Hcp Lattice Research Articles

Effect of arrangement patterns of spheres encapsulating phase change material in packed beds on thermal performance

The NaNO3-KNO3 salt has the disadvantage of poor heat transfer capacity and low heat transfer efficiency in single-tank heat storage systems. However, molten salt can be used as phase change materials (PCM) and encapsulated with highly conductive sphere-shaped metals as PCM spheres. Arranging these spheres can appropriately change the situation. In this study, a heat storage system of a multilayer packed bed of PCM spheres consisting of NaNO3-KNO3 salt and type 316 stainless steel sphere shell was designed. This system's heat transfer and heat storage performance were also investigated using CFD simulation. For the same sphere diameter, the heat storage efficiency of hexagonal close packing (HCP) was 1.1 times that of cubic close packing (CCP). With the same porosity, the heat charging efficiency for the case using the 47.5 mm sphere diameter was the highest, at 99.3 %. When the number of layers of spheres was increased from 3 to 7, the heat flux through the sphere wall and the heat storage density of PCM spheres decreased from 40.8 kW and 67.3 × 105 kJ/m3 to 35.7 kW and 59.2 × 105 kJ/m3, respectively. Therefore, the heat charging efficiency of HCP was better than that of CCP. Reducing sphere size could shorten the heat transfer time and improve the heat storage efficiency. These numerical simulation results are consistent with the commonly known fact that the volumetric heat transfer intensity in the porous media is proportional to the reciprocal particle size since the smaller the particle size, the larger the contact area between the packing and the fluid. The conclusion drawn from the present findings provides an idea for designing a PCM heat storage system with high temperature and high efficiency of molten salt. The idea could also be used as a reference for the optimal design of a packed bed reactor.

Process-induced microstructural variations in laser powder bed fusion of novel titanium alloys: A comprehensive study on volumetric energy density and alloying effects

This study explores the effect of in-situ alloying and volumetric energy density (VED) on the microstructure of Laser Powder Bed Fusion (L-PBF) fabricated Ti alloys. Pure Ti, Ti–5Cu, and Ti–5Cu–1Si (wt%) samples were printed using elemental powders with varying VEDs. This study investigates the influence of VED and Cu/Si additions on the growth restriction factor (Q) and columnar-to-equiaxed transition of the β phase. Pure Ti samples exhibited coarse, prior columnar β grains with an average diameter of 106 μm, and a grain shape factor greater than 3.0. In contrast, both Ti–Cu and Ti–Cu–Si samples displayed a significant fraction of equiaxed prior β grains with a near-spherical morphology. Additionally, Cu/Si addition refined the prior β columnar grains, reducing their average diameter to 37 μm and 25 μm in Ti–Cu and Ti–Cu–Si, respectively. Furthermore, the study reveals a strong dependence of microstructure on VED in the Ti–5Cu–1Si alloy. Higher VED promotes a more uniform distribution of solute elements and a lower thermal gradient, resulting in finer equiaxed β grains with an average diameter of 4.9 μm, compared to samples printed at lower VEDs. The addition of Cu and Si also significantly refined the lath-like α phase and decreased the c/a ratio of the Ti HCP lattice, introducing lattice microstrains in the Ti–Cu and Ti–Cu–Si alloys. These findings demonstrate the potential of in-situ alloying and VED optimization for tailoring microstructures in novel Ti alloys fabricated via L-PBF, paving the way for achieving superior mechanical properties.

Production of nickel and cobalt metal powders under autoclave conditions

This paper presents the results of studies on the reduction of dispersed cobalt and nickel metal powders from their salts in ammonia-alkaline aqueous solutions under hydrothermal autoclave conditions. A unified and environmentally friendly method for producing these powders has been developed. Hydrazine hydrate, with a 25–50 % excess of the stoichiometric ratio, was used as a reducing agent. This choice allows for obtaining metal phases that are chemically uncontaminated by decomposition products. The experiments determined the conditions for the quantitative reduction of cobalt (II) and nickel (II) ions from ammonia-alkaline aqueous solutions. The synthesis temperature for the dispersed phases ranged from 110 to 155 °C. It has been demonstrated that under the conditions used, the process is completed quantitatively within 60 min. Metal concentrations in the solutions were determined using atomic absorption spectroscopy. The results of the X-ray phase analysis confirm that cobalt forms in the HCP lattice, while nickel forms in the FCC lattice. No other phases, including X-ray amorphous phases, were observed. It was found that with an increase in the hydrothermal synthesis temperature from 110 to 155 °C, the specific surface area of cobalt increased by more than 1.5 times, and that of nickel black powders increased by approximately 2 times. Scanning electron microscopy revealed that cobalt is formed in the shape of lamellar particles with a lateral size of about 500 nm and a thickness of 50 nm, which aggregate into fractal structures. Nickel is represented by spherical particles arranged in chain-like structures. Using X-ray photoelectron spectroscopy, it was determined that the surface of the materials is covered with oxidized forms. The surface atomic concentration of cobalt (0) was approximately 2 %, while that of nickel (0) was about 25 %.

Intermetallic bulk phase Nb5Al3Cx synthesized via wet ball milling

It’s known that the Nb–Al–C system may crystallize in ternary compounds, including well studied MAX phases (Nb2AlC, Nb4AlC3) and a stable Nb3Al2C phase. This study has shown a route of synthesizing novel bulk Nb5Al3Cx intermetallic phase via wet ball milling of niobium and aluminum powders in petroleum ether and subsequent thermal treatment. The synthesized intermetallic phase has a hexagonal close-packed (hcp) lattice of space group D88 . The phase is formed at 757 °C, keeping stable at least up to ∼1130 °C. The lattice parameters are a = 7.808–7.813 Å, c = 5.270–5.279 Å, c/a = 0.675–0.676 for this phase.

Emergence of Wulff-Crystals from Atomistic Systems on the FCC and HCP Lattices

Emergence of Wulff-Crystals from Atomistic Systems on the FCC and HCP Lattices

EFFECT OF LOCAL STRESSES ON THE FRACTURE OF METALLIC MATERIALS WITH DIFFERENT LATTICES UNDER SINGLE LOADS

The study discusses the effect of the local stress field at the crack tip on the fracture behavior of coarseand ultrafine-grained materials with bcc, fcc, and hcp lattices under single loads (impact and static). The local stresses at the crack tip under impact and static loading were assessed by the hmax/t ratio, where hmax is the maximum depth of the plastic zone under the fracture surface and t is the specimen thickness. The depth of plastic zones under the fracture surface was determined using layer-by-layer etching of the surface followed by X-ray diffraction analysis. The study results showed that it is not always possible to establish an unambiguous relationship between the fracture mechanisms of metallic materials and the local stress field at the crack tip. Nevertheless, some particular relationships were established: (i) the cleavage, quasi-cleavage, or intergranular brittle fracture of materials, regardless of lattice type, is indicative of plane strain deformation, (ii) under plane stress, all materials, regardless of lattice type, exhibit ductile fracture with micropitting, and (iii) in the transition from plane strain to plane stress, most fcc materials fracture by a mixed mechanism.

Synthesis of Ti21.5V40Cr38.5 alloy by abnormal glow discharge plasma

The paper describes the structural-phase state and sorption characteristics of the Ti21.5V40Cr38.5 alloy synthesized by melting in abnormal glow discharge plasma and arc melting. X-ray diffraction patterns showed the presence of the main TiVCr phase with a body-centered cubic (bcc) lattice and Ti2O phase with a hexagonal close packed (hcp) lattice. The maximum hydrogen capacity of the samples obtained by melting in abnormal glow discharge plasma after 3 cycles was 3 wt%.

Mechanical properties of a two-phase high-entropy Fe50Mn30Co10Cr10 alloy down to ultralow temperatures

The mechanical properties comprising the stress-strain characteristics under uniaxial tensile deformation, the acoustic properties from mechanical resonance spectroscopy and—in parallel—the microstructural evolution during deformation of a nonequiatomic high-entropy alloy (HEA) Fe50Mn30Co10Cr10 have been studied in a wide temperature range, including ultralow temperatures down to 0.5 K. In the temperature range 300 to 4.2 K, a strong temperature dependence of the tensile strength occurs, hinting at the thermally activated nature of plastic deformation. Within the range of extremely low temperatures (4.2–0.5 K), however, the alloy exhibits anomalies of the yield strength, as well as discontinuous plasticity. Over the whole temperature range, the dynamic Young’s modulus of tensile deformed samples shows a reduction of absolute values compared to those of the undeformed ones, and at temperatures < 30 K a change of the temperature dependence from almost linear to power-law type. At all temperatures down to 0.5 K, the alloy’s plasticity stays as high as 50% as a consequence of a deformation driven martensitic phase transformation from fcc to hcp lattice (TRIP effect). Considering the ultralow deformation temperatures, the tensile strength reaches record values of 1513 MPa at 4.2 K, and still of 1274 MPa at 0.5 K, each being paired with significant strain hardening. These results suggest the HEA Fe50Mn30Co10Cr10 as a promising structural material for use in cryogenic environments down to extremely low temperatures.

Nanoarchitectonics of mesoporous carbon from C60/PCBM hybrid crystals for supercapacitor

Owning a highly symmetric geometry, fullerenes are popular building blocks in nanoarchitectonics. The successful preparation of fullerene superstructures with various sizes and dimensions forms the basis for their applications. However, the lack of porosity and the limited electrical conductivity of most fullerene superstructures prevent their applications in energy-related fields such as supercapacitors. Herein, hybrid crystals with a crossed-tubular morphology and a hexagonal close packed (hcp) lattice were constructed by fullerene C60 and its derivative, [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), through a modified liquid-liquid interfacial precipitation method. Subsequent calcination can greatly improve the porosity of this hybrid crystals and produce mesoporous carbon with an increased BET surface area nearly 30 times (from 13.91 to 432.41 m2 g−1), which exhibited a good performance as an electrode material in supercapacitor with a capacitance retention rate of 87.3% after 2000 cycles and specific capacitances of ∼213 F g−1 at 5 mV s−1 and ∼158 F g−1 at 1 A g−1, respectively. Compared to the crystals formed solely by fullerenes, the presence of PCBM lowered the calcination temperature and induced self-doping of oxygen. In addition, rough surfaces could be produced on the superstructures by solvent etching, which increased as well the performance of the material in supercapacitor. Our work provided a new strategy to engineer fullerene-derived functional materials for applications in energy storage and expanded the knowledge on the factors regulating their performance when using as electrode materials in supercapacitors.

Effects of nickel–cobalt material properties on glucose catalysis

The development of non-enzymatic glucose sensors becomes even more significant as the number of diabetics worldwide continues to increase. Ni–Co alloy can be fabricated by electrodeposition and used as a catalyst for glucose so that it is a suitable electrode material for non-enzymatic electrochemical glucose sensors. The results of material analyses reveal that the catalytic performance for glucose is related to the crystallographic structure of Ni–Co alloy. A Ni–Co based non-enzymatic glucose sensor with a majority of hexagonal close packed (hcp) lattice exhibits better catalytic performance and, moreover, the catalysis of glucose is a quasi-reversible redox process, either in film-type or nanowire-type alloy. However, the nanowire-type Ni–Co glucose sensor has a wider sensing range than the film-type sensor. Ni–Co glucose sensor possesses excellent anti-interference without being influenced by the chemicals coexisting with glucose. These results demonstrate the effect of the crystal structure of Ni–Co on glucose catalysis and suggest the electroplating conditions for the preparation of Ni–Co non-enzymatic glucose sensor with good catalytic performance and excellent anti-interference ability.

Microstructure evolution and constitutive modeling for mesoscaled tension of pure titanium thin sheet

The design and fabrication of micro components of pure titanium thin sheets face a challenging problem with size effects. Size effects will affect the plastic deformation mechanism of thin sheets in addition to causing unpredictable flow stresses. Although the impact of size on strength is obvious, further research is still needed, particularly for metal thin sheet with HCP lattice, to thoroughly understand the impact on deformation mechanisms. To assess the size effects on flow stress, uniaxial tensile experiments were carried out on specimens with varying thicknesses and grain sizes in this work. Additionally, Electron Backscattered Diffraction (EBSD) technology was used to analyze the dislocation evolution and twinning behavior in surface and inner grains for specimens with various thicknesses and grain sizes employing electrolytically polished specimens under different strain conditions. The results demonstrate that the feature size (t/d) has a substantial impact on the flow stress of pure titanium sheet, as seen by a rapid reduction in flow stress when t/d < 7. Microstructure analysis showed that the feature size effect has a significant impact on the deformation mechanism. Inhomogeneous strain distribution and geometrically necessary dislocation (GND) density distribution are produced by the reduction in thickness and the increase in grain size. There is a change from slip dominated to slip-twin deformation mechanism as the t/d decrease. Decreased twin nucleation stress and worsened inhomogeneous deformation are the inherent causes of twin activation as t/d decreases. When the t/d is less than 1, the dislocation starving mechanism causes a paradoxical increase in the flow stress. Furthermore, there are similarities and variances between the surface and the inner grains. Regarding GND density, there is not much of a difference between the two. However, there is a significant difference between the two in terms of twin volume fraction. The twinning volume fraction in the surface grains is much lower than that in the inner grains. To evaluate the contribution of size effect to flow stress, the variation Equation of twinning volume fraction and dislocations in surface and inner grains are quantified. These results were incorporated into a novel mesoscaled constitutive model that includes lattice friction and size-dependent grain boundaries, dislocations and twinning strengthening. The reduction of flow stress in relation to t/d is correctly predicted by the novel model, which also agrees with the experimental results. This study deepens the understanding of deformation mechanism of pure titanium thin sheets in microforming and provides numerical model for accurate forming.

The Indentation Size Effect (ISE) of Metals

The literature regarding the Reverse Indentation Size Effect (RISE) is scarce, the occurrence of which is assumed for plastic materials, including metals. The content of this article is to study the relationship between applied load and measured values of the Vickers micro-hardness of 19 metals with different types of lattices, measured with a Hanemann tester. The values of the load ranged between 0.09807 N (10 g) and 0.9807 N (100 g). The size and character of the Indentation Size Effect (ISE) were evaluated by Meyer’s power law (index n), Proportional Specimen Resistance (PSR), and Hays—Kendall methods. Meyer’s index n ranged between 1.65 for Mo and 2.44 for Ni. A correlation was found between the micro-hardness and Meyer’s index for metals with FCC and HCP lattices. The measured value of Vickers micro-hardness is influenced by the size and nature of the ISE. If this is not taken into account, it may be misleading. For this reason, we recommend using the “true hardness”, determined by the presented method.

Microstructure Evolution in Nano-sized Wiredrawing Process:

By using molecular dynamics (MD) simulation, the mechanism of nanometer-sized wiredrawing is studied. The possibility of nano-sized plastic working process and predictive microstructures are investigated. As a drawn material, pure iron (α-Fe) and magnesium (Mg) single crystals are compared. The MD models are constructed by utilizing many-body-type (FS or EAM) interatomic potentials with adequate modification for lubrication between wire and die. Dislocation slip is a principal mechanism of plastic deformation in the case of α-Fe in any drawing direction of single crystal. Besides, dislocation densities increases with increase of the wire diameter, where nucleation, annihilation and mutual behavior of dislocations are observed in detail. On the other hand, in Mg single crystal, mechanism of plastic deformation depends strongly on crystal orientation as to drawing direction. Twinning and twin boundaries can be identified by detecting a network of interface dislocations. When the c-axis of hcp lattice is inclined by 45 degrees from the drawing direction, dislocations on basal plane occur, but no twin, and therefore the minimum dislocation density is obtained.

Effect of ultrasonic forging strain processing on the surface layer microstructure and temperature-dependent mechanical properties of high nitrogen austenitic steel

The paper presents the research results on the effect of ultrasonic forging strain (UFS) processing of the high-nitrogen Cr–Mn–N austenitic stainless steel on the surface layer microstructure and tensile properties in the temperature range from −80 up to 20 °C. The high dispersion microstructure, strain aging with the formation of both CrN and Fe2N nitrides, as well as twinning were revealed in the surface layer 2–3 μm thick using a transmission electron microscope (TEM). In addition, ε-martensite with the hcp lattice was observed at a depth of more than 80 μm from the surface after UFS processing. Tensile tests showed that UFS processing had increased the yield strength. Elongation level of about 40% did not differ over the entire test temperature range. A ductile type of the fracture surfaces was observed for all test temperatures down to −80 °C.

Development of a method for calculating the LTEC anisotropy of Zr alloys

The accuracy of determining the LTEC with respect to metals and alloys with an HCP lattice based on inverse pole figures (IPF) is analyzed depending on the number of experimental points on the IPF using three averaging options: (1) taking into account the irregular arrangement of reflections on the stereographic triangle according to Morris; (2) by the multiplicity factor and (3) with the same weight of each orientation. It is shown that when evaluating the LTEC for semi-finished products with a basal texture, 17 reflections on the IPF are sufficient to provide an error of <1% when using Morris averaging and the multiplicity factor; in the case of a prismatic texture, an error of <1% is provided by all three averaging options, while Morris averaging is minimal.

In situ synchrotron radiation X-ray diffraction measurements of Fe–Mo alloy hydrides formed under high pressure and high temperature

Conventional hydrides obtained thus far contain at least one type of metal with high hydrogen affinity, which can form metal hydrides near ambient pressure. In contrast, we synthesized metal hydrides comprising only metals with low hydrogenation affinity which are metals that do not form metal hydrides MHx (M stands for metal element, x > 0.5) below 1 GPa. This is because such hydrides are rare and would demonstrate novel properties and functionalities. To provide a guideline for synthesizing such hydrides, we clarified the hydrogenation reaction processes of FexMo1−x alloys at 6 GPa and 750 °C over a wide composition range of 0.41 ≤ x ≤ 0.88, where both Fe and Mo demonstrate low hydrogen affinities. Depending on the alloy composition, the hydrogenation reaction process can be classified into three types. Two kinds of novel hydrides were synthesized herein. First, FexMo1−xH (0.50 ≤ x ≤ 0.74) with metal elements at the vertices of hexagonal close-packed (hcp) lattices, and second, Fe0.88Mo0.12H with a complex crystal structure, which is likely to be a long-period close-packed structure.

Formation of polytypes structures in Mg single crystals

We report studies of polytypes structures (PSs) formed in the microstructure of deformed Mg single crystals. PSs emerge as co-products of lattice distortion developed in dislocations’ tangles due to reactions and rearrangements of dislocations to the lower energy configurations. TEM observations indicate that entanglements containing an excess content of 〈c〉-component dislocations that maintain dislocation interactions and the evolving pattern in a confined space nucleate PSs. The distortion and rotation of the hcp lattice induce the transition to the triclinic lattice originating PSs. The overlapping strain fields from mutually interlocked dislocations and stacking faults promote local shears and shuffles that develop polytypes domains within the PSs. Ab initio and molecular statics simulations permit understanding the processes governing triclinic and polytypic transformations in Mg and the role of dislocations in PSs’ self-assembly.

Generic accommodations of an atom in the Lennard-Jones fcc and hcp rare-gas solids: A computational study

A comprehensive computational analysis of the stable accommodations of an atom in the Lennard-Jones (LJ) face-centered-cubic (fcc) and hexagonal-closest-packed (hcp) lattices is presented. For a wide range of the guest-host LJ interaction parameters and the number of host atoms removed, $N$, global optimizations are performed to find the minimum energy structures. Stable accommodations are determined using the convex hull of the energy dependence on $N$. The most stable ground accommodations are mapped onto the guest-host LJ parameter space for the hosts representing the rare-gas solids from Ne to Xe. The radial distribution function analysis is used to classify the structures found as originating from the guest placements in the lattice voids or at the node. Ten generic accommodations are identified for each lattice. Comparing to mostly polyhedral structures in fcc, the hcp ones have generally lower symmetry, which should affect the band shapes of atomic absorption spectra. Despite the well-known wrong prediction of higher hcp phase stability in the LJ model, the cases of strong accommodation preferences in one phase or another are identified. Being generic, the obtained results can be useful for a qualitative interpretation of atomic matrix isolation experiments.

Electrostatic energy of Coulomb crystals with polarized electron background.

Outer crusts of neutron stars and interiors of cool white dwarfs consist of bare atomic nuclei, arranged in a crystal lattice and immersed in a Fermi gas of degenerate electrons. We study electrostatic properties of such Coulomb crystals, taking into account the polarizability of the electron gas and considering different structures, which can form the ground state: body-centered-cubic (bcc), face-centered-cubic (fcc), hexagonal close-packed (hcp), and MgB_{2}-like lattices. At zero temperature the electrostatic energy provides a fundamental contribution to the total energy of the classical Coulomb crystal, which allows us to study structural transitions in the neutron-star crusts and crystallized white-dwarf interiors. To take the electron background polarization into account, we use the linear response theory with the electron dielectric function given either by the Thomas-Fermi approximation or by the random-phase approximation (RPA). We compare the widely used nonrelativistic (Lindhard) version of the RPA with the more general, relativistic (Jancovici) version. The results of the different approximations are compared to assess the importance of going beyond the Thomas-Fermi or Lindhard approximations. We also include the contribution of zero-point vibrations of ions into the ground-state energy. We show that the bcc lattice forms the ground state for any charge number Z of the atomic nuclei at the densities where the electrons are relativistic (ρ≳10^{6}gcm^{-3}), while at the nonrelativistic densities (ρ≲10^{6}gcm^{-3}) the fcc and hcp lattices can form the ground state. The MgB_{2}-like lattice never forms the ground state at realistic densities in the crystallized regions of degenerate stars. The RPA corrections strongly affect the boundaries between the phases. As a result, transitions between different ground-state structures depend on Z in a nontrivial way. The relativistic and quantum corrections produce less dramatic effects, moderately shifting the phase boundaries.

Formation of bcc nanocrystals in Co-based amorphous alloys

The crystallization of amorphous alloys of Co-Fe-B-(Ti, Nb) system was studied by differential scanning calorimetry, X-ray diffraction, and transmission electron microscopy. It was determined that the alloying of amorphous alloys of Co-Fe-B system with elements having a bcc lattice promotes the formation of a metastable phase with this lattice during crystallization. Nanocrystal size and the fraction of the phase with a bcc lattice depend on the concentration of alloying components. When the concentration of bcc components increases, nanocrystal size decreases. When Nb (an element with a bcc lattice) is replaced by Ti (an element with an hcp lattice), the fraction of the bcc phase decreases, and multiphase crystallization (simultaneous formation of several crystalline phases) occurs. In Co56Fe16B20Ti8 alloy, the first crystallization stage consists of two steps. During the seemingly simultaneous formation of several crystalline phases, first a small quantity of the bcc phase and then Co23B6 and fcc-Cо crystals are formed sequentially. All the results obtained indicate heterogeneous nanocrystal formation with the nucleation on structurally related ordered regions (on ordered clusters which consist of alloy components with the structure related to the crystallizing phase).