Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session N28: Focus Session: Mechanical Properties of Metals |
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Sponsoring Units: DMP Chair: Mo Li, Georgia Institute of Technology Room: LACC 506 |
Wednesday, March 23, 2005 8:00AM - 8:12AM |
N28.00001: Effect of alloying on screw dislocation structure in Mo: atomistic modelling approach with ab-initio parametrization Yu.N. Gornostyrev, N.I. Medvedeva, A.J. Freeman The plastic deformation in bcc metals is realized by the motion of screw dislocations with a complex star-like non-planar core. In this case, the direct investigation of the solute effect by first principles electronic structure calculations is a challenging problem for which we follow a combined approach that includes atomistic dislocation modelling with {\it ab-initio} parametrization of interatomic interactions. The screw dislocation core structure in Mo alloys is described within the model of atomic row displacements along a dislocation line with the interatomic row potential estimated from total energy full-potential linear muffin-tin orbital (FLMTO) calculations with the generalized gradient approximation (GGA) for the exchange-correlation potential. We demonstrate (1) that the solute effect on the dislocation structure is different for ``hard'' and ``easy'' cores and (2) that the softener addition in a ``hard'' core gives rise to a structural transformation into a configuration with a lower energy through an intermediate state. The softener solute is shown to disturb locally the three-fold symmetry of the dislocation core and the dislocation structure tends to the split planar core. [Preview Abstract] |
Wednesday, March 23, 2005 8:12AM - 8:24AM |
N28.00002: Dislocation structure and mechanical behavior of Rh$_3$X L1$_2$ intermetallic alloys: combined ab-initio-Peierls-Nabarro model approach O.Yu. Kontsevoi, Yu.N. Gornostyrev, A.J. Freeman Alloys based on Pt-group metals are promising materials for ultra-high temperature applications. Among them, Rh-based alloys are attractive due to a combination of high melting point, strength and superior oxidation resistance. Unfortunately, there is no information about dislocation properties and mechanisms driving their mechanical behavior. We analyzed the structure and mobility of dislocations in Rh$_{3}$X, where X = Ti, Zr, Hf, V, Nb, Ta, within the modified Peierls-Nabarro model with generalized stacking fault energetics calculated using the FLAPW method\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB {\bf 24}, 864 (1981)}. Superdislocations with type I core structure (APB-bounded) are preferred in Rh$_{3}$Ti and Rh$_{3}$Ta, whereas superdislocations with type II core (SISF-bounded) are predicted in Rh$_{3}$V and Rh$_{3}$Nb. An unusual superdislocation core structure (SISF-bounded type II$^\prime$ with different sequence of Shockley partials), resulting from the unstable APB energy, was found in Rh$_{3}$Hf and Rh$_{3}$Zr. Based on our analysis of dislocation structure and mobility, we provide predictions of temperature yield stress behavior of Rh-based intermetallics, and show that their dislocation properties are closely connected with features of the electronic structure and the instability of the L1$_{2}$ phase with respect to D0$_{19}$ and D0$_{24}$. [Preview Abstract] |
Wednesday, March 23, 2005 8:24AM - 8:36AM |
N28.00003: Continuum Theory of Dislocations: Cell Structure Formation Surachate Limkumnerd, James P. Sethna Line-like topological defects inside metals are called dislocations. These dislocations in late stages of hardening form patterns called \textit{cell structures}. We are developing a mesoscale theory for the formation of cell structures that systematically derives the order parameter fields and evolution laws from the conserved topological Burgers vector density or the Nye dislocation tensor. (In classical plasticity theories, describing scales large compared to these cells, one normally bypasses the complicated motions of the dislocations by supplying yield surface and plastic hardening function in order to determine the evolution of state variables.) Using Landau approach and a closure approximation, an evolution equation for the dislocation density tensor is obtained by employing simple symmetry arguments and the constraint that the elastic energy must decrease with time at fixed stress. The evolution laws lead to singularity formation at finite times, which we expect will be related to the formation of cell walls. Implementation of finite difference simulations using the upwind scheme and the results in one and higher dimensions will be discussed. [Preview Abstract] |
Wednesday, March 23, 2005 8:36AM - 8:48AM |
N28.00004: Intrinsic solid-solution softening in BCC Mo from dislocation-solute interactions Dallas Trinkle, Satish Rao, Christopher Woodward Solid solution softening observed in the group VA and group VIA transition metals has traditionally been attributed to either extrinsic effects---such as interstitial scavanging---or intrinsic effects---direct solute/dislocation interaction. We invesitgate intrinsic mechanisms using first principles methods. First, density functional theory calculates directly the interaction of Re, Hf, Os, W, Ir and Pt solutes with a straight $<111>$ screw dislocation in Mo. The local strain field associated with the dislocation core is self-consistently coupled to the long-range elastic field using the recently developed lattice-Green function boundary-condition method. The construction of simple interaction models from the {\it ab initio} data allows the extension of chemically accurate calculations to physically relevant length scales. We contrast the direct interaction energies with size- and modulus-misfits of solutes using the work of Fleischer. The misfits alone are unable to explain the presence of both softening and hardening, requiring the more complete treatment provided by {\it ab initio} methods. [Preview Abstract] |
Wednesday, March 23, 2005 8:48AM - 9:00AM |
N28.00005: How a NiAl alloy changes its stoichiometry -- special role of bulk dislocations Kevin McCarty, John P. Pierce We are studying how a NiAl alloy changes its composition when exposed to an Al flux. As Al atoms deposited on the surface diffuse into the bulk of the Ni-rich crystal, Ni atoms are displaced to the surface, where they combine with Al to form new alloy crystal. We directly observe this crystal growth by watching atomic steps advance using low-energy electron microscopy (LEEM). We find that bulk dislocations play a special role in the mass transport between the surface and the bulk. Al deposition causes the points at which bulk dislocations terminate on the (110) surface to move linearly across the surface. The dislocations provide a channel for fast mass exchange between the surface and the bulk; as they move, new crystal is left in their wake. We will discuss the relationship between the dislocation motion and the crystal equilibration. This work was supported by the Office of Basic Energy Sciences, Division of Materials Sciences of the U.S. DOE under Contract No. DE-AC04-94AL85000. [Preview Abstract] |
Wednesday, March 23, 2005 9:00AM - 9:12AM |
N28.00006: Dislocation Mobility and Cross-slip in Copper - A Molecular Dynamics Study Dan Mordehai, Guy Makov, Itzhak Kelson The dynamic properties of dislocations constitute one of the basic building blocks of any theory of plasticity. Experiments are not able yet to follow in detail the microscopic dynamic properties of the dislocation, such as dislocation motion or cross-slip, while atomistic simulations may serve as a powerful tool. Using molecular dynamics (MD) methods the dynamic properties of screw dislocations had been studied in detail for Cu, both as a function of the temperature and the applied stress. Upon applying a glide stress on the dislocation a transition from inertial to viscous motion with a stress dependent terminal velocity is observed. The experimentally observed stress dependence of the terminal velocity is reproduced quantitatively by our results [1]. Upon applying a narrowing stress on the dislocation, in a dislocation dipole structure, cross-slip occurred and the cross-slip rate in the calculations was found to be temperature and stress dependent, as expected. From these calculations the cross-slip mechanism was identified and the activation energy and volume was calculated. [1] D. Mordehai et. al. Phys. Rev. B, 67 024112 (2003) [Preview Abstract] |
Wednesday, March 23, 2005 9:12AM - 9:24AM |
N28.00007: Solute Hardening in Al-Mg: Molecular Dynamics Simulations of Single Dislocations and One-Dimensional Potential Energy Model. David Olmsted, Louis Hector, Jr., W.A. Curtin Magnesium is used as a substitutional alloying agent to improve the formability, and other properties, of aluminum in alloys such as 5xxx aluminum. Serrated flow (Portevin-Le Chatelier effect) in these alloys limit their usefulness in certain automotive applications. These serrated flow effects are believed to be dependent on Mg diffusion. In order to establish both a baseline for and a suitable model in which to study the effect of diffusion on dislocation mobility in Al-Mg alloys we have performed molecular dynamics simulations the motion of a single dislocation in Al with randomly distributed 2.5 and 5.0 at{\%} Mg. For a suitable length of dislocation, on the order of the Labusch length, we compare pinning and de-pinning of the dislocation in the molecular dynamics with a model in which a straight dislocation interacts with single Mg atoms, the small Mg-Mg interactions being ignored. We report on the results of the molecular dynamics simulations and the validation of the one-dimensional energy map model. [Preview Abstract] |
Wednesday, March 23, 2005 9:24AM - 9:36AM |
N28.00008: Study of slip bands formation in single crystal aluminum during uniaxial deformation using laser-induced photoemission technique Mingdong Cai, Lyle E. Levine, David J. Pitchure, J. T. Dickinson We report the application of the photostimulated electron emission (PSE) technique to study the slip bands formation from single crystal aluminum (99.995{\%}) during uniaxial tensile deformation. A 248-nm excimer laser (5-eV photon energy) was used as light source and the deformation was conducted with a tensile stage in ultra high vacuum working at strain rate ranging from 1$\times $10$^{-3}$ to 1$\times $10$^{-4}$ s$^{-1}$. We show that photoelectron intensities are sensitive to changes in surface morphology accompanying deformation, including slip line and band formation. In all single crystal aluminum deformed at different strain rate, the PSE intensity increases linearly with strain. Time-resolved PSE measurements show step-like increases in intensity consistent with the heterogeneous nucleation and growth of slip bands during tensile deformation. The \textit{in situ} PSE data strongly supports a recently developed dislocation dynamics model based on a percolation process. Real-time stress versus strain curves further support this model. Characterization of slip bands on the deformed surfaces was examined by atomic force microscopy (AFM). [Preview Abstract] |
Wednesday, March 23, 2005 9:36AM - 9:48AM |
N28.00009: Statistical models for the microstructural evolution in irradiated metals and alloys David J. Srolovitz, Joerg Rottler, Roberto Car The macroscopic mechanical properties of metals are intimately related to their microstructural features and their spatiotemporal evolution. We discuss a simplified statistical model for the dynamics of point defects in bcc metals that is solved through kinetic Monte Carlo (kMC) and rate equations. Self-interstitial atoms and vacancies can be produced in abundance upon irradiation with energetic particles, but they subsequently anneal due to recombination and absorption at sinks such as dislocations and grain boundaries. The model reveals a sequence of kinetic regimes that lead to a final steady state and allows us to study the size distribution of voids that form when vacancies aggregate into cluster. Here we focus on random alloys, where the point defect diffusivities are modified due to the presence of multiple exchange frenquencies. In addition, complex dealloying processes occur at sinks if the alloy components diffuse preferentially through one diffusion mechanism (self-interstitial or vacany exchange) only. We illuminate these effects with a generic kMC/rate equation model for binary alloys. [Preview Abstract] |
Wednesday, March 23, 2005 9:48AM - 10:00AM |
N28.00010: Effect of impurities on the electronic structure and stability of the A15 phase of chromium N.I. Medvedeva, O.Yu. Kontsevoi, A.J. Freeman The cubic A15 phase is well-known as an important metastable phase for {\it bcc} transition metals: it was observed in thin films and as ultra-fine particles in Cr, Mo and W alloys and its formation in O and N atmospheres is found to be more preferable compared to the {\it bcc} structure. We present first-principles FLAPW\footnote{Wimmer, Krakauer, Weinert, and Freeman, PRB {\bf 24}, 864 (1981)} results on the electronic structure, elastic constants and stability of the A15 phase in Cr and discuss the stabilizing role of light impurities. At equilibrium, the total energy of the A15 structure is only 3 mRy higher than for {\it bcc} Cr, and the calculated A15 elastic moduli do not demonstrate any shear instability. The formation of stacking faulted (twin-related) structures and A15-based phases of Cr alloyed with substitutional Re, Fe, Ni and interstitial/substitutional O, N, C was investigated taking into account full structural optimization, and the most preferable structures were found. We discuss the effect of precipitates of such binary and ternary phases on the mechanical properties of Cr and suggest that the mechanism of the ``rhenium effect'' -- namely, the improvement of ductility and strength of {\it bcc} metals upon alloying with Re -- is connected with the presence of A15-type close-packed particles. [Preview Abstract] |
Wednesday, March 23, 2005 10:00AM - 10:12AM |
N28.00011: Deformation and stress relaxation near edges in single crystal beta-NiAl during thermal oxidation Serif Uran, Marcos Grimsditch, Boyd W. Veal, Paul A. Paulikas Using micro-fluorescence and optical microscopy we have investigated the deformation and stresses that develop in the vicinity of edges (i.e. the intersection of two crystallographic faces) in single crystal beta-NiAl as it is thermally oxidized at temperatures in the range 1100-1450 Celsius. We find that the edges, initially with a radius of curvature of 2 microns, develop a significant rounding. The radius of curvature of this rounding appears to be constant at temperatures above 1250 Celsius, suggesting that the rounding takes place below this temperature. Stresses in the oxide scale show a very large decrease close to the edges and the distances over which this decrease occurs is comparable to the rounding discussed above. Data for both the deformation and stress are presented for the following pair of crystal orientations: (001) and (011), (110) and (1-10), (111) and (1-10). [Preview Abstract] |
Wednesday, March 23, 2005 10:12AM - 10:24AM |
N28.00012: Effects of high density electric currents on material processing Javier Garay, Zuhair Munir High density electric currents are common in integrated circuits and have recently been utilized as a parameter in material processing. discussion of the versatile material processing/synthesis techniques of spark plasma sintering and field activated pressure assisted synthesis. A variety of materials produced by these techniques including metals, nano-composites and intermetallics as well as possible applications of these materials as structural and functional materials are presented. The effectiveness of the electric currents are demonstrated by increased processing efficiency as well as manifested in the enhanced electrical and mechanical properties of the materials produced. In addition results from experiments performed with the aim of elucidating the current enhancing mechanism are introduced. These results show current enhanced reactivity and mass transport kinetics and help interpret the processing/synthesis findings. [Preview Abstract] |
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N28.00013: Temperature dependence of the lattice misfit in $\gamma/\gamma'$ superalloys: role of thermal expansion and composition changes O.Yu. Kontsevoi, Yu.N. Gornostyrev, A.J. Freeman, K.Yu. Khromov The magnitude of the lattice misfit $\delta$ between the $\gamma$ and $\gamma'$ phases is one of the key parameters determining the mechanical behavior, microstructure morphology and stability of $\gamma/\gamma'$ high temperature superalloys. The relative importance of two contributions to the temperature dependence $\delta(T)$ are under intense investigation, namely: (i) the difference in thermal expansion of the two phases, and (ii) the redistribution of alloying component between $\gamma$ and $\gamma'$ with the increase of temperature. We explore the role of both contributions for the Ni-Al and Ir-Nb $\gamma/\gamma'$ two-phase alloys based on \textit{ab initio} full-potential total energy and phonon spectra calculations. We demonstrate that the redistribution of the major alloy components (Al into Ni and Ni into Ni$_3$Al) gives the main contribution to $\delta(T)$ for Ni/Ni$_3$Al at $T>$600 K. For the Ir/Ir$_3$Nb system, the alloy component redistribution starts to contribute to $\delta(T)$ only at extremely high temperatures ($>$2000 K). The amplitude of these contributions can be determined by considering the shape of the $\gamma$--$\gamma'$ gap on phase diagrams. This conclusion is important for alloy design as it allows one to establish a simple relation between the alloy phase diagram and the temperature dependence $\delta(T)$. [Preview Abstract] |
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N28.00014: Anisotropic plasticity in NiAl alloy under dynamical loading Alejandro Strachan, Sheng-Nian Luo We use molecular dynamics with a first principles-based interatomic potential to characterize the orientational dependence of shock-induced plasticity in NiAl B2 alloy. For all directions studied plasticity starts with the nucleation of superpartial loops encircling 1/2$<$111$>$ slip but the subsequent events exhibit marked anisotropy. For shocks in the [110] direction we find an intricate pattern of $<$111$>${\{}110{\}} and $<$100$>${\{}110{\}} slip with the plastic wave moving at the shock velocity. In the case of [111] shocks plastic deformation is dominated by $<$100$>${\{}110{\}} slip that forms when trailing superpartials nucleate inside the initial 1/2$<$111$>$ loops. For shocks in the [100] direction (the hard direction) much stronger shocks [(uniaxial stress almost twice larger than for [110] and [111]] are required before plastic deformation is observed; we find almost simultaneous, nucleation of multiple 1/2$<$111$>$ superpartials, leading to frequent intersections that severely limit their mobility and even lead to local amorphization. In the [100] and [111] shocks we find an elastic precursor separating the leading shock front and the plastic wave. [Preview Abstract] |
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N28.00015: Vibrational Properties of point defects and dislocations in metals Babak Sadigh, Paul Schuck, Vasily Bulatov, Wilhelm Wolfer We calculate the vibrational spectra of vacancies, interestitials as well as dislocations in the fcc copper and bcc molybdenum described by interatomic potentials of the embedded-atom type. An in-depth study of the low-lying localized vibrational modes caused by the defects is presented. The concept of local atom-projected entropy within the harmonic approximation is introduced, and in this way the defect-indcued change in the thermodynamics of these metals is analyzed. [Preview Abstract] |
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