Bulletin of the American Physical Society
Fall 2009 Meeting of the Four Corners Section of the APS
Volume 54, Number 14
Friday–Saturday, October 23–24, 2009; Golden, Colorado
Session K2: Alloys |
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Chair: Gus Hart, Brigham Young University Room: Green Center 215 |
Saturday, October 24, 2009 1:50PM - 2:02PM |
K2.00001: New potential structure for jewelry application. Does it exist in Pt-Mo, Pt-Hf, or other systems? Erin Gilmartin, Jacqueline Corbitt, Gus Hart The only known intermetallic structure with an 8:1 stoichiometry is that of Pt$_8$Ti. It is intriguing that an ordered phase would occur at such low concentrations of the minority atom, but this structure occurs in about a dozen binary intermetallic systems. The formation of an ordered phase in an alloy can significantly enhance the performance of the material, particularly the hardness. We have taken a broad look at possible systems where this phase forms. Using first-principles, we calculated the stability of this structure relative to experimentally known phases for more than 70 Pt/Pd binary systems. We find the Pt$_8$Ti structure is a possible ground state in more than 20 cases. Our experimental collaborators have verified our prediction in Pt-Mo and observed order-hardening in Pt-Hf. We discuss the discovery of new ground states via the cluster expansion that are likely to be verified experimentally and their impact on Pt- and Pd-based jewelry and catalysts. [Preview Abstract] |
Saturday, October 24, 2009 2:02PM - 2:14PM |
K2.00002: Crystal structure of UNi$_{0.5}$Sb$_{2}$ Karunakar Kothapalli, Milton Torikachvili, Heinz Nakotte We report the single crystal neutron diffraction studies done to resolve the room-temperature structure of Uranium antimonide, UNi$_{0.5}$Sb$_{2}$. The time-of-flight single-crystal neutron diffraction experiments at room temperature were done on the Single Crystal Diffractometer, SCD, at Los Alamos Neutron Science Center. Previous X-ray single crystal and neutron powder diffraction studies could not unambiguously resolve the structure because of the presence of hkl/2 type reflections. The studies were done on a 2 x 1 x 0.5 mm$^{3}$ crystal and half-indexed reflections were observed corroborating the observations in previous studies. The crystal structure that accounts for all the observed reflections is determined to be tetragonal P4$_{2}$/n m c with lattice parameters a, b, c being 4.333(2) {\AA}, 4.333(2) {\AA} and 17.868(6) {\AA} respectively. A preliminary study shows no crystal structure distortion below at 10K and the compound orders antiferromagnetically. [Preview Abstract] |
Saturday, October 24, 2009 2:14PM - 2:26PM |
K2.00003: Thermal and electrical transport properties of UCu$_{4+x}$Al$_{8-x}$ Farzana Nasreen, Milton Torikachvilli, Karunakar Kothapalli, Vivien Zapf, Heinz Nakotte The UCu$_{4+x}$Al$_{8-x}$ family crystallizes in the tetragonal ThMn$_{12 }$- type structure in the range from 0.1$\le $ x$\le $ 1.95. It has been reported that the Cu-poor compounds show antiferromagnetic long-range order, followed by a transition at x=1.15 to a heavy fermion behavior. We report on the results of thermal conductivity and the Seebeck coefficient as a function of temperature (1.8-300K). Thermal conductivity data are consistent with previously published electrical resistivity data. The Seebeck coefficient measurements, S, confirm the peaks at T$_{N}$ for the antiferromagnetic compounds. We also measured electrical resistivity as function of very low temperature from 75mK to 4K and in magnetic field up to 6Tesla for UCu$_{6}$Al$_{2}$, UCu$_{5.75}$Al$_{6.25}$, UCu$_{5.5}$Al$_{6.5}$ and UCu$_{5.25}$Al$_{6.75}$. UCu$_{5.75}$Al$_{6.25 }$which was reported as non-Fermi liquid (NFL) compound shows quantum critical point induced by magnetic field. These results provide some insight about the underlying mechanisms to the apparent NFL behavior in UCu$_{5.75}$Al$_{6.25}$ compound. [Preview Abstract] |
Saturday, October 24, 2009 2:26PM - 2:38PM |
K2.00004: Platinum-Palladium Crystal Structures Weston Pratt Being able to predict Platinum-Palladium ordering is important in discovering new alloys that have commercial and industrial applications. Using direct quantum mechanical calculations coupled with a lattice-based Hamiltonian called a cluster expansion, we can predict which crystal structures are thermodynamically stable for. In addition, a Monte Carlo simulation can be used in this model to determine the order-disorder transition temperatures. Knowing which structures are thermodynamically stable and their respective transition temperatures may help develop useful platinum palladium alloys. [Preview Abstract] |
Saturday, October 24, 2009 2:38PM - 2:50PM |
K2.00005: New structures in Pd-rich ordered alloys Jacqueline Corbitt, Erin Gilmartin, Gus Hart An intriguing intermetallic structure with 8:1 stoichiometry was discovered in the 1950s in the Pt-Ti system. Since then a handful of other Pt/Pd/Ni-X binary systems have been observed to exhibit this curious structure (Pt$_8$Zr, Pd$_8$Mo,Ni$_8$Nb, etc). Precipitates of this ordered structure can significantly increase the hardness of an alloy. For jewelry applications involving Pt and Pd, international hallmarking standards require that the alloys be at least 95\% pure by weight. However, Pt- and Pd-rich alloys are often soft when purity is high if the minority atoms are disordered. Because the 8:1 structure maintains a high weight percentage of Pt/Pd, it can satisfy purity standards while increasing performance. Recent calculations and experiments suggest that the 8:1 structure may form in about 20 previously unsuspected Pt/Pd binary systems. Using first-principles calculations and cluster expansion modeling, we have performed a ground state search to find the stable structures in Pd-Nb, Pd-Cu, and Pd-Mg, and predict the temperatures at which the new structures may form. [Preview Abstract] |
Saturday, October 24, 2009 2:50PM - 3:02PM |
K2.00006: Thermodynamically stable superstructures in binary alloys Lance Nelson Adding a second metal to another can induce the formation of ordered superstructures. These ordered phases have properties that are desireable in many industrial, manufacturing and technological applications. Our goal is to find which of the thousands of possible superstructures are thermodynamically stable through the use of computational tools. Owing to the many superstructures that are possible, as well as the complex nature of some of these, DFT calculations become impractical for searching for these superstructures. We employ a cluster expansion method, which uses energy information from a relatively small number of structures and fits that information to a set of interaction types. Because the resulting expansion provides a fast way to compute energies, we can use it to calculate the energies of the thousands of other superstructures. Specifically, I discuss the use of the cluster expansion on two binary alloys: AgPd and MgZn. Palladium alloys are of interest in the fabrication of jewelry, and a stable ordered phase at some concentrations would be a breakthrough for the jewelry manufacturers. Magnesium alloys are of interest because of their strength and light weight. They are being used increasingly in the manufacturing of things such as airplanes and automobiles. A cheap alloying agent that promotes the formation of an ordered structure would be a breakthrough. [Preview Abstract] |
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