Coherent Diffraction Domains Research Articles

X-RAY DIFFRACTION STUDIES OF THE EFFECT OF PHASE COMPOSITION AND STRUCTURAL STATE CHARACTERISTICS ON THE TRIBOLOGICAL BEHAVIOR OF THE MOLYBDENUM- AND TUNGSTEN-BASED STRENGTHENING COATINGS

A comprehensive study of phase composition, structural state, parameters of fine atomic structure, mechanical and tribological properties of molybdenum-carbon and tungsten-carbon-based coatings deposited by reactive magnetron sputtering in an acetylene-argon gas mixture has been carried out. It has been shown that the resulting coatings have a nanocomposite diamond-like carbon (DLC) structure based on the metal and the metal carbide phases with close sizes of coherently diffracting domains (CDD), approximately 3–7 nm, and on hydrogenated amorphous carbon. The coating nanohardness values were 13–15 and 20–23 GPa for the Mo-DLC and W-DLC coatings, respectively. The tribological tests have demonstrated that the Mo- and W-DLC coatings can reduce friction and effectively protect the steel surfaces hardened by them both under dry friction and under boundary lubrication conditions.

Micro-strains, local stresses, and coherently diffracting domain size in shock compressed Al(100) single crystals

In situ x-ray diffraction (XRD) measurements and their analysis in Al single crystals shock compressed along the [100]-direction were utilized to examine shock wave induced microstructural heterogeneities. High-resolution XRD line profiles for the 200, 400, and 600 Al peaks were measured in uniaxial strain compression states to either 5.6 or 11.7 GPa and partial stress release to 3.5 or 6.6 GPa, respectively. Broadening of the XRD line profiles was analyzed to determine the magnitude of the longitudinal micro-strain distribution (0.195% and 0.28% full width at half maximum for 3.5 and 6.6 GPa stresses, respectively) and the size of coherently diffracting domains (CDDs) (0.125 and 0.07 μm for 3.5 and 6.6 GPa stresses, respectively). From the longitudinal micro-strain distributions, the distribution of local stress differences (or stress deviators) was obtained in the shocked state. The full width at half maximum of this distribution, a measure of the local stress inhomogeneities, is greater than half of the macroscopic stress difference for both 3.5 and 6.6 GPa peak stresses, suggesting considerable variation in local stress deviators. The CDD sizes determined here are comparable to characteristic length scales for defect-free regions determined from defect density measurements in post-shock recovery experiments. The present work represents an important step in understanding material microstructure and inhomogeneities in the shock-compressed state.

Insight Into Disorder, Stress and Strain of Radiation Damaged Pyrochlores: A Possible Mechanism for the Appearance of Defect Fluorite.

We have examined the irradiation response of a titanate and zirconate pyrochlore—both of which are well studied in the literature individually—in an attempt to define the appearance of defect fluorite in zirconate pyrochlores. To our knowledge this study is unique in that it attempts to discover the mechanism of formation by a comparison of the different systems exposed to the same conditions and then examined via a range of techniques that cover a wide length scale. The conditions of approximately 1 displacement per atom via He2+ ions were used to simulate long term waste storage conditions as outlined by previous results from Ewing in a large enough sample volume to allow for neutron diffraction, as not attempted previously. The titanate sample, used as a baseline comparison since it readily becomes amorphous under these conditions behaved as expected. In contrast, the zirconate sample accumulates tensile stress in the absence of detectable strain. We propose this is analogous to the lanthanide zirconate pyrochlores examined by Simeone et al. where they reported the appearance of defect fluorite diffraction patterns due to a reduction in grain size. Radiation damage and stress results in the grains breaking into even smaller crystallites, thus creating even smaller coherent diffraction domains. An (ErNd)2(ZrTi)2O7 pyrochlore was synthesized to examine which mechanism might dominate, amorphization or stress/strain build up. Although strain was detected in the pristine sample via Synchrotron X-ray diffraction it was not of sufficient quality to perform a full analysis on.

Низкоэнергетическая механическая обработка порошка нестехиометрического карбида титана

Introduction. The practical significance of non-stoichiometric titanium carbides TiCх in various fields of technology and in medicine is expanding. In this regard, it is important to investigate both methods of obtaining titanium carbide powder and its properties in a wide range of stoichiometry. One of the effective ways to influence the physical and mechanical properties of powder systems is its mechanical treatment. Under shock-shear action, which is realized during processing in a ball mill, mechanical energy is transferred to the powder system, as a result of which it is ground, centers with increased activity on newly formed surfaces are formed; phase transformations, crystal lattice deformation, amorphization, formation of defects, etc. are possible. The aim of this work is to study the effect of low-energy mechanical treatment in a ball mill on the structure, phase composition and parameters of the fine crystal structure of non-stoichiometric titanium carbide powder obtained by reduction of titanium oxide with carbon and calcium. Materials and methods. Powder of titanium carbide TiC, obtained by calcium carbonization of titanium oxide was investigated. The powder was treated in a drum type ball mill. The structure of the powders before and after treatment was studied using the Philips SEM 515 scanning electron microscope. The specific surface area was determined by the BET method. The phase composition and parameters of the fine crystal structure of powder materials were investigated by X-ray analyzes. Results and discussion. It was established that an increase of the time of mechanical treatment in a ball mill of a non-stoichiometric titanium carbide powder TiC0.7 leads to an increase in the specific surface area of the powder from 0.6 to 3.4 m2 / g, and the average particle size calculated from it decreases from 2 μm to 360 nm. It is shown that in the process of treatment of the non-stoichiometric titanium carbide TiC0.7 powder, its structural phase state changes. Powder particles consist of two structural components with different atomic ratio of carbon to titanium: TiC0.65 and TiC0.48. Mechanical treatment of titanium carbide powder leads to a decrease in the microstresses of the TiCx crystal lattice and the size of coherently diffracting domains (CDD) from 55 to 30 nm for the TiC0.48 phase. For the TiC0.65 phase, with an increase in the duration of mechanical treatment, as well as for TiC0.48, the size of CDD decreases, and the level of microdistortions of the crystal lattice increases. This indicates that in the process of mechanical treatment, not only the grinding of powder particles occurs, but also an increase in its defects.

Локализация деформации при диаметральном сжатии керамики ZrO-=SUB=-2-=/SUB=- (Y-=SUB=-2-=/SUB=-O-=SUB=-3-=/SUB=-)

The deformation behavior of ZrO2-Y2O3 ceramics under diametral compression was studied using the digital image correlation. Spatial-temporal patterns of strain localization along the axis of the deformable specimen εxx (x) and across the axis of the deformable specimen εyy (y) were obtained. It has been established that the accumulation of εxx and εyy deformations when testing ceramic ZrO2 (Y2O3) samples for diametral compression is non-uniform over the sample. In this case, the microstructural parameters change, such as the size of the coherently diffracting domains (CDDs) of the tetragonal phase and microstresses, the tetragonally monoclinic transformation is realized, and this localization correlates with the fixed non-uniform of the microstresses in the material volume.

Fe nanoparticles produced by electric explosion of wire for new generation of magneto-rheological fluids

Iron magnetic nanoparticles were produced by the technique of the electric explosion of a wire (EEW). The major crystalline phase (95 ± 1%) was α-Fe with lattice parameter a = 0.2863(3) nm. The size of the coherent diffraction domains of this phase was 77 ± 3 nm. The EEW MNPs presented a large saturation magnetization value, reaching about 87% of the saturation magnetization of the bulk iron. EEW NMPs demonstrated an improved magnetic performance when used in magnetorheological (MR) fluids with respect to the commercial carbonyl iron particles (CIPs) micron-sized particles studied for comparison. The MR fluids composed with the EEW nanoparticles showed larger yield stress values than those with CIP micron-sized particles, so proving that the EEW MNPs have a high potential for MR fluids applications.

EVOLUTION OF THE MICROSTRUCTURAL PARAMETERS OF COLD WORK Ti-6-Al-4V ALLOY

The aim of this work is to investigate cold worked Ti-6Al-4V (α+β) alloy. The alloy was examined by X-ray diffraction using Rietveld refinement method. MAUD software (Materials Analysis Using Diffraction) was used to analyze the microstructural parameters evolution (crystallite size, root mean square strain (r.m.s) and dislocation density. The Crystallite size is smaller in the β-phase compared to the α-phase. Microstrain and dislocation density are higher in the α-phase than those found in the β-phasefor the as received material. The microstructural parameters of Ti-6Al-4V alloy exhibit typical values of cold deformation state. The results show that the deformation process reducesthe crystallite size (coherent diffraction domains) from 520 to 210 Ȧ in the α-phase. Consequently, the r.m.s increases from 5 E-4 to 32 E-4 and the dislocation density increases from 2.92 E+10 to 4.6 E+11m-2after 85 % thickness reduction.

Evolution of thermo-physical properties and annealing of fast neutron irradiated boron carbide

Boron carbide is widely used as a neutron absorber in most nuclear reactors, in particular in fast neutron ones. The irradiation leads to a large helium production (up to 1022/cm3) together with a strong decrease of the thermal conductivity. In this paper, we have performed thermal diffusivity measurements and X-ray diffraction analyses on boron carbide samples coming from control rods of the French Phenix LMFBR reactor. The burnups range from 1021 to 8.1021/cm3. We first confirm the strong decrease of the thermal conductivity at the low burnup, together with high microstructural modifications: swelling, large micro-strains, high defects density, and disordered-like material conductivity. We observe the microstructural parameters are highly anisotropic, with high micro-strains and flattened coherent diffracting domains along the (00l) direction of the hexagonal structure. Performing heat treatments up to high temperature (2200 °C) allows us to observe the material thermal conductivity and microstructure restoration. It then appears the thermal conductivity healing is correlated to the micro-strain relaxation. We then assume the defects responsible for most of the damage are the helium bubbles and the associated stress fields.

Influence of the Surface Condition of Specimens on the Size of Coherently Diffracting Domains

Metallic sample preparation process from cutting to polishing introduces defects into the subsurface layers by plastic deformation. Methods of bulk sample characterization are usually sensitive to the microstructure and presence of defects e.g. dislocations, grain boundaries, etc. near the surface. The X-ray diffraction technique (XRD) is a unique method used primarily for phase identification and quantification of crystalline materials. The extending of the method by the whole pattern fitting using Rietveld refinement analysis enables also utilisation for other purposes such as determination of the size of coherently diffracting domains (CDD). This property quantifies the degree of disorder in material and is usually correlated with mean crystallite size, i.e. subgrain size. However, since the penetration depth of the laboratory X-ray radiation is limited to the order of tens of micrometers (depending on the material), the observed result is highly affected by the density of defects such as dislocations in the surface area of the specimen and thus by their surface condition. The effect of the different final steps of grinding and polishing on the observed size of CDD was studied and discussed for two materials - aluminum Al7075 alloy and high entropy alloy HfNbTaTiZr. The reasonable finishing for the optimal results was found for both materials.

The effect of thermal annealing on Fe/TiO2 coatings deposited with the help of RF PECVD method. Part I. Chemical and phase composition

Results of the studies on chemical structure and phase composition of both non-annealed and thermally annealed iron doped TiO2 coatings are presented. The coatings were synthesized with the help of radio frequency plasma enhanced chemical vapor deposition (RF PECVD) and characterized by iron content in the range of 0–4.76at%. In these studies, an analysis of both elemental composition and chemical bonding was performed with the help of X-ray photoelectron spectroscopy (XPS). X-ray diffraction (XRD) was applied to determine phase composition and Fourier transform infrared spectroscopy (FTIR) was used as a supplementary source of information. The obtained elemental composition data show that, apart from titanium, oxygen and iron, traces of chlorine as well as substantial amounts of carbon are present in the coatings. While chlorine has originated from plasma decomposition of TiCl4, used as a source of titanium, a presence of carbon is associated with a surface contamination resulting from photocatalytic reactions of TiO2 with the adsorbed carbon dioxide. The results of XPS and FTIR indicate that thermal annealing not only modifies phase composition of these materials, but also affects chemical bonding. Finally, XRD analysis shows that iron content has an effect on the size of coherent diffraction domains present in the films.

Investigation of the hydrozincite structure by infrared and solid-state NMR spectroscopy

To better understand lattice disorder in hydrozincite, natural hydrozincite samples and synthetic analogues were investigated by XRD, FTIR, 13 C MAS, and 13 C CPMAS NMR. The size of coherent diffraction domains ranges between ~10 nm (Synth1) and ~30 nm (Synth2). FTIR peaks from the antisymmetric CO 3 2− stretching ν3 mode were observed at 1383 and 1515 cm −1 in all samples. Peaks due to OH vibrations were observed for all the samples at 3234, 3303, and 3363 cm −1 , and were sharp only for the samples having larger crystal domains. The 13 C MAS and CPMAS NMR spectra showed a main carbon signal at 164 ppm in the Synth2 sample, while two main signals were observed at ~164 and ~168 ppm in the Synth1 sample. The intensity ratio of the latter signals were found to be independent of contact time, in the investigated range between 0.2 and 30 ms. In addition, 13 C CPMAS dynamics indicates that the Synth1 sample has shorter T 1ρ with respect to Synth2. This indicates a more effective process of spin diffusion of proton magnetization in the former due to different structural properties of Synth1 and Synth2 samples. In addition, chemical shift anisotropy analysis was attributed to a structural change in the carbonate group or hydrogen bonding for Synth1 and Synth2. This was interpreted as a deviation from the ideal structure generated by linear and planar lattice defects and/or grain boundaries.

Plasmodynamic synthesis of ultradispersed iron oxides

Results of a pilot study of direct plasmochemical synthesis of ultradispersed crystalline iron oxide phases in a high-speed pulse jet of iron-containing electrodischarge plasma that effluxes into a closed volume with the air atmosphere are presented. The synthesized powder contains all main oxide phases, including magnetite Fe3O4, maghemite γ-Fe2O3, and hematite α-Fe2O3. An important feature of the method is the formation of the metastable e-Fe2O3 phase under normal conditions. The minimum average size of coherently diffracting domains (CDD) (particles) of the magnetic phases is observed when they are synthesized at air pressure of 1.0 atm. The powders have high magnetic properties comparable with those of compact iron oxides.

Magnetic behaviour of nanocrystalline Cu–Fe–Co/Al2O3 composite powders obtained by mechanical alloying

The effects of Al2O3 addition on structural and magnetic property changes during mechanical alloying of Cu, Fe and Co element powders were studied systematically by X-ray diffraction, superconducting quantum interference device (SQUID) and magnetic force microscopy (MFM). Milling the powders for up to 10 h leads to the progressive diffusion of Co and Fe atoms into the Cu. Face-centered cubic Cu(Fe,Co) phases with coherent diffraction domains (cdd) size of ∼35 nm were prepared by mechanically alloying Cu0.5Fe0.25Co0.25 with and without Al2O3 powder (1%, 3% and 10%). The effect of crystal size on magnetic properties was studied. Mechanically alloyed metallic–ceramic composite powder showed lower saturation magnetization than the metallic system but enhanced coercive field. Milling time significantly affects the structure, composition, magnetic properties, and magnetic domains for both metallic and composite systems.

X‐ray Diffraction Study of the Effect of High‐Temperature Heat Treatment on the Microstructural Stability of Third‐Generation SiC Fibers

Third‐generation SiC fibers [High Nicalon S (HNS) and Tyranno SA3 (Ty–SA3)] were studied by X‐ray diffraction and transmission electron microscopy (TEM) after heat treatments in neutral atmosphere up to 1900°C. The microstructural changes in both materials were determined using a modified Hall–Williamson method introducing an anisotropy parameter taking into account the high density of planar defects. HNS fibers exhibit significant modifications in the coherent diffraction domains (CDD) size, which drastically increases from 24 to 70 nm in the range 1600°C–1900°C. TEM observations support these results. The residual microstrain values decrease from 0.0015 to 0.0005 between 1750°C and 1850°C. Similarly, the anisotropy parameter significantly decreases in the same temperature range. Concerning the Ty–SA3 fibers, no evolution in terms of CDD size and residual microstrain was observed. However, the anisotropy parameter decreases at 1800°C. TEM observations did not show noticeable grain growth. The grain size was found to be larger than the CDD and the planar defects density to decrease at high temperature. In both types of fibers, the CDD sizes are similar for the highest temperature heat treatments.

Grazing incidence X-ray diffraction for the study of polycrystalline layers

Different diffraction patterns measured above and below the critical angle for total reflection were recorded on a Gd doped ceria layer deposited on a Si(100) substrate in asymmetric coplanar diffraction conditions. From the analysis of diffraction patterns, it was possible to point out peculiar optical aberrations responsible for the shifts and the broadenings of Bragg peaks induced by this setup. Corrections for all these aberrations were implanted in a Rietveld programme. Using these corrections, it becomes possible to follow the evolution of the structure (chemical composition, phases) and the micro structure (strain field, micro strain due to heterogeneities between grains, size of coherent diffracting domains) versus the depth on thin films.

Are processing conditions similar in ball milling and high-pressure torsion? The case of the tetragonal-to-monoclinic phase transition in ZrO2 powders

Y 2 O 3 partially stabilized zirconium oxide powders have been mechanically processed by ball milling. A tetragonal-to-monoclinic phase transition occurred, accompanied by a refinement of the microstructure, and an increase in the dislocation density. The final phase composition, average size of coherent diffraction domains and dislocation density are comparable to those obtained after high-pressure torsion. This suggests that powder particles experience similar mechanical deformation conditions when involved in a collision or submitted to high-pressure torsion.

Mechanical processing of Fe powders

Fe powders were mechanically processed by ball milling. The average size of coherent diffraction domains and the average strain content were determined by X-ray diffraction. A mathematical model was developed to describe the microstructural refinement process as a function of the number of collisions and to estimate the amount of powder effectively processed at individual collisions. The density of dislocations was also estimated. The obtained values suggest that Fe powders can locally undergo extremely severe deformation conditions.

Rietveld refinements performed on mesoporous ceria layers at grazing incidence

Seven diffraction patterns were collected on a 100 nm Gd-doped ceria layer deposited on a silicon wafer under asymmetric reflection conditions. As the grazing-incidence angle decreases, large shifts (a few tens of degrees) and broadenings (two degrees below the critical angle) ofhklreflections are apparent in the diffraction patterns. The impact of these aberrations on the positions and profiles of the Bragg peaks is studied in detail in this work. On the basis of this analysis, diffraction patterns collected at different angles of incidence could then be refined using a unique structural model. From these refinements, the evolution of the coherent diffracting domains, the strain and the microstrain can clearly be traced as a function of depth.

Effect of pressure on kaolinite illitization

The illitization of kaolinite at increasing pressures was followed by hydrothermal experiments of kaolinite in KOH solution at 300 °C for 12 h and pressures between 500 and 3000 bar. XRD indicated a direct transformation of kaolinite into muscovite/illite with increasing pressure. However, the 27Al MAS NMR spectra showed, in addition to the muscovite/illite resonances, the presence of a signal at 61 ppm that should correspond to a secondary phase, not detected by XRD. A second series of experiments at 300 °C and 1000 bar for 1, 3 and 6 h was carried out to show direct evidence of such phase. The XRD patterns of the products clearly showed the crystallization of K–F zeolite, while the 27Al MAS NMR spectra of these samples displayed a signal at 61 ppm that must correspond, therefore, to Al in the K–F zeolite structure. In conclusion, kaolinite transformed into muscovite/illite when submitted to hydrothermal reaction in KOH solution with increasing pressure, with the formation of a secondary metastable phase called K–F zeolite, whose coherent diffraction domains were too small as to be detected by XRD.

Structural properties of biologically controlled hydrozincite: An HRTEM and NMR spectroscopic study

The microscopic properties of biomineral hydrozincite [Zn 5 (CO 3 ) 2 (OH) 6 ] from Naracauli Creek (SW Sardinia) were investigated by using X-ray diffraction (XRD), nuclear magnetic resonance spectroscopy (NMR), scanning electron microscopy (SEM), and high-resolution transmission electron microscopy (HRTEM). Because the biomineral hydrozincite turned out to significantly deviate from the ideal structure of hydrozincite, synthetic and geologic hydrozincite samples were also investigated for comparison. SEM imaging shows that biomineral hydrozincite is made of small platelet-shaped crystallites having a 20–50 nm long side at the shortest and other sides measuring hundreds of nanometers long. These are interlaced to form sheaths several micrometers long. HRTEM analysis of the biomineral samples shows an imperfectly oriented aggregation of the nanocrystals that is discussed in terms of mesocrystals. Transmission electron microscopy (TEM) and XRD analysis indicate a progressive decrease in the size of the particles in the biomineral compared to the synthetic and geologic hydrozincite samples, with coherent diffraction domains in the biomineral hydrozincite that are smaller by 30–50% than in the other samples investigated in this study. 13 C magic angle spinning (MAS) and cross polarization magic angle spinning (CPMAS) NMR spectra show more than one peak for all the investigated samples, despite the fact that carbon atoms have a unique crystallographic position in the hydrozincite structure. The additional peaks can reflect the presence of lattice defects typical of nanocrystals as indicated by the HRTEM images, where high concentration of lattice defects, such as grain boundaries and stacking modes, can be observed both in the biomineral and in the synthetic samples. Further additional peaks in the NMR spectra of biomineral samples are attributed to organic molecules, relicts of the biomineralization process, in agreement with the filaments observed in SEM images of biomineral samples.