M. J. Kramer

Nature of the cubic to rhombohedral structural transformation in (AgSbTe2)15(GeTe)85 thermoelectric material

The existence of a large thermoelectric figure of merit in (AgSbTe2)15(GeTe)85 has been known for many years. However, the nature of the crystallographic transformation in these materials from a high-temperature cubic to a low-temperature rhombohedral polymorph and its effect on electrical transport has not been clearly established. Transmission electron microscopy studies were performed that show extensive twinning in the low-temperature structure, resulting from lattice strain during the dilation along the (111) crystallographic direction. Analysis of differential scanning calorimetric studies indicates that the transformation is of second order, so that the high-temperature cubic phase is nonquenchable. High-temperature x-ray diffraction was performed to establish the transformation temperature, which was found to be complete upon heating at a temperature of 510K. Results of electrical conductivity measurements as a function of temperature on as-cast samples are discussed in terms of the observed twinning.

Experimental and ab initio molecular dynamics simulation studies of liquid Al[subscript 60]Cu[subscript 40] alloy

X-ray diffraction and ab initio molecular dynamics simulation studies of molten Al{sub 60}Cu{sub 40} have been carried out between 973 and 1323 K. The structures obtained from our simulated atomic models are fully consistent with the experimental results. The local structures of the models analyzed using Honeycutt-Andersen and Voronoi tessellation methods clearly demonstrate that as the temperatures of the liquid is lowered it becomes more ordered. While no one cluster-type dominates the local structure of this liquid, the most prevalent polyhedra in the liquid structure can be described as distorted icosahedra. No obvious correlations between the clusters observed in the liquid and known stable crystalline phases in this system were observed.

Microscopic origin of slow dynamics at the good glass forming composition range in Zr1−xCux metallic liquids

We have studied the structure and dynamics of a series of Zr1−xCux metallic liquids at temperatures of 1500 and 1300 K. We found that as the Cu composition increases, the Zr–Zr order undergoes considerable transition, which can be attributed to the different size of Zr and Cu atoms. The diffusivities of both Cu and Zr atoms become lower at the composition range corresponding to good glass forming region, i.e., 0.50≤x≤0.70. We found that the lower diffusivities for the intermediate composition range are correlated with the concentration of some specific clusters, including icosahedra, which can be screened out automatically and unbiasedly by using direct atomic dynamics analysis. Our analysis for the dynamic properties of high temperature liquids of ZrCu metallic alloys shows that the icosahedral clusters together with some other pentagon-rich Voronoi clusters are responsible for slowing dynamics. Cluster lifetime analysis indicates that the slow clusters mobility could be originated in their energy.

Development of interatomic potentials appropriate for simulation of liquid and glass properties of NiZr2 alloy

A new interatomic potential for the Ni–Zr system is presented. This potential was developed specifically to match experimental scattering data from Ni, Zr and NiZr2 liquids. Both ab initio and published thermodynamic data were used to optimise the potential to study the liquid and amorphous structure of the NiZr2 alloy. This potential has the C 16 phase, being more stable than C 11b phase in the NiZr2 alloy, consistent with experiments. The potential leads to the correct glass structure in the molecular dynamics simulation and, therefore, can be used to study the liquid–glass transformation in the NiZr2 alloy.

Origins of coercivity in the amorphous alloy Nd-Fe-Al

Results obtained from detailed studies of the microstructure and devitrification behavior, as revealed by advanced electron microscopy and synchrotron x-ray diffraction techniques, were compared with magnetic and thermal analysis characterization to obtain insight into the appreciable coercivity of nominally-amorphous melt-spun ribbon of Nd/sub 60-2/3x/Fe/sub 30-1/3x/Al/sub 10+x/(-2</spl times/<6.5). The as-spun microstructure consists of an amorphous Nd-rich, magnetically-soft matrix with clusters of approximate 1.2 nm diameter that apparently confer the hard magnetic properties to the alloy. The clusters are tentatively identified as sub-unit-cells of Nd/sub 6/Fe/sub 13-x/Al/sub 1+x/, by virtue of their unique and analogous temperature-dependent magnetic character as well as considerations of the ternary phase relationships during solidification.

Transition metal carbide formation in the Nd2Fe14B system and potential as alloying additions

Abstract Precipitates controlling grain growth in rapidly solidified Nd2Fe14B-type magnets are commercially important for inhibiting grain growth during heat treatments or thermomechanical processing. Compound additions of Group IVA, VA, and VIA transition metals along with C were added to the Nd2Fe14B (2-14-1) system. Transition metal carbide (TMC) formation was found for the Group IVA and VA transition metals. All the TMC precipitates observed formed with a 1:1 stoichiometry. To be successful as alloying additions in the 2-14-1 system, additional criteria besides carbide formation are necessary. The alloying ability of each transition metal carbide system was graded using criteria that dealt with phase stability, liquid and equilibrium solid solubility, and high temperature carbide stability. The Group IVA TMC alloys satisfied the proposed alloying criteria and were found to be excellent additions. The Group VA alloys, while forming carbides, only partially satisfied the proposed alloying criteria. Finally, the Group VIA alloys did not form TMCs.

Experimental and computer simulation determination of the structural changes occurring through the liquid-glass transition in Cu-Zr alloys

Molecular dynamics (MD) simulations were performed of the structural changes occurring through the liquid–glass transition in Cu–Zr alloys. The total scattering functions (TSF), and their associated primary diffuse scattering peak positions (K p), heights (K h) and full-widths at half maximum (K FWHM) were used as metrics to compare the simulations to high-energy X-ray scattering data. The residuals of difference between the model and experimental TSFs are ∼0.03 for the liquids and about 0.07 for the glasses. Over the compositional range studied, Zr1− x Cu x (0.1 ≤ x ≤ 0.9), K p, K h and K FWHM show a strong dependence on composition and temperature. The simulation and experimental data correlate well between each other. MD simulation revealed that the Cu–Zr bonds undergo the largest changes during cooling of the liquid, whereas the Cu–Cu bonds change the least. Changes in the partial-pair correlations are more readily seen in the second and third shells. The Voronoi polyhedra (VP) in glasses are dominated by only a few select types that are compositionally dependent. The relative concentrations of the dominant VPs rapidly change in their relative proportion in the deeply undercooled liquid. The experimentally determined region of best glass formability, x Cu ∼ 65%, shows the largest temperature dependent changes for the deeply undercooled liquid in the MD simulation. This region also exhibits very strong temperature dependence for the diffusivity and the total energy of the system. These data point to a strong topological change in the best glass-forming alloys and a concurrent change in the VP chemistry in the deeply undercooled liquid.

Oxygen-stabilized glass formation in Zr80Pt20 melt-spun ribbons

The as-quenched structure of Zr80Pt20 melt-spun ribbons containing measured oxygen contents ranging from 184 to 4737 ppm mass was studied. Ribbons containing less than 500 ppm mass oxygen are fully crystallized and consist predominantly of a metastable ordered β-Zr phase with significant solution of Pt. Increasing oxygen content to 1053 and 1547 ppm mass produces a transition to fully amorphous and to mixed amorphous and quasicrystalline structures, respectively. Samples containing 4737 ppm mass consist of quasicrystalline and crystalline phases in an amorphous matrix. The results from this study suggest a critical level of oxygen is required to promote glass formation in Zr80Pt20 melt-spun ribbons produced at a specific quench rate.

Short- and medium-range order in a Zr[subscript 73]Pt[subscript 27] glass: Experimental and simulation studies

The structure of a Zr{sub 73}Pt{sub 27} metallic glass, which forms a Zr{sub 5}Pt{sub 3} (Mn{sub 5}Si{sub 3}-type) phase having local atomic clusters with distorted icosahedral coordination during the primary crystallization, has been investigated by means of x-ray diffraction and combining ab initio molecular-dynamics (MD) and reverse Monte Carlo (RMC) simulations. The ab initio MD simulation provides an accurate description of short-range structural and chemical ordering in the glass. A three-dimensional atomistic model of 18?000 atoms for the glass structure has been generated by the RMC method utilizing both the structure factor S(k) from x-ray diffraction experiment and the partial pair-correlation functions from ab initio MD simulation. Honeycutt and Andersen index and Voronoi cell analyses, respectively, were used to characterize the short- and medium-range order in the atomistic structure models generated by ab initio MD and RMC simulations. The ab initio results show that an icosahedral type of short-range order is predominant in the glass state. Furthermore, analysis of the atomic model from the constrained RMC simulations reveals that the icosahedral-like clusters are packed in arrangements having higher-order correlations, thus establishing medium-range topological order up to two or three cluster shells.

High thermal stability of carbon-coated L10-FePt nanoparticles prepared by salt-matrix annealing

Department of Physics, University of Texas at Arlington. Ames Laboratory and Department of Materials Science and Engineering, Iowa State University. NanoTech Institute, University of Texas at Dallas.