Hcp Crystal Structure Research Articles

Stacking fault energy in FCC plutonium with multiple reference states in the modified embedded atom method

Basic concepts from the multi-reference state formalism for determining the functions for the modified embedded atom method (MEAM) are adopted to modeling elemental plutonium (Pu). In the case of elemental Pu, the focus is on the background electron density. Here we utilize a portion of the formalism that determines the structure of the background density necessary to capture correct phase ordering between fcc and ideal hcp crystal structures. The critical information comes from cold curves, that is the energy/volume relationships, for these phases. Practically speaking, the energy difference between these two phases determines the stacking fault energy of the material. At the same time, the simple monoclinic phase of elemental Pu also becomes higher in energy at the equilibrium volume of the fcc phase. The new model is based on first-principles electronic structure calculations and captures the basic phase ordering of those calculations.

Fabrication of CoZn alloy nanowire arrays: Significant improvement in magnetic properties by annealing process

Highly ordered arrays of Co 1− x Zn x (0 ≤ x ≤ 0.74) nanowires (NWs) with diameters of ∼35 nm and high length-to-diameter ratios (up to 150) were fabricated by co-electrodeposition of Co and Zn into pores of anodized aluminum oxide (AAO) templates. The Co and Zn contents of the NWs were adjusted by varying the ratio of Zn and Co ion concentrations in the electrolyte. The effect of the Zn content, electrodeposition conditions (frequency and pH) and annealing on the structural and magnetic properties (e.g., coercivity (Hc) and squareness (Sq)) of NW arrays were investigated using X-ray diffraction (XRD), scanning electron microscopy, electron diffraction, and alternating gradient force magnetometer (AGFM). XRD patterns reveal that an increase in the concentration of Zn ions of the electrolyte forces the hcp crystal structure of Co NWs to change into an amorphous phase, resulting in a significant reduction in Hc. It was found that the magnetic properties of NWs can be significantly improved by appropriate annealing process. The highest values for Hc (2050 Oe) and Sq (0.98) were obtained for NWs electrodeposited using 0.95/0.05 Co:Zn concentrations at 200 Hz and annealed at 575 °C. While the pH of electrolyte is found to have no significant effect on the structural and magnetic properties of the NW arrays, the electrodeposition frequency has considerable effects on the magnetic properties of the NW arrays. The changes in magnetic property of NWs are rooted in a competition between shape anisotropy and magnetocrystalline anisotropy in NWs.

Magnetization reversal in Co‐base nanowire arrays

AbstractThe magnetic behaviour of ordered arrays of magnetic nanowires is controlled by the combined action of anisotropy energy terms determined by geometrical shape, crystalline structure and magnetostriction, as well as by the magnetostatic interactions among them. A number of works have been reported on the role of magnetostatic interaction in arrays of nanowires, sometimes focusing on nanowires with vanishing crystalline and magnetostrictive energy terms. Here, in turn we are interested in Co‐base nanowires whose magnetocrystalline anisotropy plays a very important role. Arrays of Co‐base nanowires offer novel, sometimes unexpected behaviour that is not fully understood. Such results are connected to their magnetocrystalline anisotropy, and to their complex magnetization reversal mode. Arrays of Co and Co‐base nanowires have been fabricated by template‐assisted method, by electroplating into highly ordered self‐assembled pores of nanoporous alumina membranes. Their hcp or fcc crystal structure is confirmed to depend on length of nanowires as well as on the presence of selected elements in the composition. The magnetic behaviour of these arrays has been experimentally investigated as a function of (i) geometry (e.g. diameter and, particularly short length) and (ii) composition (e.g. adding Ni or Pd). A complementary study on its temperature dependence reveals the nature of the dominant anisotropy, while the angular dependence of hysteresis loops (e.g. coercivity and remanence) enables a deeper analysis of the magnetization reversal process. Micromagnetic reversal modes are reviewed and modelling and simulations procedures introduced. Complex rotational mechanisms are concluded to be responsible for the reversal process following micromagnetic modes distinct from those pure classically established.magnified image SEM image of an array of Co‐base nanowires.

마그네슘합금의 조직제어(組織制御)와 성형가공(成形加工) 및 스크랩 리싸이클링 기술(技術)

마그네슘 합금은 비중이 1.74로 가벼운데다 비강도는 구조용 금속 중에서 가장 크며, 방진성, 전자파 차폐성, 저용점 용 여러 가지 장점이 있어 최근 자동차 부품과 전자산업 제품 분야에서 이용이 크게 기대되는 금속이다. 그러나 마그네슘의 결정구조는 조밀 육방정이기 때문에 소성변형이 가능한 슬립면이 한정되어 있으며 압연이나 압출가공 시에는 강한 집합조직이 형성되어 상온가공이 곤란하다. 따라서 지금까지 성형성 개선을 위한 조직제어와 성형기술 분야에서 많은 연구개발이 이루어져 왔다. 본고에서는 결정립과 집합조직에 관한 미세조직의 제어방법, 용체성형, 압연 및 압출에 관한 성형가공 기술과 마그네슘 스크랩 리싸이클링 기술에 관하여 최근의 연구개발 사례를 소개한다. Recently, magnesium alloys are in the spotlight as a promising materials in the fields of automobile parts and electronic appliances due to their merits representing light weight, high specific strength, damping property, shielding of electromagnetic wave and so on. However, magnesium alloys show a poor formability at room temperature because magnesium has HCP crystal structure with limited slip planes and strong basal texture is formed during plastic deformation process such as rolling and extrusion. Therefore, many R&D efforts have been paid for improvement of formability through grain refinement, texture control and various forming technologies. This paper is giving an overview about recent achievements on control of microstructures, forming technologies and magnesium scrap recycling.

STOCHASTIC PROCESSES OF {1012} DEFORMATION TWINNING IN HEXAGONAL CLOSE-PACKED POLYCRYSTALLINE ZIRCONIUM AND MAGNESIUM

Deformation twinning is an important mechanism for metals with hexagonal close-packed (hcp) crystal structures such as titanium, magnesium, beryllium, and zirconium, as well as metals with other low-symmetry crystal structures. This article presents a multiscale constitutive model for the plastic deformation of hcp polycrystalline metals that is built upon the viscoplastic self-consistent (VPSC) polycrystal scheme. This framework includes a novel probabilistic model for predicting when, where, and which variants nucleate, a dislocation density model for crystallographic slip, and a micromechanical model for twin lamella thickening and twin reorientation. Length scales of an individual slip or twinning system and the polycrystalline aggregate are interactively connected. Here, this constitutive model is applied to high-purity magnesium (Mg) and zirconium (Zr) deformed under loading conditions where twinning is intense. Correlations are made between the formation of deformation twins, bulk stress-strain behavior and texture development, and individual grain properties such as size and crystallographic orientation.

Electrodeposited Cobalt Films: Alteration Caused by the Electrolyte pH

Cobalt (Co) films were electrodeposited on polycrystalline copper substrates at different pH values. It is observed that the crystal structure of the films converts from hexagonal close-packed (hcp) to a mixed phase of face centered cubic and hcp as the electrolyte pH decreases. The grain size calculated from the X-ray diffraction patterns decreases with the decrease of electrolyte pH. The surface of the films grown at a high pH is more uniform than that of the films grown at a low pH. The saturation magnetization and the coercivity decrease as the electrolyte pH decreases. The high coercivity value at high pH corresponds to the hcp crystal structure of the films as well as the large grain size of Co clusters. Magnetic measurements also reveal that the easy axis direction of magnetization is parallel to the film plane for all films since the higher remanent magnetization and lower saturation field are observed in parallel hysteresis loops.

Electronic structure and superconductivity of europium

We have calculated the electronic structure of Eu for the bcc, hcp, and fcc crystal structures for volumes near equilibrium up to a calculated 90GPa pressure using the augmented-plane-wave method in the local-density approximation. The frozen-core approximation was used with a semi-empirical shift of the f-states energies in the radial Schrödinger equation to move the occupied 4f valence states below the Γ1 energy and into the core. This shift of the highly localized f-states yields the correct europium phase ordering with lattice parameters and bulk moduli in good agreement with experimental data. The calculated superconductivity properties under pressure for the bcc and hcp structures are also found to agree with and follow a Tc trend similar to recent measurement by Debessai et al. [1].

Facile synthesis and characterization of Co nanoplatelets with tunable dimensions via solution reduction process

Plate-like Co nanoparticles with different sizes were synthesized by solution reduction process. The size of Co nanoplatelets can be tuned by varying the concentration of CoCl2·6H2O and the dosage of N2H4·2H2O. The Co nanoplatelets with the different size all exist in both fcc and hcp crystal structures. The normal direction of the nanoplatelets is perpendicular to the (002) planes of hcp phase or to the (111) planes of fcc phase. The saturation magnetizations of the samples are lower than the corresponding bulk value. The coercivities of the samples vary with the phase content and the particle size. The shape control mechanism was discussed.

First-principles calculations of pure elements: Equations of state and elastic stiffness constants

Using the projector-augmented wave method within the generalized gradient approximation, a systematic first-principles calculation for energy vs. volume (E–V) equations of state (EOS’s) and single crystal elastic stiffness constants (cij’s) has been performed for 76 pure elemental solids with face-centered-cubic (fcc), body-centered-cubic (bcc), and hexagonal-close-packed (hcp) crystal structures, wherein the cij’s are determined by an efficient strain–stress method, and the EOS’s are fitted by a 4-parameter Birch–Murnaghan equation upon the first-principles E–V data points. Based on the predicted EOS’s and cij’s, the phase transition pressures between bcc, fcc, and hcp structures, as well as the structural stabilities and the polycrystalline aggregate properties including bulk modulus (B), shear modulus (G), B/G ratio, and anisotropy ratio have been analyzed for pure elements and compared with available experimental data. The present systematic studies of pure elements provide not only the EOS’s and cij’s but also the benchmarks of first-principles calculations.

Relationship between silver concentration with microstructural and mechanical properties of rolled AlZn alloy

The relationship of Ag addition on microstructural and mechanical properties of rolled AlZn alloy was investigated. AlZn alloys were prepared by metal mould casting method and the Ag addition was done by Vortex technique. Microstructural characterization of AlZnAg specimens was analyzed by means of X-ray diffraction, scanning electron microscopy and transmission electron microscopy. Results show that the phases of the as-cast state alloy are solid solution zinc-rich hexagonal close-packed (hcp) crystal structure, named η-phase and α-Al solid solution with Zn dissolved into the matrix. The silver concentration in AlZn alloy influences the volume of AgZn 3 precipitates. The mechanical properties, especially the flow stress and elongation of the alloy were improved by the Ag addition. The Vortex method was used in order to diminish the process cost, generating an alloy with homogenous microstructure, less casting porosity and better mechanical properties.

Examining Co-Based Nanocrystals on Graphene Using Low-Voltage Aberration-Corrected Transmission Electron Microscopy

We present a method to produce graphene and few layer graphene sheets using solution phase chemistry, which are used as ultrathin support membranes for enhanced imaging of nanomaterials using transmission electron microscopy. We demonstrate this by decorating the surface of the graphene sheets with Co-based nanocrystals (CoCl(2) and hcp Co). Low-voltage aberration-corrected high resolution transmission electron microscopy at 80 kV is used to image the nanocrystals on the thin graphene supports. We show that electron beam irradiation causes the CoCl(2) nanocrystals to become mobile on the graphene surface and exhibit both rotational and translational motion. We provide real-time in situ monitoring with atomic resolution of the coalescence of two CoCl(2) nanocrystals on the graphene surface, driven by electron beam irradiation. The CoCl(2) nanocrystals are then annealed in vacuum and transform into Co nanocrystals with hcp crystal structure. We show that these Co nanocrystals are catalytic and electron beam irradiation leads to the etching of the graphene surface, not observed for the CoCl(2) nanocrystals.

Study of Forming Limit for Rotational Incremental Sheet Forming of Magnesium Alloy Sheet

As a lightweight material, magnesium is being increasingly used for automotive parts. However, due to a hexagonal-closed-packed (hcp) crystal structure, in which only the basal plane can move, magnesium alloy sheets exhibit a low ductility and formability at room temperature. Press forming of magnesium alloy sheets is conventionally performed at elevated temperatures of 200 °C to 250 °C and thus is known as energy consumed forming. Therefore, in view of an energy saving forming technology, we study magnesium alloy sheet forming by a rotational incremental sheet forming (RISF) at room temperature, where the rotational tool generates local heat of specimen enough to accelerate plastic deformation. The flow curves of the magnesium alloy sheet are obtained and calculated at elevated temperatures, while the yield loci of the magnesium alloy sheet are measured at room temperature. Using RISF, a square cup of 80-mm width, 80-mm length, and 25-mm height is then formed from a magnesium alloy sheet at room temperature. In addition, the strain distribution is obtained and compared with the forming limit curve (FLC) by considering the effect of the tool radius and is found to effectively predict the forming limit of a magnesium alloy sheet in RISF.

Room temperature synthesis of Li-doped MnO 2 and its electrochemical activity

A new method was developed to synthesize a uniform round-shaped Li-doped MnO 2 by ozonation of acidic MnSO 4 in the presence of Li + ions. X-ray diffraction study showed that the prepared compound was of a hexagonal-closed-packed (hcp) crystal structure with an average crystallite size of ~ 103 nm. The merging of 221/240 and 061/002 lines suggested a high microtwinning defect. Chemical composition has been explained in terms of Ruetschi's cation vacancy model. The electrochemical properties were studied by recording discharge profile in 9 M KOH at 1 mA/0.1 g and 100 Ω/0.5 g. The discharge proceeded with homogenous solid-state reduction into MnOOH followed by subsequent formation of Mn(OH) 2 via dissolution–precipitation mechanism.

Tribo-mechanical etching of structural ceramic materials during lubricated severe sliding contact

An unusual phenomenon has been observed in the contact area on the surfaces of three structural ceramic materials that were subjected to severe lubricated sliding contact with hardened steel. Silicon carbide, alumina, and partially stabilized zirconia (PSZ) were slid against a carburized 4620 steel ring in a block-on-ring test rig lubricated with polyalphaolefin synthetic oil. A step-loading protocol was used for the scuffing test. At the conclusion of the tests, the outline of grains and phases in the contact area of the ceramic materials was clearly visible, as if the surface had been etched. This behavior, what we have termed “tribo-mechanical etching,” is attributed to different mechanisms, including anisotropic mechanical and thermal deformation of each grain (SiC and Al 2O 3) and the phases (PSZ) in the ceramic material during severe sliding contact. Because of the hexagonal close-packed (hcp) crystal structure of the SiC and Al 2O 3 grains, they exhibit anisotropic mechanical and thermal behavior. Another possible mechanism for revealing the grain outlines includes uneven/unequal transfer of metal into different ceramic grains as a result of differences in the surface energy of each grain.

Experimental and Numerical Study of Electromagnetic Forming of AZ31B Magnesium Alloy Sheet

Wrought magnesium alloys are interesting materials for automotive and aeronautical industries due to their low density in comparison to steel and aluminium alloys, making them ideal candidates when designing a lower weight vehicle. However, due to their hexagonal close-packed (hcp) crystal structure, magnesium alloys exhibit low formability at room temperature. For that reason, in this study a high velocity forming process, electromagnetic forming (EMF), was used to study the formability of AZ31B magnesium alloy sheet at high strain rates. In the first stage of this work, specimens of AZ31B magnesium alloy sheet have been characterised by uniaxial tensile tests at quasi-static and dynamic strain rates at room temperature. The influence of the strain rate is outlined and the parameters of Johnson-Cook constitutive material model were fit to experimental results. In the second stage, sheets of AZ31B magnesium alloy have been biaxially deformed by electromagnetic forming process using different coil and die configurations. Deformation values measured from electromagnetically formed parts are compared to the ones achieved by conventional forming technologies. Finally, numerical study using an alternative method for computing the electromagnetic fields in the EMF process simulation, a combination of Finite Element Method (FEM) for conductor parts and Boundary Element Method (BEM) for insulators, is shown.

Controlling the crystal structure of Ni nanoparticles by the use of alkylamines

Ni nanoparticles were prepared via thermal decomposition of nickel acetate tetrahydrate in the presence of long-chain amines, which acted as both solvents and reducing agents. By tuning the reaction temperature, Ni nanostructures with either hcp or fcc crystal structure were obtained. In principle, higher temperatures favored the formation of hcp nanoparticles. The employment of additional surfactants such as 1-adamantanecarboxylic acid and trioctylphosphine-oxide facilitated the tuning of the particles’ growth limit. The size of the particles varied between 5 and 120 nm. The magnetic features of fcc-Ni nanoparticles were quite similar to the corresponding ‘bulk’ ones. On the other hand, the hcp-Ni particles showed weak magnetic features, reflected by low magnetization values, the absence of saturation magnetization and by blocking temperatures far below room temperature.

INVESTIGATION ON THE CORROSION BEHAVIOR OF HAHA-4 CLADDING BY OXIDE CHARACTERIZATION

The microstructure, the corrosion behavior and the oxide properties were examined for Zr-1.5Nb-0.4Sn-0.2Fe-0.1Cr (HANA-4) alloys which were subjected to two different final annealing temperatures: 470°C and 570°C. HANA-4 was shown to have s-enriched phase with a bcc crystal structure and Zr(Nb,Fe,Cr)₂ with a hcp crystal structure with s-enriched phase being more frequently observed compared with Zr(Nb,Fe,Cr)₂. The corrosion rate of HANA-4 was increased with an increase of the final annealing temperature in the PWR-simulating loop, 360°C pure water and 400°C steam conditions, which was correlated well with a reduction in the size of the columnar grains in the oxide/metal interface region. The oxide growth rate of HANA-4 was considerably affected by the alloy microstructure determined by the final annealing temperature.

Torsional Oscillator and Synchrotron X-Ray Experiments on SolidHe4in Aerogel

X-ray diffraction experiments show that solid 4He grown in aerogel is highly polycrystalline, with an hcp crystal structure (as in bulk) and a crystallite size of approximately 100 nm. In contrast to the expectation that the highly disordered solid will have a large supersolid fraction, torsional oscillator measurements show a behavior that is strikingly similar to high purity crystals grown from the superfluid phase. The low temperature supersolid fraction is only approximately 3 x 10(-4), and the onset temperature is approximately 100 mK.

Electrodeposition of Co–P Films from Alkaline Electrolytes

We study the influence of solution chemistry on the electrodeposition of Co–P from a citrate–glycinate alkaline electrolyte. The concentration of different cobalt complexes is calculated from chemical equilibria as a function of solution pH to examine the strength of the complexing agents, and compared with cyclic voltammetry (CV) data. CV and electrochemical quartz crystal microbalance are utilized to elucidate the effect of different P sources ( or ) on the electrodeposition process. The influence of plating conditions as well as different P sources on the deposit composition, microstructure, and magnetic properties are investigated. Both citrate and glycine are good complexing agents for cobalt, the former having a stronger effect. The addition of depolarizes cobalt reduction, while P codeposition from solutions containing is negligible. Pure cobalt and Co–P films grown from the source display a hexagonal close-packed (hcp) crystal structure and soft magnetic properties. The Co–P films grown from solutions containing display an amorphous structure with a P content above atom % in the deposit. Only when the plating current is small and the P content is low the films are crystalline with an hcp structure. These latter films display in-plane magnetic anisotropy and a relatively high coercivity.

Electrochemical fabrication of ultralow noise metallic nanowires with hcp crystalline lattice

We experimentally demonstrate that low-frequency electrical noise in silver nanowires is heavily suppressed when the crystal structure of the nanowires is hexagonal closed pack (hcp) rather than face centered cubic (fcc). Using a low-potential electrochemical method we have grown single crystalline silver nanowires with hcp crystal structure, in which the noise at room temperature is two to six orders of magnitude lower than that in the conventional fcc nanowires of the same diameter. We suggest that motion of dislocations is probably the primary source of electrical noise in metallic nanowires, which is strongly diminished in hcp crystals.