Hcp Planes Research Articles

Evaluation of structural, magnetic and concentration dependent texture variations of electrodeposited cobalt nanowires

Anodized aluminum oxide (AAO) templates have been used to synthesize Cobalt (Co) nanowires (NWs) by employing the electrodeposition technique as a function of cobalt ions concentration. Cobalt NWs show a strong dependence on cobalt ions concentration, applied during synthesis, in with reference to their structural and magnetic properties. The Co hexagonal (HCP) phase dominates X-ray diffraction, with the (002) plane oriented perpendicular to the NW long axis with lower applied deposition potential and parallel to the NW long axis at higher Co ions concentration. The reorientation of (002) HCP plane and evolution of face-cantered cubic phase (FCC) at higher concentrations of Cobalt ions results in easy axis of magnetization is switched to the long axis of NW from parallel. Co-NWs crystalline structure and magnetic characteristics might be adjusted based on Co ion concentration during synthesis.

Diffusional-displacive transformation mechanism for the β1 precipitate in a model Mg-rare-earth alloy

The β1 precipitate is a key strengthening-phase in Mg-rare-earth alloys, however, the formation mechanism concerning it has not yet been addressed. Herein, we have uncovered a diffusional-displacive dominated formation mechanism for the β1 phase, using aberration-corrected scanning transmission electron microscopy observation combined with first-principles calculations, in an aged MgSm model alloy system. Shear of the nucleated {101¯0}hcp zig-zag monolayers along a direction parallel to <001>fcc with a shear angle of ~5.26°, followed by atoms shuffling on the non-close-packed {101¯0}hcp planes, can transform the hexagonal close-packed Mg3Sm structure (β” or βH’ intermediate phase) to the face-centered-cubic structure Mg3Sm (β1 phase). Besides, the formation of the β1 phase can also be realized from the other βS’ or βL’ intermediate phase (c-bco, Mg7Sm) via solute diffusion coupled with shuffle transformation manner. A new habit plane of {101¯0}hcp // {11¯0}fcc and [0001]hcp // [110]fcc on the non-closed packed plane has been identified, which is completely different from the traditional displacive transformation mechanism usually happened on the closed-packed plane. This finding enriches the diffusional-displacive transformations.

Mechanical properties and microstructural evolution of WC-binderless and WC-Co hard materials by the heat treatment process

In this study, the heat treatment process was performed to improve the mechanical properties of WC-binderless and WC-Co hard materials pre-fabricated by PCAS process. The microstructural and mechanical properties were varied significantly by the heat treatment process with the treatment time. To increase the fracture toughness value, the heat-treatment process of the WC-binderless, WC-5 wt.% Co and WC-10 wt% Co hard materials were performed at (1,300, 1,150, and 950) °C under (5 and 48) h, respectively. Enhancement of fracture toughness that focused on the major reasons was estimated for four mechanisms. Firstly, the microstructure formation of a mechanism was investigated to demonstrate the grain coarsening behavior for variation of heat treatment time. Secondly, the fraction of the coherent interface at WC/Co to the existing in the specific plane orientation was investigated. In particular, the texture distribution of WC (0001)hcp plane and Co (111)fcc plane were discussed in the EBSD micrograph. Thirdly, the increase of the mean free path by the Co content was exhibited in relation to the variation of mechanical properties. Finally, the mechanism of the decarburization effect of WC, which occurs during the heat treatment process under specific condition were demonstrated in the XRD peaks and FE-SEM analysis. In this chemical reaction, the separated W phases were locally precipitated, resulting in an increase in fracture toughness.

Residual stress and texture control in Ti-6Al-4V wire + arc additively manufactured intersections by stress relief and rolling

Additively manufactured intersections have the theoretical risk to contain hydrostatic tensile residual stresses, which eventually cannot be thermally stress relieved. The stresses in Ti-6Al-4V wire + arc additively manufactured (WAAM) intersections are lower compared to single pass walls and stresses in continuous walls are larger compared to discontinuous walls with otherwise identical geometry. Thermal stress relief was found to virtually eliminate them.Inter-pass rolling can yield the desired grain refinement, without having any noteworthy influence on the development of residual stresses. The strain measurement itself by neutron diffraction is facilitated by the refined microstructure, because the otherwise textured microstructure produces anisotropic peak intensity, not allowing Pawley refinement. Without rolling, the 101¯1 and 101¯3 family of hcp planes are the only ones that diffract consistently in the three principal directions.

Evaluation of the influence of texture and microstructure of titanium substrates on TiO2 anodic coatings at 60 V

The current paper analyzes the influence of texture and microstructure of Ti substrates on morphology and color of titanium dioxide coatings obtained by anodic oxidation. Substrates of Ti grade 2 and Ti grade 5 with different thermo-mechanical histories were used in the shape of cylindrical bars cut in both longitudinal and transversal directions, and laminated sheets. The crystalline orientation of the surface grains in the substrate before oxidation, were determined by electron backscatter diffraction. The oxide coatings were analyzed by optical and scanning electron microscopy. The results show that different substrate grain orientations produce oxides with different colors, because of different thickness depending on their orientations. An oxide grown on a basal hcp plane, with higher atomic density is thinner than an oxide grown on a transversal hcp plane with lower atomic density. This effect is more pronounced on elongated grains. Different anodic oxidation process parameters and heat treatments of the substrate were applied in order to obtain a more uniform oxide thickness. A specific heat treatment of the substrate was the most efficient, starting from a favorable orientation of crystals, characteristic of rolling texture.

Formation mechanism of bulk nanocrystalline aluminium with multiply twinned grains by liquid quenching: A molecular dynamics simulation study

The formation mechanism of bulk nanocrystalline aluminium with multiply twinned grains has been investigated by a large-scale molecular dynamics simulation. The results show that bulk nanocrystalline aluminium can be obtained directly by quenching liquid at an appropriate cooling rate window. Most nanograins do not merge with each other in the coarsening stage due to the fast cooling rate, but are separated by high-angle grain boundaries. The nanograins exhibit a narrow grain-size range with an average diameter of 6.4nm and random crystallographic orientations. These microstructure features are consistent with some of the bulk nanocrystalline alloys prepared by melt casting in experiments. The nanograins display various twinned morphologies, and they can be described by three twin elements of parallel, cross and fivefold twinning in varying amounts. The multiply twinned nanograins come from the successive formation of twin HCP planes during the layer-by-layer growth of FCC stacking blocks, and the growth kinetics plays a critical role in the grain morphology. This formation mechanism is different from that of fivefold deformation and annealing twins in the nanocrystalline metals.

Ballistic transport of spin waves incident from cobalt leads across cobalt–gadolinium alloy nanojunctions

Calculations are presented for the scattering and ballistic transport of spin waves (SW) incident from cobalt leads, on ultrathin ferrimagnetic cobalt–gadolinium ‥Co][Co(1−c)Gd(c)]ℓ[Co‥ nanojunction systems. The nanojunction [Co(1−c)Gd(c)]ℓ itself is a randomly disordered alloy of thickness ℓ hcp lattice planes between matching hcp planes of the Co leads, at known stable concentrations c≤0.5 for this alloy system. To compute the spin dynamics, and the SW scattering and ballistic transport, this alloy nanojunction is modeled in the virtual crystal approximation (VCA), valid in particular at the length scale of the nanojunction for submicroscopic SW wavelengths. The phase field matching theory (PFMT) is applied to compute the localized and resonant magnons on the nanojunction. These magnons, characteristic of the embedded nanostructure, propagate in its symmetry plane with spin precession amplitudes that decay or match the spin wave states in the semi-infinite leads. The eigenvectors of these magnon modes are calculated for certain cases to illustrate the spin precession configurations on the nanojunction. The VCA-PFMT approach is also used to calculate the reflection and transmission spectra for the spin waves incident from the Co leads on the nanojunction. The results demonstrate resonance assisted maxima for the ballistic SW transmission spectra due to interactions between the incident spin waves and the nanojunction magnon modes. These properties are general for variable nanojunction thicknesses and alloy stable concentrations c≤0.5. In particular, the positions of the resonance assisted maxima of spin wave transmission can be modified with nanojunction thickness and alloy concentration.

An Attempt to Image Chemical Ordering in Close-Packed Layer of Mg&amp;ndash;Zn&amp;ndash;Y 18R Long-Period Stacking-Ordered Structure by Scanning Tunneling Microscopy

For the first time, nanostructures of 18R (Mg85Zn6Y9) LPSO (long-period stacking-ordered) were examined using STM (scanning tunneling microscopy), which unambiguously confirmed the proposed model of the atomic structure of the LPSO phase in which the Zn­Y clusters are arranged in a two-dimensional hexagonal lattice in the (0001)hcp plane. The typical domain size of this hexagonal arrangement was estimated to range from several nanometers to a few tens of nanometers. In addition, the structures at the domain boundaries of the hexagonal arrangement were imaged and clarified by STM. [doi:10.2320/matertrans.M2013123]

Aqueous Electrodeposition and Mechanism of Iron Group-Vanadium Ternary Alloys

Composition of ternary CoFeV deposits can be varied and tailored by control of the [Fe]/[Fe + Co] solution molar ratios and solution pH. Near 2V Permendur compositions were obtained with Bs of 2.35T and Bs of 2.39T for DC and PC deposits, respectively. Annealing deposits in vacuum at 820oC for 2 hours increased Br to equivalent 2V Permunder. X-ray diffraction patterns show microstructures of mixed HCP, FCC and BCC planes, depending on deposit composition. Co-rich deposits had HCP preferred orientation but as Fe content increases HCP plane disappeared and Fe BCC plane became the preferred orientation. Surface morphology changed from a needle-like structure in Co-rich deposits to nodular appearance with increasing Fe content. A hydrogen atom mechanism for the reduction of the oxovanadium, Co(II) and Fe(II) within the heterodinuclear and heterotrinuclear biscitrate complexes is proposed. This mechanism is also applicable to aqueous electrodeposition of IG-Mo and IG-W alloys.

Growth Mechanism of Single Crystal Nanowires of fcc Metals (Ag, Cu, Ni) and hcp Metal (Co) Electrodeposited

Using X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM), we have studied the growth of the single crystalline nanowires of Ag, Cu, Ni and Co metals in the small pores (50 nm). We find that the preferential growth surface of the single crystalline nanowires is on the atomically rough surfaces such as fcc(110) and hcp. We have proposed a new model to explain the growth of the nanowires of fcc and hcp metals. In this model we argue that the preferential growth should depend on the number of sites for dehydration of hydrated metal ions on a metal surface. The dehydration occurs only at the apex site of a protruding surface atom since the apex site of the protruding surface atom has an enhanced electrical field. The sites for the dehydration on the fcc(110) and hcp atomically rough planes are in number much larger than those on the fcc(111) and hcp(0001) atomically smooth surfaces, thus leading to the preferential growth on the fcc(110) and hcp planes.

Correlation of crystallographic structure and magnetic properties of electrodeposited Co-rich Co-Pt films

Co-rich Co-Pt alloy films have been grown by electrodeposition on Au buffer layer. Detailed texture measurements have been performed in order to understand the dependence of magnetic properties on phase composition and texture. X-ray analysis in Bragg-Brentano geometry shows that films seem to consist of hexagonal closed packed (hcp) or fcc phase. Due to similar lattice spacing of the densely packed planes of both possible phases, the (111) fcc and (002) hcp planes of Co-Pt and also the higher order reflections coincide. As a result, the information supplied by X-ray diffraction was of limited value. Hence pole figure measurements are used which determine the directional distribution of planes with a selected spacing. Here we present an approach to use pole figure measurements for phase identification. We show that this method allow a direct correlation with the magnetic properties, in particular with the magnetocrystalline anisotropy and magnetic texture.

Atomistic simulation of the fcc–hcp transition in single-crystal Al under uniaxial loading

The dynamic behavior of the single-crystal Al under [001] uniaxial strain is simulated by classic molecular dynamics. The fcc–hcp structural transition is successfully observed when the loading pressure reaches about 90 GPa, and the reverse transition is also found with hysteresis. The mechanism and morphology evolution of both the forward and backward transitions are analyzed in detail. It is found in the process of the structural transition that the (010)fcc or (100)fcc planes transit into (0001)hcp planes, and the twins of the hcp phase along the (112)-plane appear, whose boundaries finally become along the (110)-plane. Besides, we find the twinning (along the (110)fcc planes) in the hcp phase prior to the back transition (hcp–fcc). Our simulations show the coexistence of fcc and hcp phases over a wide range of pressures, and finally, the phase transition is evaluated by using the radial distribution functions.

Soliton theory of incommensurate phases in LiCsSO4 crystals

An extended version of the soliton theory is derived and used to explain the observed low-temperature phase transitions in LiCsSO4 which are assumed to be driven by the planar orientation of SO4 tetrahedrons. Two possible states of this orientation are expressed as S=1/2 spin states. Within the framework of the hcp Ising model, the phase transitions are described in terms of two order parameters representing spin-density wave along two mutually orthogonal directions in the basic hcp plane. The first order parameter describes the modulated phase below the temperature, T1. The ground-state configuration of the incommmensurate phase is a soliton lattice of domain walls which get narrower as the temperature decreases till the lock-in point, T2. The second order parameter describes the Curie second-order phase transition at T* and represents a collinear coherence of the planar turns of the tetrahedrons SO4 in each of the single magnetic planes, being the building units of the domain walls.

Visualization and analysis of structural ordering during crystallization of a gold nanoparticle

In this molecular dynamics study, we provide an insight into the crystallization of a liquid droplet by following the structural ordering of the surface and core atoms in a 10 179 atom gold nanoparticle quenched from the melt down to $298\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We use the $Q6$ order parameter and ring statistics to measure the degree of crystallization during quenching and a topological structure measure based on planar graphs to visualize the time evolution of structural ordering of the nanoparticle core at the temperature where significant crystallization first occurs $(949\phantom{\rule{0.3em}{0ex}}\mathrm{K})$. Our results indicate that crystallization of the surface precedes that of the core and, in fact, provides a template for the ordering of the core region. There is also evidence presented that hcp twin planes form directly below the region where the edges of two (111) surface planes meet. At $298\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, the nanoparticle forms a defected icosahedral structure, and we observe the formation of fcc fragments and hcp planes characteristic of an icosahedral structure in the core as a function of temperature.

Frustrated magnetism in superconducting hexagonal Fe: Calculation of inter-atomic pair exchange interactions

We calculate magnetic exchange interaction parameters in hcp Fe using magnetic force theorem in density functional theory in a range of volumes, which covers the region of stability of the superconducting state. The calculations are done for a paramagnetic state using disordered local moment model within a local fixed spin-moment constraint. It is found that exchange interaction parameters strongly favor antiferromagnetic inter-plane stacking of the basal hcp planes. At the same time, the nearest neighbor in-plane exchange interaction parameters are also strongly antiferromagnetic, which should lead to magnetic frustrations in the planar triangular lattice. Eventual consequences of the magnetic frustration effects on the interplay between magnetism and superconductivity in hcp Fe under pressure are discussed.

The growth of noble metals in (112¯0)-oriented hexagonal close-packed nano-films by epitaxy on Nb(001)

The morphology and crystal structure of noble metal nano-films deposited on oxygen contaminated and oxygen-free Nb(001) surfaces have been studied with angle-resolved ultraviolet photoelectron spectroscopy, X-ray photo-electron diffraction, and reflection high energy electron diffraction. In the both cases a deposited noble metal film aligns its 〈211〉 direction with the [110] direction of the Nb(001) surface. But, while a noble metal grows on an oxygen contaminated Nb(001) surface with the hexagonal close-packed (hcp) (111) planes parallel to the surface (i.e. in the (111)-oriented face centred cubic phase (fcc)), on a non-contaminated Nb(001) it grows with its hcp planes perpendicular to the surface. The latter happens because in the initial stages of the epitaxy the first two monolayers (MLs) of the noble metal grow pseudomorphically on a contamination-free Nb(001). The pseudomorphic layer is strongly extended parallel to the Nb(001) surface in comparison to its natural fcc (001) plane. As a consequence of the atomic volume conservation principle the out-of-plane lattice of the pseudomorphic layer is contracted. Thus, its body centred tetragonal (110) planes, which stay perpendicular to the surface, contract into denser-packed planes, i.e. in hcp ones. In the direction perpendicular to the surface, where the substrate does not have a direct influence on the film, the pseudomorphic layer relaxes into its natural close-packed phase, i.e. into hcp atomic planes. These planes appear as soon as the third pseudomorphic ML begins to grow. The stacking axis of the planes lies in the (100) surface of Nb and is locked by it. The fact that thick nano-films of Cu (up to 50 MLs), Ag and Au (up to 100 MLs) grow in the (112¯0)-oriented hcp phase can be attributed to a much better fit of the hcp than of fcc stacking sequence to the four-fold symmetry of the Nb(001) surface.

Mechanisms and Epitaxial Relationships between Close-Packed and BCC Lattices in Block Copolymer Solutions

The epitaxial relationships between two close-packed lattices (face-centered cubic (fcc) and hexagonally close packed (hcp)) and the body-centered cubic (bcc) lattice of spherical micelles were investigated by small-angle X-ray scattering (SAXS) in two in situ shear cells. Two symmetric poly(styrene-b-isoprene) diblock copolymers with block molecular weights of 8.0 × 103 and 7.0 × 103 g/mol, and 1.5 × 104 and 1.5 × 104 g/mol, respectively, were employed. Thermoreversible fcc → bcc order−order transitions were identified in three styrene-selective solvents, dimethyl phthalate, diethyl phthalate and dibutyl phthalate, and in two isoprene-selective solvents, tetradecane and squalane. Upon shearing an fcc solution, a mixture of highly oriented fcc and hcp crystals was produced, due to the random stacking of the {111} planes along the shear gradient. A bcc phase was grown epitaxially from each hcp/fcc mixture by heating to a temperature within the bcc window without shear. By employing two shear cells, access to scattering along both the gradient axis and the vorticity axis was achieved. This proved crucial to elucidating both the fcc → bcc and hcp → bcc transformations. These mechanisms are related to the Bain distortion and the Burgers mechanism, respectively, which are well-established in simple metals. Interestingly, the close-packed planes in fcc/hcp ({111}fcc and {0002}hcp planes) and bcc ({110} planes) were preserved during these transformations, but bcc unit cells with nine distinct orientations were produced. These reflect particular orientation relationships (ORs) between the parent fcc/hcp and the newly formed bcc crystals, which correspond to the Kurdjumov-Sachs, Burgers, Nishiyama-Wassermann, and Pitsche-Schrader ORs that are prevalent in metals.

Local structures in medium-sized Lennard-Jones clusters: Monte Carlo simulations

The Lennard-Jones clusters composed of approximately 200 atoms were simulated using canonical Monte Carlo method at the reduced temperatures between ${T}^{*}=0.05$ and ${T}^{*}=0.49.$ Local arrangements of atoms within the clusters were identified by means of a specially invented algorithm based on the classification of five types of regular coordination polyhedra corresponding to face-centered cubic (fcc), hexagonal close-packed (hcp), icosahedral (ic), pentagonal direct-packed (pdp), and body-centered cubic (bcc) local structures. Clusters cut off from three types (fcc, hcp, and bcc) of perfect crystals show a structural transition with gradually increasing temperature in the temperature range corresponding to solid clusters. The spectrum of 201 atomic clusters formed at the temperature ${T}^{*}=0.25$ from a starting configuration with random atom positions is characterized by a variety of local structures, with quantitative dominance of hcp and pdp structural units and absence of bcc units. Most of these clusters are polyicosahedral, very often showing a triangular or tetrahedral arrangement of ic units. In some clusters noncrystallographic ic and pdp atom arrangement is absent. In such cases spatial separation of two crystallographic phases can be observed in the form of fcc and hcp domains or as sandwiched parallel fcc and hcp planes.

Martensitic transformation in Fe-Mn-Si based alloys

The stacking fault energy (SFE) or the stacking fault probability plays an important role in the determination of the critical driving force ΔGγ→e for the martensitic transformation fcc(γ)→hcp(e) in ternary Fe-Mn-Si alloys, ΔGγ→e increases with the content of Mn and decreases with that of Si. Thermodynamical prediction of Ms in ternary Fe-Mn-Si alloys is established. The fcc(γ)→hcp(e) martensitic transformation in Fe-Mn-Si is semi-thermoelastic and the nucleation process does not strongly depend on soft modes. Nucleation of e-martensite is not dominated by a pole mechanism. Based on the phenomenological theory of martensite crystallography, a shuffle on (0001)hcp planes is required when d111≠d0002. The derived principal strains for the Bain distortion are smaller, i.e. more reasonable than the values given by Christian. Alloying elements which strengthen the austenite, lower the SFE of the γ phase and reduce the TγN temperature may be beneficial to the shape memory effect (SME) of Fe-Mn-Si based alloys.

Microstructure-dependent viscosity in concentrated suspensions of soft spheres

Monodisperse colloidal suspensions of polymethylmethacrylate spheres swollen in benzyl alcohol have been rheologically examined under applied steady and oscillatory shear while simultaneously monitoring microstructure via light scattering. In concentrated samples, long-lived nonequilibrium microstructures can be induced, corresponding to random hexagonal-close-packed planes (hcp) stacked in the direction of the shear gradient. The direction of closest packing within each hcp plane can be oriented along either the vorticity or direction of flow. Creep and creep recovery measurements have been examined for each of these two orientations as a function of particle concentration and stress. Results indicate a strain-dependent dissipative process correlated with changes sample microstructure, while dynamic measurements of the storage modulus show no significant difference between microstructures. We argue that instantaneous viscosities can be measured and show how they are correlated with changes in particle microstructure. The observed elastic response in these suspensions will be shown to be due to local microstructure and particle deformation.