Laves Phase C15 Research Articles

Formation mechanism and thermal stability of C15-Laves phase in a Hf-containing Co-based superalloy

Topologically close-packed (TCP) phases such as μ and Laves phases formed during solidification has an important influence on the mechanical properties of superalloys. Understanding the formation and elimination mechanism of TCP phases is the key to promote the development of superalloys. In this study, C15-Laves phase was first found in a Hf-containing Co-based superalloy, meanwhile, the formation mechanism and thermal stability of the C15-Laves phase were clarified. The results showed that the formation of C15-Laves phase was associated with the segregation behavior of alloying elements during solidification: Co, W and Cr segregated to the dendrite core, and Hf, Ta, Ti, Mo, Ni, Al, Si segregated to the interdendritic region. In the final stage of solidification, the C15-Laves phase enriched in Hf, Ta and Ti formed, and it had similar lattice parameters to the C15-Co2Hf phase. The incipient melting point of C15-Laves phase was between 1170 ℃ and 1180 ℃, and it gradually decomposed to form the Hf-rich MC carbide during solution treatment. The large formation energy differences between the MC carbide and C15-Laves phase promoted a rapid elimination of the C15-Laves phase, which reduced solution treatment time of the alloy. The results can provide a theoretical basis and experimental data support for the composition design and heat treatment process optimization of multicomponent novel Co-based superalloys.

The effect of titanium and silicon addition on phase equilibrium and mechanical properties of CoCrFeMnNi-based high entropy alloy

The effects of Ti and Si addition on the phase equilibrium and mechanical properties of the equiatomic CoCrFeMnNi high entropy alloy (Cantor alloy) were investigated. The phase equilibrium at 1000 °C was determined from the result of X-ray diffraction and electron probe micro-analysis. Ti addition stabilizes the sigma phase, A12 phase and C14-Laves phase, while Si addition stabilizes the A13 phase. The phase relationships were represented by projection onto (Co, Fe, Mn, Ni)–Cr–X(Ti or Si) isothermal ternary cross-section at 1000 °C in Co–Cr–Fe–Mn–Ni–X senary system. Tensile tests were conducted on Cantor-based fcc single solid solution alloys with Ti or Si dissolution at room temperature. The 0.2% yield strength and ultimate tensile strength increased with either element addition. The Ti-added alloy showed higher strength than the Si-added alloy. The difference in ductility in the alloys is related to their strain hardening behavior in the higher strain range.Graphic abstract

Microstructure and mechanical properties of Ti3V2NbAlxNiy low-density refractory multielement alloys

This study aimed to develop a novel series of low-density refractory high-entropy alloys (RHEAs) by precipitation strengthening. The design ideas of high-entropy alloys and titanium alloys were combined to develop Ti3V2NbAl0.5, Ti3V2NbNi0.5, and Ti3V2NbAl0.5Ni0.5 refractory multielement alloys. These alloys displayed low densities of 5.39, 5.83, and 5.55 g/cm3, respectively. The effects of Al and Ni addition on the microstructure and mechanical properties were investigated. Thereinto, the Ti3V2NbAl0.5 with a body-centered cubic structure had a yield strength of 760 MPa, and the compressive strain exceeded 50%. The Ti3V2NbNi0.5 alloy strengthened by large C15 Laves-phase particles in the interdendritic regions exhibited a high yield strength of 1130 MPa but with a limited compressive strain of 20%. By contrast, the Ti3V2NbAl0.5Ni0.5 alloy had a high yield strength of 1250 MPa and acceptable strain of 40%, benefiting from the precipitation of fine C14 Laves-phase particles and twined B19’ martensite. Moreover, the specific yield strength (SYS) of the Ti3V2NbAl0.5Ni0.5 alloy was 223 kPa·m3kg−1, superior to most other reported RHEAs at room temperature. It remained a high SYS of 198 and 54 kPa·m3kg−1 at 700 and 800 °C, respectively, which shows a superior balance between its density and mechanical properties across a wide temperature range.

Annealing-induced microstructure and properties evolution of refractory MoFeCrTiWAlNb3 eutectic high-entropy alloy coating by laser cladding

A refractory high-entropy alloy (HEA) coating with a composition close to MoFeCrTiWAlNb3 was fabricated by laser cladding on M2 high-speed steel. The as-cladded coating is mainly constituted by body-centered-cubic (BCC) solid solution, MC carbide and C14-Laves phase, and exhibits typical lamellar eutectic. To explore its performance at elevated temperature, these samples were annealed at 950 °C for 6 h–10 h in addition to a 4 h annealing treatment in the temperature range of 750 °C–950 °C. The results show that when the coating is annealed at 750 °C, its microstructure remains basically unchanged, and the hardness can reach 635 HV0.2. However, the hardness drops sharply due to the dissolution of the eutectic Laves phase as the annealing temperature increases. Interestingly, after annealing at 950 °C for 6 h–10 h, the hardness rises as the number of precipitated Laves phase and σ phase increases. The hardness and volume loss of the coating annealed at 950 °C for 8 h are 639 HV0.2 and 0.74 mm3, respectively, indicating that the coating has excellent high temperature softening resistance.

Correlation between the structure and thermomagnetic properties of pseudo-binary (Tb,Er)Ni2 solid solutions

Magnetic and thermodynamic properties, the isothermal magnetic entropy change ΔSmag is among them, of the polycrystalline pseudo-binary Tb1-xErxNi2 (x = 0.25, 0.5, and 0.75) solid solutions are studied in accordance with the erbium substitution for terbium, which causes changing the crystal structure. The Tb1-xErxNi2 solid solutions at room temperature were found to have the C15 Laves-phase superstructure rather than the C15 structure typical of the parent compounds, TbNi2 and ErNi2. Moreover, as was shown by magnetic measurements, the substitution of Er for Tb in Tb1-xErxNi2 results in the decrease in the ordering temperature of the pseudo-binary compounds, which corresponds to the second-order ferromagnet to paramagnet magnetic phase transition; in the case of x = 0.25 and x = 0.75, the decrease is from 29.0 to 13.6 K, respectively.The isothermal magnetic entropy change, which is estimated to characterize the magnetocaloric properties of the pseudo-binary compounds, was analyzed using the thermomagnetic Maxwell relation and in terms of the Landau theory of phase transitions. The maximum magnetic entropy change of the Tb0·75Er0·25Ni2, Tb0·5Er0·5Ni2 and for Tb0·25Er0·75Ni2 compositions reaches 6.6 (12.7), 6.9 (13.8) and 7.5 (14.9) J/kgK for a magnetic field change of 2 (5) T, respectively.The relatively large reversible and close values of ΔSmag allow us to state that these pseudo-binary solid solutions show promise as materials for magnetic refrigerators operating within a specific low-temperature range.

Magnetic and magnetoelastic properties of rare earth intermetallides based on TbFe2

The paper presents the results of the study of magnetic and magnetostrictive properties of rare-earth intermetallides based on TbFe2 compound: Tb1-xZrxFe2, Tb1–xSmxFe2 and (Tb1–xYx)0.8Sm0.2Fe2. These alloys have a cubic crystal structure of the Laves phase C15 throughout the substitution region. For the system of alloys Tb1–xZrxFe2 the single-phase area is limited to the replacement parameter 0 < x < 0.2 and 0.8 < x < 1.0. The compounds of the Tb1–xSmxFe2 system were studied via X-ray dilatometry. The magnetostriction of alloys within the Tb1–xZrxFe2 and (Tb1–xYx)0.8Sm0.2Fe2 systems was studied in the temperature range of 80-320 K in magnetic fields of up to 17 kOe via a strain-gauge method. It was found that each system has the phenomenon of sign inversion of magnetostriction constants: in the system Tb1–xZrxFe2 in the area of replacement parameter x = 0.8, in Tb1–xSmxFe2 - in the area x = 0.45, and in the system of alloys (Tb1–xYx)08Sm02Fe2 at x = 0.6. It was demonstrated that magnetic compensation and spin reorientation phenomena occur in these systems. The obtained results are discussed in the model of alloys with competing exchange interactions.

High-capacity Zr-based AB2-type alloys as metal hydride battery anodes

This work focused on the structural and electrochemical properties of AB2-type Zr/Ti-based metal hydride anode alloys. The as-cast and annealed Ti0·24Zr0·76Ni1·1Mn0·7V0.2 alloys were characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electrochemical characterization (EC). X-ray diffraction (XRD) results indicated the presence of homogenous C14 Laves phase in the annealed alloy, and the major C15 and C14 phases coexisted with the TiNi and Zr9Ni11 secondary phases in the as-cast alloy. SEM images showed the phase compositions in these alloys and confirmed the significant decrease in the secondary phases of the annealed alloy. During EC, the as-cast alloy showed better activation performance than the annealed alloy. This result was related to the catalytic effect of the Ti-/Zr-related secondary phases during electrochemical reactions. Annealing increased the maximum discharge capacity from ∼350 mAh/g for the as-cast alloy to ∼400 mAh/g for the annealed alloy during activation. The annealed alloy also showed considerable cycling stability and a slightly high discharge rate. We developed a C14-Laves phase metal hydride alloy showing a discharge capacity ∼400 mAh/g at a 100 mA/g rate (C/4) suitable for electric vehicle (EV) application.

MoFe1.5CrTiWAlNbx refractory high-entropy alloy coating fabricated by laser cladding

To obtain excellent performance of cutting-tool-coating materials, the MoFe1.5CrTiWAlNbx (x = 1.5, 2, 2.5, 3; x values in molar ratio) high-entropy alloy (HEA) coating was designed and fabricated on M2 tool steel by laser cladding. The microstructure, phase structure, microhardness and wear resistance were investigated by XRD, SEM, TEM, microhardness tester and wear machine, respectively. The experimental results show that the coating mainly consists of BCC, (Nb, Ti) C as well as C14-Laves phase. When x = 1 and 2, respectively, the microstructure of the coating is composed of cellular crystal, on which exists particle carbide; as x = 2.5 and 3, respectively, the microstructure transforms into the columnar crystal, on which a number of particle carbide occurs. The microhardness of the coating increases gradually as the increase of Nb content, when x = 3, the microhardness reaches a maximum of HV 910, which is far higher than that of substrate, and the wear resistance becomes best. The wear mechanism of coating is mainly abrasive wear.

First-principles investigations of effects of solute elements on stability and electronic structure of laves phase/matrix interface in Ni-based superalloys

It is acknowledged that solute elements like Ru exert great influence on precipitation of deleterious topologically close-packed (TCP) phases. In this work, by means of first principle calculation based on density functional theory (DFT), the interface structure between C14-laves phase and matrix γ phase has been established for the first time, and stable interfacial termination type was achieved. Effects of solute atoms like Ru on interface stability, electronic structure, and bond characteristics were further calculated. It has been proved that, unlike other elements, Ru can notably elevate the interfacial energy of C14-laves/γ system, lower interface stability, and increase the difficulty of nucleation of C14-laves phase; on the contrary, W will decrease the interfacial energy of C14-laves/γ and make it easier for C14-laves phase to nucleate; Re, Mo, and Cr will also increase the interfacial energy, but nowhere near the effect of Ru. Besides, in comparison with other solute elements, C14-laves/γ substituted by Ru has the smallest degree of interfacial charge accumulation, lowest interfacial atomic bonding strength, and poorest interfacial stability. All the reasons discussed above lead to the blocking effect of Ru on the nucleation of C14-laves phase.

Predictions of thermomagnetic properties of Laves phase compounds: TbAl2, GdAl2 and SmAl2 performed with ATOMIC MATTERS MFA computation system

Recent calculations of properties of TbAl2 GdAl2 and SmAl2 single crystals, performed with our new computation system called ATOMIC MATTERS MFA are presented. We applied localized electron approach to describe the thermal evolution of Fine Electronic Structure of Tb3+, Gd3+ and Sm3+ ions over a wide temperature range and estimate Magnetocaloric Effect (MCE). Thermomagnetic properties of TbAl2, GdAl2 and SmAl2 were calculated based on the fine electronic structure of the 4f8, 4f7 and 4f5 electronic configuration of the Tb3+ and Gd3+ and Sm3+ ions, respectively. Our calculations yielded: magnetic moment value and direction; single-crystalline magnetization curves in zero field and in external magnetic field applied in various directions m(T,Bext); the 4f-electronic components of specific heat c4f(T,Bext); and temperature dependence of the magnetic entropy and isothermal entropy change with external magnetic field – ΔS(T,Bext). The cubic universal CEF parameters values used for all CEF calculations was taken from literature and recalculated for universal cubic parameters set for the RAl2 series: A4 = +7.164 Ka04 and A6 = −1.038 Ka06. Magnetic properties were found to be anisotropic due to cubic Laves phase C15 crystal structure symmetry. These studies reveal the importance of multipolar charge interactions when describing thermomagnetic properties of real 4f electronic systems and the effectiveness of an applied self-consistent molecular field in calculations for magnetic phase transition simulation.

Improving the microstructure and mechanical properties of Zr-Ti alloy by nickel addition

The microstructure and mechanical properties of (Zr-Ti)1-χNiχ(χ =0, 1, 3, and 5 wt.%) alloys were studied. With the addition of nickel (Ni), the phase composition of alloys changes from α phase to α phase + hexagonal close packed (hcp) C14 Laves-phase. The thickness of lamellar α phase decreases with increasing Ni content. Optical microscopy and transmission electron microscopy results indicated that C14 Laves-phase is formed on the boundary. The Zr-Ti-3wt.%Ni alloys exhibits better ultimate compressive strength and compressive plasticity (1715 MPa and 9.70%) than that of Zr-Ti alloys (1598 MPa and 12.40%). The decrease of the average thickness of the lamellar α-phases (from 0.183 μm to 0.065 μm) leads to the improvement of the strength of the alloy. Additionally, the solid solution strengthening and the second phase strengthening are also important reasons for the improvement of the strength.

Magnetism of CoCrFeNiZrx eutectic high-entropy alloys

We have investigated the magnetism of CoCrFeNiZrx (x = 0.4–0.5) eutectic high-entropy alloys (HEAs) in relation to their microstructure by XRD, SEM, magnetization, specific heat and electrical resistivity measurements. Two structural phases develop in the CoCrFeNiZrx HEAs, a Zr-free fcc solid solution and a Zr-containing C15 Laves-phase intermetallic compound, where in both phases magnetic transition elements Co, Cr, Fe and Ni substitute each other in a random-like manner. Relative fractions of the two phases change substantially with the Zr concentration. Microstructurally, the two phases form alternatively grown fine lamellar structure typical of eutectic alloys. In addition, bulky dendrites of the fcc solid solution form in large number for the x = 0.40 concentration and in small number for x = 0.45. Two magnetic structures coexist in the CoCrFeNiZrx HEAs. The first is a disordered ferromagnetic (FM) phase that develops in the interior of large dendrites of the fcc solid solution and in some larger lamellas of this phase. The second phase is superparamagnetic-like and originates from the remaining spins of the fcc solid solution fraction, located at the surfaces and in the interfaces, and from all spins of the C15 Laves-phase fraction. The relative magnitude of the disordered FM-state magnetization with respect to the superparamagnetic magnetization in the samples with different Zr concentration is predominantly determined by the volume fraction of the fcc bulky dendrites. There is no one-to-one relation between the two magnetic structures on one side and the two structural components of the CoCrFeNiZrx eutectic mixtures on the other side. The magnetism of multi-phase HEAs containing magnetic elements cannot be described as a compositional average of magnetic properties of the constituent phases.

Microstructure and Hydrogen Storage Properties of Ti1V0.9Cr1.1 Alloy with Addition of x wt % Zr (x = 0, 2, 4, 8, and 12)

The effect of adding Zr on microstructure and hydrogen storage properties of BCC Ti1V0.9Cr1.1 synthesized by arc melting was studied. The microstructures of samples with Zr were multiphase with a main BCC phase and secondary Laves phases C15 and C14. The abundance of secondary phases increased with increasing amount of zirconium. We found that addition of Zr greatly enhanced the first hydrogenation kinetics. The addition of 4 wt % of Zr produced fast kinetics and high hydrogen storage capacity. Addition of higher amount of Zr had for effect of decreasing the hydrogen capacity. The reduction in hydrogen capacity might be due to the increased secondary phase abundance. The effect of air exposure was also studied. It was found that, for the sample with 12 wt % of Zr, exposure to the air resulted in appearance of an incubation time in the first hydrogenation and a slight reduction of hydrogen capacity.

First-principles and model simulation of all-optical spin reversal

All-optical spin switching is a potential trailblazer for information storage and communication at an unprecedented fast rate and free of magnetic fields. However, the current wisdom is largely based on semiempirical models of effective magnetic fields and heat pulses, so it is difficult to provide high-speed design protocols for actual devices. Here, we carry out a massively parallel first-principles and model calculation for thirteen spin systems and magnetic layers, free of any effective field, to establish a simpler and alternative paradigm of laser-induced ultrafast spin reversal and to point out a path to a full-integrated photospintronic device. It is the interplay of the optical selection rule and sublattice spin orderings that underlines seemingly irreconcilable helicity-dependent/independent switchings. Using realistic experimental parameters, we predict that strong ferrimagnets, in particular, Laves phase C15 rare-earth alloys, meet the telecommunication energy requirement of 10 fJ, thus allowing a cost-effective subpicosecond laser to switch spin in the GHz region.

Modelling of phase equilibria in the Hf-V system below room temperature

Phase transformation from orthorhombic HfV2 structure to cubic C15 Laves phase structure, which occurs during heating at about 114 K, is well known. In this contribution, a thermodynamic description of this phenomenon is provided supported by ab initio calculations. We utilize the third generation of thermodynamic database extending the Scientific Group Thermodata Europe (SGTE) unary data to zero Kelvin and demonstrate that it may be also applied to intermetallic phases. The data from a recent thermodynamic assessment of the Hf-V system (valid for temperatures above 298.15 K) were used and extended to zero Kelvin by the same method as it was used for unary data. Under the assumption of validity of harmonic approximation and electronic contribution to the heat capacity, the thermodynamics of C15 and orthorhombic phase were described. With the help of ab initio approach, we demonstrate that the HfV2 orthorhombic phase and C15 Laves phase are mechanically stable at 0 K and thanks to entropy stabilization they are in equilibrium with pure element phases in the temperature region of structural change.

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.

Point defects and Zn-doping in defective Laves phase C15 MgCu2: A first-principles study

Employing first-principles calculations, the point defect in C15 Laves phase MgCu2 is studied. The formation enthalpies of point defects and defective C15 MgCu2 compound show that the dominating defect structure is MgCu and CuMg antisite defect on Mg-rich and Cu-rich side of off-stoichiometry, respectively. The concentration of various point defects as a function of composition of MgCu2 is analyzed from grand canonical statistics. The calculated local geometrical variation around defect demonstrates an apparent size effect, and the electronic densities of states and charge density difference distributions indicate the gradually weaker bonding from MgCu to VCu and from CuMg to VMg systems. Furthermore, doping of Zn atom in defective MgCu2 has been investigated. It is found that Zn has a stronger preference for occupying the Cu sublattice than for Mg sublattice because of the relative lower formation enthalpy. The electronic structure further reveals that the stability of Zn-doped system is associated with the stronger covalent bonds between Cu–Cu and Zn–Cu atoms.

Microstructure Characterization and Fracture Toughness of Laves Phase-Based Cr–Nb–Ti Alloys

Three Laves phase-based alloys with nominal compositions of Cr2Nb–xTi (x = 20, 30, 40, in at%) have been prepared through vacuum non-consumable arc melting. The results show that the microstructures of Cr2Nb-(20, 30) Ti alloys are composed of the primary Laves phase C15–Cr2(Nb,Ti) and bcc solid solution phase, while the microstructure of Cr2Nb–40Ti alloy is developed with the eutectic phases C15–Cr2(Nb,Ti)/bcc solid solution. The measured fracture toughness of ternary Laves phase C15–Cr2(Nb,Ti) is about 3.0 MPa m1/2, much larger than 1.4 MPa m1/2 for binary Laves phase Cr2Nb. Meanwhile, the fracture toughness of Cr2Nb–xTi (x = 20, 30, 40) alloys increases with increasing Ti content and reaches 10.6 MPa m1/2 in Cr2Nb–40Ti alloy. The eutectic microstructure and addition of Ti in Cr2Nb are found to be effective in toughening Laves phase-based alloys.

Permanent magnetism of intermetallic compounds between light and heavy transition-metal elements

First-principle calculations are used to investigate the intrinsic magnetic properties of intermetallic alloys of the type XMn, where X is a 4d or 5d element and M is Fe or Co. Emphasis is on the hexagonal C14 Laves-phase 1:2 and 1:5 alloys, the latter crystallizing in the CaCu5 structure. These series are of interest in permanent magnetism from fundamental and practical viewpoints, respectively. In the former, the unit cells form a prototypical motif where a heavy atom with high spin–orbit coupling and magnetocrystalline anisotropy is surrounded by many somewhat smaller M atoms with high magnetization, and the latter are Laves-phase derivatives of renewed interest in permanent magnetism. Our DFT calculations predict magnetic moments, magnetizations and anisotropies, as well as formation energies. The results are analyzed across the 4d and 5d series, especially with respect to hybridization effects between 3d and 4d/5d bands.

Magnetic properties of C15 the laves-phase compound SmRu2

We report the magnetic properties of the polycrystalline C15 Laves-phase rare-earth compound SmRu2. The sample of SmRu2 was made by using an arc-melt method and was annealed under 500 °C in vacuumed quartz tube for 50 hours. The magnetic susceptibility χ of SmRu2 follows a Curie-Weiss law at temperature above 80 K. The effective paramagnetic moment p eff is 1.39 µ B /Sm-atom, and the Weiss temperature θ has a positive value of 42 K. The low-temperature χ for T < 60 K increases rapidly with decreasing temperature. In the resistivity ρ and the specific heat C p , we found an inflection and a jump, respectively, at about 55 K. Furthermore, we observed a hysteresis in the magnetic field dependence of the magnetization at 2 K. These experimental results suggest that the C15 Laves-phase compound SmRu2 is a ferromagnetic material with T C ∼ 55 K.