic needle-like lamellar bundles are reproduced in the simulations, and the effects of polymer diffusivity, anisotropy and interfacial energy on growth kinetics and morphology will be discussed.
O-20: Phase-field Study on the Transformation of Lath Martensite in Steel: Mingyu Cho1; Pilryung Cha1; 1Kookmin University
Lath martensite is a characteristic structure in quenched steels with a low or negligible carbon content, maraging steels and interstitial free steels. The transformation of a parent austenite grain into lath martensite was characterized as a grain subdivision on different length scales. On a coarse scale, the austenite grain breaks down to several packets, each containing extended parallel blocks. Each block is further subdivided by laths, which are narrow units with a width in the submicrometer range. In this study, we analyze the origin of lath martensite formation by using volume-conserving multi-phase field model with micro-elasticity. Although various deformation mechanisms such as Bain, Kurdjumov–Sachs, Nishiyama-Wassermann and Pitch mechanisms have been proposed for cubic to tetragonal martensitic transformation, only Bain deformation mechanism has been considered in previous phase field models. In this study, we consider all the formation mechanisms and reveal a plausible origin for the microstructure of lath martensite.
O-21: Phase Field Modeling of Grain Boundary Grooving and Migration due to Electric Field and Thermal Gradient: Supriyo Chakraborty1; Praveen Kumar2; Abhik Choudhury2; 1Ohio State University; 2Indian Institute of Science
Grain-boundary migration, void formation as well as associated hillock formation are important mechanisms which lead to the failure of interconnects in the microelectronic packages. An understanding of the underlying physics of each of the phenomena can allow better design of interconnects. We formulate a phase-field model for studying the phenomena of grain-boundary grooving under the combined influence of pure diffusion controlled transport, electric current and thermal gradient. We separately investigate the contributions of each of the stimuli towards the process of grain-boundary migration and hillock formation, by performing phase field simulations as well as comparing with analytical theories. Additionally, we qualitatively reproduce the phenomena observed in experiments on polycrystalline metals, wherein electromigration and thermomigration may act in unison or against each other towards their contributions in grooving,hillocking and void growth.
O-23: Predicting the Enthalpy of Hydrogen Dissolution in Niobium Using First Principles: Arvind Ramachandran1; Houlong Zhuang1; Klaus Lackner1; 1Arizona State University
The dependence of the enthalpy of hydrogen dissolution (∆H_H) in niobium on the concentration of hydrogen in the alpha phase of niobium (c_H) has been studied through past experiments. Important inferences about the interactions between interstitial hydrogen atoms in niobium have been drawn from those experiments. In this work, we use density functional theory (DFT) calculations to predict the variation of ∆H_H with c_H. We treated the different possible hydrogen atom arrangements in a supercell using importance sampling and developed the underlying statistical mechanics formalism for this problem. The predicted trend for the variation of ∆H_H with c_H is in good agreement with the experimental results. The method developed for treating this problem is general and can be extended to the treatment of other systems with interstitial atoms in metals.
O-24: Prediction of Permeability Tensor for Columnar Dendritic Structures: Phase-field and Lattice Boltzmann Simulations: Yasumasa Mitsuyama1; Tomohiro Takaki1; Shinji Sakane1; Yasushi Shibuta2; Munekazu Ohno3; 1Kyoto Institute of Technology; 2The University of Tokyo; 3Hokkaido University
Accurate prediction of permeability tensor for interdendritic flow is crucial for highly accurate simulation of macrosegregation formed during solidification of alloys. However, there is not enough information regarding the permeability tensor of interdendritic flow. In our previous study, we performed permeability predictions of flow normal to columnar dendrites [Acta Mater., 164 (2019) 237-249]. We also enabled the permeability prediction of flow along an arbitrary direction for columnar dendrites. However, our previous studies were for cell-like dendrites, whose secondary arms were not so developed. In this study, we investigate the permeability tensor for columnar dendritic structures with fully developed secondary arms. Here, we employ the phase-field and lattice Boltzmann methods to predict the permeability. The simulations are performed by parallel GPU computing using a GPU supercomputer TSUBAME3.0 at Tokyo Institute of Technology.
O-25: Probing the Interactions Between Interstitial Hydrogen Atoms in Niobium Through Density Functional Theory Calculations: Arvind Ramachandran1; Houlong Zhuang1; Klaus Lackner1; 1Arizona State University
Experiments that study the absorption of hydrogen in niobium have revealed specific properties about the interactions between interstitial hydrogen atoms. It has been reported in the past that there are long-range attractive and short-range repulsive interactions between hydrogen atoms in niobium. Further, it has been suggested that the interaction between the hydrogen atoms in niobium are of many-body nature. In this work, we use density functional theory (DFT) calculations to study the interactions between interstitial hydrogen atoms in niobium, to test the validity of the experimental inferences made about these interactions and understand their origin starting from first principles. We find that the interactions between hydrogen atoms in niobium can be understood as a combination of an indirect image interaction that is attractive and a direct interaction that is repulsive. Further, through DFT calculations, we demonstrate the many-body nature of these interactions.
O-26: Reduction Kinetics Analysis of Fe2O3 in the Case of Carbon Precipitation: Yangxin Chen1; Liangying Wen1; Shengfu Zhang1; Jiao Cao1; 1Chongqing University
The reduction kinetics of Fe2O3(purity 99.9%) under the condition of carbon separation was investigated by thermogravimetric isotherm with atmosphere controlled reduction experiment.TG curves were recorded during the experiment, and conversion rate, reaction rate, reduction rate and other parameters were calculated through experimental data.The results showed that the carbon evolution process was accelerated in the FeO→Fe stage, and the weight of samples was obviously increased.Through lnln method,the reaction mechanism function is G(a)=[-ln(1-a)]1/2, and the reaction activation energy is 43.4kJ/mol.
O-27: Role of Defects, Interfaces in FCC-BCC Massive Transformation in Iron Using Molecular Dynamics Simulation: Pawan Tripathi1; Somnath Bhowmick1; 1Indian Institute of Technology, Kanpur, India
Austenite(FCC) to ferrite(BCC) transformation in iron is of great significance as it plays an important role in determining the microstructure and subsequently material properties. In the present work, the massive transformation from high temperature(FCC) phase to low temperature(BCC) phase is simulated using MD simulation. The simulations are performed by creating an FCC region, sandwiched between two BCC regions (interface formed according to Nishiyama-Wassermanorientation relationship, with (110) of BCC oriented parallel to (111) of FCC) in temperature range of 1000 to 1200 K. Orientation between the FCC and BCC phase was changed via tilting [111] direction of the FCC with respect to the [110] of the BCC region and is found to play a major role in transformation kinetics. Mechanism responsible for the transformation from high temperature FCC phase to low temperature BCC phase in different orientations was explained. Effect of point defects on massive transformation was also analysed.
O-28: Solute Partition at Solid-liquid Interface of Binary Alloy by Molecular Dynamics Simulation: Kensho Ueno1; Yasushi Shibuta1; 1The University of Tokyo
We present a simple approach to calculate the solute partition at the solid–liquid interface of binary alloy from molecular dynamics simulation. Solidus and liquidus compositions are successfully derived from the equilibrium composition at the solid–liquid interface, which agrees well with equilibrium solid and liquid composition from the conventional Metropolis Monte-Carlo simulation. It is significant in this study to present a simple approach of the estimation of partition-based properties including interfacial dynamics since it is not straightforward for conventional Monte-Carlo simulations to treat the solute partition and the interfacial dynamics simultaneously. Our simple technique can be expanded to the examinations of various interfacial properties easily. For example, composition dependence of the solid–liquid interfacial energy of the binary alloy can be examined on the basis of compositions including solute partition.
O-29: Study of Drying during Straight Grate Process and Its Factors - Analysis with CFD: Feng Cao1; 1Central South University
Straight grate , one of the main equipment for the oxidized pellets making, whose drying data during the process is hard to obtain. A two-dimensional unsteady mathematical model of green pellet drying straight grate process subjected to turbulent flow, which consider evaporation and condensation was developed based on porous media theory for obtain the internor mechanism of the change in the process of drying. Employed with FLUENT software and C language programming via custom code , and finally validated through the experiment. Using the mathematical model , Taking drying pellet production per unit of hot air energy consumption as optimization target, the influence of factors such as wind temperature, wind speed, the pellet layer thickness and time distribution of two drying stages on the drying effect of straight grate , was explored by orthogonal experiment method.
O-30: Studying the Nb-H System Using Density Functional Theory Calculations: Arvind Ramachandran1; Houlong Zhuang1; Klaus Lackner1; 1Arizona State University
Experiments that study the absorption of hydrogen in niobium have revealed specific properties about the interactions between interstitial hydrogen atoms. From the experimental enthalpy of hydrogen dissolution (∆H_H) in niobium, it has been inferred that there are long-range attractive and short-range repulsive interactions between hydrogen atoms in niobium. Further, it has been suggested that the interaction between the hydrogen atoms in niobium are of many-body nature. In this work, we use density functional theory (DFT) calculations to study the interactions between interstitial hydrogen atoms in niobium, understand their origin starting from first principles, and predict the variation of ∆H_H with c_H. We find that the interactions between hydrogen atoms in niobium are indeed of many-body nature, and can be understood as a combination of an attractive indirect image interaction and a repulsive direct interaction. The predicted trend for the variation of ∆H_H with c_H is in good agreement with experiments.
O-31: Surface Diffusion in Metallic Glasses Using Atomistic Simulations: Ajay Annamareddy1; Paul Voyles1; John Perepezko1; Dane Morgan1; 1University of Wisconsin
Atoms on the surface are loosely bonded to their neighbors, leading to their non-localization and accompanying high surface mobility even in the condensed phase. Here, we study the enhancement of surface diffusion (DS) in model metallic glasses by MD simulations. First, we focus on the prefactor and activation energy in Arrhenius fits to glassy-state diffusion. The activation energies of the surface (QS) and bulk (QV) diffusion are related, as expected, by QS ≈ 0.5 × QV, contributing to the observed increase in DS compared to DV. However, the surface also has orders of magnitude lower pre-exponential factors than the bulk. We explore how structural and cooperative motion changes between the bulk and surface might explain the changes in prefactor. We also investigate the suppression of the surface glass-transition-temperature from variations in D and relaxation times. Finally, we present an estimate of the thickness of the region of enhanced surface diffusion.
O-32: Thermodynamic Stability and Kinetics of Nb3Ge, Nb3Al, and Nb3Ga A15 Phases: Ajinkya Hire1; Hannah Bayard1; Chris Orozco1; Biswas Rijal1; Lilong Zhu1; Ryan Porter2; Zeming Sun2; Matthias Liepe2; Michele Manuel1; Richard Hennig1; 1University of Florida; 2Cornell Univerity
In superconducting radio frequency(SRF) cavities, Nb3Sn coating leads to shorter cavity lengths, higher operating temperatures and lower power consumption due to its high superheating-field. Nb3X(X=Ge,Ga,Al) A15-phases have higher superconducting temperature and field as compared to Nb3Sn and can be potential alternatives to Nb3Sn for SRF applications. In this work, we investigate the thermodynamic-stability and synthesis condition of these phases. We use density functional theory(DFT) and a cluster expansion-based approach to calculate the thermodynamic-stability range of these phases. We also perform diffusion couple experiments to validate the solubility limits and determine the interdiffusion-coefficients, an essential parameter in the synthesis of these phases. Furthermore, the superconducting properties of these materials are also determined using DFT and Eliashberg calculations. We find that Nb-antisite defects are the predominant defects which stabilize Nb3X at off-stoichiometric compositions. We also find differences between our calculated composition-ranges and the ranges in literature at which these phases are stable.