Bcc Crystalline Structure Research Articles

Molecular dynamics simulations of the temperature effect in the hardness on Cr and CrN films

Three-dimensional molecular dynamics (MD) simulations of nanoindentation technique was carried out for Cr and CrN thin films that present BCC and FCC crystalline structures respectively. Structures were oriented in the plane (100) and placed on silicon substrates. A pair wise potential was employed for simulating the interaction between atoms of each layer and a repulsive radial potential was used for representing a spherical tip indenting the sample. Mechanical properties of these two materials were obtained varying the temperature from 300K to 1000K with steps of 100K. The hardness and elastic parameters were found for each temperature, showing a better mechanical response for films at low temperature. Structural changes evolution was observed presenting vacancies and slips as the temperature was increased. A temperature smoothing occurred because of the long range of slips and vacancies propagation. Then, the interatomic force decreases as the kinetic energy of the particles involved in nanoindentation process increases.

Oxygen Diffusion in an Nb-Ta Alloy Measured by Mechanical Spectroscopy

When metals that present bcc crystalline structure receive the addition of interstitial atoms as oxygen, nitrogen, hydrogen and carbon, they undergo significant changes in their physical properties because they are able to dissolve great amounts of those interstitial elements, and thus form solid solutions. Niobium and most of its alloys possess a bcc crystalline structure and, because Brazil is the largest world exporter of this metal, it is fundamental to understand the interaction mechanisms between interstitial elements and niobium or its alloys. In this study, mechanical spectroscopy (internal friction) measurements were performed on Nb-8.9wt%Ta alloys containing oxygen in solid solution. The experimental results presented complex internal friction spectra. With the addition of substitutional solute, interactions between the two types of solutes (substitutional and interstitial) were observed, considering that the random distribution of the interstitial atoms was affected by the presence of substitutional atoms. Interstitial diffusion coefficients, pre-exponential factors and activation energies were calculated for oxygen in this alloy.

Side-by-Side In(OH)(3) and In(2)O(3) Nanotubes: Synthesis and Optical Properties.

A simple and mild wet-chemical approach was developed for the synthesis of one-dimensional (1D) In(OH)3 nanostructures. By calcining the 1D In(OH)3 nanocrystals in air at 250 °C, 1D In2O3 nanocrystals with the same morphology were obtained. TEM results show that both 1D In(OH)3 and 1D In2O3 are composed of uniform nanotube bundles. SAED and XRD patterns indicate that 1D In(OH)3 and 1D In2O3 nanostructures are single crystalline and possess the same bcc crystalline structure as the bulk In(OH)3 and In2O3, respectively. TGA/DTA analyses of the precursor In(OH)3 and the final product In2O3 confirm the existence of CTAB molecules, and its content is about 6%. The optical absorption band edge of 1D In2O3 exhibits an evident blueshift with respect to that of the commercial In2O3 powders, which is caused by the increasing energy gap resulted from decreasing the grain size. A relatively strong and broad purple-blue emission band centered at 440 nm was observed in the room temperature PL spectrum of 1D In2O3 nanotube bundles, which was mainly attributed to the existence of the oxygen vacancies.

Modeling of induced empirical constitutive relations on materials with FCC, BCC, and HCP crystalline structures: severe plastic deformation

In this study, empirical constitutive relations of materials with different crystalline structures through severe plastic deformation are introduced. Here, for each material, an optimized empirical relation is chosen by fitting some empirical relations on the results achieved from a dislocation-based constitutive model. In this work, four modes of empirical relations are fitted on the results of modified Estrin–Toth–Molinari–Brechet constitutive model for four materials with different crystalline structures (Al, Cu, Ta, and Zr). The obtained relations for the materials can be usable in commercial finite element codes.

Sonoelectrochemical Synthesis of FeCo Nanoparticles: Study of the Effects of Baths Composition on Process Efficiency and Particles Features

This study describes synthesis of FeCo nanoparticles by using a method which couples electrodeposition of metals with the employment of high power ultrasound. A 20 kH titanium alloy horn ultrasound generator, a “sonoelectrode”, generated short current pulses which were triggered and followed by ultrasonic pulses. The primary role of ultrasound is to induce cavitation phenomenon in the electrolyte and the ablation of the metallic nuclei from the cathodic surface. A rest time restores the initial conditions in the electrolyte close to the sonoelectrode. The final product is a suspension of nanoparticles with high purity and surface/volume ratio, which can be controlled by varying process parameters like time management and current density. The effects of baths components on chemical, morphological and structural features of produced nanoparticles were investigated and evaluated. Nanopowders were characterized by TEM, XRF, XRD and X-EDS; results showed that (i) process efficiency was mainly affected by combined effects of organic additive and supporting electrolyte while (ii) chemical composition of produced nanoparticles was influenced by metal salt anions; (iii) nanoparticles with prevalent bcc crystalline structure were formed, and with (iv) an average grain size of 15÷25 nm; finally (v) morphological and structural features of nanoparticles were not influenced by baths composition. Keywords: Sonoelectrochemistry, nanoparticles, iron-cobalt alloy, process efficiency, chemical composition

Diffusion of Nitrogen in Ti-13V-11Cr-3Al Alloys

Metals with a bcc crystalline structure such as Ti-13V-11Cr-3Al alloys have their physical properties significantly changed through the addition of interstitial elements such as oxygen and nitrogen. These metals can dissolve substantial amounts of interstitial elements forming solid solutions. Mechanical spectroscopy measurements constitute a powerful tool for studying interactions of these interstitial elements with other elements that make up the alloy. From these measurements, it is possible to obtain information regarding diffusion, interstitial concentration, interaction between interstitials, and other imperfections of the crystalline lattice. In this paper, Ti-13V-11Cr-3Al alloys with several amount of nitrogen, in a solid solution, were studied using mechanical spectroscopy (internal friction) measurements. The results presented complex internal friction spectra which were resolved in a series of constituent Debye peaks corresponding to different interactions and interstitial diffusion coefficients. Pre-exponential factors and activation energies were calculated for nitrogen in theses alloys.

Microstructure Characterization by EBSD of Hot Rolled High-Silicon Steel

Non oriented electrical steels are soft magnetic materials used in the core of electrical motors. No preferential anisotropy of the electrical texture in the rolling plane is desired. Nowadays these special steels are mainly alloyed with Si, Al and some additives to improve the magnetic properties and to reach a good of formability. For (Si, Al)-concentrations higher than 2 wt.% the α- γ-α phase transformation is suppressed, resulting in a bcc crystalline structure from liquidus to room temperature. These electrical steels, which will be discussed in the paper, exhibit the lowest values of the magnetic losses. Hot rolling of FeSi electrical steels has been found to be one of the fundamental steps in producing these materials with optimum properties. The resulting properties, as well known, are determined by the type of magnetic textures and the structural inhomogeneities. Electron Backscattered Diffraction (EBSD) is a reliable tool for microstructural and texture characterization of different materials. Two compositions of electrical steel are studied by optical microscopy and EBSD, with special attention paid to characterize the grain morphology and its texture through thickness.

Nitrogen Diffusion in the Nb-2.0wt%Ti Measured by Mechanical Spectroscopy

Metals that present bcc crystalline structure, when receiving addition of interstitial atoms as oxygen, nitrogen, hydrogen and carbon, undergo significant changes in their physical properties, being able to dissolve great amounts of those interstitial elements, thus forming solid solutions. Niobium and most of its alloys possess bcc crystalline structure and, as Brazil is the largest world exporter of this metal, it is fundamental to understand the interaction mechanisms between interstitial elements and niobium or its alloys. In this paper, mechanical spectroscopy (internal friction) measurements were performed in Nb-2.0wt%Ti alloys containing nitrogen in solid solution. The experimental results presented complex internal friction spectra and with the addition of substitutional solute, it was observed interactions between the two types of solutes (substitutional and interstitial), considering that the random distribution of the interstitial atoms was affected by the presence of substitutional atoms. Interstitial diffusion coefficients, pre-exponential factors and activation energies were calculated for nitrogen in the Nb-2.0wt%Ti alloys.

Sonoelectrochemical (20 kHz) production of Co65Fe35 alloy nanoparticles from Aotani solutions

This paper describes the production of alloy nanoparticles of Co:Fe ratio 65:35 from Aotani solutions in the presence of high power ultrasound (20 kHz). The production of this new type of alloy nanoparticles was performed potentiostatically and galvanostatically at (298 ± 1) K using a newly designed experimental set-up i.e. a ‘sonoelectrode’ producing short applied current pulses triggered and followed immediately by ultrasonic pulses. It was shown that cathode efficiencies decreased with increasing current densities and high nanoparticle yields were obtained at low current densities. Morphological and structural studies of the produced nanoparticles were performed by TEM, SEM, XRD, and SAED, and showed that the strongly aggregated Co65Fe35 alloy nanoparticles were predominantly formed, with prevalent body-centered cubic bcc crystalline structure; no redissolution of the nanoaggregates was observed and no separate Fe and Co metallic nanoparticles were produced sonoelectrochemically. The experimental value of the lattice parameter for bcc Co–Fe alloy was 2.85 A and was in excellent agreement with literature values.

Magnetism and disorder in BCC AlCuFe intermetallics

We present here a systematic study of the structural and magnetic properties of a series of alloys around a central composition Al 50Cu 20Fe 30, obtained by mechanical alloying. The samples have BCC crystalline structure with partial B2 order. Thermal annealing, through differential scanning calorimetry measurements, just removes some defects but does not change the crystalline structure. AC susceptibility measurements show that in spite of their structural similarity these samples have quite different magnetic behaviors. Furthermore, most of the samples show a complex magnetic evolution with temperature. Some samples have a cluster glass-like behavior at low temperatures and a superparamagnetic-like one at higher temperatures. A model of magnetic clusters originating in composition fluctuations across the alloy is proposed to explain the observed magnetic properties.

Structural and magnetic properties of mechanically alloyed AlCuFe intermetallics

We present here a study of the structural and magnetic properties of a series of mechanically alloyed intermetallics around a central composition Al 5 0 Cu 2 0 Fe 3 0 , varying the Cu and Fe content. All the samples have bcccrystalline structure with partial B2 order and show no evidence of a magnetic splitting in the Mossbauer spectra. Thermal annealing, removes some defects but do not change this structural feature. All the studied alloys show, at room temperature, a hysteresis loop that may be interpreted in terms of two magnetic components coming respectively from superparamagnetic and ferromagnetic-like regions. The parameters characterizing thess magnetic properties change with composition but no systematic behavior could be deduced.

Textural Characterisation of Iron-Promoted Raney Nickel Catalysts Synthesised by Mechanical Alloying

Mesoporous binary AlxNiy and ternary AlxNiyFez Raney-type catalysts were synthesised by a two-step procedure involving two main processes, i.e. (i) mechanical metal alloying and (ii) alkaline aluminium leaching. Pure metallic powders of Al, Ni and Fe (if required) were first mechanically alloyed in an attrition mill and then subjected to KOH leaching to selectively remove part of the aluminium atoms. After a slow drying process, a fine, nanostructured slit-shaped material was obtained. Substrate characterisation involved studies by atomic absorption (AA), X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDXS) in the SEM and nitrogen physisorption. An intermetallic β- or B2-(AlNi) phase with a metastable bcc crystalline structure was formed as a non-equilibrium phase after the metal alloying process. Because of aluminium removal, the β-(AlNi) phase was transformed into the more stable nickel fcc structure. In this work, some important physicochemical proper...

Instability of Alexander-McTague crystals and its implication for nucleation.

We show that the argument of Alexander and McTague, that the bcc crystalline structure is favored in those crystallization processes where the first-order character is not too pronounced, is not correct. We find that any solution that satisfies the Alexander-McTague condition is not stable. We investigate the implication of this result for nucleation near the pseudospinodal in near-meanfield systems.

Structural study of FeSi nanostructured materials

The mechanical alloying (MA) of Fe and Si powder with ball milling conditions corresponding to an injected shock power of 1.3 W g1, leads to an expansion of the A2 crystalline disordered solid solution phase region up to 27.5 at.% Si (the equilibrium value is 9 at.% Si). In this composition range, an amorphous phase is also detected. In the mechanically alloyed states, Mössbauer spectroscopy reveals that for a Si content less than 15 at.%, the Fe and Si atoms are randomly distributed according to a binomial law in agreement with a bcc structure. For higher Si content, even the X-ray diffraction (XRD) patterns reveal the bcc crystalline structure, the Mössbauer spectra show that the random distribution of the Fe and Si atoms in the crystalline lattice can no longer be maintained.

Magnetic and structural properties of rf-sputtered Co/Fe and Co/Cr multilayers

Multilayers alternating cobalt with different spacers such as iron and chromium at nanometric scale have been deposited by rf diode sputtering. The structures have been characterized in situ by kinetic ellipsometry and ex situ by grazing x-ray reflection, x-ray diffraction, Auger profile analysis, and transmission electron microscopy. Nuclear magnetic resonance (NMR) and high-field SQUID magnetometry have been used to determine the magnetic properties. In Co/Fe multilayers the structure strongly depends on the cobalt thickness tCo. For tCo below 2 nm, the cobalt layers exhibit a bcc crystalline structure, and the crystalline coherence extends over many periods. For tCo above 2 nm, for Co a mixed fcc-hcp phase has been observed. In this case the crystalline coherence is destroyed from one iron layer to the other. The NMR frequency (198 MHz), and the magnetic moment (1.58 μB/atom) of bcc cobalt have been evaluated. A similar behavior is observed in Co/Cr multilayers, the bcc phase disappearing around tCo=1.5 nm. Moreover, interdiffusion is observed at the Cr-on-Co interface: The samples have zero magnetization up to tCo=0.7 nm. Preliminary experiments show some antiferromagnetic coupling between Co layers. <lz>

A Comparative Study of Short Range Order in Fe-Cr and Fe-V Alloys Around Equiatomic Composition

ABSTRACTConfigurational energies have been calculated for equiatomic Fe-Cr and Fe-V alloys possessing the high temperature bcc crystalline structure, within a first principles electronic band structure approach. In agreement with experimental facts, a tendency towards order, with a B2 ordered structure of CsCI type, is found for FeV whereas phase separation characterizes FeCr. These results suggest that the nature of short range order in the high temperature bcc solid solution is not the primary driving force for describing the structural transformation from bcc to sigma which takes place in both alloys upon decreasing temperature.

Evolution of Microscopic Bond-Orientational Order at Freezing Transitions in Supercooled One-Component Plasmas

We report the first microscopic observation by Monte Carlo simulation ( N =1458) of how the particle correlations and bond-orientational orders develop in supercooled one-component plasmas at the freezing transitions. It is found that a formation of particle layers expedites a subsequent evolution into a metastable state and that the final states have bcc-crystalline layered structures with an admixture of a few intralayer interstitials.