Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session G15: Mechanical Properties of Metals |
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Sponsoring Units: DCMP Chair: Michael J. Mehl, Naval Research Laboratory Room: Baltimore Convention Center 311 |
Tuesday, March 14, 2006 8:00AM - 8:12AM |
G15.00001: Thermodynamic and transport properties of beryllium-copper alloys for less than 10 wt.{\%} Cu. Jason Cooley, Jason Lashley, W. Larry Hults, Tim Tucker We have investigated the heat capacity and electrical conductivity of various beryllium-copper alloys in the composition range of 0 to 10 wt.{\%} Cu. [Preview Abstract] |
Tuesday, March 14, 2006 8:12AM - 8:24AM |
G15.00002: Elastic properties of monoclinic $Gd_{5}(Si_{2}Ge_{2})$ O. Svitelskiy, A. Suslov, T.A. Lograsso, D.L. Schlagel, V.K. Pecharsky, K.A. Gschneidner, Jr. $Gd_{5}(Si_{2}Ge_{2})$ is an alloy characterized by a giant magnetocaloric effect $(\Delta T/\Delta B \sim 8K/2T)$ and a colossal strain (up to 10,000ppm) that exist due to a magnetic- martensitic phase transition occurring close to the room temperature. Because of these unique properties, $Gd_{5}(Si_{2} Ge_{2})$ has a potential for energy efficient environmentally friendly refrigeration and actuation uses. We show the results of the first-ever ultrasonic study of a single crystal of this alloy. Sound speeds measured in a number of different geometries allowed us to determine the full elastic constant tensor for this material in the monoclinic phase. The anisotropy of elasticity of the crystal is demonstrated by the polar plots of the speed of sound, Young's modulus and linear compressibility in all three main crystallographic planes. The pure modes directions are determined. The bulk modulus is estimated at $2.58 \times 10^{11}~dynes/cm^{2}$ and the Debye temperature at 250 K. Work in Ames is supported by the US DOE. Work in NHMFL is supported by the In-House Research Program, NSF and State of Florida. [Preview Abstract] |
Tuesday, March 14, 2006 8:24AM - 8:36AM |
G15.00003: Explanation of the discrepancy between the theoretical and experimental yield stresses in bcc metals Roman Groger, Vaclav Vitek We propose a mesoscopic model that is capable to explain the factor of 2 to 3 discrepancy between experimentally measured low-temperature yield stresses of bcc metals and those typically determined by atomistic simulations of an isolated screw dislocation. The model suggests that the reason for this discrepancy is that in reality the glide of the screw dislocations is cooperative, as commonly observed at low and intermediate temperatures. In this case the stress needed to move the most highly stressed screw dislocation in an array of dislocations and simultaneously operate a dislocation source is estimated to be about a factor of 2 to 3 lower than the Peierls stress of an individual screw dislocation. Since the above-mentioned discrepancy has been encountered not only in bcc metals but also, for example, for the prism slip in hexagonal crystals, the proposed model is likely to hold more generally. [Preview Abstract] |
Tuesday, March 14, 2006 8:36AM - 8:48AM |
G15.00004: Optical Investigation of the Effects of Substrate Orientation on Oxidation of Single Crystal beta-nickel aluminum (NiAl) Serif Uran, Marcos H. Grimsditch, Boyd W. Veal, Paul, A. Paulikas Nondestructive optical techniques have been used to study the oxidation of low-index crystal faces of single-crystal beta-nickel aluminum (NiAl) as a function of temperature. Using these techniques, residual stress, phase composition, and thickness of the scales were determined at various temperatures. The oxidation of the three low-index surfaces, (001), (110) and (111) exhibit discernible differences. Consistently lower stress values are observed on (001) surfaces. Fluorescence and Raman results indicate a higher concentration of theta-aluminum oxide (Al2O3) on (001) faces and lower concentrations on (111) faces at all temperatures between 800 and 1200 Celsius degrees. Although the residual stresses are higher on (110) faces, these are the only surfaces which maintain adherence up to 1450 Celsius degrees. All surfaces exhibit a stress anomaly between 1200 and 1250 Celsius degrees. [Preview Abstract] |
Tuesday, March 14, 2006 8:48AM - 9:00AM |
G15.00005: Short-Range Order Effects on the Thermodynamic Properties of the $\rm Ni_{3}Al$ Alloy Alex Antonelli, Bernardo Meirelles, Caetano Miranda The reasons for the substantial changes induced by disorder in the thermodynamic properties of the $\rm Ni_{3}Al$ alloy still remain controversial. While several computational studies have found a significant increase in both vibrational entropy and volume as the alloy becomes disordered, there is also evidence that these properties remain unchanged upon disordering. In this work, we present a computational study of the excess vibrational entropy and excess volume of the disordered $\rm Ni_ {3}Al$ alloy, in which, we focused on the interplay between short-range order and long-range order effects on these properties. Using the Simulated Annealing optimization technique, we obtained several structures that exhibit no long- range order, but have different degrees of short-range order. Excess vibrational entropies were computed using an empirical potential, while excess volumes were obtained using both: an empirical potential and \textit{ab initio} techniques. Our calculations indicate that, both excess vibrational entropy and excess volume are substantially reduced in alloys that have no long-range order but have some degree of local order. Both methodologies used to model the alloy give rise to similar results for the excess volume, indicating that the short-range effects are not an artifact of how the system is modelled. [Preview Abstract] |
Tuesday, March 14, 2006 9:00AM - 9:12AM |
G15.00006: Molecular Dynamics Study of Self-Interstitial Atoms in BCC Metals: Crystallization in Mo and ferromagnetic Fe Daniel Finkenstadt, Dimitrios Papaconstantopoulos, Michael Mehl Among point defects, the self-interstitial atom (SIA) is less well investigated than its relative, the atomic vacancy. Experimentally, the SIA is important for understanding of embrittlement in irradiated materials, however there are other processes in which the SIA may also be important, namely, the solidification and crystallization, at the atomic scale, of matter from the amorphous state. Amorphous structure may be treated within the context of density functional theory as a network of SIA and vacancies. We will present tight-binding molecular dynamics (TBMD) studies of interactions between SIA, vacancies, surfaces and interfaces, within bcc-based metals, with focus on Mo and ferromagnetic Fe. We will show, via TBMD simulation, that structural transformation and mass transfer are facilitated and made rapid by the presence of a SIA. Results are compared, as a benchmark, to previous plane-wave pseudopotential results for SIA formation energies. [Preview Abstract] |
Tuesday, March 14, 2006 9:12AM - 9:24AM |
G15.00007: Atomic displacements in binary alloys Yevgeniy Puzyrev, J.S. Faulkner, G. E. Ice, C.J. Sparks Molecular dynamics calculations are used to model the thermal and static displacements in copper and copper-gold alloys. Adjusted embedded atom potentials that give very accurate atomic sizes for the pure metals are used. The agreement with experiments supports the argument that this is a reliable approach for predicting displacements, and will be useful in interpreting experimental data on atomic displacements in metals and alloys. The positions of the atoms in a binary alloy at temperature $T$ at all times $t$,${\rm {\bf R}}_i \left( t \right)$, can be found using molecular dynamics (MD)\footnote{D. C. Rapaport, \textit{The Art of Molecular Dynamics Simulation}, \textbf{2}nd edition (Cambridge University Press, Cambridge, U. K., 2004)} if the forces on the atoms are known. It is still necessary to use supercells, but they can contain thousands of atoms. The set of ${\rm {\bf R}}_i \left( t \right)$ contains all the information about both static and thermal displacements. Of course, MD is a classical theory, and the only quantum mechanical effects appear in the construction of the forces. [Preview Abstract] |
Tuesday, March 14, 2006 9:24AM - 9:36AM |
G15.00008: First-Principles Predictions of Ordered Structures in Fe-Ni Alloys S. V. Barabash, V. Blum, A. Zunger The observed ordered fcc Fe-Ni compounds are L1$_2$ FeNi$_3$ and (possibly metastable) L1$_0$ FeNi. Little is known about ordering tendencies in Fe-rich fcc alloys (stable at high $T$ at all compositions), despite the immense technological importance of such materials. We studied the $T=0$ and finite-$T$ ordering in fcc Fe-Ni alloys, employing the mixed-basis cluster expansion approach built on {\em ab initio} input formation enthalpies. We address explicitly the fact that Fe-Ni alloys may occur in either of three distinct ferromagnetic phases: high-spin (HS) fcc, low-spin (LS) fcc, and bcc phases. We distinguish between fcc- and bcc- based structures by analyzing the coordination of the nearest neighbors, and between HS and LS fcc phases by analyzing the total magnetic moment and performing series of fixed-moment calculations. We cluster expand the properties of HS ferromagnetic fcc alloys, providing a special attention to the unstable HS fcc Fe end- point. We further compare the stability of our studied HS fcc compound structures with alternative LS fcc and with bcc compound structures. For Fe-rich alloys, we find that due to instability of HS fcc Fe with respect to tetragonal deformations, the alloys have an exceptionally strong tendency to form (100) superlattices. For Ni-rich alloys, we predict new unsuspected ordered Fe-Ni compounds. [Preview Abstract] |
Tuesday, March 14, 2006 9:36AM - 9:48AM |
G15.00009: Interplay between diffusive and displacive transformations: A Phase-Field approach Rajeev Ahluwalia, Mathieu Bouville The phase-field method has been extensively used to study martensitic (displacive) transformations. However they have been studied ``in isolation.'' We study the competition between the diffusionless martensitic transformation and a diffusive phase transition which involves phase separation. Phase separation occurs if diffusivity is high enough whereas martensite will form only if little diffusion can occur. A complete description of martensite-forming materials should incorporate both difuusive as well as diffusionless modes of structural change. We use phase-field simulations to study the interplay between these diffusive and displacive phase transformations at different temperatures. TTT (temperature-time-transformation) diagrams can thus be obtained. We study the role of the temperature-dependence of diffusivity on the microstructure and TTT diagrams, by strengthening or weakening this dependence keeping other parameters unchanged. [Preview Abstract] |
Tuesday, March 14, 2006 9:48AM - 10:00AM |
G15.00010: Lattice Dynamics of Hydrogenated Austenitic Steels Sergey Danilkin We investigated hydrogen vibrations in of Fe-18Cr-10Ni and Fe-25Cr-20Ni austenitic steels doped in H gas atmosphere at pressures up to 7 GPa with maximum H content about H/Me=0.9. In case of less stable Fe-18Cr-10Ni alloy formation of the martensite phase was observed after hydrogen treatment [1]. Inelastic neutron scattering measurements were performed with spectrometers FANS at NIST and TOSCA at ISIS. Experiments show broad H peak in energy spectrum of scattered neutrons at frequencies above the metal band modes. Vibrational energy of H atoms in studied steels decreases from 132 meV at H/Me=0.0033 to 111 meV at H/Me=0.9 due to lattice dilatation [2]. The energy dependence of H modes on interatomic distance is compared with experimental data on other Me-H systems. The broadening of the H modes at H contents from 0.003 to 0.4 - where the single broad peak is observed - is most probably connected with the Me-H force constant disorder. At higher H contents - where H-peak has the two-component structure - the H-H interaction becomes important resulting in the dispersion of the optical phonon branches. [1]. M. Hoelzel, S.A. Danilkin, H. Ehrenberg, et al., Mat. Sci. Eng., 2004, v. A384, p. 255. [2]. M. Hoelzel, V. Rajevac, S.A. Danilkin, et al., J of Phys. -- Cond. Mat., 2005, v. 17 (23), p. 3537. [Preview Abstract] |
Tuesday, March 14, 2006 10:00AM - 10:12AM |
G15.00011: \textit{Ab initio} calculations of MAX phases M$_{2}$TlC (M = Ti, Zr, Hf), and M$_{2}$GaN (M = Ti, V, Cr) Sanjay Khare, Sunil Patil, Jacob Warner MAX phases have been a subject of interest recently [cf. M. W. Barsoum Prog. Solid St. Chem. 28, 201 (2000).] because of their useful mechanical, electrical and thermal properties. Here we have studied two groups of M$_{2}$AX : (i) M = Ti, V, Cr, A = Ga and X = N and (ii) M = Ti, Zr, Hf, A = Tl and X = C. We calculated the lattice parameters, bulk modulus B and local electronic density of states (LDOS) of these phases using first-principles total energy calculations. Our computed lattice structural parameters match the experimental values within 5{\%} for all six materials. Values for B were computed to be (i) 158, 170, and 180 GPa and (ii) 125, 120, and 131 GPa for the first and second group respectively. These values suggest that Ti$_{2}$TlC, Zr$_{2}$TlC and Hf$_{2}$TlC maybe the softest of all the MAX phases explored so far. The total density of states shows that all six materials are conducting. The major features in LDOS are i) the hybridization of the M d orbitals with X p orbitals and (ii) M d orbitals with A p orbitals. [Preview Abstract] |
Tuesday, March 14, 2006 10:12AM - 10:24AM |
G15.00012: Thermal transport and heat capacity of Ti$_{3}$AlCN and Ti$_{2}$AlC$_{0.5}$N$_{0.5}$ T. H. Scabarozi, M. Barsoum, A. Ganguly, S. Gupta, S.E. Lofland, J.D. Hettinger, P. Finkel We report an investigation of the specific heat and the thermal transport of a subset of the so-called MAX-phase family of materials Ti$_{3}$AlCN and Ti$_{2}$AlC$_{0.5}$N$_{0.5}$.The thermal transport results are analyzed to investigate the impact of mixing C and N on the X-sites. The heat capacity results are investigated to determine if this mixing influences the density of electronic states. We find that these mixtures have larger thermal conductivities than Ti$_{3}$AlC$_{2}$, Ti$_{2}$AlN or Ti$_{2}$AlC (Ti$_{3}$AlN$_{2}$ has not yet been synthesized). This increase has been attributed to an increase in the phonon thermal conductivity due to solid solution hardening. An increased elastic modulus is consistent with this result and the fact that the Debye temperature increases in the mixture and is in excess of 700K. [Preview Abstract] |
Tuesday, March 14, 2006 10:24AM - 10:36AM |
G15.00013: Electrical and structural properties of Tantalum nitride thinfilms near metal-insulator transition Lei Yu, Rakesh Singh, John Rowell, Nathan Newman, Lin Gu The resistivity of thin-film Ta$_{x}$N can be tuned through the metal-insulator (MI) transition by adjusting the nitrogen partial pressure during the reactive sputtering process. Recent study show that NbTiN/Ta$_{x}$N/NbTiN structures with Ta$_{x}$N tuned in this range can produce Josephson junctions with a high IcRn product and other device parameters required for the next generation of superconductive RSFQ circuits. Despite the practical possibilities, very little is known about transport over the small length scales associated with transversing the barrier. To address this issue, we focus on the transport and structural properties horizontally through NbTiN/Ta$_{x}$N/NbTiN structures and transversely across single Ta$_{x}$N films for a wide range of Ta$_{x}$N thicknesses. When TaxN films are deposited onto oxidized Si wafers, the resistivity (measured in the direction perpendicular to film growth) increases as films become thinner (from 500 nm to 20 nm). Surprisingly, the converse is found for NbTiN/Ta$_{x}$N/NbTiN structures. The conductivity of TaxN near the MI transition has temperature dependence of $\sigma(T)=\sigma(0)+AT^{n}$, common in disordered metallic systems. Our analysis of transport has allowed us to discern the role of percolation transport and the change in material parameter as a function of film thickness. [Preview Abstract] |
Tuesday, March 14, 2006 10:36AM - 10:48AM |
G15.00014: Alpha-plutonium's low-temperature elastic constants J. B. Betts, A. Migliori, H. Ledbetter, D. Dooley, D.A. Miller Using resonant-ultrasound spectroscopy, we measured alpha-plutonium's polycrystal elastic constants between 18 and 344 K. All elastic constants -- bulk, shear, extension, longitudinal moduli and Poisson ratio -- behave smoothly during cooling, indicating no significant phase transition: electronic, magnetic, or structural. Both principal elastic constants (bulk and shear) increase about 30{\%} upon cooling from 300 to 0 K, a large change among metals, which we attribute to 5$f$-electron delocalization. From the low-temperature elastic constants, we computed that the Debye temperature equals 205 K, exceeding significantly most previous estimates. From the bulk-modulus/temperature slope \textit{dB}/\textit{dT}, we computed that the Gruneisen parameter equals 5.1, intermediate among previous estimates using other approaches. Alpha-plutonium shows an unusually high shear-modulus/bulk-modulus ratio $G$/$B$, thus a low Poisson ratio: 0.18. Within 0.5{\%}, the Poisson ratio shows temperature invariance; its small negative slope being opposite expectation. Again, we attribute this exceptional behavior to 5$f$-electron localization. [Preview Abstract] |
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