Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session P23: Structural Materials and Hydrogen |
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Sponsoring Units: DCMP Chair: David Singh, Oak Ridge National Laboratory Room: Colorado Convention Center 110 |
Wednesday, March 7, 2007 11:15AM - 11:27AM |
P23.00001: Thin Film Synthesis and Characterization of MAX-Phase Compounds T. H. Scabarozi, W. Tambussi, J. D. Hettinger, S. E. Lofland, M. W. Barsoum We present the synthesis and characterization of thin film MAX-phase compounds. Thin film synthesis was performed by magnetron sputtering from compound and elemental target materials on 2-inch c-axis sapphire wafers. A series of experiments were carried out where parameters of temperature, gas flow, pressure, and cathode power were varied. Films were characterized by Raman spectroscopy, electron microscopy, X-ray diffraction, and atomic force microscopy. Most films were readily synthesized with multiple-phases which were hexagonal or cubic. All phases were epitaxial, with growth along the (000l) and (111) direction for hexagonal and cubic compounds, respectively. With careful control of temperature and stoichiometry, single phase films were produced. Surprisingly, we synthesized Ti$_{2}$AlC by diffusion of Al from the substrate while only sputtering Ti and C. [Preview Abstract] |
Wednesday, March 7, 2007 11:27AM - 11:39AM |
P23.00002: Composition Dependence of Elastic properties in M$_{2}$AX Materials T. Scabarozi, S. E. Lofland, J. D. Hettinger, S. Amini, P. Finkel, M. Barsoum We report on correlations between thermal expansion, elastic modulii, thermal transport, specific heat, and electrical transport measurements of materials within the MAX-phase family. Elastic modulus measurements are made using an ultrasonic time of flight technique. Thermal expansion measurements are made using high-temperature x-ray diffractions. We see a clear variation in elastic properties in materials of the form M$_{2}$AC(M=Ti) with A=S having the largest elastic modulus of all M$_{2}$AX materials measured to date. It also has the largest Debye temperature as measured from specific heat. The phonon contribution to the thermal conductivity is relatively large, similar in size to the thermal transport resulting from charge carriers. The elastic modulus approaches that found in Ti$_{3}$SiC$_{2}$, a M$_{3}$AX$_{2}$ material. The overall goal of this work is to correlate measurements of these properties varying M and A to unravel the role of both lattice constituents in determining the elastic properties of this class of materials. [Preview Abstract] |
Wednesday, March 7, 2007 11:39AM - 11:51AM |
P23.00003: Structural phase transformations in Ti$_{3}$Al$_{2}$Nb system, a first-principles approach Mahdi Sanati, Damien West First-principles method is employed to determine the vibrational entropy and Gibbs free energy as a function of temperature of the homogenous Ti$_{3}$Al$_{2}$Nb system. Calculated energies at T=0 K show instabilities in ternary B2 Ti$_{3}$Al$_{2}$Nb alloy against the $\omega $ and B8$_{2}$ structures. We show that at high temperatures the B2 phase is stabilized by the vibrational entropy. The transition temperatures for B2 $\to \quad \omega $ and B2 $\to $ B8$_{2}$ have been calculated and are in excellent agreement with experiment. [Preview Abstract] |
Wednesday, March 7, 2007 11:51AM - 12:03PM |
P23.00004: Precipitation behavior of sigma phase in 304 and 430 stainless steels as hot-rolled at 800$^{\circ}$C Chih-Chun Hsieh, Dong-Yih Lin, Weite Wu The effect of various reduction ratios on the precipitation of sigma phase in 304 and 430 stainless steels as hot-rolled at 800$^{o}$C have been investigated in this study. The sigma phase showed a dendrite --like morphology in the as received materials. A hot rolling process changed the morphology of sigma phase from dendrite-like to globular, especially at higher reduction ratio. The amounts of sigma phase in the stainless steels increased gradually at 800$^{\circ}$C with the increasing the reduction ratios from 0 to 75{\%}. The XRD analyses showed that a higher reduction ratio also enhanced the conversion of the delta (110) to sigma (542). [Preview Abstract] |
Wednesday, March 7, 2007 12:03PM - 12:15PM |
P23.00005: Intrinsic mechanism of the solid solution softening and hardening in bcc transition metal alloys: combined first principles calculations with atomic row modeling N. I. Medvedeva, Yu. N. Gornostyrev, A. J. Freeman The solute--dislocation interaction is of great interest since it determines one of the important strengthening mechanisms in alloys. It is still unclear why some alloying elements lead to hardening but others give rise to softening at low temperature. To reveal the intrinsic mechanisms in solution softening/hardening, the interaction of $d$ transition metal additions with dislocations in bcc metals was studied by using a combined approach including the atomic row model with {\it ab-initio} parametrization of interatomic interactions. We found opposite trends in solute-dislocation interaction for the Groups V and VI bcc metals. Additions with extra valence electrons, which enhance the double kink nucleation and result in softening in the Group VI metals, cannot lead to softening in the Group V metals and vice versa; additions with fewer electrons may give rise to softening in the Group V metals but lead to strong hardening in the Group VI metals. We demonstrate that the electronic structure, rather than atomic size or shear modulus misfits, plays an important role in the softening/hardening effects in bcc transition metal alloys. [Preview Abstract] |
Wednesday, March 7, 2007 12:15PM - 12:27PM |
P23.00006: Phase field model for recrystallization kinetics S. Sreekala, Mikko Haataja In the recrystallization process, dislocation-free grains grow at the expense of highly deformed matrix. We introduce a phase-field model to study the isothermal recrystallization process as a phase transformation driven by the stored elastic energy which is explicitly non-local due to the long-ranged dislocation strain fields. The dislocations are represented by a coarse-grained Burgers vector density in two spatial dimensions. We have used this model to study the influence of several spatially distinct dislocation distributions (random, cellular and power-law correlated) on the growth kinetics of the recrystallized grain. Our results show that random dislocation distribution produces isotropic growth, whereas the other two distributions show anisotropic and irregular growth as seen in experiments. Also, the growth rate for the random and power-law correlated dislocation distribution follows the JMAK theory rather closely, while the highly anisotropic cell structure shows significant deviations.~ We demonstrate that the deviations arise from the presence of non-local elastic strain fields. [Preview Abstract] |
Wednesday, March 7, 2007 12:27PM - 12:39PM |
P23.00007: Possible Mechanism of the Pseudogap Formation in Intermetallic Compound Al$_3$V Susumu Miyahara, Kazuo Tsumuraya The pseudogaps in compounds give a unique electronic character in the materials such as the ones in the skutterudites. The Al$_3$V compound with DO$_{22}$ structure has also a deep pseudogap that has been explained by the presence of the covalent Al-V and Al- Al bonds in the compound [1]. We propose another possibility of the formation of the pseudogap in the DO$_{22}$ compound using a density functional method. We introduce a Peierls distortions along c-axis of the double-stacked ordered unit cell, we calculate the density of states and check the gap formation. We will apply the mechanism to the other systems with pseudogap. [1] M. Kraj\v{c}\'{\i} and J. Hafner, J. Phys.: Condens. Matter, 14, 1865 (2002). [Preview Abstract] |
Wednesday, March 7, 2007 12:39PM - 12:51PM |
P23.00008: Thermal expansion and magnetostriction in RAl$_3$ (R = Tm,Yb,Lu) single crystals S.L. Bud'ko, J. Frederick, P.C. Canfield, G.M. Schmiedeshoff We present temperature dependent thermal expansion and low temperature longitudinal magnetostriction measurements taken using a capacitance dilatometer [G.M. Schmiedeshoff et al., RSI, in press] in a PPMS-14 instrument for several cubic RAl$_3$ (R = rare earth) compounds. Quantum oscillations in the magnetostriction were observed in LuAl$_3$ and YbAl$_3$, including few new frequences for the latter. Data on qualitative changes in TmAl$_3$ thermal expansion in presence of the longitudinal magnetic field will be presented and discussed. [Preview Abstract] |
Wednesday, March 7, 2007 12:51PM - 1:03PM |
P23.00009: Theoretical Prediction of Activities in Dilute .$\gamma $-Ni(Al) Solid Solution at Elevated Temperatures Yong Jiang, John Smith, Anthony Evans For the prediction and interpretation of high-temperature diffusion and related phenomena in multiplayer systems, knowledge of thermodynamic activities is essential. For example, interfacial structures and adhesion strengths of .$\gamma $-Ni(Al)/Al2O3 at elevated temperatures strongly depend on Al activities. In this study, we present a method for predicting activity coefficients and hence activities in dilute .$\gamma $-Ni(Al) solid solutions from first-principles. Both thermal lattice vibration and electronic contributions to free energies are considered and compared. Vibrational contributions tend to dominate the temperature dependencies of the free energies: though electron thermal effects are significant. Calculations show opposing temperature trends for the formation enthalpies and entropies, leading to a partial cancellation of their role in the overall energetics. Nevertheless, their remaining temperature effects are strong. Over the temperature range, 400 K $<$ T $<$ 1700 K, the Al activity coefficient varies by 15 orders of magnitude, due to the relative strength of Al-Ni and Al-Al bonds. The Ni activity coefficient only varies less than 4{\%} over the same range. Calculational results compare well with available experimental data. The thermodynamic principles elucidated from the calculations are used to provide a fundamental interpretation. [Preview Abstract] |
Wednesday, March 7, 2007 1:03PM - 1:15PM |
P23.00010: Ab Initio Study of the Effect of Solute Atoms on Stacking Fault Energy in Aluminum Yue Qi, Raja Mishra The stacking fault energy (SFE) in binary and ternary alloys of Al with common alloying elements was studied using density function theory. Among these alloying elements, Fe further increases the SFE and Ge reduces the SFE of Al. The elements increasing the directional inhomogeneity in the electronic charge distribution of the FCC structure correlates with the increasing SFE. The maximum value of charge difference on the fault plane, Max($\Delta \rho )$, is used to characterize how much electron has been redistributed due to the stacking fault formation, and the SFE monotonically increases with Max($\Delta \rho )$. [Preview Abstract] |
Wednesday, March 7, 2007 1:15PM - 1:27PM |
P23.00011: Thermal stability and diffusion of defects on an Al(100) Francesca Baletto, Nicola Marzari Understanding and controlling homoepitaxial metal-on-metal growth is a key challenge in surface physics due to its great technological interest. We use a combination of classical and ab-initio techniques, including molecular dynamics and transition-state finding, to identify the dominant mechanisms of diffusion for adatoms and vacancies on an Al(100) surface. We find that exchange and concerted exchange mechanisms, well known in adatom diffusion, play also a central role in vacancy diffusion. In addition, they lead to a distinct phase transition localized in the first layer as temperature is raised. [Preview Abstract] |
Wednesday, March 7, 2007 1:27PM - 1:39PM |
P23.00012: Phonons in nickel and aluminum at elevated temperatures from neutron scattering Max Kresch, Olivier Delaire, Rebecca Stevens, Jiao Lin, Brent Fultz Measurements of the neutron scattering from elemental nickel were made at 10\,K, 300\,K, 575\,K, 875\,K and 1275\,K, and from elemental aluminum at 10\,K, 150\,K, 300\,K, 525\,K and 775\,K. From the scattering, the phonon densities-of-states (DOS) were calculated, and subsequently fit to Born von K\'{a}rm\'{a}n models of the lattice dynamics. Comparing to previous measurements of thermal expansion, and elastic moduli, we found a small, negative anharmonic contribution to the entropy in both cases. For nickel, we used this to place new bounds on the high temperature magnetic entropy. In both metals, the DOS displayed significant broadening at elevated temperatures. This anharmonic broadening was quantified, and shown to increase approximately as T$^2$. [Preview Abstract] |
Wednesday, March 7, 2007 1:39PM - 1:51PM |
P23.00013: A first principles calculation and statistical mechanics modeling of defects in Al-H system Min Ji, Cai-Zhuang Wang, Kai-Ming Ho The behavior of defects and hydrogen in Al was investigated by first principles calculations and statistical mechanics modeling. The formation energy of different defects in Al+H system such as Al vacancy, H in institution and multiple H in Al vacancy were calculated by first principles method. Defect concentration in thermodynamical equilibrium was studied by total free energy calculation including configuration entropy and defect-defect interaction from low concentration limit to hydride limit. In our grand canonical ensemble model, hydrogen chemical potential under different environment plays an important role in determing the defect concentration and properties in Al-H system. [Preview Abstract] |
Wednesday, March 7, 2007 1:51PM - 2:03PM |
P23.00014: Elastic Constants of Rare Earth and Transition Metal Di-Hydrides C. S. Snow, J. A. Knapp, J. F. Browning Determinations of the elastic constants of rare earth (RE=Er, La,) and some transition metal (TM=Sc, Ti, Zr) di-hydrides are extremely difficult. Single crystals of these di-hydrided metals can not be obtained because they break up into fine powders due to the large stresses in the materials caused by the crystallographic changes upon hydriding. However, polycrystalline thin films of these hydrided materials can be grown and are stable over a wide temperature and pressure range. In order to determine the elastic constants of thin metal di-hydride films ab-initio electronic structure calculations using the VASP code have been carried out. These calculations are then compared to bulk and shear moduli measured by a nano-indentation technique. Details and results of the calculations and measurements of the elastic constants of rare earth and transition metal di-hydride films will be presented and a discussion of future applications of this technique will be given. [Preview Abstract] |
Wednesday, March 7, 2007 2:03PM - 2:15PM |
P23.00015: Ab initio electronic and lattice dynamical properties of cerium dihydride Tanju Gurel, Resul Eryigit The rare-earth metal hydrides are interesting systems because of the dramatic structural and electronic changes due to the hydrogen absorption and desorption. Among them, cerium dihydride (CeH$_{2}$) is one of the less studied rare-earth metal-hydride. To have a better understanding, we have performed an ab initio study of electronic and lattice dynamical properties of CeH$_{2}$ by using pseudopotential density functional theory within local density approximation (LDA) and a plane-wave basis. Electronic band structure of CeH$_ {2}$ have been obtained within LDA and as well as GW approximation. Lattice dynamical properties are calculated using density functional perturbation theory. The phonon spectrum is found to contain a set of high-frequency ($\sim$ 850-1000 cm$^{-1}$) optical bands, mostly hydrogen related, and low frequency cerium related acoustic modes climbing to 160 cm$^ {-1}$ at the zone boundary. [Preview Abstract] |
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