Hexagonal Laves Phase Research Articles

The crystal structures and magnetic properties of YbMg0.75In1.25 and Yb6Mg6.41In5.59

Abstract The quasi-binary system YbMg2-YbIn2 was studied around the equiatomic composition. In contrast to the ordered rare earth (RE) phases REMgIn (ZrNiAl type), ytterbium forms phases with different structures and pronounced Mg/In mixing (M sites). The structures of YbMg0.75In1.25 (CaLiSn type, P3m1, a = 501.95(7), c = 1087.3(2) pm, wR2 = 0.0490, 790 F 2 values, 32 variables) and Yb6Mg6.41In5.59 (Yb6Ir5Ga7 type, P63/mcm, a = 1060.77(14), c = 970.27(16) pm, wR2 = 0.0484, 701 F 2 values, 26 variables) were refined from single-crystal X-ray diffractometer data. YbMg0.75In1.25 is an AlB2 superstructure with a tripling of the subcell. The magnesium and indium atoms form three differently puckered layers of M 6 hexagons. The Yb6Mg6.41In5.59 structure is derived from the hexagonal Laves phase YbMg2 (MgZn2 type, P63/mmc). A klassengleiche symmetry reduction leads to four crystallographically independent M sites for the rows of corner- and face-sharing tetrahedra which allow a composition close to the equiatomic one. The M–M distances in both structures cover the broad range from 289 to 331 pm, comparable to the sums of the covalent radii. Temperature dependent magnetic susceptibility studies of the polycrystalline YbMg0.75In1.25 and Yb12Mg13In11 samples indicate Pauli paramagnetism with room temperature values of 2.8(1) × 10−3 emu mol−1 (YbMg0.75In1.25) and 5.2(1) × 10−3 emu mol−1 (Yb12Mg13In11).

Математическое моделирование взаимодействия водорода с многокомпонентными ИМС на основе гексагональных фаз Лавес

To calculate the properties of previously unstudied intermetallic compounds (IMC), a statistical model developed in our laboratory was applied, which uses already known literature data on the thermodynamic parameters of absorption / desorption reactions of hydrogen for intermetallic compositions with the structure of hexagonal Laves phases. For the alloys, hydrogen absorption and desorption isotherms were plotted. Comparison of calculated and experimental data was carried out to assess the correctness of the forecast model.

Stability of the atomic arrangement in Laves phases Be2Fe1–xOsx (0 ≤ x ≤ 0.75) and Be2Fe1–xRux (x[formula omitted] 0.5)

The binary Be2Fe, which crystallizes in the C14-structure type hexagonal Laves phase, can dissolve a large amount of Os as a third element, without changing the crystal structure (about ¾ of Fe can be replaced by Os). A study of the homogeneity range for the Be2Fe1–xOsx Laves phase, shows an increase of the lattice parameters with an increase of the Os content. When Ru substitutes Fe in Be2Fe, a temperature- and composition-dependent transformation from the C14 to the cubic C15 Laves phase is observed, with a narrow homogeneity range. Formation energy calculations revealed that the C14-structure type is energetically favored when Os substitutes Fe in Be2Fe, while a structural phase transition is observed between Be2Fe (C14) and Be2Fe0.5Ru0.5 (C15), all in good agreement with the experimental results. A strong charge transfer from Be to Os or Ru is observed from the analysis of electron density within the Quantum Theory of Atoms in Molecules (QTAIM) framework. A formation of multiatomic bonds is found from the electron localizability approach. Be2Fe1–xOsx undergoes a transition to a disordered ferromagnetic state at TC= 350 K and behaves like a ferromagnet with a non-collinear magnetic structure (not all domains polarized along field). Be2Fe1–xRux orders ferromagnetically below TC= 340 K and the very narrow hysteresis curves show that Be2Fe1–xRux is a soft ferromagnet.

Structural investigation and hydrogenation properties of TiZrXMnFeNi (X = Cr, Mo, and W) high entropy alloys

In this study, we design and synthesize three high entropy alloys with a composition of TiZrXMnFeNi (X = Cr, Mo, and W), using semi-empirical thermodynamic parameters. These alloys have the same value of VEC= 6.49 and mixing entropy of 14.89 J·K−1·mol−1. The main crystal structure in all samples is C14 hexagonal Laves phase, with Mo and W-rich precipitation in TiZrMoMnFeNi and TiZrWMnFeNi, respectively. Thermodynamic properties of hydrogen absorption/desorption, the effect of temperature and applied hydrogen pressure, cyclic stability up to 50 cycles, and calculation of apparent activation energy have been investigated in detail. By substituting Cr with Mo and, W atoms in the alloy composition, the hydrogen absorption capacity decreases (from 1.4 to 1.2, and 0.8 wt% respectively, at 268 K), and the hydrogen absorption rate increases markedly. Additionally, the rate-controlling step of hydrogen absorption changes from diffusion to nucleation and growth. The results indicate that the substitution of heavier atom in this composition and thus increasing the Ω parameter leads to the formation of preferential nucleation sites and facilitate hydrogen absorption.

The new high-pressure hexagonal Laves phase of the YbZn2 compound

The new high-pressure hexagonal Laves phase of the YbZn2 compound

Influence of Heat Treatment and High-Pressure Torsion on Phase Transformations in TiZrHfMoCr High-Entropy Alloy

The study focused on a 21.99 at.%Ti–22.49 at.%Zr–20.35 at.%Hf–17.45 at.%Mo–17.73 at.%Cr). Analytical techniques such as X-ray diffraction, scanning electron microscopy as well as X-ray absorption spectroscopy were employed to investigate the alloy’s structure, phase transformations, and properties. The alloy in the as-cast state contained three phases, namely the body-centred cubic (A2) phase, hexagonal Laves phase (C14), and cubic Laves phase (C15). The alloy has been annealed for a long time at different temperatures. It led to the disappearance of the hexagonal Laves phase, leaving behind two primary phases, namely the cubic Laves phase (C15) and the body-centered cubic phase (A2). At 1200 °C, the A2 phase almost disappeared, resulting in a practically single-phase sample. After a high-pressure torsion (HPT) treatment, the hexagonal Laves phase disappeared entirely, while the A2 and C15 phases remained. The grain size of the A2 and C15 phases was refined after HPT and grains were elongated, and their configuration resembled a layered structure. The high hardness of the A2 and C15 + C14 phases accounted for this behavior. The lattice parameters in the A2 and C15 phases after HPT treatment approached those observed after prolonged annealing at 1000 °C, indicating that the composition of these phases after short-term high-pressure torsion at ambient temperature is equivalent to the composition of these phases after long tempering at 1000 °C. The rate of diffusion-like mass transfer during severe plastic deformation was estimated to be many orders of magnitude higher than that for conventional bulk diffusion at the HPT treatment temperature and similar to that at elevated temperatures above 1000 °C. X-ray absorption spectroscopy results obtained at K-edges of Ti, Cr, Zr, and Mo as well as at the L3-edge of Hf indicated that the local environment around metal atoms before HPT was similar to that after HPT. However, the static disorder increased after HPT, which could be attributed to an increased specific amount of metal atoms in the disordered grain boundary layers after HPT-driven grain refinement.

A third generation CalPhaD assessment of the Fe–Mn–Ti system part I: The binary subsystems Fe–Mn, Fe–Ti and Mn–Ti

The binary systems Fe–Mn, Fe–Ti and Mn–Ti play remarkable roles in the development of numerous structural and functional materials. As their properties are closely related to the thermodynamic behavior of the phases in the corresponding alloys, the calculation of phase equilibria is crucial for a sustainable and straightforward alloy development. In that course, thermodynamic descriptions within the framework of 3rd generation CalPhaD databases of the binary systems Fe–Mn, Fe–Ti and Mn–Ti were developed. To ensure physically meaningful estimations of the individual thermodynamic properties, advanced sublattice models for several phases, such as A12 (α-Mn), A13 (β-Mn) and C14 (hexagonal Laves phase) were applied. The modelling was supported by heat-capacity measurements for the C14–Mn2Ti phase, to provide a thermodynamically profound basis for a subsequent description of the phase relations. Through the combination of the advanced models and reliable thermodynamic data for each system, thermodynamic descriptions could be derived, which facilitate reliable calculations from 0 K to far beyond the melting point of the individual alloys.

Microstructural verification of the theoretically designed novel eutectic multi-principal element alloy

This letter proposes the theoretical design methodology based on the proportional mixing of binary eutectics for predicting novel eutectic multi-principal element alloys (EMPEAs). It was then experimentally verified by manufacturing a dual-phase eutectic Zr61.9AlTiVCr alloy having a density of 6.04 g/cc. Zirconium was identified as the eutectic forming element in this system, notably having only 3 binary eutectics in a quinary system. The alloy developed by vacuum arc melting revealed the formation of an irregular lamellar eutectic microstructure consisting of cubic and hexagonal Laves phases. The as-cast alloy had a superior hardness of HV527, with 780 MPa compressive strength. This study paves the way for designing new EMPEAs without depending on any simulation software.

Phase stability of TiAl-based BCC high entropy alloys

The demand for materials that withstand harsh conditions in high-performance applications has increased drastically in recent years. Due to the predicted outstanding properties of high entropy alloys (HEAs) at elevated temperatures, this class of materials has attracted enormous scientific attention. This paper assesses the phase stability of TiAl-based HEAs, namely TiAlNbV–Mo and TiAlNbV–Mn systems, based on first-principles calculations using Density Functional Theory. We advocate that mixing energies of the HEAs with respect to their possible products offers a suitable way to make predictions about possible decomposition processes. Phase stability is first evaluated at 0 K, followed by the inclusion of the stabilizing effect of the configurational entropy at different temperatures. Additionally, also the effect of vibrational entropy is estimated within the harmonic Debye model. The predicted phase stabilities are discussed in light of existing experimental results showing microstructural evolution before and after heat treatments. Overall, TiAlNbV–Mo, exhibiting a body-centered cucic lattice, has been identified as a kinetically stabilized HEA, whereas TiAlNbV–Mn decomposes into a body-centered cubic phase and the hexagonal Laves phases.

Microstructure and Mechanical Properties of Spark Plasma Sintered CoCrFeNiNbX High-Entropy Alloys with Si Addition

Three mechanically alloyed (MA) and spark plasma sintered (SPS) CoCrFeNiNbX (X = 5, 20, and 35 at.%) alloys with an addition of 5 at.% of SiC were investigated. The face-centered cubic (FCC) high-entropy solid solution, NbC carbides, and hexagonal Laves phase already developed during MA. In addition, the SPS compacting led to the formation of oxide particles in all alloys, and the Cr7C3 carbides in the Nb5 alloy. The fraction of the FCC solid solution decreased with increasing Nb concentration at the expense of the NbC carbide and the Laves phase. Long-term annealing at 800 °C led to the disappearance of the Cr7C3 carbide in the Nb5 alloy, and new oxides-Ni6Nb6O, Cr2O3, and CrNbO4-were formed. At laboratory temperature, the Nb5 alloy, containing only the FCC matrix and carbide particles, was relatively strong and very ductile. At a higher Nb content (Nb20 and Nb35), the alloys became brittle. After annealing for 100 h at 800 °C, the Nb5 alloy conserved its plasticity and the Nb20 and Nb35 alloys maintained or even increased their brittleness. When tested at 800 °C, the Nb5 and Nb20 alloys deformed almost identically (CYS ~450 MPa, UTS ~500 MPa, plasticity ~18%), whereas the Nb35 alloy was much stronger (CYS of 1695 MPa, UCS of 1817 MPa) and preserved comparable plasticity.

Novel as-cast NiAlCoFeNb dual-phase high-entropy alloys with high hardness

• A novel class of NiAlCoFeNb HEAs were synthesized by vacuum arc melting. • These HEAs consist of NiAl-rich B2 phase and FeNb-rich hexagonal Laves phase. • The hardness increases with the incremental volume fraction of Laves phase. • The equiatomic Ni 20 Al 20 Co 20 Fe 20 Nb 20 HEA has ultrahigh hardness of ∼ 818 HV. The microstructure, phase constitution and microhardness of Ni 20 Al 20 Co 20 Fe 20 Nb 20 , Ni 25 Al 25 Co 20 Fe 20 Nb 10 and Ni 30 Al 30 Co 20 Fe 15 Nb 5 high-entropy alloys (HEAs) synthesized by vacuum arc melting have been investigated. These as-cast HEAs have a dual-phase structure including intermetallic NiAl-rich B2 phase and FeNb-rich hexagonal Laves phase, and the Nb increase leads to more fraction of Laves phase. Ultrahigh hardness of ∼ 818 HV was achieved in the equiatomic Ni 20 Al 20 Co 20 Fe 20 Nb 20 dual-phase HEA, which is consisted of 61.8 vol% Laves phase and 38.2 vol% B2 phase.

Mg2MnGa3 – An orthorhombically distorted superstructure variant of the hexagonal Laves phase MgZn2

Abstract The Laves phase Mg2MnGa3 was synthesized from the elements by arc-melting and subsequent annealing in a silica ampoule at T = 670 K. The structure of Mg2MnGa3 was refined from single-crystal X-ray diffractometer data: URe2 type, Cmcm, a = 543.24(1), b = 869.59(3), c = 858.58(2) pm, wR2 = 0.0556, 273 F 2 values and 24 variables. The manganese and gallium atoms form a three-dimensional network of corner- and face-sharing MnGa3 tetrahedra that derive as a ternary ordering variant from the hexagonal Laves phase MgZn2. The structures of the distortion and coloring variants, i.e., MgZn2, URe2, Mg2Cu3Si and Mg2MnGa3 are discussed on the basis of a Bärnighausen tree. The electronic structure calculation data indicate that in addition to the metallic type of bonding an additional covalent interaction appears between the Ga–Ga and Mn–Ga atoms.

Notable hydrogen storage in Ti–Zr–V–Cr–Ni high entropy alloy

In the present investigation, we discussed the synthesis, structural and hydrogen storage behavior of high-entropy Ti–Zr–V–Cr–Ni equiatomic intermetallic alloy. This alloy was synthesized by arc melting in an argon atmosphere where base pressure was in the order of 10−5 atm before purging with argon gas. The X-ray diffraction study revealed that the alloy is C14 type hexagonal Laves phase. The pressure composition isotherms (PCI) of this alloy were investigated with pressure ranges at 0–40 atmosphere. The total hydrogen storage capacities were found to be 1.52 wt%. The reversible hydrogen storage capacity was quite stable and only slight decreases in the storage capacity was observed after 10 cycles during hydrogen soaking. The demonstrations of hydrogen storage capacity of the Ti–Zr–V–Cr–Ni equiatomic alloy were thus established, indicating the future potential of developing this class of high entropy intermetallic based materials for hydrogen storage.

Structural and In Situ X-ray Diffraction Study of Hydrogenation of CaxMg1−xNi2 (0 ≤ x ≤ 1)

In the quasi-binary system CaNi2-MgNi2 solid-solutions CaxMg1−xNi2 (0 ≤ x ≤ 1) were prepared from the elements. They crystallize in the hexagonal Laves phase type (MgNi2, C36) for x ≤ 0.33 (P63/mmc, a = 482.51(7) pm, c = 1582.1(3) pm for x = 0, a = 482.59(3), c = 1583.1(1) for x = 0.33) and in the cubic Laves phase type (MgCu2, C15) for 0.33 < x (Fd3¯m, a = 697.12(3) pm for x = 0.5, a = 705.11(2) pm for x = 0.67, a = 724.80(2) pm for x = 1). After hydrogenation in an autoclave the X-ray diffraction patterns changed completely. Reflections assigned to CaNiH3, and Ni and Rietveld refinement confirmed this. The hydrogenation properties of CaxMg1−xNi2 (0 ≤ x ≤ 1) compounds were also studied in situ by X-ray powder diffraction. In situ X-ray powder diffraction of CaxMg1−xNi2 (0 ≤ x ≤ 1) compounds under 0.3 MPa hydrogen gas flow (15 sccm), data collected on a Rigaku SmartLab diffractometer in an Anton Paar XRK 900 Reactor Chamber using Cu-Kα1 radiation. Scanning electron microscopy and EDX spectroscopy confirmed the entitled materials and elemental composition, respectively. From the Transmission electron microscopy and Selected area electron diffraction concluded that the CaxMg1−xNi2 (0 ≤ x ≤ 1) compounds were crystalline.

Phase constitution of the noble metal thin-film complex solid solution system Ag-Ir-Pd-Pt-Ru in dependence of elemental compositions and annealing temperatures

Multiple-principal element alloys hold great promise for multifunctional material discovery (e.g., for novel electrocatalysts based on complex solid solutions) in a virtually unlimited compositional space. Here, the phase constitution of the noble metal system Ag-Ir-Pd-Pt-Ru was investigated over a large compositional range in the quinary composition space and for different annealing temperatures from 600 to 900 °C using thin-film materials libraries. Composition-dependent X-ray diffraction mapping of the as-deposited thin-film materials library indicates different phases being present across the composition space (face-centered cubic (fcc), hexagonal close packed (hcp) and mixed fcc + hcp), which are strongly dependent on the Ru content. In general, low Ru contents promote the fcc phase, whereas high Ru contents favor the formation of an hcp solid-solution phase. Furthermore, a temperature-induced phase transformation study was carried out for a selected measurement area of fcc-Ag5Ir8Pd56Pt8Ru23. With increasing temperature, the initial fcc phase transforms to an intermediate C14-type Laves phase at 360 °C, and then to hcp when the temperature reaches 510 °C. The formation and disappearance of the hexagonal Laves phase, which covers a wide temperature range, plays a crucial role of bridging the fcc to hcp phase transition. The obtained composition, phase and temperature data are transformed into phase maps which could be used to guide theoretical studies and lay a basis for tuning the functional properties of these materials.

Stable high-entropy TiZrHfNbVCrMoMnFeCoNiAl Laves phase

Recent researches in the field of high-entropy materials reveal an intriguing result: the multi-component systems combining the chemical elements with diverse valencies and radii can form single-phase solid solutions with the structure of intermetallic Laves phases. Here we report the fabrication of hexagonal Laves phase (C14, prototype MgZn2) in duodenary TiZrHfNbVCrMoMnFeCoNiAl alloy. The phase is stable after the thermal treatment at 973 K for 50 h. To characterize the material, we examine its electrical conductivity and magnetic properties. The measurements reveal that the Laves phase is a Curie-Weiss paramagnet, which demonstrates metallic conduction and a pronounced Kondo-like anomaly at 80 K. Analysis of experimental data as well as ab initio calculations suggests that chemical complexity and compositional disorder cause strong s-d band scattering and thus the rather high density of d-states in the conduction band.

Structure of (ZrTi)0.5(VCrFe(Ni0.9Cu0.1))0.5- and (ZrTi)0.5(VMoFeNi)0.5-Based Deuterides

We have synthesized (ZrTi)0.5(VCrFe(Ni0.9Cu0.1))0.5 and (ZrTi)0.5(VMoFeNi)0.5 pseudobinary high-entropy compounds and related deuterides. According to neutron and X-ray diffraction results, the deuterides have the same hexagonal Laves phase structure as the parent alloys. We have determined the position of the deuterium atoms and their atomic position coordinates. The results indicate that the deuterium atoms reside predominantly in positions 24l and 12k2, typical of hydrogen in hexagonal Laves phases.

First-principles analysis of precipitation in Mg-Zn alloys

Precipitation in Mg-Zn alloys was analyzed by means of first principles calculations. Formation energies of symmetrically distinct hcp Mg1-xZnx (0 < x < 1) configurations were determined and potential candidates for Guinier-Preston zones coherent with the matrix were identified from the convex hull. The most likely structures were ranked depending on the interface energy along the basal plane. In addition, the formation energy and vibrational entropic contributions of several phases reported experimentally (Mg4Zn7, MgZn2 cubic, MgZn2 hexagonal, Mg21Zn25 and Mg2Zn11) were calculated. The formation energies of Mg4Zn7, MgZn2 cubic, and MgZn2 hexagonal Laves phases were very close because they were formed by different arrangements of rhombohedral and hexagonal lattice units. It was concluded that \beta_1^' precipitates were formed by a mixture of all of them. Nevertheless, the differences in the geometrical arrangements led to variations in the entropic energy contributions which determined the high temperature stability. It was found that the MgZn2 hexagonal Laves phase is the most stable phase at high temperature and, thus, \beta_1^' precipitates tend to transform into the \beta_2^' (MgZn2 hexagonal) precipitates with higher aging temperature or longer aging times. Finally, the equilibrium \beta phase (Mg21Zn25) was found to be a long-range order that precipitates the last one on account of the kinetic processes necessary to trigger the transformation from a short-range order phase \beta_2^' to \beta .

Effect of niobium alloying on the microstructure, phase stability and mechanical properties of CoCrFeNi2.1Nbx high entropy alloys: Experimentation and thermodynamic modeling

A systematic study was carried out to understand the effect of Nb addition on the microstructure, phase stability, and mechanical properties in a series of cast and aged CoCrFeNi2.1Nbx (x = 0, 0.2, 0.4, 0.74) high entropy alloys (HEAs). The base alloy without Nb addition showed a fully face-centered cubic (FCC) phase, whereas Nb addition was found to promote the formation of the hexagonal Laves phase. The cast HEAs were further aged at temperatures varying from 800 °C to 1200 °C. Aging of the different Nb alloyed HEAs at 800 °C and 1000 °C resulted in the precipitation of D019 structured ε-phase having fine acicular morphology. In contrast, the HEAs aged at 1200 °C did not reveal the presence of ε precipitates. These experimental observations were critically assessed with the thermodynamic predictions. The fine precipitate network in the CoCrFeNi2.1Nb0.2 and CoCrFeNi2.1Nb0.4 HEAs improved the hardness and tensile strength considerably. However, the CoCrFeNi2.1Nb0.74 HEA showed considerable embrittlement due to the presence of a high fraction of the Laves phase. Present results indicated considerable potential of achieving superior mechanical properties in these cast HEAs even without involving thermo-mechanical processing.

Extracting [Pd@Sn9]4– and [Rh@Pb9]4– Clusters from their Binary Alloys Using “Metal Scissors”

The search for novel ternary intermetallic compounds with specific structures is still a challenge. We found that the two‐step synthesis giving a typical alloy in the first step followed by the reaction of this alloy with alkali metals is a promising route. The intermetallic compounds K12Pd0.47Sn17 and K4RhPb9 were synthesized by high‐temperature reactions of preformed Pd‐Sn and Rh‐Pb alloys with K acting as “metal scissors” and were characterized by means of single crystal and powder X‐ray diffractometry. The salt‐like ternary phases K12Pd0.47Sn17 and K4RhPb9 contain novel endohedrally filled intermetalloid clusters [Pd@Sn9]4– and [Rh@Pb9]4–, respectively. The crystal packing of the products corresponds to filled variants of the binary Zintl phases K12Sn17 and K4Pb9, respectively. The crystal structure of K12Pd0.47Sn17 can be regarded as a hierarchical replacement variant of the hexagonal Laves phase MgZn2, with [Pd@Sn9]4–/[Sn9]4– and [Sn4]4– on Mg and Zn positions, respectively, whereas the packing of [Rh@Pb9]4– clusters in K4RhPb9 is hcp. K12Pd0.47Sn17 was characterized by Raman spectroscopy. For the first time Raman modes typical for endohedrally filled [Pd@Sn9]4– clusters are observed and in accordance with quantum‐chemical calculations. In addition Raman spectroscopy shows also the presence of filled Pd@Sn9 clusters in a phase of nominal composition “Na12Pd2Sn17”. The results are discussed with respect to the volume increase due the incorporation of transition metal atoms.