Short-range Order Domains Research Articles

Improved Structural Description of Different γ-Al2O3 Materials Using Disordered δ5-Al2O3 Phase via X-ray Pair Distribution Function Analysis

Extensive research has been conducted in the past on the crystallographic characteristics of γ-Al2O3 support materials due to their advantageous properties in heterogeneous catalysis. While their structure is most commonly described as spinel, their intrinsic disorder and nanostructure have prompted alternative models involving tetragonal space groups, supercells, or occupancy of non-spinel positions. X-ray pair distribution function (PDF) analysis has further postulated the existence of short-range order domains with structural remnants from boehmite precursors from which γ-Al2O3 is commonly prepared via calcination. In this PDF study, we now show that a recently theoretically found monoclinic δ5-Al2O3 phase is, in fact, best suited for describing the structure of different commercial Al2O3 supports, as well as a self-prepared and an industrial Ni/Al2O3 methanation catalyst. Furthermore, in situ experiments under catalytic cycling in the methanation reaction demonstrate that the nanoscale structure of this δ5 phase is preserved during cycling, pointing towards the high stability of the therein-represented disorder. A complete description of the disordered Al2O3 support structure is crucial in the field of heterogeneous catalysis in order to distinguish disorder within the bulk support from additional interfacial restructuring processes such as surface oxidation or spinel formation due to nanoparticle–support interactions during catalytic cycling in in situ scattering experiments.

Destruction of mesoscopic chemically modulated domains in single phase high entropy alloy via plastic deformation

Chemically modulated mesoscopic domains in a fcc single phase CrMnFeCoNi equi-atomic high entropy alloy (HEA) are detected by small angle diffraction performed at a synchrotron radiation facility, whereas the mesoscopic domains cannot be detected by conventional X-ray diffraction and 2D mappings of energy dispersive X-ray spectroscopy by scanning electron microscopy and scanning transmission electron microscopy. The mesoscopic domains are deformed and shrieked, and finally destructed by plastic deformation, which is supported by the comprehensive observations/measurements, such as electrical resistivity, Vickers hardness, electron backscattering diffraction, and hard X-ray photoemission spectroscopy. The destruction of the mesoscopic domains causes the decrease in electrical resistivity via plastic deformation, so called K-effect, which is completely opposite to the normal trend of metals. We confirmed that the presence and the size of local chemical ordering or short-range order domains in the single phased HEA, and furthermore, Cr and Mn are related to form the domains.

Short-range order strengthening in boron-doped high-entropy alloys for cryogenic applications

Boron doping with an adequate concentration is highly desirable for alloy development because it profoundly improves the material's interface cohesion via interfacial segregation. However, scientific and applicable potentials of soluble boron that resides at the alloy internal grains are generally overlooked. Here we report a strategy for overcoming the typically low strengths of face-centered cubic high-entropy alloys (HEAs) through exploiting soluble boron instead of the interfacial boron. We find that soluble boron increases stress/strain field at the recrystallized HEA grain structure, leading to the generation of short-range order (SRO) in those deformation structure under load at 77 K. The highly increased degree of SRO at planar dislocation slip band that forms during straining, proved by electron microscopy and synchrotron X-ray diffraction, strengthens a typical non-equimolar Fe40Mn40Co10Cr10 (at%) HEA, particularly increasing yield strengths by ∼32%, to ∼1.1 GPa compared to those of boron-free reference materials with similar grain sizes. The advent of deformation-induced SRO domains causes severe lattice distortion (specifically, contraction), leading to the increased cryogenic yield strength of ∼210 MPa, but generating micro-voids at grain boundaries. This study on deformation-induced SRO via boron advances the fundamental understanding of SRO impacts on HEA grains and mechanical properties at cryogenic temperatures, which may pave a general pathway for developing a wide range of ultrastrong alloys for cryogenic applications.

Investigation of the low-temperature mechanical behavior of elastomers and their carbon nanotube composites using microindentation

The micromechanical properties of epoxy resin elastomers and their carbon nanotube composites were studied using a microhardness tester equipped with low-temperature chamber. X-ray diffraction analysis indicated that all specimens were free of any crystalline components and were amorphous with only short-range order domains. The Vickers microhardness of all samples has been estimated in the temperature range 230–300 K. The measurements demonstrated that at room temperature these materials are elastomers (notably, they are in high-elastic state) and on cooling in the range of 250–270 K the glass transition takes place. Analysis of the temperature dependence of microhardness suggested that the thermomechanical and relaxation properties of the materials studied are consistent with a rheological model of a standard linear solid where the relaxation time (or viscosity) depends exponentially on the temperature in accordance with the Arrhenius equation for the rate of thermally activated process. Empirical estimates for the nonrelaxed and relaxed Young’s moduli and also for the activation energy (U = 0.75 eV) and the period of attempts (τ0 = 10–12 s) of the molecular process which determines the relaxation properties and the glasstransition of the materials have been obtained. The addition of carbon nanotubes into elastomeric epoxy resin had no effect on its micromechanical characteristics as measured by the microhardness tester. It is shown that the conventional microindentation method is an efficient tool of investigating the thermomechanical properties of elastomers nearby and below the glass transition temperature.

Precipitation of coherent Ni2(Cr, W) superlattice in an Ni–Cr–W superalloy

It is demonstrated that a nanometer-sized Ni2(Cr, W) superlattice with a Pt2Mo-type structure can precipitate in an Ni–Cr–W alloy by means of a simple aging treatment at 550°C. The dark-field image of short-range order domains has been found for the first time experimentally. The mechanism of short-range order to long-range order transformation has been revealed based on transmission electron microscopy result and static concentration waves theory and found to be continuous ordering. The randomness of the transformation of static concentration waves leads to equiprobable occurrence of the different variants. The transformation of short-range order to long-range order gives rise to the Pt2Mo-type Ni2(Cr, W) superlattice. The interfaces between Ni2(Cr, W) and Ni-based matrix and the different variants of Ni2(Cr, W) have been investigated by high resolution transmission electron microscopy. The results reveal that the interfaces between Ni2(Cr, W) and surrounding matrix are coherent at the atomic scale.

Friction Stir Welding of Bulk Metallic Glass Vitreloy 106a

A Zr58.5Nb2.8Cu15.6Ni12.8Al10.3 (Vitreloy 106a) bulk metallic glass (BMG) was successfully welded by friction stir welding (FSW) with a fixed polycrystalline cubic boron nitride (PCBN) pin tool below its crystallization temperature. The microstructure was analyzed by X-ray diffraction (XRD) to evaluate the crystallization of the weld. The reduced radial distribution function (RDF), short-range order (SRO) domain size, and atomic pair distribution function (PDF) were analyzed to evaluate the effect of FSW at atomic scale. As a result, the SRO domain size was reduced for the weld surface, while increased for the weld nugget.

Thermodynamic calculation and experimental investigation of glass formation in Zr–Ni–Ti alloy system

Based on the undercooling theory resulting from the existence of multicomponent chemical short-range order domains, the glass forming range (GFR) in Zr–Ni–Ti alloy system was predicted by thermodynamic calculation. In order to verify the calculated results, a series of alloys Zr 90− x Ni x Ti 10 ( x = 25, 30, 35, 40, 45, 50, 55 at.%) were designed to conduct melt-spinning experiments. The X-ray diffraction (XRD) results show that a fully amorphous structure can be obtained in as-quenched Zr 90− x Ni x Ti 10 ( x = 25, 30, 35, 40, 45 at.%) alloys, while Zr 40Ni 50Ti 10 and Zr 35Ni 55Ti 10 alloys are partially amorphous and fully crystalline, respectively. These experimental results are consistent with the predicted GFR. In addition, the glass forming ability (GFA) was investigated by differential scanning calorimetry (DSC) experiments and evaluated by a thermodynamic parameter, the reduced undercooling Δ T r.

Direct observations of nucleation in a nondilute multicomponent alloy

The chemical pathways leading to gamma-prime(L1_2)-nucleation from nondilute Ni-5.2 Al-14.2 Cr at.%, gamma(f.c.c.), at 873 K are followed with radial distribution functions and isoconcentration surface analyses of direct-space atom-probe tomographic images. Although Cr atoms initially are randomly distributed, a distribution of congruent Ni3Al short-range order domains (SRO), <R> =0.6 nm, results from Al diffusion during quenching. Domain site occupancy develops as their number density increases leading to Al-rich phase separation by gamma-prime-nucleation, <R>=0.75 nm, after SRO occurs.

Nanofibrillar Structure and Molecular Mobility in Spider Dragline Silk

Nephila edulis dragline silk has been investigated by elastic and quasielastic neutron scattering techniques. The data support a three-phase model of nanofibrils, composed of crystalline and short-range order domains, which are embedded in an amorphous matrix of random protein chains. A meridional superlattice peak in D2O-exchanged silk is tentatively assigned to a smectic β-sheet structure in the short-range order domains. Water is absorbed preferentially by the amorphous matrix. The onset of molecular mobility in hydrated silk at about 200 K is attributed to water molecules, which move on the picosecond time scale. Coexisting slower motions on the nanosecond time scale are possibly due to polymer chain dynamics. Native silk at rest appears to behave similar to a glass below 300 K.

Chemical short-range order domain in bulk amorphous alloy and the prediction of glass forming ability

Short-range order domains of face central cubic Zr 2 Ni (F-Zr 2 Ni) and tetragonal Zr 2 Ni (T-Zr 2 Ni) type structure with a size about 1—3 nanometers were observed in bulk amorphous Zr 52.5 Cu 17.9 Ni 14.6 Al 10 Ti 5 alloy by using HREM and nano-beam electron diffraction technique. A new thermodynamic model was formulated based on the concept of chemical short-range order (SCRO). The molar fractions of CSRO and thermodynamic properties in Ni-Zr, Cu-Zr, Al-Zr, Al-Ni, Zr-Ni-Al and Zr-Ni-Cu were calculated. According to the principle of maximum Δ G CSRO , the optimum glass forming ability (GFA) compositions were predicted in binary and ternary alloys. These results were proved to be valid by the experimental data of crystallizing activation energy, Δ T x and XRD patterns. The TTT curves of Zr-Ni-Cu alloys calculated based on CSRO model shows that the lowest critical cooling rate GFA is in the order of 100 K/s, which is close to the practical cooling rate for the preparation of Zr-based BMG alloys.

Concept of chemical short range order domain and the glass forming ability in multicomponent liquid

The concept of multicomponent chemical short-range order (MCSRO) domain is systematically developed by the experimental investigation of Zr–Ti–Cu–Ni–Al bulk metallic glass (BMG) and thermodynamic modeling and calculation. The existence of MCSRO domains in Zr-based BMG is verified by the observations of high-resolution transmission electron microscopy (HRTEM) images and the analysis of nano-beam electron diffraction patterns. The size of the nano-beam used in this work is 0.5 nm in diameter. Thermodynamic evaluation of the melt composed of multiple-MCSRO domains and glass-forming ability (GFA) based on the concept of MCSRO domains has also been conducted. It is indicated that the thermodynamic calculation of the GFA based on MCSRO model is consistent with the experimental data of crystallization activation energy and glass transition temperature for Ni-Zr and Zr-Cu binary alloys, and with supercooled liquid region (ΔTx) for Zr–Ni–Al ternary alloy. The existence of MCSRO domain lowers the free energy of the melt (ΔGMCSRO), resulting in a large undercooling and a larger energy barrier to the nucleation of a critical crystalline nucleus. Large ΔGMCSRO, low melting point as well as co-existence of multiple MCSRO domains are valid criterion for the valuation of GFA.

Influence of the liquid states on the crystallization process of nanocrystal-forming Zr–Cu–Pd–Al metallic glasses

Rapidly solidified ribbons of nanocrystal-forming Zr–Cu–Pd–Al metallic glasses were prepared at various liquid temperatures (TL). Differential scanning calorimetry (DSC) traces show clearly the influence of the liquid states on the thermal properties and crystallization process. Namely, with increasing TL, the exothermal peaks of the DSC traces shift to higher temperatures, the super-cooled-liquid region ΔTx increases, and the decomposition of the metastable compound Zr2(Cu, Pd) becomes more difficult. These results suggest that the liquid state strongly controls the crystallization process of the nanocrystal-forming metallic glasses. This behavior may originate from the variation of the quenched-in nuclei, which highly depends on the short-range-order domains in liquid with different TL. We suggest that the stronger attractive interaction in Zr–Pd, which exhibits large negative mixing enthalpy, leads to the short-range order domains.

Anisotropic amplitudes of adsorbate bending vibrations from x-ray photoelectron diffraction: A multiple-scattering evaluation.

The C 1s photoelectron angular emission from a disordered CO layer on Ni(110) at 300 K (coverage approximately 0.7 monolayer) was studied for polar angles up to 80\ifmmode^\circ\else\textdegree\fi{} and several azimuths. The experimental intensity distributions are compared to multiple-scattering cluster calculations which include the possibility of anisotropic bending vibrations of the adsorbed molecule based on a new formalism for thermal averaging in the diffraction theory. From the azimuth-dependent width of the forward scattering peak we have determined bending vibraional amplitudes of 12\ifmmode^\circ\else\textdegree\fi{} and 17\ifmmode^\circ\else\textdegree\fi{} for the [11\ifmmode\bar\else\textasciimacron\fi{}0] and [001] azimuths, respectively. The optimum agreement between experiment and theory over the whole range of polar angles is obtained for a CO layer with short-range order domains and equal occupation of on-top and short-bridge sites.

Digital image processing for high resolution electron microscopy

Digital image processing has become an important tool for the interpretation and analysis of high resolution electron micrographs. Image processing allows to remove unwanted contrast-effects from the image, which are caused by uncontrollable exterior factors. This allows the maximum amount of essential image detail to be extracted. The HREM image processing techniques that will be discussed are: real space lattice averaging, image subtraction methods, Fourier filtering techniques, and cross-correlation for pattern recognition. Their application will be illustrated for typical interpretation problems in experimental high resolution electron microscopy, such as the nature of stacking fault tetrahedra in silicon, catalyst particle localisation in zeolites, lateral extent of wavy antiphase boundaries in Cu3Pd, short range order domains in Au4Cr, and GaAs/AlAs interfaces. Digitale Bildverarbeitung ist zu einem wichtigen Mittel für die Interpretation und Analyse von hochaufgelösten Elektronenmikrogrammen geworden. Bildverarbeitung erlaubt die Beseitigung unerwünschter Kontrasteffekte aus dem Bild, die durch unkontrollierbare äußere Faktoren verursacht werden. Dies erlaubt die Extraktion des Maximalgehalts der wesentlichen Bilddetails. Die hier diskutierten HREM-Bildverarbeitungstechniken sind: Realraumgittermittelung, Bildsubtraktionsmethoden, Fourierfilterungstechniken und Kreuzkorrelationen für Mustererkennung. Ihre Anwendung wird für typische Interpretationsprobleme bei der experimentellen Hochauflösungsmikroskopie illustriert, zum Beispiel für die Art der Stapelfehlertetraeder in Silizium, Katalyseteilchenlokalisierung in Zeoliten, laterale Ausdehnung von welligen Antiphasengrenzen in Cu3Pd, Nahordnungsdomänen in Au4Cr und GaAs/AlAs-Grenzflächen.