Hexagonal Close-packed Structure Research Articles

High entropy alloys with hexagonal close-packed structure derived from thin film combinatorial approach

A thin film combinatorial synthesis approach was utilized to develop a compositional and structural library for the OsRuWCo alloy system. The survey of this alloy space produced 24 unique compositions and their corresponding crystal structures, which represent a phase map where amorphous and single-phase hexagonal close-packed alloys were identified. Based on the identification of a promising candidate film region, a new high entropy alloy based on OsRuWCo, but with non-equiatomic composition, was synthesized in bulk form. The alloy exhibited a single-phase hexagonal close-packed structure in the as-cast state. Three derivatives from this system were also developed by considering the heat of mixing, atomic size, and binary solubility. These new alloys are based on OsRuWCoIr and OsRuWCoFe, both of which exhibit single-phase hexagonal close-packed as-cast structures, as determined by X-ray diffraction and electron microscopy. Additionally, this large compositional space was utilized to evaluate conventional parameters that describe high entropy alloys (∆Smix, ∆Hmix, Ω, δ, and VEC), and these were found to elucidate the criteria for solid solution formation in this system. Trends illustrating the evolution from amorphous to crystalline phases are demonstrated and discussed. It is concluded that in this particular case, the dominant factors promoting crystallinity are thermodynamic or electronic, rather than geometric.

Effects of Al on crack propagation in titanium alloys and the governing toughening mechanism

In this work, molecular dynamics simulation is performed to investigate the effect of Al element on the crack propagation along the {101‾1} twin boundary in Ti alloys with different contents and distribution of Al element during the uniaxial tensile deformation. The results show that increasing Al content obviously promotes the percentage of face-centered cubic (FCC) structure during the deformation process, which hinders the micro-crack propagation and enhances the plasticity and toughness of the Ti–Al alloys. The FCC bands transform from the hexagonal close-packed (HCP) matrix satisfying the basal-type (B-type) orientation relationship (OR) represented by (0001)HCP||(111)FCC and [12‾10]HCP||[11‾0]FCC. The increase of Al element reduces the basal stacking fault (BSF) energy of the HCP structure and thus contributes to the formation of BSFs, which grow into FCC bands with the B-type orientation by the slip of Shockley partial dislocations. The {101‾1} pyramidal SF activated at the crack tip evolves into FCC grains in the Ti–Al alloys following the prismatic-type (P-type) OR with the HCP matrix represented by (101‾0)HCP||(110)FCC and [12‾10]HCP||[11‾0]FCC, during which atomic shear and shuffle is involved.

Synergistic effect of Nb and Mo alloying on the microstructure and mechanical properties of CoCrFeNi high entropy alloy

Improving the strength of face-centered cubic high entropy alloy, CoCrFeNi, has been a research focus in recent years. In this paper, to further enhance its strength for engineering applications, a series of CoCrFeNi-(Nb, Mo) high entropy alloys were prepared using vacuum arc-melting. The synergistic effect of alloying by Nb and Mo on microstructure and mechanical properties of CoCrFeNi-based high entropy alloy were investigated. The criterion Md‾ was used to evaluate the phase evolution for current alloy system. Through synergistic alloying with Nb and Mo, a new kind of Laves phase with hexagonal close-packed structure was formed. The Laves phase is semi-coherent with the face-centered cubic matrix. Furthermore, through adjusting the ratio of Nb and Mo, fine lamellar structure Laves phase was obtained. The synergistic precipitation strengthening and solid-solution effect lead to high yield and fracture strength of 426 MPa and 714 MPa, maintaining high elongation of 17.4%. Fracture observation shows that the face-centered cubic matrix of the alloy system is ductile fracture, and the Laves phase is cleavage fracture. The results of this research would provide a basis for strengthening face-centered cubic high entropy alloy through synergistic alloying effect.

Ordered isoporous membranes from ionic diblock copolymers via SNIPS: Optimizing effective factors with a structural survey

The membrane formation directly through the self-assembly and nonsolvent induced phase separation (SNIPS) method without the aid of foreign additives has been more challenging in the case of ionic block copolymers, like polystyrene-block-poly (acrylic acid) (PS-b-PAA), in comparison with non-ionic amphiphilic systems. Herein, the influence of governing factors on the structure tuning, including physicochemical characteristics of polymer, solvent and nonsolvent, casting parameters and process conditions to prepare additive-free and ordered isoporous membranes was studied. Due to the high degree of ionization and hydrophilicity of PAA blocks, micellization in the form of “reverse micelles” was aimed to induce more stable micelles assembly. The well-ordered micelles assembly was generated by the suitable selection of solvent (1,4-dioxane), strongly segregated characteristic of PS-b-PAA (high χN) and proper adjustment of casting parameters including polymer concentration and solvent evaporation rate and time. The uniformity and regularity of the micelles assembly during phase inversion step was preserved owing to the high freezing point of solvent. Highly permeable asymmetric isoporous membranes prepared over a wide range of evaporation times (40–120 s) using relatively low polymer concentrations (15 wt%) with adjustable and monodispersed pore-size in the range of sub-10 up to 22 nm. FESEM and AFM images in addition to DLS measurements and solution SAXS analysis were applied to demonstrate the hexagonal close-packed (HCP) structure of micelles assembly and partially hexagonal arrangement of pores at the membrane selective-layer. It was also demonstrated that corona (PS) of adjacent micelles on the membrane surface fused together to form a continuous membrane matrix, while surface-pores were the nanoscale voids, formed between the densely packed spherical reverse micelles practically immobilized by immersion in low-temperature water bath. Such additive-free, asymmetric, iso- and nano-porous membranes have attractive potential practical applications in biomedical and pharmaceutical industries for size-based separations, in addition to their conventional applications in water purification.

Experimental and numerical investigations of Ni–Co–SiO2 alloy films deposited by magnetic-field-assisted jet plating

A new method of magnetic-field-assisted jet plating is presented to manufacture Ni-Co-SiO2 alloy films. The influence of different concentrations of Co2+ ions of the electrolyte is investigated with and without magnetic field to study the resulting properties of the deposits. The texture orientation, surface morphology, magnetic properties and corrosion resistance of the alloy films were characterized. The results show that with increasing Co2+ concentration in conventional jet-plating, the surface morphology changes from a granular crystal structure to a needle-like structure at 30 g/L caused by the hexagonal close-packed (HCP) structure of a large Co content. Differently, the assistance of the magnetic field leads to lower Co content in the films, even at a high Co2+ concentration of 30 g/L. The deposit layer remains in the face-centered cubic (FCC) structure and shows a granular morphology. The magnetic field in general leads to grain refinement and inhibits the abnormal Ni-Co co-deposition. The Ni–Co–SiO2 alloy films obtained by magnetic-field-assisted jet plating have a smooth and dense surface. The best soft magnetic properties and corrosion resistance are obtained at a Co2+ concentration of 20 g/L. The coercivity is as low as 7.5 Oe, and the corrosion current density is as low as 1.12 μA·cm−2 in 3.5 wt% NaCl solution without agitation and at room temperature, clearly showing the advantages of the method for preparing superior soft magnetic materials. In addition, a physical model of magnetic-field-assisted jet plating is established. The magnetic forces and the resulting electrolyte flow are analyzed with the help of numerical simulations, and the influence of the magnetic field on the deposition process is discussed from the perspective of magnetohydrodynamics.

Hydrogenation treatment under several gigapascals assists diffusionless transformation in a face-centered cubic steel

The use of hydrogen in iron and steel has the potential to improve mechanical properties via altering the phase stability and dislocation behavior. When hydrogen is introduced under several gigapascals, a stoichiometric composition of hydrogen can be introduced for steel compositions. In this study, a face-centered cubic (fcc) stainless steel was hydrogenated under several gigapascals. When the steel was not hydrogenated, the microstructure after depressurization was an fcc with a hexagonal close-packed (hcp) structure. In contrast, the hydrogenation treatment resulted in a fine lath body-centered cubic (bcc) structure arising from diffusionless transformation. In particular, the bcc phase formed through the following transformation sequence: fcc → hcp → dhcp (double hexagonal close-packed phase) → bcc. That is, the use of hydrogenation treatment realized fine microstructure evolution through a new type of diffusionless transformation sequence, which is expected to be used in future alloy design strategies for developing high-strength steels.

Phase Decomposition and Strengthening in Hfnbtatizr High Entropy Alloy from First-Principles Calculations

Phase decomposition influences significantly the mechanical properties of high entropy alloys (HEAs). Prediction of the phase decomposition of HEA is greatly hindered by the hyper-dimensional composition space of the alloys. In the present work, we propose to represent the HEAs as various pseudo-binary alloys of which the temperature dependent free energies as functions of compositions may be readily calculated by using first-principles methods in combination with thermodynamic models. With the calculated free energies, the phase diagrams of the pseudo-binary alloys may be constructed and the phase decomposition can be predicted. This procedure is applied to Hf-Nb-Ta-Ti-Zr alloy with body-centered cubic (BCC) structure. We predict that the equiatomic HfNbTaTiZr HEA suffers from phase decomposition below critical temperature of 1298 K. The HEA decomposes most favorably to BCC NbTa-rich and HfZr-rich phases. The BCC HfZr-rich phase transfers to a hexagonal close-packed structure (HCP) phase at low temperature. The predicted compositions of the decomposed phases are in good agreement with experiment and Thermal-Calc modeling. Furthermore, the effect of the phase decomposition on the strength of the HEA is evaluated by considering the solid-solution and precipitation strengthening mechanisms. The precipitation strengthening effect is stronger than the solid-solution strengthening at the low annealing temperature but becomes weaker at high annealing temperature.

Martensitic transformation in CrCoNi medium-entropy alloy at cryogenic temperature

We investigated the in situ deformation behavior of the CrCoNi medium-entropy alloy at a cryogenic temperature of 140 K and compared it with deformation at room temperature. The sample exhibited higher strength and larger ductility at the cryogenic temperature. The CrCoNi alloy remained single-phase face-centered cubic at room temperature, while deformation at 140 K resulted in a martensitic transformation to the hexagonal close-packed structure. The phase transformation, an additional deformation mechanism to stacking faults, twinning, and dislocation slip, resulted in a higher work hardening at cryogenic temperature. The study addresses the structure metastability in the CrCoNi alloy, which led to the formation of epsilon-martensite from the intrinsic stacking faults.

Origin of phase stability in Fe with long-period stacking order as an intermediate phase in cyclic γ−ε martensitic transformation

A class of Fe-Mn-Si--based alloys exhibit a reversible martensitic transformation between the $\ensuremath{\gamma}$ phase with a face-centered cubic (fcc) structure and an $\ensuremath{\epsilon}$ phase with a hexagonal close-packed (hcp) structure. During the deformation-induced $\ensuremath{\gamma}\text{\ensuremath{-}}\ensuremath{\epsilon}$ transformation, we identified a phase that is different from the $\ensuremath{\epsilon}$ phase. In this phase, the electron diffraction spots are located at the 1/3 positions that correspond to the ${0002}$ plane of the $\ensuremath{\epsilon}$ (hcp) phase with 2H structure, which suggests long-period stacking order (LPSO). To understand the stacking pattern and explore the possible existence of an LPSO phase as an intermediate between the $\ensuremath{\gamma}$ and $\ensuremath{\epsilon}$ phases, the phase stability of various structural polytypes of iron was examined using first-principles calculations with a spin-polarized form of the generalized gradient approximation in density functional theory. We found that an antiferromagnetic ordered $6{\mathrm{H}}_{2}$ structure is the most stable among the candidate LPSO structures and is energetically closest to the $\ensuremath{\epsilon}$ phase, which suggests that the observed LPSO-like phase adopts the $6{\mathrm{H}}_{2}$ structure. Furthermore, we determined that the phase stability can be attributed to the valley depth in the density of states, close to the Fermi level.

A novel NbTaW0.5 (Mo2C)x refractory high-entropy alloy with excellent mechanical properties

In this study, a refractory high-entropy alloy (RHEA) NbTaW0.5 was selected as the master alloy and carbide Mo2C was added to prepare the NbTaW0.5(Mo2C)x (x = 0–0.25) RHEAs. The effect of Mo2C addition on the phase composition, microstructure evolution, and mechanical properties of the as-cast and annealed RHEA was investigated. The microstructure and mechanical properties of the as-cast and isothermally annealed (1673 K/12 h) NbTaW0.5(Mo2C)x RHEAs were studied. The results showed that the as-cast RHEAs consisted of a disordered body-centered cubic solid solution phase and a second M2C carbide phase with hexagonal close-packed (HCP) crystal structure. The HCP–M2C carbides were enriched with Nb, Ta, and C elements and tended to achieve a similar eutectic morphology in the interdendritic regions. After isothermal annealing, orthorhombic carbides were formed near the primary carbides, which apparently depleted the carbon supersaturation of the matrix. The compression mechanical properties of as-cast alloys were determined at room temperature, 1473 K, and 1673 K. The yield strength of NbTaW0.5(Mo2C)0.2 RHEA exceeded 1000 MPa at 1473 K, and even reached 697 MPa at 1673 K, far exceeding some of the currently reported RHEAs. These RHEAs had good plasticity and showed excellent application prospects.

A Review on Capturing Twin Nucleation in Crystal Plasticity for Hexagonal Metals

Owing to its ability to incorporate Schmid’s law at each integration point, crystal plasticity has proven a powerful tool to simulate and predict the slip behavior at the grain level and the ensuing heterogeneous stress/strain localization and texture evolution at the macroscopic level. Unfortunately, notwithstanding substantial efforts during the last three decades, this remarkable capability has not been replicated for materials where twinning becomes a noticeable deformation mechanism, namely in the case of low-stacking fault energy cubic, orthorhombic, and hexagonal close-packed structures. The culprit lies in the widely adopted unphysical pseudo-slip approach for capturing twin formation. While the slip is diffuse, twinning is a localized event that occurs as a drastic burst of a confined number of partial twinning dislocations establishing an interface that pursues growth through a thread of perfect twinning dislocations in the sense of bicrystallography. Moreover, at earlier stages, twin nucleation may require atomic diffusion (Shuffling) and faceting, generally demanding higher stress levels not necessarily on the twin shear plane, while triaxiality at adequate sites might be needed or preferred such as lower grain boundary misorientations or other twin boundaries. Identifying a mathematical framework in the constitutive equations for capturing these twin formation sensitivities has been a daunting challenge for crystal plasticity modelers, which has stalled ameliorating the design of key hexagonal materials for futuristic climate change-related industries. This paper reviews existing approaches to incorporating twinning in crystal plasticity models, discusses their capabilities, addresses their limitations, and suggests prospective views to fill gaps. The incorporation of a new physics-based twin nucleation criterion in crystal plasticity models holds groundbreaking potential for substantial progress in the field of computational material science.

Nature of electronic topological transition and superconductivity in bismuth under high pressure from ab initio random structure searching

We have predicted the hexagonal close-packed (hcp) structure of bismuth (Bi) using ab initio random structure searching (AIRSS) at extreme conditions. The calculation, which included spin–orbit coupling, shows that the hcp structure is thermodynamically and dynamically stable at high pressure. The electronic band structure calculations suggest the downshifting of the flat band through compression due to Lifshitz transitions. The Fermi surface shape of hcp Bi produces the metallicity in this material. The electron localization function reveals a weak bonding of Bi. The solutions of electronic topological transition and a soft-mode of phonon dispersion provide the possibility for prediction and reduction of the superconducting transition temperature.

Microstructural evolution of a polycrystalline cobalt during tensile deformation

The microstructural evolution of cobalt during tensile deformation was investigated via electron back scatter diffraction and transmission electron microscope. The undeformed sample has an almost pure hexagonal closed-packed structure, and there exist a large number of special boundaries with a 70.5°/<112‾0> misorientation. Twinning dominates the tensile deformation process, especially the {101‾2} twins. When the strain reaches 15%, the volume fraction of the twins reaches up to 61%. Meanwhile, {101‾2}−{101‾2} double twinning and even {101‾2}−{101‾2}−{101‾2} triple twinning are triggered, which work together with primary twins and martensite phase transformation to refine the grains. In addition, two different formation modes of special boundaries with misorientation of 39°/<112‾0> are observed. One is formed by partial dislocations slipping, and the other is formed by {101‾1}−{101‾2} double twinning.

High-pressure magnetic properties of antiferromagnetic samarium up to 30 GPa using a SQUID-based vibrating coil magnetometer

Samarium (Sm) has antiferromagnetic (AFM) ordering at the hexagonal site at ${T}_{\mathrm{N}}(\text{hex})=106$ K, and at the cubic site at ${T}_{\mathrm{N}}(\text{cub})=14$ K at ambient pressure. The structural transition from the so-called Sm-type structure to the double hexagonal close-packed (dhcp) structure occurs at approximately 6 GPa. According to electrical resistance measurements, the shift of ${T}_{\mathrm{N}}$(hex) toward the low-temperature side occurs simultaneously with the shifting of ${T}_{\mathrm{N}}$(cub) toward the high-temperature side with increasing pressure. We conducted dc magnetic measurements on Sm at high pressure up to 30 GPa using a SQUID-based vibrating-coil-magnetometer to pursue ${T}_{\mathrm{N}}$(cub) at high pressure: The magnetic measurements revealed stepwise anomalies below 2 GPa. A ferromagnetic (FM) anomaly was observed in the dhcp structure, suggesting that spins at both structural sites formed the FM magnetization. In the face-centered cubic (fcc) structure at above 12 GPa, a reduction in the magnetic signal occurred. In the distorted fcc structure at above 20 GPa, sufficient suppression of the FM moment was observed and afterward the diamagnetic signal suggesting the possibility of superconductivity was observed.

Two-Dimensional Frank−Kasper Z Phase with One Unit-Cell Thickness

Z phase is one of the three basic units by which the Frank-Kasper (F-K) phases are generally assembled. Compared to the other two basic units, that is, A15 and C15 structures, the Z structure is rarely experimentally observed because of a relatively large volume ratio among the constituents to inhibit its formation. Moreover, the discovered Z structures are generally the three-dimensional ordered Gibbs bulk phases to conform to their thermodynamic stability. Here, we confirmed the existence of a metastable two-dimensional F-K Z phase that has only one unit-cell height in the crystallography in a model Mg-Sm-Zn system, using atomic-scale scanning transmission electron microscopy combined with the first-principles calculations. Self-adapted atomic shuffling can convert the simple hexagonal close-packed structure to the topologically close-packed F-K Z phase. This finding provides new insight into understanding the formation mechanism and clustering behavior of the F-K phases and even quasicrystals in general condensed matters.

Search for Crystal Structure Candidates by Applying the Generalized Scaled Hypersphere Search Method to Volume of Unit Cell Containing Quasi Rigid Body Approximated Atoms and Molecules

The volume function was employed to predict crystal structures. Face-centered cubic and hexagonal close-packed structures were identified as minima on the function. The optimization of the function with the actual ethane crystal resulted in largely similar orientations of the molecules. Searching on the function from a random structure resulted in some minima, including a structure similar to the actual structure.

Distinct chemical fixation of CO2 enabled by exotic gold nanoclusters

Atomically precise metal nanoclusters, especially the metal nanoclusters with an exotic core structure, have given rise to a great deal of interest in catalysis, attributing to their well-defined structures at the atomic level and consequently unique electronic properties. Herein, the catalytic performances of three gold nanoclusters, such as Au38S2(S-Adm)20 with a body-centered cubic (bcc) kernel structure, Au30(S-Adm)18 with a hexagonal close-packed (hcp) core structure, and Au21(S-Adm)15 with a face-centered cubic (fcc) kernel structure, were attempted for the CO2 cycloaddition with epoxides toward cyclic carbonates. Due to the excess positive charge with a strong Lewis acidity and large chemical adsorption capacity, the bcc-Au38S2(S-Adm)20 nanocluster outperformed the hcp-Au30(S-Adm)18 and fcc-Au21(S-Adm)15 nanoclusters. Additionally, the synergistic effect between the gold nanocluster and co-catalyst played a crucial role in CO2 cycloaddition.

Thermodynamics of lattice vibrations in non-cubic crystals: the zinc structure revisited.

A phenomenological model of anisotropic lattice vibrations is proposed, using a temperature-dependent spectral cutoff and varying Debye temperatures for the vibrational normal components. The internal lattice energy, entropy and Debye-Waller B factors of non-cubic elemental crystals are derived. The formalism developed is non-perturbative, based on temperature-dependent linear dispersion relations for the normal modes. The Debye temperatures of the vibrational normal components differ in anisotropic crystals; their temperature dependence and the varying spectral cutoff can be inferred from the experimental lattice heat capacity and B factors by least-squares regression. The zero-point internal energy of the phonons is related to the low-temperature limits of the mean-squared vibrational amplitudes of the lattice measured by X-ray and γ-ray diffraction. A specific example is discussed, the thermodynamic variables of the hexagonal close-packed zinc structure, including the temperature evolution of the B factors of zinc. In this case, the lattice vibrations are partitioned into axial and basal normal components, which admit largely differing B factors and Debye temperatures. The second-order B factors defining the non-Gaussian contribution to the Debye-Waller damping factors of zinc are obtained as well. Anharmonicity of the oscillator potential and deviations from the uniform phonon frequency distribution of the Debye theory are modeled effectively by the temperature dependence of the spectral cutoff and Debye temperatures.

Stability and equation of state of face-centered cubic and hexagonal close packed phases of argon under pressure

The compression of argon is measured between 10 K and 296 K up to 20 GPa and and up to 114 GPa at 296 K in diamond anvil cells. Three samples conditioning are used: (1) single crystal sample directly compressed between the anvils, (2) powder sample directly compressed between the anvils, (3) single crystal sample compressed in a pressure medium. A partial transformation of the face-centered cubic (fcc) phase to a hexagonal close-packed (hcp) structure is observed above 4.2–13 GPa. Hcp phase forms through stacking faults in fcc-Ar and its amount depends on pressurizing conditions and starting fcc-Ar microstructure. The quasi-hydrostatic equation of state of the fcc phase is well described by a quasi-harmonic Mie–Grüneisen–Debye formalism, with the following 0 K parameters for Rydberg-Vinet equation: V_0 = 38.0 Å^3/at, K_0 = 2.65 GPa, K'_0 = 7.423. Under the current experimental conditions, non-hydrostaticity affects measured P–V points mostly at moderate pressure (le 20 GPa).

Fischer–Tropsch Synthesis for Light Olefins from Syngas: A Review of Catalyst Development

Light olefins as one the most important building blocks in chemical industry can be produced via Fischer–Tropsch synthesis (FTS) from syngas. FT synthesis conducted at high temperature would lead to light paraffins, carbon dioxide, methane, and C5+ longer chain hydrocarbons. The present work focuses on providing a critical review on the light olefin production using Fischer–Tropsch synthesis. The effects of metals, promoters and supports as the most influential parameters on the catalytic performance of catalysts are discussed meticulously. Fe and Co as the main active metals in FT catalysts are investigated in terms of pore size, crystal size, and crystal phase for obtaining desirable light olefin selectivity. Larger pore size of Fe-based catalysts is suggested to increase olefin selectivity via suppressing 1-olefin readsorption and secondary reactions. Iron carbide as the most probable phase of Fe-based catalysts is proposed for light olefin generation via FTS. Smaller crystal size of Co active metal leads to higher olefin selectivity. Hexagonal close-packed (HCP) structure of Co has higher FTS activity than face-centered cubic (FCC) structure. Transition from Co to Co3C is mainly proposed for formation of light olefins over Co-based catalysts. Moreover, various catalysts’ deactivation routes are reviewed. Additionally, techno-economic assessment of FTS plants in terms of different costs including capital expenditure and minimum fuel selling price are presented based on the most recent literature. Finally, the potential for global environmental impacts associated with FTS plants including atmospheric and toxicological impacts is considered via lifecycle assessment (LCA).