Changes In Atomic Order Research Articles

Suppression of crystallization in a Ca-based bulk metallic glass by compression

We have studied the crystallization kinetics of Ca40Mg25Cu35 bulk metallic glass under isothermal annealing in the supercooled liquid region, with and without applied stress, by in-situ high-energy synchrotron x-ray diffraction. We observed that without stress, crystallization started from the very beginning of isothermal annealing. However, when a uniaxial compressive stress was applied during isothermal annealing, no change in atomic order was observed at the beginning. Thereafter, the glassy structure relaxed, but still without noticeable evidence of crystallization. It is suggested that the applied compressive stress restricted the atomic mobility, which suppressed crystallization.

Molecular origins of anisotropic shock propagation in crystalline and amorphous polyethylene

Molecular dynamics simulations are used to analyze shock propagation in amorphous and crystalline polyethylene. Results for the shock velocity ${U}_{s}$ are compared to predictions from Pastine's equation of state and hydrostatic theory. The results agree with Pastine at high impact velocities. At low velocities the yield stress becomes important, increasing the shock velocity and leading to anisotropy in the crystalline response. Detailed analysis of changes in atomic order reveals the origin of the anisotropic response. For shock along the polymer backbone, an elastic front is followed by a plastic front where chains buckle with a characteristic wavelength. Shock perpendicular to the chain backbone can produce plastic deformation or transitions to different orthorhombic or monoclinic structures, depending on the impact speed and direction. Tensile loading does not produce stable shocks: Amorphous systems craze and fracture while for crystals the front broadens linearly with time.

Multiscale structural changes of atomic order in severely deformed industrial aluminum

The regularities of multiscale structural changes in the atomic order of the aluminum alloy AD-1 after a severe cold plastic deformation by conventional rolling in smooth rolls or in rolls with relief recesses favorable for shear deformation have been investigated. It has been found that there are four types of structural fractions that differ in scale and perfection of atomic order: crystallographic planes with a long-range order; nanoscale fragments of the planes (D = 100–300 A) with an incipient long-range order; smaller groups of atoms (D = 20–30 A) of amorphized structure; and the least ordered structural fraction of intercluster medium, keeping only a short-range atomic order (2–3 interatomic distances, 10 A). The presence of diffuse halo bands in the region of intense Debye lines indicates phase transitions of the order → disorder type with the formation of one to three groups of amorphous clusters with the dominance, in the nanometer scale, of the atomic order characteristic of the family of planes (111), (220), and (311) of crystalline aluminum. We have found a dynamic phase transition with the changing crystallographic order of aluminum, with the matrix structure of a face-centered cubic (FCC) lattice, in the form of nanosized local groups of atoms, that is, the deformation clusters of aluminum with a simple cubic K6 lattice. In the case of conventional rolling, the development of large clusters 50–500 A in size is observed; however, in the use of rolls with relief recesses, the difference in the sizes of the clusters is one half as much: 50–250 A. Based on the analysis of the integrated intensity of incoherent X-ray scattering by the samples, we have elucidated the nature of the lowest measured density for the sample subjected to conventional rolling, which consists in the volume concentration of disorderly arranged atoms, the highest of the compared structures, which indicates the formation therein of the greatest amount of fluctuation “voids.”

Atomic order and martensitic transformation entropy change in Ni–Co–Mn–In metamagnetic shape memory alloys

The martensitic transformation entropy changes, ΔStr, measured from the stress-induced transformation in Ni–Co–Mn–In crystals with different degrees of long range L21 atomic order, follow a single temperature dependence and show good agreement with the theoretical Bragg–Williams approximation. The strong influence of the atomic order on ΔStr is attributed to the large shift in the transformation temperatures caused by the change in atomic order and the natural temperature dependence of the austenite magnetization. The effects of associated changes in magnetic order have a negligible contribution to ΔStr. Additional mechanical measurements performed under 1.6T do not show any notable changes in ΔStr.

Atomistic mechanism of cyclic phase transitions in Nd–Fe–B based intermetallics

Mechanical milling induced atomic disorder and the mechanism of the cyclic crystal → amorphous → crystal phase transitions in a Nd 2Fe 14B intermetallic – based Nd 10Fe 85B 5 alloy were investigated. Least square first shell fitting of Extended X-ray Absorption Fine-Structure (EXAFS) data of mechanically milled samples revealed that phase evolution during mechanical milling of Nd 10Fe 85B 5 alloy occurred in two stages as a result of two dynamical effects: milling induced forced atomic mixing and enhanced diffusion due to continuous production of point defects. In the first stage of milling, atomic mixing of Nd and Fe atoms dominated, resulting in extensive chemical and structural disorder in the initially ordered intermetallic, followed by amorphization. The second stage of milling was characterized by a remarkable increase in point defect density. Enhanced diffusion resulted in redistribution of alloy atoms and annihilation of these point defects leading to the precipitation of α-Fe nanocrystals in the amorphous matrix. Thus the time evolution of defect structures formed during mechanical milling and their atomic scale interactions led to the cyclic phase transitions in the Nd 10Fe 85B 5 alloy. These milling induced changes in atomic order were found to strongly influence magnetic properties. Crystallization of mechanically milled alloy and effect of annealing parameters on magnetic properties was also analyzed.

Toward Understanding the Chloroquine Action at the Molecular Level in Antimalarial Therapy − X-ray Absorption Studies in Acetic Acid Solution

The local atomic structure around the central iron of the synthetic soluble analog of malarial pigment in acetic acid solution and with addition of chloroquine as found by X-ray absorption spectroscopy is reported. The special interest was drawn to the axial linkage between the central iron atom of the ferriprotoporphyrin IX (FePPIX) coordinated axially to the propionate group of the adjacent FePPIX. This kind of bonding is typical for hematin anhydride. Detailed analysis revealed differences in oxygen coordination sphere (part of dimer linkage bond) between synthetic equivalent of hemozoin in the powder state and dissolved in acetic acid and water at different concentrations mimicking the physiological condition of the parasite's food vacuole. The results of performed studies suggest that the molecular structure of synthetic analogue of hemozoin is no longer dimer-like in acidic solution. Further changes in atomic order around Fe are seen after addition of the antimalarial drug chloroquine.

Tuning martensitic transformation and magnetoresistance effect by low temperature annealing in Ni45Co5Mn36.6In13.4 alloys

We studied the influence of post-annealing on magnetic and transport properties in Ni45Co5Mn36.6In13.4 alloys. Our results demonstrate that post-annealing at low temperatures, ⩽300 °C, can lead to a significant change in magnetic properties, martensitic temperature (TM) and magnetoresistance effect through structure relaxations and possible change in atomic order. It was found that TM shifts from 314 to 283 K but the strong metamagnetic behaviour is still retained when the sample is annealed at 300 °C for 3 h. Thereupon, a large magnetoresistance effect over an extended temperature range can be achieved through controlling the heat treatment conditions. Meanwhile, the thermal stability of the novel composition is also disclosed through the investigations on low temperature annealing effect.

X-Ray Absorption Studies of Ge Layers Buried in Silicon Crystal

Polarization-dependent X-ray absorption spectroscopy was used to study the local microstructure of Ge layers buried in silicon. The layers with thickness from 6 to 20 monolayers of Ge were grown by molecular beam epitaxy on Si substrate and were covered by Si (20 nm). To investigate the morphology of grown structures, X-ray absorption near edge structure and extended X-ray absorption fine structure analysis of the Ge K-edge was done. The performed qualitative analysis proves that X-ray absorption spectra are very sensitive to the local order in the formed structures and are sources of unique information about morphology of the buried Ge layers. Using these techniques we were able to observe the changes in atomic order around the Ge atoms in investigated buried layers and compare the formed atomic order with that in crystalline Ge. A substantial increase in intensity, broadening and chemical shift of the X-ray absorption near edge structure spectrum for 8 ML were observed. It can be related to the increase in density of electron states caused by increase in the localization of the states due to potential appearing at the Ge island boundaries and indicated the formation of quantum dots. The observed in-plane modulations of radial distribution and out-of-plane for different layers were discussed.

A lattice gas model for the Zener relaxation

The Zener relaxation is an anelastic relaxation process in disordered crystals due to stress-induced changes of atomic order. We present a model calculation of this process for a simple cubic lattice gas which is non-interacting except for the exclusion of multiple occupancies. The relaxation results from the stress-induced formation and dissolution of bonds between (paired) lattice gas atoms on nearest-neighbour sites. Relaxation spectra are obtained for the compressibility and the shear compliance . The spectra are proportional to , where c is the occupation probability of a given site. The frequency dependence of the spectra is determined by the jump rate of the lattice gas atoms. Compared with a spectrum for a single relaxation time, the spectrum of is moderately broadened, whereas the spectrum of shows a sizable broadening. The present results are applicable to both interstitial and substitutional alloys.

The effect of mechanical milling on the structure and magnetic properties of Fe2Sc Laves phase

Abstract The Fe2Sc intermetallic compound with C14 Laves phase structure was mechanically milled. The structural evolution and the change in atomic order were followed by X-ray diffraction and Mossbauer spectroscopy. The influence of mechanical milling on the magnetic properties was studied by Mossbauer spectroscopy and magnetization measurement. The results indicated that ball milling induced a partial chemical disorder followed by an amorphization. The partial chemical disorder and the amorphization lead to an increase in magnetic moment of Fe in the materials. The amorphous phase exhibits a ferromagnetic behavior with Curie temperature of about 250 K, which is much lower than that of 540 K for the original compound. The crystallization temperature of the amorphous phase is about 873 K, as determined by DSC measurement.

Observation of a temporal evolution of defected epitaxial erbium silicide layers at room temperature

We present angle-resolved photoemission spectra of the √3×√3R30° ErSi 2−x (0001) surface (x∼0.3) under ultra-high vacuum conditions at room temperature. An ordering process in the atomic structure of the topmost layers is clearly observed over several hours or days. This change in atomic order is confirmed by low-energy electron diffraction. It is particularly marked for ultra-thin silicide layers grown by solid phase epitaxy or coevaporation in the 500°–600°C temperature range.

Electrode stability of LaNiMn hydride-forming materials prepared by conventional and rapid quenching techniques

Electrochemical charge-discharge-cycling measurements are made on electrode samples prepared from alloy powders of composition LaNi 4.5Mn 0.5. It is observed that by using the rapid quenching technique, the electrode durability of this hydride-forming compound can be improved. X-Ray diffraction energy-dispersive X-ray analysis as well as charge-discharge curves of the electrodes show that this improvement in electrode properties is accompanied by an improvement in material homogeneity as well as changes in atomic order of the crystal structure.

Observation of rotational anisotropy in the second-harmonic generation from a metal surface

We show that crystalline metal surfaces, in particular the Cu(111) surface, exhibit strong second-harmonic (SH) rotational anisotropy. The form of this anisotropy is fully predicted by the current theory of SH from the surface and near-surface bulk of centrosymmetric media. This SH anisotropy allows measurements of changes in atomic order, e.g., phase transitions, on metal surfaces. Since the anisotropic contribution can only arise from interband transitions, it provides greater SH sensitivity to distinguishing adsorbate binding sites.

A Mossbauer study of local atomic order in AuFe alloys

The first atomic order parameter, alpha 1, has been determined for a series of Au100-xFex alloys (x=5, 10, 15 and 20) from an analysis of room-temperature Mossbauer spectra. All alloys in the as-rolled state exhibit positive values of alpha 1, indicative of clustering of Fe atoms within the Au matrix. Changes in atomic order are introduced with heat treatment. At low annealing temperatures, negligible change is observed in the values of alpha 1, due to the slow diffusion rates, but with increasing aging temperature the value of alpha 1 decreases and finally becomes negative, indicative of atomic short-range order. The lack of change of alpha 1 with heat treatment at aging temperatures greater than about 200 degrees C and about 330 degrees C, respectively, in Au95Fe5 and Au90Fe10 is due to the limitation imposed by the quench time with substantial reordering occurring during the quench. The metallurgical state of AuFe alloys annealed at high temperatures is therefore primarily dependent on the quench time. The diffusion process in AuFe alloys is shown to be predominantly via a single-vacancy diffusion mechanism.