Laves Phase Structure Research Articles

Magnetostructural phase transitions and magnetocaloric effect in Tb-Dy-Ho-Co-Al alloys with a Laves phase structure

The influence of simultaneous substitution within the rare earth (R) and Co sublattices on the structural, magnetic, and magnetocaloric properties of the Laves phase RCo2-type compounds is studied. Main attention is devoted to the studies of the magnetostructural phase transitions and the transition types with respect to the alloy composition. Multicomponent alloys Tbx(Dy0.5Ho0.5)1−xCo2 and Tbx(Dy0.5Ho0.5)1−xCo1.75Al0.25 were prepared with the use of high purity metals. Majority of the Tbx(Dy0.5Ho0.5)1−xCo2 alloys exhibit magnetic transitions of the first-order type and a large magnetocaloric effect. The substitution of Al for Co in Tbx(Dy0.5Ho0.5)1−xCo2 increases the Curie temperature (TC) but changes the transition type from first-to the second-order. The discussion of the physical mechanisms behind the observed phenomena is given on the basis of the first principles electronic-structure calculations taking into account both the atomic disorder and the magnetic disorder effects at finite temperatures. The advantage of Al-containing materials is that sufficiently high magnetocaloric effect values are preserved at T > TC.

Formation and fine-structures of nano-precipitates in ZIRLO

Transmission electron microscopy and energy dispersive X-ray spectroscopy have been used for crystallographic and compositional characterization of nano-precipitates distributed within the α-Zr matrix of the ZIRLO alloy. Intermetallic nano-particles with the C14 Laves-phase structure were found to be the dominant precipitate phase typically larger than 60 nm in sizes, which is assigned to the Zr(Fe,Nb)2 formula with Fe and Nb being mixed on the second sublattice sites in the bracket. Stacking faults were also observed in the ternary Laves phase, together with dislocations presenting on prism planes of the matrix solid solution. Besides, some fine spherical β-Nb particles about 30 nm in size were also found to precipitate within the α-Zr grains, with the α/β interface being crystallographically coherent.

Structure, magnetic and magnetothermal properties of the non-stoichiometric ErCo2Mnx alloys

The structure, magnetic and magnetothermal properties and heat capacity of the ErCo2Mnx (0 ≤ x ≤ 1.4) non-stoichiometric alloys have been studied. It is shown that the alloys with x ≤ 0.8 are almost single-phase with the MgCu2-type Laves-phase structure, while the alloys for x ≥ 1.0 contain considerable amount of the cubic Th6Mn23-type phase. Alloying with manganese leads to a substantial increase in the exchange interactions. For the binary ErCo2 compound, the magnetization shows a first-order magnetic phase transition with the Curie temperature TC = 35 K. For all the Mn-containing alloys studied, the transition is found to be the second-order type; the Curie temperature in ErCo2Mnx sharply growths with increasing x and reaches its maximum value 212 K for the ErCo2Mn0.6 alloy. Using the magnetization data and the results of the heat capacity measurements, we estimated the magnetocaloric effect for these compounds. The obtained results are discussed on the assumption of a strong variation of the 3d-metal magnetic moments upon alloying with Mn.

Influence of magnetic flux density and cooling rate on orientation behavior of Tb0.27Dy0.73Fe1.95 alloy during solidification process

The rare-earth giant magnetostrictive material Tb0.27Dy0.73Fe1.95 is one of the most important functional magnetic materials. Their superior properties include high saturation magnetostrictive coefficient at room temperature, high electromechanical coupling coefficients, high output power, fast response, high energy density, and non-contact drive. Thus, they can be used to build sensors, precision machinery, magnetomechanical transducers, and adaptive vibration-control systems. In this material, the magnetic phase (Tb, Dy)Fe2 has a typical MgCu2-type cubic Laves phase structure and exhibits different magnetostrictive properties along different crystal orientations. The 111 direction of this phase is the easy magnetization axis, along which the linear magnetostriction is higher than other directions. Thus, researchers have focused on preparing (Tb, Dy)Fe2 with a crystallographic orientation along or close to the 111 direction. Generally, the directional solidification method is used to prepare the Tb0.27Dy0.73Fe1.95 alloy. However, a crystal orientated along the 110 or 112 direction is always obtained and both of these directions require a high external magnetic field for improved magnetostrictive performance. The 111 preferred growth orientation can be acquired using seed crystal technology. However, the relatively low growth velocity can cause the appearance of the linear (Tb, Dy)Fe3 phase which induces a high brittleness of the material. Therefore, new methods to prepare Tb0.27Dy0.73Fe1.95 products with high 111 orientation at higher growth velocity are required. In this paper, we solidify the Tb0.27Dy0.73Fe1.95 alloys under various high magnetic field and cooling rate conditions. We study the effects of the magnetic flux density and cooling rate on the crystal orientation of the (Tb, Dy)Fe2 phase and the magnetization behavior of the alloys. It is found that after field-treated solidification, a high 111 orientation of (Tb, Dy)Fe2 along the magnetic field direction can be produced. As a consequence, the magnetostriction without applying stress remarkably increases. By increasing the magnetic flux density applied during the solidification of the Tb0.27Dy0.73Fe1.95 alloys, the 111 orientation of (Tb, Dy)Fe2 could be obtained at higher cooling rates. Ranging from 4 T to 10 T, with increasing cooling rate the magnetic flux density, at which the 111 or 110 orientation of (Tb, Dy)Fe2 occurs, increases or decreases, respectively. The saturated magnetization of the alloys increases with increasing cooling rate. The application of the magnetic fields does not affect the saturated magnetization.

Hydrogen occlusion behaviors of ZrVFe alloys under mild hydrogen pressures and lowering of their hydrogen desorption temperatures

Four kinds of ZrVFe alloys with the C14 Laves phase structure had high hydrogen storage capacity under mild pressures, and those alloys would be promising for widespread application such as recovery of hydrogen derived from water splitting. Addition of titanium to ZrVFe lowered hydrogen desorption temperatures through increase of equilibrium pressures. Substitution of ferrovanadium for vanadium did not deteriorate hydrogen occlusion performance of ZrVFe, and provided with lower cost alloys. Another way to reduce the cost would be elimination of purifying step of vanadium prepared by the thermite process, but remaining aluminum proved to bring about lower hydrogen capacity, as evidenced by comparative experiments including a ZrVFe alloy with aluminum intendedly added. Every alloy had fairly good cycling durability except the one contaminated with aluminum.

Influence of Valence Electron Concentration on Laves Phases: Structures and Phase Stability of Pseudo‐Binary MgZn2–xPdx

AbstractA series of pseudo‐binary compounds MgZn2–xPdx (0.15 ≤ x ≤ 1.0) were synthesized and structurally characterized to understand the role of valence electron concentration (vec) on the prototype Laves phase MgZn2 with Pd‐substitution. Three distinctive phase regions were observed with respect to Pd content, all exhibiting fundamental Laves phase structures: 0.1 ≤ x ≤ 0.3 (MgNi2‐type, hP24; MgZn1.80Pd0.20(2)), 0.4 ≤ x ≤ 0.6 (MgCu2‐type, cF24; MgZn1.59Pd0.41(2)), and 0.62 ≤ x ≤ 0.8 (MgZn2‐type, hP12: MgZn1.37Pd0.63(2)). Refinements from single‐crystal X‐ray diffraction indicated nearly statistical distributions of Pd and Zn atoms among the majority atom sites in these structures. Interestingly, the MgZn2‐type structure re‐emerges in MgZn2–xPdx at x ≈ 0.7 with the refined composition MgZn1.37(2)Pd0.63 and a c/a ratio of 1.59 compared to 1.64 for binary MgZn2. Electronic structure calculations on a model “MgZn1.25Pd0.75” yielded a density of states (DOS) curve showing enhancement of a pseudogap at the Fermi level as a result of electronic stabilization due to the Pd addition. Moreover, integrated crystal orbital Hamilton population (ICOHP) values show significant increases of orbital interactions for (Zn,Pd)–(Zn,Pd) atom pairs within the majority atom substructure, i.e., within the Kagomé nets as well as between a Kagomé net and an apical site, from binary MgZn2 to the ternary “MgZn1.25Pd0.75”. Multi‐centered bonding is evident from electron localization function (ELF) plots for “MgZn1.25Pd0.75”, an outcome which is in accordance with analysis of other Laves phases.

Microstructure and Wear Behavior of CoCrFeMnNbNi High-Entropy Alloy Coating by TIG Cladding

Alloy cladding coatings are widely prepared on the surface of tools and machines. High-entropy alloys are potential replacements of nickel-, iron-, and cobalt-base alloys in machining due to their excellent strength and toughness. In this work, CoCrFeMnNbNi HEA coating was produced on AISI 304 steel by tungsten inert gas cladding. The microstructure and wear behavior of the cladding coating were studied by X-ray diffraction, scanning electron microscopy, energy dispersive spectrometer, microhardness tester, pin-on-ring wear tester, and 3D confocal laser scanning microscope. The microstructure showed up as a nanoscale lamellar structure matrix which is a face-centered-cubic solid solution and niobium-rich Laves phase. The microhardness of the cladding coating is greater than the structure. The cladding coating has excellent wear resistance under the condition of dry sliding wear, and the microploughing in the worn cladding coating is shallower and finer than the worn structure, which is related to composition changes caused by forming the nanoscale lamellar structure of Laves phase.

Experimental Study of the Magnetocaloric Effect in the Pseudo Binary Laves-Phase Compounds

Polycrystalline samples of DyCo 2, HoCo 2, ErCo 2 and their solid solutions Dy1-xHoxCo2, Dy1-xErxCo2, Ho1-xErxCo2 have been studied using X-ray diffraction analysis, heat capacity and magnetic measurements. The X-ray diffraction performed at room temperature allowed to reveal that all solid solutions solidify with the formation of the C15 cubic Laves-phase structure corresponding to the Fd3m space group. All binary compounds and their solid solutions are typical ferrimagnets and are magnetically ordered at temperature below 140 K. In all doped solid solutions an decrease of the magnetic ordering temperature is observed. At high temperatures, all samples are Curie-Weiss paramagnets. The magnetization behaviour and the magnetic transition are analyzed in terms of Landau theory. The magnetocaloric effect has been estimated using the heat capacity data.Comparison of the influence of doping effect in pseudo-binary compounds on their magnetic and magnetocaloric properties for all solid solutions is presented.

Progress in Visualizing Atomic Size Effects with DFT-Chemical Pressure Analysis: From Isolated Atoms to Trends in AB5 Intermetallics.

The notion of atomic size poses an important challenge to chemical theory: empirical evidence has long established that atoms have spatial requirements, which are summarized in tables of covalent, ionic, metallic, and van der Waals radii. Considerations based on these radii play a central role in the design and interpretation of experiments, but few methods are available to directly support arguments based on atomic size using electronic structure methods. Recently, we described an approach to elucidating atomic size effects using theoretical calculations: the DFT-Chemical Pressure analysis, which visualizes the local pressures arising in crystal structures from the interactions of atomic size and electronic effects. Using this approach, a variety of structural phenomena in intermetallic phases have already been understood in terms that provide guidance to new synthetic experiments. However, the applicability of the DFT-CP method to the broad range of the structures encountered in the solid state is limited by two issues: (1) the difficulty of interpreting the intense pressure features that appear in atomic core regions and (2) the need to divide space among pairs of interacting atoms in a meaningful way. In this article, we describe general solutions to these issues. In addressing the first issue, we explore the CP analysis of a test case in which no core pressures would be expected to arise: isolated atoms in large boxes. Our calculations reveal that intense core pressures do indeed arise in these virtually pressure-less model systems and allow us to trace the issue to the shifts in the voxel positions relative to atomic centers upon expanding and contracting the unit cell. A compensatory grid unwarping procedure is introduced to remedy this artifact. The second issue revolves around the difficulty of interpreting the pressure map in terms of interatomic interactions in a way that respects the size differences of the atoms and avoids artificial geometrical constraints. In approaching this challenge, we have developed a scheme for allocating the grid pressures to contacts inspired by the Hirshfeld charge analysis. Here, each voxel is allocated to the contact between the two atoms whose free atom electron densities show the largest values at that position. In this way, the differing sizes of atoms are naturally included in the division of space without resorting to empirical radii. The use of the improved DFT-CP method is illustrated through analyses of the applicability of radius ratio arguments to Laves phase structures and the structural preferences of AB5 intermetallics between the CaCu5 and AuBe5 structure types.

Magnetic and magnetocaloric properties of rare earth intermetallic compounds HoCo2−xNix (x=0.75 and 1.25)

Magnetic and magnetocaloric properties of polycrystalline rare earth intermetallic compounds HoCo2−xNix (x=0.75 and 1.25) have been investigated. Both HoCo1.25Ni0.75 and HoCo0.75Ni1.25 compounds crystallize in MgCu2 type, cubic Laves phase structure (space group Fd3¯m, no. 227) and order ferromagnetically at ~40K and ~36K (TC) respectively. Field dependent magnetization at 5K shows soft ferromagnetic behavior. The saturation magnetization value is about 8.5μB/f.u. for both compounds. Magnetocaloric effect in HoCo2-xNix (x=0.75 and 1.25) compounds has been evaluated in terms of isothermal magnetic entropy change (ΔSm) that peaks around TC with maximum values of about −16.7J/kgK and −15.2J/kgK, respectively, for a field change of 70kOe. Substitution of Ni at the Co-site of HoCo2 broadens the magnetic transition and enhances relative cooling power which makes these materials suitable for low temperature magnetic refrigeration applications.

Stability of self-interstitial clusters with C15 Laves phase structure in iron

The energetics and stability of self-interstitial clusters with C15 Laves phase structure in iron are investigated by Density Functional Theory (DFT) calculations. First, the properties of bulk C15 are examined. The C15 structure is shown to be mechanically and dynamically stable. Second, the influence of the calculation scheme on the energy difference between C15, ring and loop configurations of di-, tri-, tetra- and octa-interstitial clusters is studied. These calculations confirm that, according to DFT, the C15 structure has by far the lowest energy of all known configurations of tetra- and octa-interstitial clusters in bcc-Fe.

Magnetic, magnetocaloric and magnetoresistive properties of cubic Laves phase HoAl2 single crystal

We report the magnetization (M) and magnetoresistance (MR) results of HoAl2 single crystals oriented along the 〈100〉 and 〈110〉 directions. Although HoAl2 has cubic Laves phase structure, a large anisotropy is observed in M and MR below the Curie temperature (TC). A satisfactory correlation between magnetic entropy change (ΔSM) and MR could be established along 〈110〉 and also 〈100〉, except for the temperature (T) region around which spin reorientation takes place. Large inverse magnetocaloric effect is observed at low T, which is associated with the spin reorientation process in the 〈100〉 direction. A theoretical model based on the Landau theory of phase transition can describe the T-variation of −ΔSM for T > TC.

Stability of self-interstitial clusters with C15 Laves phase structure in iron

Stability of self-interstitial clusters with C15 Laves phase structure in iron

Comprehensive Macroscopic Investigation on Hexagonal C14 Laves-Type Ru-Based Superconductors ARu2 (A = Lu, Y, Sc) with Effective Electron Correlation

We discovered that LuRu2 with a hexagonal C14 Laves phase structure exhibited a superconducting transition at 2.23 K. The magnetization, electronic resistivity, and specific heat measurements for LuRu2 and the C14 Laves phases YRu2 and ScRu2, superconductors with the same number of valence electrons, reveal that the compounds are type-II weak-coupling BCS superconductors with nodeless but probably slightly anisotropic superconducting gaps. A noteworthy finding of these experiments is that the compounds are correlated electron systems, as suggested by their Wilson ratios of approximately 2 and the large enhancement in their electronic specific heat coefficients compared with those obtained by GGA calculation. Many Ru-containing compounds show correlated electron phenomena, but they are the first examples of a Ru-based correlated electron system with neither nonmetallic nor metalloid elements as constituent elements, probably owing to the covalent bond character of Ru peculiar to Laves phases. The relations...

Crystal structure and magnetic properties of RE(Mn,In)2 (RE = Ho, Er, Tm)

Crystal structure and basic magnetic properties of the compounds RE(Mn,In)2 (RE=Ho, Tm, Er) were investigated. X-ray powder diffraction revealed the MgZn2-type of structure, space group P63/mmc, a=5.691(1), c=9.265(2) Å for HoMn1.25In0.75, and a=5.607(1), c=9.176(2) Å, for TmMn1.26In0.74; MgCu2-type, space group Fd3¯ m; a=7.556(1) Å for ErMn1.85In0.15. The positions of the Mg atoms in the Laves phase structures are occupied by rare-earth atoms, and the positions of the small atoms (Cu or Zn) are occupied by a statistical mixture of Mn and In atoms. The unit cell expands by the In substitution, and crossing the critical limit for the Mn–Mn spacing can allow the formation of Mn moments. This can contribute to the magnetic randomness and presumably glassy behaviour.

β-Mn-Type Co8+xZn12–x as a Defect Cubic Laves Phase: Site Preferences, Magnetism, and Electronic Structure

The results of crystallographic analysis, magnetic characterization, and theoretical assessment of β-Mn-type Co-Zn intermetallics prepared using high-temperature methods are presented. These β-Mn Co-Zn phases crystallize in the space group P4(1)32 [Pearson symbol cP20; a = 6.3555(7)-6.3220(7)], and their stoichiometry may be expressed as Co(8+x)Zn(12-x) [1.7(2) < x < 2.2(2)]. According to a combination of single-crystal X-ray diffraction, neutron powder diffraction, and scanning electron microscopy, atomic site occupancies establish clear preferences for Co atoms in the 8c sites and Zn atoms in the 12d sites, with all additional Co atoms replacing some Zn atoms, a result that can be rationalized by electronic structure calculations. Magnetic measurements and neutron powder diffraction of an equimolar Co:Zn sample confirm ferromagnetism in this phase with a Curie temperature of ∼420 K. Neutron powder diffraction and electronic structure calculations using the local spin density approximation indicate that the spontaneous magnetization of this phase arises exclusively from local moments at the Co atoms. Inspection of the atomic arrangements of Co(8+x)Zn(12-x) reveals that the β-Mn aristotype may be derived from an ordered defect, cubic Laves phase (MgCu2-type) structure. Structural optimization procedures using the Vienna ab initio simulation package (VASP) and starting from the undistorted, defect Laves phase structure achieved energy minimization at the observed β-Mn structure type, a result that offers greater insight into the β-Mn structure type and establishes a closer relationship with the corresponding α-Mn structure (cI58).

Effect of partial Gd substitution on the magnetic and magnetocaloric properties in Dy-Ho-Gd-Co multicomponent compounds

The magnetic and magnetocaloric properties of polycrystalline (Dy0.9Ho0.1)1−xGdxCo2 (0.0 ≤ x ≤ 0.15) solid solutions were investigated experimentally using magnetic measurements; the Landau theory was also applied to clarify peculiarities of the magnetic phase transition in the compounds. X-ray diffraction patterns taken at room temperature exhibit the existence of the C15 cubic Laves phase structure for all samples under study. The lattice parameter increased from 0.7186 for x = 0.0 to 0.7194 nm for x = 0.15 and the Curie temperature TC increases from 130 to 183 K as the Gd content increases to x = 0.15. The isothermal entropy change ΔSmag was calculated according to magnetic measurements using the thermodynamic Maxwell's relation. A decrease in entropy change with the increase of Gd content has been observed. Under an external field change of from 0 to 3 T, the maximum entropy change decreased from 8.1 for x = 0.0 to 4.3 J kg−1 K−1 for x = 0.15. The effect of increasing the Gd amount in the (Dy0.9Ho0.1)1−xGdxCo2 solid solutions on their magnetic and magnetocaloric properties is discussed.

Effect on Magnetic Properties of Al Atoms in YFe2 and YCo2

From the theoretical point of view, magnetic properties of pseudo-binary compounds Y(Fe, Al)2 and Y(Co, Al)2 with the cubic Laves phase structure are reviewed. Assuming an ordered atomic structure, the electronic structures in these compounds are calculated in the self-consistent augmented-plane-wave (APW) method, and the effect of the Al atoms on the magnetic properties is discussed.

High-Pressure Synthesis and Superconductivity of the Laves Phase Compound Ca(Al,Si)2 Composed of Truncated Tetrahedral Cages Ca@(Al,Si)12

The Zintl compound CaAl2Si2 peritectically decomposes to a new ternary cubic Laves phase Ca(Al,Si)2 and an Al-Si eutectic at temperatures above 750 °C under a pressure of 13 GPa. The ternary Laves phase compound can also be prepared as solid solutions Ca(Al(1-x)Si(x))2 (0.35 ≤ x ≤ 0.75) directly from the ternary mixtures under high-pressure and high-temperature conditions. The cubic Laves phase structure can be regarded as a type of clathrate compound composed of face-sharing truncated tetrahedral cages with Ca atoms at the center, Ca@(Al,Si)12. The compound with a stoichiometric composition CaAlSi exhibits superconductivity with a transition temperature of 2.6 K. This is the first superconducting Laves phase compound composed solely of commonly found elements.

Structure, Magnetic Properties and Magnetocaloric Effect of Dy-Doped CeFe&lt;sub&gt;2&lt;/sub&gt; Alloys

Ce1-xDyxFe1.9(x = 0, 0.02, 0.05, 0.1) alloys were prepared successfully by means of arc-melting and the following homogenization. XRD patterns show all alloys are indexed as MgCu2-type C15 cubic Laves phase structure. With the Dy substitution of Ce, the Curie temperatures from ferro-to para-magnetism become gradually enlarged. The values of for a magnetic-field change from 0-5 T at 234, 246, 268, 303 K are 31.7, 26.4, 21.8, and 17.2 J kg-1K-1for x = 0, 0.02, 0.05, and 1, respectively.