C15 Structure Research Articles

Formation mechanism and oxidation performance for a novel Mo(Si,Al)2 coating prepared using a modified pack cementation strategy

Using a modified Si-Al deposition strategy in the process of halide-activated pack cementation, conventional and novel Mo(Si,Al)2 coatings are fabricated on molybdenum substrate to assess their distinct formation mechanism and oxidation performance. The results show that the conventional coatings are composed of an inner layer containing pure C11b MoSi2 and an outer layer containing C11b MoSi2 with a little Al. By contrast, the new coating is composed of pure C22 Al8Mo3 as the inner layer and C40 Mo(Si,Al)2 as the outer layer (unexpectedly low Al percentage of 13 at.%). Notably, the two kinds of coatings have different microstructures and formation characteristics. The former's synthesis pathway includes the sequential formation of C22 Al8Mo3, C40 Mo(Si,Al)2, and C11b MoSi2, accompanied by a reduction in the proportion of solidified Al in Mo(Si,Al)2. The latter's synthesis path includes the transformation from C11b MoSi2 to C40 Mo(Si,Al)2, and the Al content is always maintained at the highest level with the coating growth. Meanwhile, the phase transition between Al8Mo3 and Mo(Si,Al)2 is an irreversible process and neither coating forms the C54 structure of Mo(Si,Al)2 due to the low temperature. Besides, the different oxidation mechanisms are elucidated on basic of the two deposition procedures and corresponding to oxidation performance. Hence, the newly developed Mo(Si,Al)2 coating of first silicon and then aluminum in the process of pack-cementation can be effectively used to fabricate a protective α-Al2O3 barrier layer at a temperature of 1300 °C.

Superconductivity in Ternary Germanite TaAl x Ge2-x with a C40 chiral structure

Abstract We report a new family of chiral intermetallic superconductors TaAl x Ge2-x . The mother compound TaGe2 has a C40-type chiral hexagonal crystal structure with a pair of enantiomorphic space groups of P6222 and P6422. By substituting Ge with Al, TaAl x Ge2-x polycrystals with the C40 structure were synthesized with Al substitution x from 0 to 0.8. Magnetic susceptibility, magnetization curves and electrical resistivity revealed that TaAl x Ge2-x with x of 0.2 to 0.4 was a type-II superconductor with a superconducting transition temperature T c of 2.0 to 2.2 K. The superconductivity disappeared at with x less than 0.2 or was largely suppressed at with x of 0.5 or more, although all the measurements were performed at temperatures above 1.8 K. An emergence of superconductivity is discussed in terms of the lattice constants changes with the Al substitution.

The effect of pressure on the physical properties of ScFe2 Laves phases

The physical properties of ScFe2 Laves phases with cubic (C15) and hexagonal (C14, C36) structures under ambient and high pressures are studied using the first-principles method and provide a comprehensive and in-depth analysis of the magnetic, elastic and dynamical properties. The spin magnetic moments of three structures decrease as pressure increases, and in the C36 structure, the spin reversal of Fe (6h) moment near 90 GPa is found. The critical pressures of magnetic collapse for the C14, C15 and C36 structures are 90, 160, and 170 GPa, respectively. The C15 structure is the most thermodynamically stable phase below 44.4 GPa, and a phase transition from C15 to C14 structure is observed. In the thermodynamic stability pressure range, with 0–44.4 GPa for C15 structure and 44.4–90 GPa for C14 structure, the C15 and C14 structures are both dynamically and mechanically stable. The C14 and C15 structures under high pressure are ductile structures with significant differences in elastic properties and anisotropy. Finally, the Debye temperature and sound velocity under high pressure are discussed in detail.

Study of magnetic properties of the fullerene C36 structure by Monte Carlo simulations

Study of magnetic properties of the fullerene C36 structure by Monte Carlo simulations

Structural Evolution and Hydrogen Sorption Properties of YxNi2−yMny (0.825 ≤ x ≤ 0.95, 0.1 ≤ y ≤ 0.3) Laves Phase Compounds

The YxNi2−yMny system was investigated in the region 0.825 ≤ x ≤ 0.95, 0.1 ≤ y ≤ 0.3. The alloys were synthesized by induction melting and corresponding annealing. The substitution of Mn for Ni (y = 0.1) favors the formation of a C15 structure with disordered Y vacancies against the superstructure of Y0.95Ni2. For y = 0.2 and 0.3, Mn can substitute in both Y and Ni sites. Single-phase compounds with a C15 structure can be formed by adjusting both the Y and Mn contents. Their hydrogen absorption–desorption properties were measured by pressure–composition isotherm (PCI) measurements at 150 °C, and the hydrides were characterized at room temperature by X-ray diffraction and TG–DSC experiments. The PCIs show two plateaus corresponding to the formation of crystalline and amorphous hydrides. The heating of the amorphous hydrides leads to an endothermic desorption at first and then a recrystallization into Y(Ni, Mn)3 and YHx phases. At higher temperatures, the Y hydride desorbs, and a recombination into a Y(Ni, Mn)2 Laves phase compound is observed. For y = 0.1, vacancy formation in the Y site and partial Mn substitution in the Ni site enhance the structural stability and suppress the hydrogen-induced amorphization (HIA). However, for a larger Mn content (y ≥ 0.2), Mn substitutes also in the Y sites at the expense of Y vacancies. This yields worse structural stability upon hydrogenation than for y = 0.1, as the mean ratio r(Y, Mn)/r(Ni/Mn) becomes larger than for y = 0.1 r(Y, ☐)/r(Ni/Mn).

Phase stability and thermo-physical properties of Laves-Co<sub>2</sub>(Hf Ta) alloys

There exists still the controversy over the stable structure of Laves-phase Co<sub>2</sub>(Hf Ta) alloys with the C14, C15 or C36 structures. In this study, the stability, electronic and thermodynamic properties of Laves-phase Co<sub>2</sub>(Hf Ta) are investigated. In order to fully understand the influence of magnetic state and temperature on phase stability, we systematically study the free energy change at finite temperature, elastic stability, and phonon dispersion. The low Curie temperature can be estimated, which suggests that the Co<sub>2</sub>(Hf Ta) alloys possess the paramagnetic state in a wide temperature range. Results indicate that the lattice vibration and electronic excitation have an important effect on the phase stability. The ground state of Co<sub>2</sub>Hf compound has a C14-type structure, while the ground state of Co<sub>2</sub>Ta has a C36-type structure, without the effect of temperature. After doping Hf with different concentrations (0.25, 0.50, 0.75) into Co<sub>2</sub>Ta, the most stable structure still possesses the C36-type structure. After considering the contribution of vibration entropy and electron entropy, the relatively stable structures of Co<sub>2</sub>Hf and Co<sub>2</sub>Ta undergo the C36 and C14 phase transition, respectively. In addition, the thermodynamic properties, including Debye temperature, heat capacity, and vibration entropy, which vary with pressure and temperature, are studied. The electronic properties of Co<sub>2</sub>Hf and Co<sub>2</sub>Ta compounds are analyzed by the charge difference and density of states. The similar electronic density of states between different phases suggest that the Lave phases have the similar stability. The Hf-Co bonding with a certain direction is revealed. Our results are of great significance in understanding the structure and properties of Co<sub>2</sub>Hf and Co<sub>2</sub>Ta compounds.

Induced formation of nano precipitates and improve microstructure and mechanical properties by ultrasonic wave in the solid-liquid two-phase area of as-cast Ti48Al2Cr2Nb 2.5 C alloy

To refine the microstructure and form Ti3Al2 and C14 nano particles, and then improve strength and strain simultaneously, the Ti48Al2Cr2Nb 2.5 C alloy has been prepared under ultrasonic wave. Size of lamellar colony, lamellar spacing, phase composition, compressive properties and the related mechanisms have been studied. Results show that size of lamellar colony decreases from 40.7 to 33.0 µm and lamellar spacing decreases from 1.5 to 0.8 µm. Average length-diameter ratio of Ti2AlC decreases from 5.4 to 1.9 with increasing the ultrasonic treatment time from 20 to 100 s. Two kinds of nanoparticles form. One is hexagonal Ti3Al2 phase, which has incoherent relationship with γ phase. Another is Ti(Al, Cr)2 phase with C14 structure. The Ti2AlC and α grain are refined by undercooling nucleation and dendrite fragmentation in solid-liquid two-phase area. The active dislocations and vacancy under ultrasonic promote the diffusion of Ti and Cr, and then induce the Ti3Al2 and Ti(Al, Cr)2 formed during solid-phase transformation. Compressive results show that the strength increases from 1901 to 2532 MPa, and the strain simultaneously increases from 27 to 34% with increasing the time from 0 to 100 s. The lattice distortion caused by the difference in thermal expansion coefficient between the Ti3Al2, Ti(Al, Cr)2 and matrix and the microstructure refined by ultrasonic jointly hinder dislocation slip to strengthen the alloy. The refinement and uniform distribution of Ti2AlC hinder crack propagation to improve toughness.

The Effects of the Al and Zr Contents on the Microstructure Evolution of Light-Weight AlxNbTiVZry High Entropy Alloy.

To investigate the comprehensive effects of the Al and Zr element contents on the microstructure evolution of the AlNbTiVZr series light-weight refractory high entropy alloys (HEAs), five samples were studied. Samples with different compositions were designated Al1.5NbTiVZr, Al1.5NbTiVZr0.5, AlNbTiVZr, AlNbTiVZr0.5, and Al0.5NbTiVZr0.5. The results demonstrated that the actual density of the studied HEA samples ranged from 5.291 to 5.826 g·cm-3. The microstructure of these HEAs contains a solid solution phase with a BCC structure and a Laves phase. The Laves phase was further identified as the ZrAlV intermetallic compound by TEM observations. The microstructure of the AlNbTiVZr series HEAs was affected by both the Al and Zr element contents, whereas the Zr element showed a more dominant effect due to Zr atoms occupying the core position of the ZrAlV Laves phase (C14 structure). Therefore, the as-cast Al0.5NbTiVZr0.5 sample exhibits the best room temperature compression property with a compression strength (σp) of 1783 MPa and an engineering strain of 28.8% due to having the lowest ZrAlV intermetallic compound area fraction (0.7%), as characterized by the EBSD technique.

High-temperature phase stability and phase transformations of Niobium-Chromium Laves phase: Experimental and first-principles calculation studies

The high-temperature phase stability of the NbCr2 Laves phase has not yet been established beyond disagreement. The well accepted C14 phase field has been eliminated from the Nb-Cr phase diagram in a recent study that claimed that the NbCr2 alloy shows the C15 structure up to melting. The present study was conducted to address these ambiguities. The experimental investigations show the presence of C14, C15, and C36 structures in the as-cast alloy. Differential scanning calorimetry studies of the as-cast alloy show the transformation of the eutectic mixture of bcc + NbCr2 to single-phase NbCr2 at 1338 K. An in-situ X-ray diffraction (XRD) study of as-cast alloy shows a phase transformation of C36 → C14 below 1273 K. Density functional theory (DFT) calculations confirm two phase transformations: (a) a metastable transformation of C36 → C14 at 783 K and (b) an equilibrium transformation of C15 → C14 at 1971 K, which was validated by high temperature DTA and TEM studies. Further, extended DFT calculations on supercells with four compositions of NbCr1.9, NbCr2, NbCr2.1, and NbCr2.2 also corroborated the presence of the C14 phase, and consequently, the Nb-Cr phase diagram is modified. This finding resolves the controversy regarding the existence of an equilibrium C14 phase field before melting.

The Influence of Hydrogen on Structural and Magnetic Transformations in RMn2Hx Hydrides with Laves Phase C15 and C14 Structures—A Review

Laves phases crystallize in simple structures and are very common intermetallic phases that can form from combinations of elements throughout the periodic table, giving a huge number of known examples. A special feature of AB2 or AB5 phases is the ability to absorb hydrogen. This study attempts to collect, systematize and summarize the knowledge about RMn2Hx (R: Tb, Gd, Ho, Dy, Er, Sm, Nd and Y) hydrides available in the literature that is mainly related to structural and magnetic transformations. Due to the enormous wealth of data, the analysis focused on hydrides with x < 4.5 H/f.u., i.e., hydrides obtained at relatively low pressure (less than a few bars). The hydrides obtained in this way can be treated as potential hydrogen stores, which undoubtedly accounts for their current attractiveness.

Highly deformable Laves phase in a high entropy alloy

By replacing Nb with (Hf+Nb+Ta) in CoCrFeNi2.1(Nb)0.2 high entropy alloy (HEA), which contained hard and brittle hexagonal C14 Laves phase, a novel CoCrFeNi2.1(HfNbTa)0.2 HEA was designed. Heavy cold-rolling complemented by microstructural and nanoindentation studies produced evidence of the surprisingly soft and ductile attributes of the Laves phase in the CoCrFeNi2.1(HfNbTa)0.2 HEA, in striking contrast to the almost universal brittle behavior of the Laves phases. Crystallographic and chemical analyses of the Laves phase in the CoCrFeNi2.1(HfNbTa)0.2 HEA confirmed the cubic C15 structure, realized by the site occupancy preferences of the constituent elements. The unusual deformability of the Laves phase was attributed to the spectacular propensity for nano-twin formation on the {111}<112> system, presumably resulting from its increased compositional complexity. Meanwhile, the results should open a pathway for designing highly deformable Laves phases in HEAs.

High-field magnetic and magnetocaloric properties of pseudo-binary Er1−xHoxNi2 (x = 0.25–0.75) solid solutions

Hydrogen is quickly becoming a desired type of fuel, however, the energy and cost required for liquefaction using today’s cooling technology is excessively high. Magnetic cooling based on the magnetocaloric effect is an energy-efficient and environmentally friendly alternative to commonly used vapor compression, but improvements in refrigerants are crucial for this technology to succeed. Polycrystalline Er1−xHoxNi2 (x = 0.25, 0.5, 0.75) Laves-phase solid solutions obtained by the arc-melting method have been investigated due to their potential for low-temperature refrigerants. Er0.75Ho0.25Ni2 and Er0.5Ho0.5Ni2 crystallize in cubic Laves phase superstructure (space group F-43 m), while Er0.25Ho0.75Ni2, similarly to the initial ErNi2 and HoNi2 binary compounds, crystallizes with the formation of the regular cubic C15 structure (space group Fd-3 m). To evaluate how the structure affects magnetic and magnetocaloric properties, studies in a wide magnetic field range, up to 14 T, were conducted. Measurements show all samples obey the second-order magnetic phase transition from ferromagnetic to paramagnetic state, and their Curie temperatures increase with increasing Ho content from 8 K for Er0.75Ho0.25Ni2 to 12.3 K for Er0.25Ho0.75Ni2. At higher temperatures, all solid solutions are Curie-Weiss paramagnets. However, it has been observed that the formation of the superstructure with lower translational symmetry seems to affect magnetic moments and magnetic entropy values. Er0.25Ho0.75Ni2, with the regular cubic C15 structure, showed the highest entropy changes of 43.5 J/kgK around 12 K, while Er0.75Ho0.25Ni2 and Er0.5Ho0.5Ni2, with cubic superstructure, provided ∼30 % lower results of 30.3 and 35.1 J/kgK, around 8 and 10 K, respectively, for magnetic field change of 14 T. Nevertheless, relatively large and reversible values of magnetic entropy prove that these compounds can be promising candidates for magnetic cooling operating within the low-temperature range needed for hydrogen liquefaction.

Hydrogen and nitrogen absorption properties of Ce-added Zr2(Fe1–xNix) alloys

To improve the nitriding resistance of Zr2Fe alloy, the Ce-added Zr2(Fe1–xNix) (x = 0, 0.15, 0.3, 0.5) alloys were prepared by magnetic levitation melting method. The effects of Ni substitution for Fe on the phase structure, hydrogen and nitrogen absorption properties of the alloys were investigated by X-ray diffraction, scanning electron microscopy and p-c isotherm measurements. The experimental results show that Ni substitution can effectively inhibit the formation of both α-Zr and ZrFe2 phases and promote the formation of C16 structure Zr2(Fe, Ni) phase, causing Ni-substituted alloys to exhibit low nitrogen absorption rate and capacity. At 623 K under 0.5 MPa nitrogen pressure, the nitrogen absorption capacity of Ce-added Zr2(Fe0.5Ni0.5) alloy reaches to 0.8 mL/g, much lower than that of Zr2Fe alloy (1.5 mL/g). Ni substitution decreases the crystal cell volume of the C16 Zr2(Fe, Ni) phase, resulting in an increase in the hydrogen absorption equilibrium pressure. At 623 K under 0.05 MPa hydrogen pressure, the hydrogen absorption capacity decreases from 1.46 wt% of Zr2Fe alloy to 1.41 wt% of Ce-added Zr2(Fe0.5Ni0.5) alloy.

HRTEM Characterization of an Age-Hardened Mg–Ca Binary Alloy

A Mg–2.8 mass% Ca binary alloy was subjected to a homogenization treatment. The three-dimensional morphology and coherency of the Mg2Ca phase (with C14 structure), which precipitated in the primary α-Mg dendrite, were investigated for the specimens aged at 423 K. The Mg2Ca precipitate exhibited a hexagonal plate-like morphology, with a planar surface that is parallel to the (0001)α basal plane and sides that are parallel to the {-1010}α columnar plane of the α-Mg matrix. The coarsening Mg2Ca precipitate retained coherency with the α-Mg matrix for aging time of 3 h, which corresponded to the peak-aged condition. Furthermore, during the coarsening process, misfit dislocations were introduced on the planar surface of the precipitates, and the coherent interface became a semi-coherent interface for aging time of 100 h, which corresponded to the over-aged condition. The Mg2Ca precipitates evolved from a hexagonal plate-like morphology to a rectangular morphology and then to a polygonal morphology during aging treatment at 423 K.

Experimental investigation and thermodynamic description of the Fe–Hf–Nb system

According to solidification microstructure of as-cast alloys and phase constituents of annealed samples, the liquidus surface projection and phase relationships at 1373 and 1273 K in the Fe–Hf–Nb system were constructed. A ternary phase τ with AlCuHf-structure was found, and the composition ranges of Nb in τ at 1373 and 1273 K were ∼ 20.5–30.8 at.% and ∼ 20.8–30.2 at.%, respectively. Moreover, the maximum solubilities of Nb in FeHf2 was determined to be ∼ 16.9 at.% and ∼ 16.0 at.% at 1373 and 1273 K, and the maximum solubilities of Hf in μ was measured to be ∼ 4.6 at% and ∼ 3.3 at.% at 1373 and 1273 K. Fe2Hf and Fe2Nb with the same C14 structure formed a homogeneous solid solution from Fe2Hf to Fe2Nb at 1373 and 1273 K. Seven primary solidification regions in the liquidus surface projection and two three-phase regions and seven two-phase regions in the isothermal sections at 1373 and 1273 K were experimentally determined. On the basis of experimental data, the thermodynamic parameters of individual phases in the Fe–Hf–Nb system were optimized by CALculation of PHAse Diagram (CALPHAD) method. Four solution phases (liquid, bcc, fcc, and hcp) were described as the substitutional solution. The intermetallic phases FeHf2, λ1, λ2, λ3 and τ were modeled as (Fe)1(Hf,Nb)2, (Fe,Hf,Nb)2(Fe,Hf,Nb)1, (Fe)2(Hf)1, (Fe)2(Hf,Nb)1, and (Fe,Hf,Nb)1(Fe,Hf,Nb)1 by a two-sublattice model, respectively. μ was described as (Fe,Hf,Nb)1(Hf,Nb)4(Fe,Hf,Nb)2(Fe,Hf,Nb)6 by a four-sublattice model. A set of self-consistent thermodynamic parameters of the Fe–Hf–Nb system was obtained.

The stability and properties of the PtFe2 Laves phases

The structural stability of PtFe2 Laves phase with cubic and hexagonal structures has been investigated with the help of the first-principles methods, followed by targeted calculations of electronic, magnetic, lattice-dynamics and mechanical properties. The formation enthalpies of different magnetic configurations are calculated and their ferromagnetic (FM) states are found to be thermodynamically stable. However, there is strong competition for the FM states of the three structures, although the cubic C15 structure has the lowest energy. These FM states are dynamically stable owing to the spontaneous magnetization. The total magnetic moments of the FM state for these three structures are close to each other at 0 GPa, and the spin-polarized electronic DOS is investigated to promote insight into the magnetic properties. The elastic constants and modulus show that all three structures with FM state are mechanically stable, and the hexagonal C14 structure has the largest bulk modulus, but its shear modulus and Young's modulus are the lowest. Finally, the elastic anisotropy was quantified by several anisotropy indexes. The results reported here reveal that the anisotropy is in a sequence of C15 > C14 > C36.

Experimental investigation and thermodynamic description of the Co–Nb–Ti system

The solidification microstructures of as-cast alloys and the equilibria microstructure of annealed alloys at 1273 K in the Co–Nb–Ti system were determined by scanning electron microscopy with energy dispersive spectroscopy (SEM/EDS) and X-ray diffraction (XRD), and the liquidus surface projection and partial isothermal section at 1273 K were constructed. Nine primary solidification regions, bcc(Nb,Ti), fcc(Co), Co3Ti, CoTi, CoTi2, λ1, λ2, λ3, and μ and five ternary invariant reactions, liq. → λ1 + λ2 + μ, liq. → CoTi + λ2 + μ, liq. → bcc(Nb,Ti) + CoTi + μ, liq. + fcc(Co) + λ3 → Co3Ti, and liq. + CoTi → bcc(Nb,Ti) + CoTi2 were experimentally determined or deduced in the liquidus surface projection. Three three-phase regions, CoTi + λ2 + μ, bcc(Nb,Ti) + CoTi + μ, and bcc(Nb,Ti) + CoTi + CoTi2 and six two-phase regions, Co3Ti + λ3, λ2 + μ, CoTi + λ2, CoTi + μ, bcc(Nb,Ti) + μ, and bcc(Nb,Ti) + CoTi at 1273 K were experimentally determined. The continuous compound λ2 with a C15 structure from the Co–Nb side to Co–Ti side was formed at 1273 K. The solubilities of Ti in μ and Nb in CoTi were determined to be ~11.1 at% and ~18.4 at% at 1273 K, respectively. And no new phases were found. According to the available experimental data from the present work and literatures, the Co–Nb–Ti system was optimized by CALPHAD (CALculatiuon of PHAse Diagram) method. Four solution phases (liquid, bcc, fcc, and hcp) were described using the substitutional solution model. A two-sublattice model (Co,Nb,Ti)2(Co,Nb,Ti)1 was used to described three compounds λ1, λ2, and λ3 according to their crystal structures and homogeneity ranges. Co7Nb2 and CoTi2 were treated as Co7(Nb,Ti)2 and Co(Nb,Ti)2 by a two-sublattice model, respectively. μ was treated as (Co,Nb,Ti)1(Nb,Ti)4(Co,Nb,Ti)2(Co,Nb,Ti)6 by a four-sublattice model. Co3Ti with L12 structure and CoTi with B2 structure were treated as the ordered phases of fcc and bcc solutions using the thermodynamic models (Co,Nb,Ti)0.75(Co,Nb,Ti)0.25Va1 and (Co,Nb,Ti,Va)0.5(Co,Nb,Ti,Va)0.5Va3, respectively. A set of self-consistent thermodynamic parameters of the Co–Nb–Ti system was obtained.

First principles calculation on electronic structures and mechanical properties of TiCrTaV high–entropy alloy

The formability, electronic structures, Debye temperatures and mechanical properties of TiVCrTa multi-component alloys with different crystal structures are investigated with first principles methods based on plane-wave pseudopotential theory. Results show that the three structures exhibit ductility. The C15 structure (Ti41Cr25Ta12V22) is highly stable and has strong compressibility resistance along the X-axis. And the generation of the C15 structure increase the B value, and thus improves the deformation resistance of the material. The BCC2 structure (Ti27Cr27Ta18V28) partially improves the wear resistance of the material. The BCC1 structure (Ti23Cr25Ta23V29) has strong elastic anisotropy. The largest E value of the two BCC structures is observed in the [111] direction, and the lowest in the [100] direction. The opposite is observed in the C15 structure. The BCC2 structure has a strong covalent bond and thermodynamic stability. The C15 structure has the strongest electron interaction and best atomic bonding and most obvious metallic bonding characteristics.

Atomic site occupancy of alloying elements and Laves phase stability in γ-γ′ Co-base superalloys

The atomic site occupancy of alloying elements in the ZrCo2 and HfCo2 based Laves phases formed in the Co-9Al-9W-2Zr and Co-9Al-9W-2Hf alloys was experimentally determined by the combined techniques of energy-dispersive X-ray spectroscopy mapping with atomic resolution and the atom location by channeling enhanced microanalysis (ALCHEMI) method. The results show that both Al and W occupy the Zr site in the C15 and the Hf site in the C36 crystal structure. The atomic locations of Al and W were further confirmed by comparing calculated inelastic cross-sections with experimental ALCHEMI results. Using the determined atomic site occupancies of alloying elements, special quasirandom structure solid solution models with 192 atoms were constructed and employed in first-principles calculations. It is found theoretically that the energy of formation of the C15 structure is always lower than that of the C36 structure at 0 K in both ZrCo2 and HfCo2 phases, no matter whether Al and W are incorporated or not. However, further ab initio molecular dynamics simulations suggest that lattice vibration at finite temperature contributes significantly to the phase stability, stabilizing the C36 structure compared to the C15 type for the (Hf, Al, W)Co2 phase at finite temperature, which fits well with the experimental findings.

Phases and phase equilibria in the Fe–Al–Zr system

AbstractIsothermal sections at 800, 1000, and 1150 °C as well as a tentative partial liquidus surface of the ternary Fe–Al–Zr system were established by means of electron-probe microanalysis, X-ray diffraction, differential thermal analysis, and light-optical as well as scanning electron microscopy. The most prominent features of the ternary phase diagram are the extended homogeneity ranges of the Laves phases. By continuous substitution of Fe by Al, the structure changes three times starting from the cubic C15 structure of Fe2Zr to hexagonal C14 (λ1) back to cubic C15 (λ2) and again to hexagonal C14 (Al2Zr). The various Laves phase fields are separated by very small two-phase fields. Besides the Laves phases λ1and λ2, three more ternary intermetallic phases were found, whose homogeneity ranges have been determined for the first time. In addition, new results concerning the homogeneity ranges of intermetallic phases in the binary subsystems Fe–Al and Al–Zr are reported. The solubilities of the third components in the binary phases are found to be generally very low with the exception of the Laves phases. As a consequence, extended two-phase fields between the Fe(Al) solid solution and Laves phase or between Fe(Al) solid solution and the ternary ThMn12-type phase τ1are formed.