Hcp Crystals Research Articles

Burton-Cabrera-Frank theory for the influence of off-cut direction on the growth of hcp crystals

Burton-Cabrera-Frank theory for the influence of off-cut direction on the growth of hcp crystals

Crystal-Phase- and B-Content-Dependent Electrochemical Behavior of Pd─B Nanocrystals toward Oxygen Reduction Reaction.

The manipulation of crystal phases in metal-nonmetal interstitial alloy nanostructures has attracted considerable attention due to the formation of unique electronic structures and surface atomic arrangements, resulting in unprecedented catalytic performances. However, achieving simultaneous control over crystal phase and nonmetal elements in metal-nonmetal interstitial alloy nanostructures has remained a formidable challenge. Here, a novel synthesis approach is presented for Pd─B interstitial alloy nanocrystals (NCs) that allows investigation of the crystal-phase- and B-content-dependent catalytic performance. Through comparison of the oxygen reduction reaction (ORR) properties of Pd─BX interstitial alloy NCs with different crystal phases and B contents, achieved by precise control of reaction temperature and time, the influences of crystal phase and B contents in the Pd─BX interstitial alloy NCs on ORR are precisely investigated. The hexagonal closed packed (hcp) PdB0.5 NCs exhibit superior catalytic activity, with mass activities reaching 2.58 A mg-1, surpassing Pd/C by 10.3 times, attributed to synergistic effects by the hcp crystal phase and relatively high B contents. This study not only provides a novel approach to fabricate interstitial alloy nanostructures with unconventional crystal phases and finely controlled nonmetal elements but also elucidates the importance of crystal phase and nonmetal element content in optimizing electrocatalytic efficiency.

Ab initio investigation of electronic structure and magnetic transformation during the bcc to hcp transition in Fe induced by pressure

The pressure-induced structural transition from a body-centered cubic (bcc) to a hexagonal close-packed (hcp) crystal in Fe is studied through ab initio electronic structure calculations, combining with the thermodynamic and kinetic phase-transition methods. According to the energy-volume relationship and thermodynamic criterion, the reasonable phase transition pressure is calculated to be about 13.88 GPa. The SSNEB method is employed to examine kinetic structural and barrier changes along bcc-hcp transition under various pressures. The transformation path involves the deformation of (110)bcc into the tightly packed (0001)hcp, accompanied by the relative slip of (110)bcc along the [1̅10]bcc direction, leading to the formation of the hcp structure. Our results indicate a reduction in the transition barrier of bcc-hcp with increasing pressure. Under the thermodynamic phase transition pressure, the bcc-hcp phase transition barrier is still as high as 140 meV, implying that pure hydrostatic pressure is not a sufficient condition to drive the phase transition. In addition, different from the continuous change of volume, the magnetism of the structure undergoes a magnetic mutation at the transition state, and the phase transition process involves complex magnetic transitions.

MOLECULAR SIMULATION OF WATER STRUCTURE IN NARROW SLITLIKE PORES

The structure of water in narrow slitlike pores has been studied by the methods of molecular dynamics simulation. Pores with interwall distances of 6.2–15.5 Å have been considered. Water structures resulting from spontaneous crystallization upon cooling to T = 300 K have been clarified on the basis of twoand three-dimensional order parameters. It has been shown that the observed structures can be described as sections of FCC or HCP crystals.

Semi-classical Anharmonic Correlated Einstein Model Debye-Waller Factors and EXAFS of Hcp Crystals

Semi-classical Anharmonic Correlated Einstein Model Debye-Waller Factors and EXAFS of Hcp Crystals

Self Assembly of Patchy Anisotropic Particle Forming Free Standing Monolayer Film (Adv. Theory Simul. 3/2023)

The phase diagram of prolate ellipsoidal particles shows the co-existence of two types of HCP crystals with the gas particles. Through thermodynamic calculation it is established that a free-standing monolayer having an HCP arrangement co-exist with the gas particles in a small region of the phase diagram. This is reported by Sujin B. Babu and co-workers in article number 2200666.

Template-induced crystallization of charged colloids: a molecular dynamics study.

By using a large enough number of particles and implementing a parallel algorithm on the CUDA platform, we have performed brute-force molecular dynamics simulations to study the template-induced heterogeneous crystallization in charged colloids. Six kinds of templates, whose patterns include the planes of fcc(100), fcc(110), fcc(111), bcc(100), bcc(110) and bcc(111), have been implanted into the middle of the simulation box. Except the fcc(111) template, whose structure benefits not only fcc but also hcp crystals resulting in a similar behavior to homogeneous crystallization, bcc-type templates favor the formation of bcc crystals and bcc-like precursors while fcc-type templates favor the formation of fcc crystals and fcc-like precursors. Therefore, for fcc(100) and fcc(110) templates, heterogeneous crystallization will definitely result in a fcc crystallite. However, the results of heterogeneous crystallization that are induced by bcc-type templates are subtly different at different state points. At the state points where the interaction strength of charged colloids is weak and the fcc phase is thermodynamically stable, the bcc crystals formed with the promotion of bcc-type templates are not stable so as to tend to transform into fcc or hcp crystals. When the interaction strength of charged colloids is high, the predominant bcc crystals formed with the promotion of bcc-type templates can always persist within the time scale of simulation although not bcc but fcc crystals are thermodynamically stable.

Atomic-scale study of He ion irradiation-induced clustering in α-Zirconium

Zircaloy-4 alloy specimens consisting of α-Zr grains were irradiated on a tandem accelerator with 5 MeV He ions to a fluence of 5 × 1021 ions m − 2 at different temperatures. Defect clusters and He bubbles were observed in the TEM micrographs of the irradiated microstructure. The small defect clusters and He bubbles were found to exhibit spherical shapes whereas the large defect clusters showed plate-like shapes. Molecular dynamics simulations and crystallography analyses revealed that the defect clusters are mainly composed of Zr interstitials. During He-ion irradiation, the clustering of Zr dumbbell interstitials results in the formation of small and spherical defect clusters. With further irradiation, the Zr interstitials prefer to occupy the octahedral sites that align along the {12¯14} planes with other site interstitials, forming cluster sections. These initial planar clusters then grow by extending along the <101¯0> directions and by forming more cluster sections along the <12¯16> directions, leading to large plate-like defect clusters on the {1¯21¯1}planes. He bubbles nucleate by the clustering of He atoms and vacancies that are formed by kicking out the lattice atoms, and then grow by the migration and coalescence of helium-vacancy clusters. Small He atoms have many interstitial sites to occupy in the α-Zr HCP crystal, allowing He bubbles to grow to spherical shapes. In addition, grain boundaries induce the accumulation of primary point defects and He atoms and high irradiation temperature accelerates diffusion, and hence larger defect clusters and He bubbles form in the grain boundary regions and at higher irradiation temperature.

Prismatic-to-basal plastic slip transition in zirconium

We model three-dimensional dislocation ensembles in Zr pillars using newly developed mobility laws for dislocations on prismatic and basal planes. The effects of the loading orientation and temperature on the micro-scale mechanical response of single crystals are systematically explored. Easy dislocation glide is observed on prismatic planes, while hard glide occurs on basal planes. By selection of sample temperature and loading orientation, we show that the transition from prismatic to basal glide of dislocation ensembles can be controlled. We also show that the presence of dislocation cross-slip strongly promotes prismatic glide as a result of prismatic/basal cross-slip energy asymmetry. Cross-slip does not alter the occurrence of plastic slip transition but only induces a shift towards higher temperatures. The preponderance of plastic slip on basal or prismatic planes is found to be mediated by a transitional dislocation, which is composed of glissile segments on parallel prismatic planes connected by glissile super-jogs on basal planes. These findings provide a mechanistic understanding of experiments and highlight the significance of transitional prismatic-basal dislocations on the macroscopic characteristics of plasticity in HCP crystals.

Tensile deformation behaviours of polycrystalline Cu80Ni20 alloy: insights from molecular dynamics simulations

ABSTRACT The polycrystalline Cu80Ni20 sample was simulated by molecular dynamics (MD) approach. This sample consists of the fcc and hcp crystals alternated with disordered (d-) structures. Under the uniaxial tensile test, the sample exhibits elastic and plastic behaviours. The fcc and hcp crystals disintegrate into the d-structures with increasing strain but small clusters of fcc and hcp crystals still exist at the large strain. A very small number of d-atoms recrystallises into the fcc and hcp crystals during the tensile deformation. The distances between atoms are stretched gradually during the tensile loading. The growth and coalescence of big simplexes with R S ≥ 2.0 Å occur only in the d-structures. The shear band (SB) is formed during the tensile deformation. The big simplexes coalesce and propagate in the SB, causing the crack propagation.

On the micromechanics of void mediated failure in HCP crystals

Three-dimensional voided cell calculations are carried out to investigate the effect of initially flat voids on the effective behavior of hexagonal crystals. The crystalline matrix is described using a set of constitutive relations that account for slip and twinning. In some materials, the limiting case of penny-shaped cracks better represents incipient void formation. Cell boundary conditions are such that internal necking of the intervoid ligament or void impingement are the only possible failure modes. The overall behavior of the cell is analyzed under various stress states and two loading orientations, with special attention given to the evolution of damage-relevant microstructural variables. Particular emphasis is placed on crystallographic aspects of void growth and coalescence, which often prove difficult to represent using phenomenological anisotropic plasticity models.

Analysis of Dimensional Change in Zr-2.5%Nb CANDU Pressure Tube Material aged at 300-400°C using Neutron Diffraction

The primary boundary of the CANDU reactor consists of Zr-2.5%Nb alloy pressure tubes and SA106 alloy feeder pipes. The pressure tubes are exposed to fast neutron (E&gt;1Mev) irradiation during reactor operation, and experience dimensional changes. The actual dimensional change in the pressure tube during in-service operation is greater than the design estimate. It is not fully understood what is responsible for the dimensional changes in the pressure tubes. In this study, to accelerate the aging effect on the pressure tube material, aging was performed at 300-400°C for up to 20,000 hours and neutron diffraction was used to track variation in lattice spacing, thereby identifying the reason for the dimensional change. The analysis result showed that α-Zr, the main phase of Zr-2.5%Nb, had little dimensional change at 350°C or lower. On the other hand, the aging treatment at 400°C resulted in anisotropic expansion in the α-Zr of the HCP crystal, of 0.02% and 0.08% in the (1010) and (0002) directions, respectively. It was confirmed that above 350oC the β-phase in the Zr-2.5%Nb alloy was decomposed to precipitate β-Nb . The driving force of the lattice expansion may be due to the diffusing out tendency of the super saturated Nb in α-Zr and due to the residual cold working effect applied during the manufacturing process. The enhancing effects of fast neutron irradiation on lattice diffusion are discussed in detail.

Anharmonic effects and EXAFS of hcp crystals studied based on high-order expanded Debye-Waller factors including dispersion effects

The anharmonic effects and extended X-ray absorption fine structure (EXAFS) of hcp crystals have been studied based on the high-order expanded Debye-Waller factors. The theory is derived based on the many-body perturbation approach and cumulant expansion up to the fourth order with using the linear anharmonic oscillator model to account for the dispersion involving more contributions of phonons from the first Brillouin zone. The derived anharmonic effective potential includes the many-body effect by a simple measure avoiding the intensive full lattice dynamical calculations. The Morse potential is assumed to describe the single-pair atomic interaction. The derived analytical expressions for cumulants describe the anharmonic effects and are applied to the anharmonic EXAFS contributing to the accurate structural determination. The advantages and efficiencies of the present theory are illustrated by good agreement of numerical results for all considered quantities of Zn in hcp phase with experiment.

Investigation of cutting force, cutting stress, subsurface deformation, and phase change during the macro and nano cutting process of OFHC copper

In this paper, the FEM and MD method are used to establish macro and nano cutting model of OFHC copper, and the cutting force, cutting stress and subsurface deformation and phase change are compared and analyzed under the macro cutting depth (0.2mm, 0.3mm, 0.4mm) and nano cutting depth (0.8nm, 1.2nm, 1.6nm). Meanwhile, experiments are used to verify the simulated cutting force. The conclusions are below: The change trends of macro and nano cutting force are similar, and the ratios of the cutting forces in x and y directions at different cutting depths are similar. In addition, there are lateral chip both in the macro and nano cutting processes, showing a high degree of consistency. However, the nano cutting stress reached 25 times of that in the macro state. Then it was found that the area of unrecoverable plastic deformation on the surface increased with the increase of the cutting depth during the macro cutting process, but the existence depth of pure plastic deformation remained unchanged within 0.05mm. Although in the nano cutting process, the proportion of FCC crystals converted to HCP crystals remains unchanged as the cutting depth increases, but the existence range is increasing. Finally, the amplitude and change trend of experimental and the simulated cutting force are highly consistent, which verifying the rationality of the macro simulated cutting model.

Anisotropic yield surfaces of additively manufactured metals simulated with crystal plasticity

The mechanical anisotropy created by additive manufacturing (AM) is not yet fully understood and can depend on many factors, such as powder material, manufacturing technology and printing parameters. In this work, the anisotropic mechanical properties of as-built, laser powder bed fusion (LPBF) austenitic stainless steel 316L and titanium alloy Ti-6Al-4V are investigated through crystal plasticity simulations. Periodic representative volume elements (RVEs) are used that are specific to each material. The RVE for austenitic stainless steel consists of FCC crystals with a crystallographic texture measured by X-ray diffraction. The α′ martensite microstructure of Ti-6Al-4V is captured with a multi-scale RVE, including internal lamellar structures, using HCP crystals and a synthetically generated texture. For both materials, the crystal plasticity parameters are calibrated against tensile tests carried out on dog-bone specimens printed in different orientations. The RVEs, calibrated to experiments, are applied in virtual material testing and subjected to multiple load cases to generate the Hill-48 and Yld2004-18p yield surfaces of the materials.

A crystal plasticity model for porous HCP crystals in titanium alloys under multiaxial loading conditions

In this paper, an efficient and effective crystal plasticity model is proposed for porous HCP crystals subject to a variety of multiaxial loading conditions. These conditions include (i) uniaxial, biaxial, and triaxial, (ii) tension and compression, (iii) low and high triaxiality, and (iv) axisymmetric and non-axisymmetric loadings. The framework is based on a combination of variational homogenization, phenomenological extensions, and assumptions motivated by observations from the high-fidelity micromechanical analysis. A novel penalty-free algorithm is employed to reach and maintain a specified stress state while performing representative volume element (RVE)-based crystal plasticity finite element (CPFE) analysis with porosity. The RVE studies demonstrate that the initial porosity, crystallographic orientation, and stress states have a significant effect on the homogenized mechanical responses of microscopically porous RVEs. The proposed porous crystal plasticity model is developed and calibrated using a database generated from the results of micromechanical RVE analysis. The calibrated porous model is reasonably effective in predicting the response of porous crystalline RVEs.

The roles of Rh crystal phase and facet in syngas conversion to ethanol

Design of Rh catalyst by controlling crystal phase and facet is significant for providing higher mass-specific activity and selectivity for ethanol synthesis from syngas. We demonstrate here the roles of crystal phase and facet in syngas conversion to ethanol on different crystal facets of fcc and hcp Rh by DFT calculations and microkinetic modeling. Compared to hcp Rh, fcc Rh not only exhibits higher intrinsic activity for CH and ethanol formation, but also exposes much denser active facets and inhibits methanol, especially, the mainly exposed (111). Further, for the Fe-modified Rh catalysts, fcc Rh exhibits better activity and selectivity toward ethanol, which is still superior to hcp Rh. Thus, Rh-based catalysts should be focused on the fcc crystal phase instead of hcp crystal phase. The findings in this work are significantly beneficial to the rational design of Rh-based catalysts for CO hydrogenation to desired ethanol.

An initio study of influence of substitution of Sc with Al on intrinsic mechanical properties of hexagonal high-entropy alloys Hf0.25Ti0.25Zr0.25Sc0.25−xAlx (x ≤ 15%)

The intrinsic mechanical properties of novel hexagonal high entropy alloys (HEAs) Hf0.25Ti0.25Zr0.25Sc0.25−xAlx (x ≤ 15%) have been studied using first-principles theory based on special quasi-random structure, and influence of Al content is stressed. The derived formation enthalpy and elastic constants indicate the thermodynamical and mechanical stability of the studied alloys. With substitution of Al for Sc, bulk modulus, shear modulus and Young's modulus tend to decrease, while HEAs become more ductile. The estimated elastic anisotropies of alloys show an ascending trend with increasing Al content, and the shear and tensile Young's moduli along several typical directions have been obtained for the studied HCP crystal. Furthermore, the studied ideal strength of HCP HEAs suggest that the ideal tensile strength (ITS) of HEAs occurs in [112̅0] direction, and decreases from 5.34 GPa to 3.80 GPa with increasing Al content for x goes from 0.00 to 0.15, whereas corresponding critical tensile strain increases from 0.07 to 0.08. The ideal shear strength (ISS) of HEAs takes place in (101̅0)< 112̅0 > shear system. With increasing Al content for x goes from 0.00 to 0.15, the ISS decrease from 3.71 GPa to 2.46 GPa, the corresponding critical shear strains increase also from 0.10 to 0.11. So HEAs are more ductile as increasing Al concentration at expense of both ITS and ISS. It is worth noting that the initial slopes of tensile and shear stress–strain are in good agreement with the tensile and shear modulus from elastic parameter, also exhibiting descending trend with Al content. The critical resolved shear stresses (CRSS) estimated from the ITS for HEAs is smaller than the corresponding ISS, suggesting that slip is preference to cleavage. The shearability and half-width of the dislocation are also evaluated and discussed. Furthermore, the detailed electronic structure in bond length evolution is further studied, from feature of the Al-M atom pairs due to addition of Al element, the mechanism for enhancement of ductility for HfTiZrSc1−xAlx is uncovered, all of these provide more details for more adequate understanding deeply the intrinsic mechanism of mechanical properties of alloys HfTiZrSc1−xAlx.

Analysis of Nanoscratch Mechanism of C-Plane Sapphire with the Aid of Molecular Dynamics Simulation of Hcp Crystal.

In this study, single groove nanoscratch experiments using a friction force microscope (FFM) with a monocrystalline diamond tip were conducted on a c-plane sapphire wafer to analyze the ductile-regime removal and deformation mechanism including the anisotropy. Various characteristics, such as scratch force, depth, and specific energy for each representative scratch direction on the c-plane of sapphire, were manifested by the FFM, and the results of the specific scratch energy showed a trend of six-fold symmetry on taking lower values than those of the other scratch directions when the scratch directions correspond to the basal slip directions as . Since this can be due to the effect of most probably basal slip or less probably basal twinning on the c-plane, a molecular dynamics (MD) simulation of zinc, which is one of the hexagonal close-packed (hcp) crystals with similar slip/twining systems, was attempted to clarify the phenomena. The comparison results between the nanoscratch experiment and the MD simulation revealed that both the specific scratch energy and the burr height were minimized when scratched in the direction of the basal slip. Therefore, it was found that both the machining efficiency and the accuracy could be improved by scratching in the direction of the basal slip in the single groove nanoscratch of c-plane sapphire.

Understanding the role of local texture variation on slip activity in a two-phase titanium alloy

During plastic deformation of engineering alloys with a hcp crystal structure, slip systems with different critical resolved shear stresses can be activated. In the case of two-phase titanium alloys such as Ti-6Al-4V, it is well established that various a→ and c→+a→ type slip systems can be activated in the α-phase, which dominates this alloy. However, their relative likelihood is less well established particularly when comparing prismatic a→ and basal a→ slip which are known to have similar critical resolved shear stress values. By combining EBSD-based grain orientation mapping and high-resolution digital image correlation, grain specific shear strain mapping and Burgers vector direction analysis was carried out after small levels of plasticity in two differently microtextured Ti-6Al-4V samples. This enabled the different types of strain heterogeneity and strain patterns to be linked to the underlying microstructure and microtexture. The detailed analysis shows that the dominance of a particular a→ type slip mode greatly varies with the local texture and that shear strain patterns extend across many grains when soft macrozones (clusters of similarly orientated grains) are present. The work highlights that the relative displacement ratio analysis significantly improves slip trace analysis in hcp crystals by reducing the cases of ambiguous solutions and that grain neighbourhood can have a greater impact on slip system activation than Schmid factor.