Bulletin of the American Physical Society
2008 APS March Meeting
Volume 53, Number 2
Monday–Friday, March 10–14, 2008; New Orleans, Louisiana
Session A40: Phase Stability and Phase Transitions |
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Sponsoring Units: DCMP Chair: Stefano Curtarolo, Duke University Room: Morial Convention Center 232 |
Monday, March 10, 2008 8:00AM - 8:12AM |
A40.00001: First-principles solution to the problem of Mo lattice stability Igor Abrikosov, Christian Asker, Arkady Mikhaylushkin, Anatoly Belonoshko The energy differences between the ground state body-centred structure (bcc) and closed-packed face-centred structure (fcc) structures for transition metals in the middle of the series show unusually large disagreements when they are obtained by the thermochemical approach based on the analysis of experimental data or by first-principles electronic structure calculations. Considering a typical example, the lattice stability of Mo, we present a solution to this long-standing problem. In contrast to conventional total energy calculations within Density Functional Theory framework, we carry out \textit{ab initio }molecular dynamics simulations for the two phases at high temperature. We show that at these conditions both bcc and fcc structures of Mo are dynamically stable, and the difference in their configurational energies decreases dramatically as compared to the zero temperature result, approaching the value derived by means of the thermochmical approach. We show that the main contribution to the effect comes from the modification of the canonical band structure for bcc and fcc phases due to lattice vibrations at high temperature, and discuss consequences of our finding for future first-principles simulations of phase stability. [Preview Abstract] |
Monday, March 10, 2008 8:12AM - 8:24AM |
A40.00002: Link between structural and mechanical stability of fcc- and bcc-based ordered Mg-Li alloys. Maje Phasha, Phuti Ngoepe, Hasani Chauke, Duc Nguyen-Manh, David Pettifor The first principles pseudopotential calculations based on the Perdew-Burke-Ernzerhof (PBE) form of generalized gradient approximation (GGA) within density functional theory (DFT) have been used to successfully investigate the electronic and elastic properties of cubic-based Mg-Li alloys. The heats of formation, Jones-type analysis and mechanical elasticity were utilized in predicting structural stability profile, and their results consistent with each other. An interesting correlation between shear modulus (C$\prime )$ and the predicted energy differences of corresponding bcc and fcc ordered compounds relative to hcp Mg and Li lattices is observed. [Preview Abstract] |
Monday, March 10, 2008 8:24AM - 8:36AM |
A40.00003: Density-Functional and CALPHAD Studies of U-Zr Alloys Alexander Landa, Per Soderlind, Patrice Turchi, Levente Vitos, Andrei Ruban The U-Zr alloy alloys have been recognized as a fuel for liquid-metal fast breeder reactors. First-principles methods are employed to study ground-state properties of U-Zr alloys for the most important phases observed experimentally, namely \textit{$\gamma $} (bcc) and \textit{$\delta $} (C32). Effective interatomic interactions obtained from the screened GPM, incorporating KKR-ASA-CPA, have been applied in MC simulations to derive the \textit{$\gamma $}-phase miscibility gap. EMTO-CPA method has been applied to study properties of the open \textit{$\delta $}-phase. Results of \textit{ab initi}o calculations are compared with experimental data and CALPHAD assessment. Then, the CALPHAD assessed U-Zr phase diagram is contrasted with the one predicted with the input from \textit{ab initio}. This work shows that an overall validity of a combined \textit{ab initio}--CALPHAD approach to thermodynamic properties exists, and that the knowledge and quantitative output gained from quantum mechanics on phase stability and its relation to $f$-bonding, can be used to explore other actinide-based systems, for which experimental data are sparse or lacking. This work performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. [Preview Abstract] |
Monday, March 10, 2008 8:36AM - 8:48AM |
A40.00004: Phase Stability Studies of Ni3Al and Pt3Al Structures Hasani Chauke, Ralf Drautz, Benoit Minisini, Phuti Ngoepe, David Pettifor The structural instability of cubic L12 against the non-cubic DO'C and tP16 Pt3Al have been investigated in direct comparison with Ni3Al, using plane-wave pseudopotential methods within the local density approximation. We predict that the deleterious DO'C is more stable than cubic ductile L12 Pt3Al, in agreement with the experimental observation of the LT DO'C phase and HT L12 phase. In contrast to the Pt-Al, the L12 Ni3Al is ground state and is consistent with the experimental phase diagram. The transformation from L12 into DO'C has been investigated more explicitly, where the transformation path along DO'C leads to a stable phase at displacement parameter u=0.041?. However the Ni3Al phase, is stable at u=0.0 corresponding to the cubic L12 phase, as expected. The phonon dispersion spectra have been used to confirm the relative structural trend where a soft mode was detected for L12 and DO'C which is not found in tP16 Pt3Al. [Preview Abstract] |
Monday, March 10, 2008 8:48AM - 9:00AM |
A40.00005: Where are Nature's missing structures Gus L.W. Hart Our society's environmental and economic progress depends on the development of high-performance materials such as lightweight alloys, high-energy-density battery materials, recyclable motor vehicle and building components, and energy-efficient lighting. Meeting these needs requires us to understand the central role of crystal structure in a material's properties. Despite more than 50 years of progress in first- principles calculations, it is still impossible in most materials to infer ground-state properties purely from a knowledge of their atomic components--a situation described as `scandalous' in the well-known essay by Maddox. Many methods attempt to predict crystal structures and compound stability, but here I take a different tack--to infer the existence of structures on the basis of combinatorics and geometric simplicity. The method identifies `least random' structures, for which the energy is an extremum (maximum or minimum). Although the key to the generic nature of the approach is energy minimization, the extrema are found in a chemistry-independent way. [Preview Abstract] |
Monday, March 10, 2008 9:00AM - 9:12AM |
A40.00006: Fully {\it ab initio} melting curve of aluminium up to 350 GPa Johann Bouchet, Francois Bottin, Gerald Jomard We performed {\it ab initio} molecular dynamics simulations to compute the melting curve of aluminium. Both the two-phase (TP) and heat-until melt (HUM) methods have been considered. The former describes an heterogeneous mechanism with a well-defined melting temperature T$_{\rm m}$. On the other hand, in the HUM the crystal melts homogeneously and can be overheated above T$_{\rm m}$. The limit of superheating is the critical temperature T$_{\rm LS}$. Calculations are carried out in the NVT ensemble up to 350 GPa with supercells of different sizes. For each method, we present the convergence of the melting curves as a function of the number of atoms, and compare our results with previous calculations and experiments. We also discuss the evolution of the degree of overheating with respect to the pressure. At last, by means of NPT simulations we also compute volume change on melting, which is in good agreement with previous free energy calculations. [Preview Abstract] |
Monday, March 10, 2008 9:12AM - 9:24AM |
A40.00007: Free energies from ab initio calculations for liquid Mg Raquel Lizarraga, Carl Greeff We performed free energy calculations for liquid metals from {\it ab initio} potential surfaces by means of Monte Carlo methods that involve sampling on the potential surface defined by a reference system. This allows for large gains in efficiency because the random walk is carried out on the much faster reference potential, and the {\it ab initio} potential is only evaluated on a small subset of uncorrelated configurations. This is highly desirable since direct free energy calculations for liquid metals from ab initio potential surfaces are very computationally intensive. Our calculations on liquid magnesium shows that we can obtain free energies accurate at the meV/atom level with only 100 evaluations of the {\it ab initio} total energies. [Preview Abstract] |
Monday, March 10, 2008 9:24AM - 9:36AM |
A40.00008: Ultrafast melting and solidification of Ag studied by time-resolved third harmonic generation Wai Lun Chan, Virginia McCreary, Alexie Lagoutchev, Yinon Ashkenazy, Kwangu Kang, David Cahill, Robert Averback We study the transformation of Ag between the solid and liquid phases using pump-probe femtosecond laser experiments. A pump with fluences of 100-500 mJ cm$^{-2}$ is used to heat the sample while third harmonic generation of light from a time-delayed probe is used to determine the structure as a function of time. For the melting experiment, we find that the solid begins to melt before the phonons are thermalized by the electrons, according to the two-temperature model. In addition, we find by using Ag thin films with different thicknesses, that the depth of heat deposition is less than 100nm. Both observations suggest that the established two-temperature model is insufficient to explain the melting kinetics in Ag by the femto-second laser. For the solidification experiment, we are able to determine the solidification velocity as a function of undercooling down to half of the melting temperature. The results are compared to predictions form molecular dynamics simulations. [Preview Abstract] |
Monday, March 10, 2008 9:36AM - 9:48AM |
A40.00009: Revisiting the segregation driving forces rule: the Coupled Three Effects Model J\'{e}r\^{o}me Creuze, Isabelle Braems, Fabienne Berthier, Christine Mottet, Guy Tr\'{e}glia, Bernard Legrand Separating the surface segregation enthalpy into three elementary contributions (cohesive, alloy and size) has been proposed by many authors, but rarely tested quantitatively. Such a separation rule, derived from a tight-binding Hamiltonian years ago, has yielded very satisfying results for various environments (surfaces, grain boundaries and clusters) for the Cu-Ag system and for many other alloys, but recently stumbled over the Co-Pt system. We propose a new approach based on a systematic study of the permutation enthalpies, both in the bulk and in the surface, as a function of the mixed interaction involved in the $N$-body interatomic potentials derived from the electronic structure. We then show that both the disagreement observed for Co-Pt and the agreement mentioned for Cu-Ag can be explained by the variation of the effective pair interactions in the surface and by the existence of coupling coefficients between the three effects. Finally, we introduce a new decomposition, the Coupled Three Effects Model (CTEM), that is valid for systems with both size and cohesive effects. [Preview Abstract] |
Monday, March 10, 2008 9:48AM - 10:00AM |
A40.00010: Ambient-temperature Conditioning as a Probe of Double-C Transformation Mechanisms in Pu-2.0 at. {\%} Ga. Jason R. Jeffries, K.J.M. Blobaum, M.A. Wall, A.J. Schwartz The gallium-stabilized Pu-2.0 at. {\%} Ga alloy undergoes a partial or incomplete low-temperature martensitic transformation from the metastable delta phase to the monoclinic alpha-prime phase near -120 $^{\circ}$C. This transformation has been shown to occur isothermally and it displays anomalous double-C kinetics in a time-temperature-transformation diagram. While the underlying mechanisms responsible for the double-C behavior are currently unresolved, recent experiments suggest that a conditioning treatment influences the upper-C. As such, the effects of the conditioning treatment can provide valuable insight into the mechanisms dominating the phase transition. A differential scanning calorimeter (DSC) is used to investigate the effects of conditioning temperature and time upon the delta/alpha-prime transition. The results will be discussed as they pertain to radiation damage, nucleation, embryo formation, or phase-field stability. [Preview Abstract] |
Monday, March 10, 2008 10:00AM - 10:12AM |
A40.00011: New Transformation Path of Shape Memory NiTi N. Hatcher, O. Yu. Kontsevoi, A.J. Freeman The detailed mechanism of structural evolution during the martensitic transformation in NiTi is not fully understood. To fully characterize the transformation path and to precisely determine the structure of its martensitic phase, we employ the highly-precise all-electron full-potential linearized augmented plane wave (FLAPW) method\footnote{Wimmer,\,Krakauer,\,Weinert,\,and\,Freeman,\,Phys.\,Rev.\,B,\,{\bf 24},\,864\,(1981)}, and identify the governing processes of the martensitic transformations between the B2, B19, R, B19$'$ and recently proposed B33 and BCO phases. Several precursor phenomena for displacive phase transformations in the B2, B19, and R phases are identified, i.e. soft C$_{44}$ and C$'$ elastic constants, regions of Fermi surface nesting, and instabilities towards electronic topological transitions. By means of generalized stacking fault calculations, we find that the B2 phase has a low resistance to $\langle100\rangle\{110\}$ shear. A new, barrrierless transformation path from B2 to B19$'$ is established by calculating a bilayer $\langle100\rangle\{110\}$ shear with full structural optimization which leads to a monoclinic intermediate phase at $1/2$a displacement; a relaxation of this structure's monoclinic angle results in the B19$'$ phase. [Preview Abstract] |
Monday, March 10, 2008 10:12AM - 10:24AM |
A40.00012: Magneto-Resistance at the Pre-Martensite Transition in Ni$_{2}$MnGa C. P. Opeil, J. C. Lashley, J. L. Smith A magneto-resistance and angle resolved photoemission (ARPES) study of the pre-martensite phase of the ferromagnetic shape memory alloy single crystal Ni$_{2}$MnGa, reveals a temperature (235 \underline {$>$} T \underline {$>$} 190 K) and field dependent (0 -- 1 T) positive/negative magneto-resistance slope. Previous inelastic neutron scattering experiments (Zheludev et al., PRB \textbf{51}, 1995) on this Heusler alloy indicate a phonon branch [110]-TA$_{2}$ softening in the pre-martensite phase along \textbf{q} = (1/3, 1/3, 0).~ This phonon softening combine with our ARPES data show that significant depletion of states (pseudo gap) occur at the premartensitic transition temperature. Recent results (Shapiro et al., EPL \textbf{77}, 2007) reveal phasons associated with the charge density wave (CDW) resulting from Fermi surface (FS) nesting.~ Our experimental results will be discussed in light of electron-phonon coupling. [Preview Abstract] |
Monday, March 10, 2008 10:24AM - 10:36AM |
A40.00013: Optical properties of ErH$_{2+x}$: First principles calculations and experimental measurements. Clark Snow, Thomas Mattsson Rare earth and transition metal hydrides exhibit many interesting physical phenomena, from metal-semiconductor transitions to transparency changes at RT as a function of hydrogen content. Electrical resistivity measurements by P. Vajda (1) indicate that ErH$_{2+x}$ undergoes a metal-semiconductor transition between 240-290K and an antiferromagnetic transition between 1.75-2.3K depending on hydrogen content. These same transitions should also cause profound changes in the optical properties. This work will present first principles calculations of the optical properties of ErH$_{2+x}$ from 300-600K where x ranges from -0.2 to 0.2. The calculations will be compared to experimental results on thin films of ErH$_{2+x}$ which were obtained as a function of temperature from 10-600K and hydrogen content. \newline [1] P. Vajda and J.N. Daou, Phys. Rev. B., Vol. 49, p. 3275 (1994). [Preview Abstract] |
Monday, March 10, 2008 10:36AM - 10:48AM |
A40.00014: Role of defects and impurities on the thermal stability of transition-metal nitrides and carbides L. Tsetseris, N. Kalfagiannis, S. Logothetidis, S. T. Pantelides Transition-metal nitrides and carbides are used in a variety of applications because of their renowned hardness and stability. Here, we present the results of first-principles calculations on point defects and impurities in the prototype systems of TiN and TiC, and in HfN and ZrN. We find features which are common to all systems, while we unravel also key differences. In cases, for example N interstitials in TiN, the interaction between defects is attractive and it favors the formation of defect complexes. Moreover, we show that the atomic-scale mechanisms of interaction and migration of point defects and their complexes can account for various changes of transition metal nitrides and carbides after annealing at widely different temperatures. Finally, we discuss the fundamentals of trapping and release of the most common impurities in TiN. The work was supported in part by the McMinn Endowment at Vanderbilt University, AFOSR MURI Grant FA9550-05-1-0306, and GSRT-PENED- 03ED613. [Preview Abstract] |
Monday, March 10, 2008 10:48AM - 11:00AM |
A40.00015: Atom Transport in Random Close Packed Metal Alloys under Thermal Forcing Yong W. Kim Alloy making entails disparate pyro-metallurgical pathways, contributing to variability in elemental composition profiles. Surface segregation by constituent elements is a long-standing example. Thermal cycling and exposures to intense fluxes of energetic particles and photons, as in fission and fusion reactors, force the movement of composition profiles. Transport properties thus become dependent of materials' history. We note that a non-crystalline alloy specimen is a randomly close packed assembly of atoms, and, as such, contains a distribution of residual nano-crystallites. The primary effect of varied forcing mechanisms is to convert atoms bound in nano-crystallites into those of glassy configurations. Spatial fluctuation is increased, specific to elemental species. In this paper we present a modeling of thermal conversion of crystallite atoms into a glassy state. The distribution function of nano-crystallites by size at room temperature is modeled by the distribution of nano-clusters formed from an atomic vapor plume.[Kim, Lee, Belony, Rev. Sci. Instr. \textbf{17}, 10F115 (2006)] At a given temperature, equilibrium dissociation of a nano-crystallite into `glassy' atoms is treated by the law of mass action. The equation of state is fashioned after the thermal expansion of the specimen volume with respect to a reference. A large set of simultaneous dissociation equations is solved iteratively. Work supported in part by NSF-DMR(Metals). [Preview Abstract] |
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