Bulletin of the American Physical Society
APS March Meeting 2010
Volume 55, Number 2
Monday–Friday, March 15–19, 2010; Portland, Oregon
Session A15: Structural Materials, Defects, Deformation |
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Sponsoring Units: DCMP Chair: Lin-Lin Wang, University of Illinois at Urbana-Champaign Room: B114 |
Monday, March 15, 2010 8:00AM - 8:12AM |
A15.00001: Modeling Point Defects in Austenite using Density Functional Theory Ron Gibala, William Counts, Chris Wolverton Austenite is an fcc based, paramagnetic phase of iron that is an important component in many commercial steels. Modeling the paramagnetic state using DFT is problematic due to the disordered magnetic structure. Here, we use DFT to investigate the properties of pure and defected fcc Fe in a number of different (collinear) magnetic orderings in an effort to find a suitable model for austenite. We compare the properties of the following magnetic orderings: ferromagnetic, non-magnetic, and various anti-ferromagnetic arrangements. We investigate the formation and binding energies of vacancy, carbon, and hydrogen point defects in fcc Fe with each potential magnetic state. Where possible, experimental results were used as a metric to judge each magnetic phase. We find that an antiferromagnetic phase containing alternating double layers of spin up and spin down (AFM-DL) is the best surrogate for paramagnetism in austenite. [Preview Abstract] |
Monday, March 15, 2010 8:12AM - 8:24AM |
A15.00002: Ab-initio and thermodynamic description of interaction of hydrogen with vacancies in fcc iron Roman Nazarov, Tilmann Hickel, Joerg Neugebauer Several mechanisms of hydrogen embrittlement are associated with a significant increase of the vacancy concentration in a H-rich atmosphere. These superabundant vacancies can form vacancies clusters or even microvoids in regions of high stress (for example crack tips), facilitate the formation of brittle phases and reduce elastic properties of crystalline structure. In order to reveal the physics of this phenomenon we have employed density-functional theory (DFT) together with thermodynamic concepts. Our systematic comparison of isolated and hydrogen loaded vacancies in fcc iron with various magnetic configurations reveals that hydrogen reduces the formation energy of a vacancy. This decrease can be significant, as up to 6 hydrogen atoms can be incorporated into a vacancy. Based on our ab-initio results we developed a thermodynamic model which determines the concentrations of vacancies, of hydrogen in different interstitial positions and of vacancy-hydrogen complexes as a function of pressure, temperature and external hydrogen chemical potential. Applying this model we find dramatically increased vacancy concentrations and total hydrogen concentration in fcc iron if the material is exposed to a H-rich atmosphere. [Preview Abstract] |
Monday, March 15, 2010 8:24AM - 8:36AM |
A15.00003: Characteristic Picture of Fe-Based Disordered Alloys: {\em Ab Initio} Study S. J. Kang, Miyoung Kim, Young-Kyun Kwon We use {\em ab initio} density fuctional theory to investigate the stability, elastic properties and thermal expansion behaviors of various Fe-based alloys. We focus on systems with cubic unit cell containing between 54 and 128 atoms constructed on underlying body-centered cubic Fe structure with 1 to 20~\% of Al, Si, and/or Mn substitutions. We calculate the formation energy of each of the energetically stable disordered structures by performing the geometry relaxation. Physical properties of such alloys are determined by their crystalline structures, and the concentration and atomic configuration of subtituted atoms. It is found that their mechanical characters depend on the disorderness of substituted atoms. We study the effect of manganese, which prevents the Fe-based alloy from forming secondary phase exhibiting poor mechanical property. Unusual mechanical properties are found in certain disordered Fe-Si and Fe-Al alloys. Moreover, their thermal expension behaviors and their effect on structures and atomic configurations are also investigated by molecular dynamics simulations. Most of our results are comparable to experimental data available and their unknown physical reasons will be discussed. [Preview Abstract] |
Monday, March 15, 2010 8:36AM - 8:48AM |
A15.00004: Liquid-metal embrittlement in Iron: surface energy reduction of Fe(110) upon Hg, Pb and Bi adsorption Lin-Lin Wang, Thierry Auger, Duane D. Johnson Recent crack propagation measurements in the presence of Hg, Pb, and Bi liquid overlayers have shown that Hg is the strongest embrittler. These observations are in distinct contrast to previous theoretical predictions for submonolayer coverage, where Hg decreases the Fe(110) surface energy the least amongst the three liquid metals. Using density functional theory with molecular dynamics, we quantitatively reproduce the experimental observation, and show that there is a crossover in the reduction of surface energy depending on coverage. We analyze the electronic structure effects responsible for the reversal trend in the surface energy reduction. The Hg-Fe interaction involves significant hybridization between Hg 5d bands and Fe 3d bands for liquid-like overlayer, while, for Pb and Bi, the interaction with Fe is best described via polarization, which diminishes in strength with more than one monolayer coverage. [Preview Abstract] |
Monday, March 15, 2010 8:48AM - 9:00AM |
A15.00005: Multi-scale modelling of the structure and mobility of small defect clusters in iron Mihai-Cosmin Marinica, Francois Willaime The mobilities of self-interstitial atoms (SIA) and their clusters in metals, especially body-centered cubic metals, are one of the main issues in multiscale models for the prediction of the microstructure evolution that these materials undergo under irradiation. In iron configurations made of non-parallel dumbbells and with a reduced mobility have been recently identified [1]. These results showed that non-conventional configurations and finite temperature effects must be taken into account [1]. We address these two points more thoroughly using on the one hand the activation relaxation technique [2], an eigenvector following method for systematic search of saddle points and transition pathways on a given potential energy surface, and on the other hand lattice dynamics calculations. For the most stable configurations, we have identified their migration mechanism. However, some clusters with low saddle point energies have to be considered in the kinetics of the system, although they are not linked to the most stable ones. Lattice dynamics free energy show that at high temperature configurations with $<$111$>$ dumbbells and/or non-parallel dumbbells are favoured [1]. The low frequency modes at the origin of this are analyzed. 1. D.A Terentyev \textit{et al}, Phys. Rev. Lett. 100 (2008)145503. 2. G.T. Barkema \textit{et al}, Phys. Rev. Lett. 77 (1995) 4358; E. Cances \textit{et al}, J. Chem. Phys. 130 (2009) 114711. [Preview Abstract] |
Monday, March 15, 2010 9:00AM - 9:12AM |
A15.00006: Predicting Dislocation Climb and Creep from Explicit Atomistic Details Mukul Kabir, Timothy T. Lau, David Rodney, Sidney Yip, Krystyn J. Van Vliet We report kinetic Monte Carlo (kMC) simulations of dislocation climb in a heavily deformed body-centered-cubic Fe comprising a supersaturation of vacancies. This approach explicitly incorporates the effect of nonlinear interactions between these point and line defects on vacancy migration barriers, and enables predictions of diffusivity and climb over relevant timescales at elevated temperatures. Vacancy migration barriers rapidly decrease inside the dislocation core and thus self-diffusivity locally increases. For a given uniaxial stress, We employ kMC to calculate climb velocities and thereby the macroscopic creep rates. The extracted stress exponent for steady-state creep and its variation with temperature agree well with experiment. Finally, these atomistically informed kinetic simulations demonstrate the stress dependence of creep activation energy in such metals that is attributable to complex point-line defect interactions. [Preview Abstract] |
Monday, March 15, 2010 9:12AM - 9:24AM |
A15.00007: Grain boundaries and glasses: birds of a feather Hao Zhang, David Srolovitz, Jack Douglas, James Warren Polycrystalline materials can be viewed as composites of crystalline ``grains'' separated from one another by thin ``amorphous'' grain boundary (GB) regions. While GBs have been exhaustively investigated at low temperatures (T), where these regions are relatively ordered, much less is known about them at higher T where they exhibit structural disorder, and where characterization methods are limited. The time and spatial scales accessible to molecular dynamics (MD) simulation are appropriate for investigating the dynamical and structural properties of GB at elevated T and we exploit MD to explore basic aspects of GB dynamics as a function of T. It has long been hypothesized, based on the processing characteristics of polycrystalline materials, that GBs have features in common with glass-forming liquids. We find remarkable support for this suggestion, as evidenced by string-like collective motion, transient caging of atom motion, and non-Arrhenius T dependence of GB mobility. Evidently, the frustration caused by the inability of atoms in the GB region to simultaneously order with respect to competing grains is responsible for this similarity. The paradigm that grains are encapsulated by a ``frustrated fluid'' provides a powerful conceptual model of polycrystalline materials. [Preview Abstract] |
Monday, March 15, 2010 9:24AM - 9:36AM |
A15.00008: Computing ab initio free energy contributions of point defects B. Grabowski, L. Ismer, T. Hickel, J. Neugebauer A common assumption when computing defect concentrations is that the dominant entropy contribution is due to configurational entropy. Other entropy contributions such as harmonic and anharmonic lattice vibrations are assumed to be second order effects and are computationally expensive to calculate. Thus, such contributions have been rarely considered in defect calculations. With the increasing capability of ab initio approaches to e.g. provide accurate free energies to macroscopic approaches (e.g. CALPHAD), the inclusion of the aforementioned smaller entropy contributions will become more and more important. We have therefore developed a hierarchical scheme to coarse grain the configurations space allowing to efficiently calculate harmonic and anharmonic contributions to vacancy formation [PRB 79, 134106 (2009)]. In the present talk we will discuss the application of this approach to vacancies in aluminum. It turns out that the entropy of vacancy formation is significantly affected by anharmonicity. [Preview Abstract] |
Monday, March 15, 2010 9:36AM - 9:48AM |
A15.00009: Atomistic simulation of plastic deformation in metallic nanowires Min Ji, Cai-Zhuang Wang, Kai-ming Ho Plastic deformation in metallic nanowires was studied by atomistic simulations using empirical interatomic potentials based on embedded atom method (EAM). Several factors affecting the results of the deformation simulation such as temperature effect and the accuracy of the EAM potentials have been investigated. Deformation structures of Cu, Al and Au {100} nanowires after their yield point were analyzed and compared with each other. We found that metals with different stacking fault (SF) energy behavior differently during the plastic deformation at nanoscale. Furthermore deformation behavior of the nanowires under compression and tension were also compared. [Preview Abstract] |
Monday, March 15, 2010 9:48AM - 10:00AM |
A15.00010: First principles study of thermodynamic, structural and elastic properties of eutectic Ti-Fe alloys Li-Fang Zhu, Martin Fri{\'a}k, J{\"o}rg Neugebauer Ti-based alloys have been suggested for commercial applications with a great potential due to their high strength and good corrosion resistance. The strength of these materials can be even further increased if bulk nano-structured eutectic alloys are produced. Motivated by experimental results showing eutectic Fe-Ti alloys decomposing into the FeTi compound with B2 structure and $\beta $-Ti alloys with varying Ti concentration, Ti-Fe alloys covering a broad range of Ti concentrations were studied using density functional theory within generalized gradient approximation. Our formation energies correctly predict the experimentally observed phases and explain their stability in terms of a sensitive concentration dependence of the density of states at the Fermi level. Further, single-crystalline elastic constants as well as polycrystalline moduli are predicted employing Hershey's homogenization. Based on these results we discuss the effect of local lattice strain on the thermodynamic phase stability and elastic properties in nano-structured eutectics. [Preview Abstract] |
Monday, March 15, 2010 10:00AM - 10:12AM |
A15.00011: The impact of kinetics and the thresholds of twinning in \emph{bcc} tantalum Kyle Caspersen, Robert Rudd, Mike Surh, David Richards, Jim Glosli, Fred Streitz The kinetics of micro-structural evolution (i.e. phase transitions and twinning) is not well understood. For small strain rates the effect of kinetics is negligible due to the rapid rate at which these evolutions occur; however, for the large strain rates that can occur under dynamic loading conditions the micro-structure, and hence the strength of the material, may depend on the details of the kinetics. Therefore, in this work we investigated kinetics of one particular micro-structural evolution process--- twinning in \emph{bcc} tantalum. To perform this investigation we performed a series of Molecular Dynamics simulations over a range of temperatures and pressures to determine the twinning threshold and understand the kinetics. We present here the results of these simulations, where one notable result was that the twinning threshold had an unexpected dependence on temperature. [Preview Abstract] |
Monday, March 15, 2010 10:12AM - 10:24AM |
A15.00012: Embedded-atom method potential for niobium Michael Fellinger, Hyoungki Park, John Wilkins Large-scale simulations of plastic deformation and phase transformations require classical interatomic potentials. We construct a force-matched [1] embedded-atom method potential [2] for niobium as the first step in alloy potential development. The program {\it potfit} [3] produces a reliable and transferable potential by optimizing the model parameters to DFT forces, energies, and stresses. The model accurately describes properties related to the fitting data, and also produces excellent results for quantities outside the fitting range. Structural, elastic, defect, and thermal properties compare well with DFT and experiment, e.g., surface energies are within 4\% of DFT values, generalized stacking-fault energies are within 10\% of DFT values, and the melting temperature is within 2\% of the experimental value. \\[4pt] [1] F. Ercolessi and J.~B. Adams, Europhys. Lett. {\bf 26}, 583 (1994).\\[0pt] [2] M.~S. Daw and M.~I. Baskes, Phys. Rev. Lett. {\bf 50}, 1285 (1983).\\[0pt] [3] P. Brommer and F. G\"{a}hler, Modelling Simul. Mater. Sci. Eng. {\bf 15}, 295 (2007). [Preview Abstract] |
Monday, March 15, 2010 10:24AM - 10:36AM |
A15.00013: Role of the defect core in energetics of vacancies in aluminum Vikram Gavini Electronic structure calculations at macroscopic scales are employed to investigate the crucial role of a defect-core in the energetics of vacancies in aluminum. We find that vacancy core-energy is significantly influenced by the state of deformation at the vacancy-core, especially volumetric strains. Insights from the core electronic structure and computed displacement fields show that this dependence on volumetric strains is closely related to the changing nature of the core-structure under volumetric deformations. These results are in sharp contrast to mechanics descriptions based on elastic interactions that often consider defect core-energies as an inconsequential constant. Upon studying the influence of various macroscopic deformations, which include volumetric, uniaxial, biaxial and shear deformations, on the formation energies of vacancies, we show that volumetric deformations play a dominant role in governing the energetics of these defects. Further, by plotting formation energies of vacancies and di-vacancies against the volumetric strain corresponding to any macroscopic deformation, we find that all variations in formation energies collapse on to a \emph{universal} curve. Implications of these results in the context of dynamic failure in metals due to spalling are analyzed. [Preview Abstract] |
Monday, March 15, 2010 10:36AM - 10:48AM |
A15.00014: The formation and structure of the oxide and hydroxide chemisorbed phases at the aluminum surface, and relevance to hydrogen embrittlement Michael Francis, Robert Kelly, Matthew Neurock Aluminum alloys used in aerospace structures are susceptible to environmentally assisted cracking (EAC) induced by hydrogen embrittlement (HE) (Gangloff and Ives 1990). Crack growth experiments have demonstrated a linear relation between the relative humidity of the environment and crack growth rates, indicating the importance of water (Speidel and Hyatt 1972). While the presence of water has been demonstrated to be necessary for EAC of aluminum, crack growth rates have been linked to the diffusivity of hydrogen in aluminum (Gangloff 2003) and hydrogen densities at the crack tip as high as Al2H have been observed (Young and Scully 1998). While the mechanism by which hydrogen embrittles aluminum is yet not well understood, without the entry of hydrogen into the aluminum matrix, embrittlement would not occur. While at the crack tip high hydrogen concentrations exist, the solubility of hydrogen in aluminum is normal near 1 ppm (Wolverton 2004). In this work combined first principles and kinetic Monte Carlo methods will be used to examine the oxide and hydroxide structure resulting from exposure of aluminum to H2O or O2 and relevance to hydrogen entry as well as EAC is discussed. [Preview Abstract] |
Monday, March 15, 2010 10:48AM - 11:00AM |
A15.00015: Ab initio investigation of grain boundary cohesion in Al alloys Shengjun Zhang, Oleg Y. Kontsevoi, A.J. Freeman, G.B. Olson Strength and hardness of aluminum alloys can be substantially increased by alloying with Mg, Zn, Cu, Si, and other elements. The main drawback of Al alloys is their susceptibility to stress corrosion cracking, which is caused by alloying impurities segregated at grain boundaries. We investigated the embrittling and cohesion-enhancing effects of impurities on a $\Sigma 5(012)[100]$ grain boundary in Al by means of the full-potential linearized augmented plane-wave (FLAPW) method within the framework of the Rice-Wang thermodynamic model and within the ab initio tensile test approach. We calculated segregation energies, analyzed local atomic configurations, electronic structures and spatial charge density distributions around segregated impurities, and identified the roles of atomic size and the bonding behavior of the impurity with the surrounding Al atoms. The results show that He, H and Na are strong embrittlers, Zn is a weak embrittler, while Sc, B, Cu and Mg are cohesion enhancers. We further evaluated the effect of co-alloying with two or more elements on grain boundary strength. This work provides a fundamental basis for the design of high strength Al alloys. [Preview Abstract] |
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