Experimental and ab initio molecular dynamics study of the structure and physical properties of liquid GeTe

GeTe is a prototypical phase-change material employed in data storage devices. In this work, the atomic structure of liquid GeTe is studied by x-ray and neutron diffraction in the temperature range from 1197 to 998 K. The dynamic viscosity is measured from 1273 to 953 K, which is 55 K below the solidification point, using an oscillating-cup viscometer. The density of liquid GeTe between 1293 and 973 K is determined by the high-energy γ-ray attenuation method. The experiments are complemented with ab initio molecular dynamics (AIMD) simulations based on density functional theory (DFT). Compatibility of the AIMD-DFT models with the diffraction data is proven by simultaneous fitting of all data sets in the frame of the reverse Monte Carlo simulation technique. It is shown that octahedral order dominates in liquid GeTe, although tetrahedral structures are also present. The viscosity of the equilibrium and weakly undercooled liquid GeTe obeys the Arrhenius law with a small activation energy of the order of 0.3 eV, which is indicative of a highly fragile liquid. The calculated density of states and electronic wave functions point to the existence of a pseudogap and localized electron states within the gap in the equilibrium liquid near the melting point as well as in the undercooled liquid

TEM study of the martensitic phases in the ductile DyCu and YCu intermetallic compounds

DyCu and YCu are representatives of the family of CsCI-type B2 rare earth intermetallic compounds that exhibit high room temperature ductility. Structure, orientation relationship, and morphology of the martensites in the equiatomic compounds DyCu and YCu are examined using transmission electron microscopy (TEM). TEM studies show that the martensite structures in DyCu and YCu alloys are virtually identical. The martensite is of orthorhombic CrB-type B33 structure with lattice parameters a = 0.38 nm, b = 1.22 nm, and c = 0.40 nm. (02 (1) over bar) twins were observed in the B33 DyCu and YCu martensites. The orientation relationship of B33 and B2 phases is (11 (1) over bar)[112]B33 parallel to (110)[001]B2. The simulated electron diffraction patterns of the B33 phase are consistent with those of experimental observations. TEM investigations also reveal that a dominant orthorhombic FeB-type B27 martensite with lattice parameters a = 0.71 nm, b = 0.45 nm, and c = 0.54 nm exists in YCu alloy. (I (1) over barI) twins were observed in the B27 YCu martensite. The formation mechanism of B2 to B33 and B2 to B27 phase transformation is discussed

Precipitation of T₁ and θ′ Phase in Al-4Cu-1Li-0.25Mn During Age Hardening: Microstructural Investigation and Phase-Field Simulation

Experimental and phase field studies of age hardening response of a high purity Al-4Cu-1Li-0.25Mn-alloy (mass %) during isothermal aging are conducted. In the experiments, two hardening phases are identified: the tetragonal θ′ (Al2Cu) phase and the hexagonal T1 (Al2CuLi) phase. Both are plate shaped and of nm size. They are analyzed with respect to the development of their size, number density and volume fraction during aging by applying different analysis techniques in TEM in combination with quantitative microstructural analysis. 3D phase-field simulations of formation and growth of θ′ phase are performed in which the full interfacial, chemical and elastic energy contributions are taken into account. 2D simulations of T1 phase are also investigated using multi-component diffusion without elasticity. This is a first step toward a complex phase-field study of T1 phase in the ternary alloy. The comparison between experimental and simulated data shows similar trends. The still unsaturated volume fraction indicates that the precipitates are in the growth stage and that the coarsening/ripening stage has not yet been reached

Giant thermal expansion and α-precipitation pathways in Ti-Alloys

Ti-Alloys represent the principal structural materials in both aerospace development and metallic biomaterials. Key to optimizing their mechanical and functional behaviour is in-depth know-how of their phases and the complex interplay of diffusive vs. displacive phase transformations to permit the tailoring of intricate microstructures across a wide spectrum of configurations. Here, we report on structural changes and phase transformations of Ti-Nb alloys during heating by in situ synchrotron diffraction. These materials exhibit anisotropic thermal expansion yielding some of the largest linear expansion coefficients (+ 163.9×10-6 to-95.1×10-6 °C-1) ever reported. Moreover, we describe two pathways leading to the precipitation of the α-phase mediated by diffusion-based orthorhombic structures, α″lean and α″iso. Via coupling the lattice parameters to composition both phases evolve into α through rejection of Nb. These findings have the potential to promote new microstructural design approaches for Ti-Nb alloys and β-stabilized Ti-Alloys in general

An Artificial SEI Layer Based on an Inorganic Coordination Polymer with Self-Healing Ability for Long-Lived Rechargeable Lithium-Metal Batteries

Upon immersion of a lithium (Li) anode into a diluted 0.05 to 0.20 M dimethoxyethane solution of the phosphoric-acid derivative (CF$_{3}$CH$_{2}$O)$_{2}$P(O)OH (HBFEP), an artificial solid-electrolyte interphase (SEI) is generated on the Li-metal surface. Hence, HBFEP reacts on the surface to the corresponding Li salt (LiBFEP), which is a Li-ion conducting inorganic coordination polymer. This film exhibits – due to the reversibly breaking ionic bonds – self-healing ability upon cycling-induced volume expansion of Li. The presence of LiBFEP as the major component in the artificial SEI is proven by ATR-IR and XPS measurements. SEM characterization of HBFEP-treated Li samples reveals porous layers on top of the Li surface with at least 3 μm thickness. Li−Li symmetrical cells with HBFEP-modified Li electrodes show a three- to almost fourfold cycle-lifetime increase at 0.1 mA cm$^{-2}$ in a demanding model electrolyte that facilitates fast battery failure (1 M LiOTf in TEGDME). Hence, the LiBFEP-enriched layer apparently acts as a Li-ion conducting protection barrier between Li and the electrolyte, enhancing the rechargeability of Li electrodes

Ti-Al composite wires with high specific strength

An alternative deformation technique was applied to a composite made of titanium and an aluminium alloy in order to achieve severe plastic deformation. This involves accumulative swaging and bundling. Furthermore, it allows uniform deformation of a composite material while producing a wire which can be further used easily. Detailed analysis concerning the control of the deformation process, mesostructural and microstructural features and tensile testing was carried out on the as produced wires. A strong grain refinement to a grain size of 250–500 nm accompanied by a decrease in h111i fibre texture component and a change from low angle to high angle grain boundary characteristics is observed in the Al alloy. A strong increase in the mechanical properties in terms of ultimate tensile strength ranging from 600 to 930 MPa being equivalent to a specific strength of up to 223 MPa/g/cm3 was achieved

Local texture measurements with high-energy synchrotron radiation on NiAl deformed in torsion

Plastic deformation leads to crystallographic preferred orientations (texture) of the grains in a polycrystalline sample. Therefore, the study of these textures gives informations about the slip systems activated during the deformation. In this study the deformation of polycrystalline NiAl was done by torsion under confining pressure leading to crack-free samples with a well-defined strain gradient. NiAl, an ordered intermetallic alloy with B2 structure, is a potential material candidate for high-temperature applications. Polycrystalline NiAl cylindrical samples with two different initial textures were deformed in torsion tests at 1000 K and 1273 K, respectively, in a Paterson-type rock deformation machine [1] under 400 MPa argon confining pressure. The diameter and height of the samples were 10 mm. The applied torsion leads to a simple shear in the tangential direction in a plane normal to the torsion axis. The shear strain and the shear strain rate in the samples increase linearly from zero at the torsion axis to a maximum ( ) at the sample edge. To investigate the local textures between the torsion axis and the edge, small pins with a diameter of 1 mm were prepared in the radial direction for each of the four deformed samples Quantitative texture measurements were performed with high-energy (100 keV) synchrotron radiation at the beamline BW5 [2], The incident monochromatic beam was defined by a slit system to 1 mm x 2 mm. The small pins were mounted in the Eulerian cradle parallel to the rotation axis ω. An image plate detector was positioned perpendicularly to the diffracted beam at a distance from the sample of about 1.3 m. Thus, the Debye-Scherrer rings with the indices (100), The texture was measured as a function of the shear strain at five different positions between γ = 0 and 3. The samples deformed at 1273 K showed a poor grain statistics due to a large grain size. The corresponding pole figures are not shown here. The torsion deformation at 1000 K leads to much smaller grains. The corresponding (100) pole figures are shown for γ = 1.5; 2.3 and 3 and two different initial texture