C54 Phase Research Articles

Quantum-mechanical calculations of the difference of structural energies C14 and bcc phases C14 Fe and Mo and formation energy of ferromagnetic C14 Fe2Mo Laves phase

The structural energies of iron and molybdenum in the BCC and C14 Laves phases in the Fe – Mo system, formation enthalpies and bulk elastic moduli are calculated for further use in the calculations of the BCC / BCC+C14Fe2Mo Laves phase boundary at low (room) temperatures.

Wettability and solubility behavior of Fe–B–C binder on (Cr–Ti–C)-reinforced composites

Iron-base matrix composites containing Cr-20Ti-10C reinforcement fabricated by infiltrating at 1200°C to 1280°C for 30 to 60 mins are investigated in this work. Peculiarities of the formation of the interfaces between Cr-20Ti-10C hard alloy and Fe-3.1B-0.1C liquid alloy (wt. pct) are determined. It includes the study of the solubility between the iron-base binder and the reinforcement combined with the investigation of the contact angles of the liquid phase formed at infiltration temperature by sessile drop method. X-ray analysis as well as optical and scanning electron microscopy is employed to investigate the binder/particulate interfaces. The interfacial zones produced as a result of contact interaction consist of (Cr,Ti)7C3 and (Ti,Cr)3C phases embedded in α-Fe3(C,B) eutectic alloyed with Cr and Ti. The origin of the formation of the interfaces is probably related to the partial dissolution of the low-melting-temperature particulate phase identified as (Cr,Ti)7C3 in the molten Fe–B–C binder. When raising the infiltrating temperature and prolonging the infiltrating period, wetting properties of Cr-20Ti-10C improve. This fact opens the possibility to replace copper-base alloys as binder by a cheaper and stronger iron-base alloy.

The δ-six-stream spherical harmonic expansion adding method for solar radiative transfer

A δ-six-stream spherical harmonic expansion adding method (δ-6SDA) for solar radiative transfer is proposed. δ-6SDA has similar accuracy as δ-six-stream discrete ordinates method (δ-6DOM) but it is almost three orders of magnitude faster than δ-6DOM in flux calculation. In addition, δ-6SDA is extended to calculate the azimuthally dependent intensity in this study. The truncated-plus-single-scattering (TMS) correction is introduced to improve accuracy. Two single-layer cases with the Cloud C1 phase function and Henyey-Greenstein phase function are considered to investigate the accuracy of δ-6SDA for intensity calculation. The relative errors of these two cases for δ-6SDA are generally less than 2% and 5%, respectively. The root-mean-square errors for δ-6SDA are generally smaller than δ-6DOM. The computing time of δ-6SDA is 3–4 orders of magnitude less than δ-6DOM. Due to its high accuracy and efficiency, δ-6SDA can be widely used in remote sensing for simulating satellite observations.

Duplex phase hexagonal-cubic multi-principal element alloys with high hardness

We report on the influence of Mn additions on the microstructure and properties of (CoCrCuTi)100−xMnx multiprincipal element alloys (MPEAs) containing 5, 10, 15, and 20% Mn prepared by arc-melting. It was observed that the alloys containing less than 10 at.% Mn contained BCC dendrites and a Cu-rich face centered cubic (FCC) interdendritic matrix. In contrast, alloys containing Mn ≥ 10 at.%, consisted of a hexagonal dendritic Laves phase (C14, hexagonal, P63/mmc) with small amounts of Cr-rich and Ti-rich dendrites, and a Cu-rich FCC interdendritic matrix. Isopleths of the CoCrCuMnTi system were generated using the ThermoCalc software with TCHEA3 database in order to elucidate phase formation in these alloys. In addition, the particular composition Co22Cr18Cu20Mn16Ti24 was found to consist of Laves C14 phase hexagonal dendrites in a Cu-rich FCC interdendritic matrix with very small dispersed Ti-rich dendrites. Selected area electron diffraction of the dendritic phase in Co22Cr18Cu20Mn16Ti24 was found to have lattice parameters approximated to be a = 4.8Å and c = 7.5Å based on fitting the d-spacing of the indexed patterns to previously published data on the Cr2Ti intermetallic phase. The Vickers mircohardness of the hexagonal dendritic phase of Co22Cr18Cu20Mn16Ti24 was found to be 996.6 ± 57.2 HV 0.01, while the Cu-rich interdendritic matrix has hardness of 457.3 ± 7.0 HV 0.01 and overall hardness of both regions equaling 480.2 ± 21.4 HV 1. The overall Rockwell hardness was found to be 43 ± 2 HRC. The compressive strength of Co22Cr18Cu20Mn16Ti24 was measured to be 1430 MPa, while wear rate was measured as 2.64 × 10−5 mm3/m·N.

Structure of Cubic Al73.8Pd13.6Fe12.6 Phase with High Al Content

A cubic ternary phase Al73.8Pd13.6Fe12.6 (designated C′ phase), with very high Al content (Al/TM = 2.82, TM denotes transition metal) was prepared by spark plasma sintering (SPS). Its crystal structure was determined by combing single-crystal X-ray diffraction (SXRD) and scanning electron microscope (SEM) equipped with energy dispersive X-ray spectroscopy (EDS) measurements. The crystal structure of the new phase can be described with a small unit cell (a = 7.6403(2) Å; space group Pm3, No. 200) as that of Al2.63Rh (a = 7.6692(1) Å; space group P23, No. 195) while different from those of the reported Al39Pd21Fe2 (a = 15.515(1) Å; space group Fm3, No. 202) and Al69Pd17Fe14 (a = 15.3982(2) Å; space group Im3, No. 204) compounds, which both adopt a double length unit cell in the Al–Pd–Fe system. The mechanism of distributing more Al atoms in the new phase was compared with that of the Al2.63Rh phase by analyzing their site symmetry and the corresponding site of occupancies (SOF). Furthermore, relations of the C′ phase to the reported Al69Pd17Fe14 (designated C1 phase) and Al39Pd21Fe2 (designated C2 phase) phases were investigated by analyzing their building units with the “nanocluster” method in the ToposPro package.

Chemical diversity of iron species and structure evolution during the oxidation of C14 Laves phase Zr(Fe,Nb)2 in subcritical environment

C14 Laves phase Zr(Fe,Nb)2 was fabricated and oxidized in a subcritical environment. Electron energy loss spectroscopy was used to study the migration and chemical state evolution of the elements. The oxidation process was accompanied by the formation of multilayered oxide scale, consisting of a crystalline oxide layer, amorphous oxide layer (Fe-poor) and Zr-Nb-Fe-O amorphous layer. The formation of closely bonded, semi-free and metallic iron species as well as the out diffusion of Fe contribute to the multilayered structure growth. The amorphization of the C14 phase occurred with the incorporation of oxygen, which preferentially bonded with Zr and Nb.

Microstructure and Mechanical Properties of (TiZrHf)50(NiCoCu)50 High Entropy Alloys

The microstructure and mechanical properties of pseudo-binary (TiZrHf)50(NiCoCu)50 high entropy alloys (HEAs) with B2 matrix were investigated in this study. The studied HEAs includes Ti40Zr10Ni40Co5Cu5, Ti30Zr20Ni30Co10Cu10, Ti25Zr25Ni16.67Co16.67Cu16.67, Ti30Zr10Hf10Ni30Co10Cu10, Ti20Zr15Hf15Ni20Co15Cu15, and Ti16.67Zr16.67Hf16.67Ni16.67Co16.67Cu16.67. XRD and SEM analyses revealed that all studied HEAs were a mixture of B2 and C15 intermetallic phases, instead of a single solid solution, due to their highly negative enthalpy of mixing and the relatively large difference in atomic radii. The B2 matrix was found capable of forming a solid solution of all six elements in a wide composition range. The lattice constants of B2 and C15 phases increase linearly with increasing the difference in atomic radii. Additionally, the microhardness of these HEAs increases from 409 to 595 HV with the increasing difference in atomic radii as well. Compression tests demonstrated substitution of Zr with Hf can improve the ductility. In all studied HEAs, the Ti30Zr10Hf10Ni30Co10Cu10 HEA achieves a great balance between yield strength, ultimate compressive strength, and fracture strain of 1574 MPa, 1933 MPa, and 6.5%, respectively.

Single-Atom-Thick Active Layers Realized in Nanolaminated Ti3(AlxCu1-x)C2 and Its Artificial Enzyme Behavior.

A Ti3(AlxCu1-x)C2 phase with Cu atoms with a degree of ordering in the A plane is synthesized through the A site replacement reaction in CuCl2 molten salt. The weakly bonded single-atom-thick Cu layers in a Ti3(AlxCu1-x)C2 MAX phase provide actives sites for catalysis chemistry. As-synthesized Ti3(AlxCu1-x)C2 presents unusual peroxidase-like catalytic activity similar to that of natural enzymes. A fabricated Ti3(AlxCu1-x)C2/chitosan/glassy carbon electrode biosensor prototype also exhibits a low detection limit in the electrochemical sensing of H2O2. These results have broad implications for property tailoring in a nanolaminated MAX phase by replacing the A site with late transition elements.

Phase Equilibria Among γ/TCP/GCP Phases in Nb-Poor Region of Ni-Nb-Co Ternary System at Elevated Temperatures

Phase equilibria among the γ-(Ni, Co), topologically close-packed and geometrically close-packed phases in the Ni-Nb-Co ternary system were investigated using x-ray diffraction, transmission electron microscopy, and electron probe microanalysis for equilibrated bulk alloys at 1100 and 1200 °C. A two-phase equilibrium between the γ (A1) and D0a phases (Ni3Nb) is confirmed to exist on the Ni-rich side with the Ni/(Ni + Co) ratio above ~ 0.8, which agrees well with literature data. The phases in equilibrium with the A1 phase change into a ternary-compound D019 phase ((Ni, Co)3Nb) and an mC18 phase (Co7Nb2) with increasing Co content. The C15 phase (Co2Nb) is not in equilibrium with the A1 phase. The three-phase tie-triangle of the A1/mC18/D019 moves toward the Co-rich side with decreasing temperature at an equivalent Nb concentration, while that of A1/D019/D0a almost remains at the chemical composition of Ni-20Co-20Nb in the temperature range investigated. Because the mC18 phase is in equilibrium with the A1 phase up to 1086 °C in the Co-Nb binary system, Ni addition should stabilize the mC18 phase and expand the mC18 phase region toward high temperatures.

Frank–Kasper Phases Identified in PDMS‐b‐PTFEA Copolymers with High Conformational Asymmetry

In the search for the formation of Frank-Kasper phases from diblock copolymer self-assembly, a series of compositionally asymmetric poly(dimethylsiloxane)-b-poly(2,2,2-triflouroethyl acrylate)s (PDMS-b-PTFEAs) are synthesized to produce PDMS-rich phases with PDMS volume fractions (fPDMS ) ranging from 0.746 to 0.869. As determined by small-angle X-ray scattering analysis, the Frank-Kasper σ and C14 phases are identified at fPDMS = 0.796 and 0.851, respectively, plausibly due to high conformational asymmetry (ε ≈ 2.20) between the two blocks. Intriguingly, the σ phase develops during heating from a short-range liquid-like packing (LLP) state, whereas the C14 phase is achieved at room temperature, which are both followed by a disordering at higher temperatures. Based on thermal experiments from a super cooled disordered state, the findings further provide compelling evidence of an LLP-hexagonally packed cylinder-σ transition and a direct pathway to the C14 phase during heating from an LLP state.

Studies of Zr-based C15 type metal hydride battery anode alloys prepared by rapid solidification

The objective of the present work was to study an interrelation between the conditions of the rapid solidification casting and electrochemical properties of a C15 Laves-type Zr-based metal hydride battery anode alloy. A 7-component overstoichiometric with B/A>2 alloy (Ni + Mn + V + Fe/Zr + Ti + La = 2.078) was Zr- and Ni-rich. Melt spinning was performed using as cast mother ingot at different rotation speeds of 5, 16.5, and 33 Hz with a 20-cm diameter Cu cooling wheel. The phase composition and morphology of the ribbons were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM) along with X-ray energy dispersive spectroscopy (EDS). Partial Mn vaporization took place during the melt spinning and became more pronounced with the increase in the quenching rate at higher wheel rotation speeds. The SEM study showed a successful refinement and a more homogeneous distribution of La in the melt-spun alloys. However, the alloys demonstrated a more difficult activation performance as compared to the as-cast sample which was associated with introduction of Zr into the composition of the secondary La-and Ni-rich phase when quenching the melt. Presence of Zr in the secondary phase results in inferior rate of its hydrogenation. The formed during the rapid solidification (La,Zr) (Ni,Mn) secondary phase also contained a significant amount of oxygen additionally worsening the activation performance. The grain size gradually decreased from 3.5 μm in the as-cast alloy to approximately 250 nm for the melt spun sample obtained at the highest cooling rate. The two-phase alloys contained a mixture of two Laves type intermetallics, C15 and C14 ones. The abundance of the secondary C14 phase decreased with increase of the cooling rate as compared to the as-cast sample. The electrochemical performance was significantly affected by the compositional and microstructural changes associated with the variation in the cooling rate. The highest electrochemical discharge capacity of 414 mAh/g was obtained for the 16.5 Hz melt spun rate because of an optimized composition of the Laves phase. Furthermore, the diffusion coefficient of hydrogen in this alloy was the highest in a full state of charge, which indicates that faster hydrogen diffusion and shorter hydrogen diffusion paths were achieved via an application of the rapid solidification process. An optimization of the rapid solidification conditions is required to improve an activation behavior of the studied Laves types metal hydride anode alloys.

Long-range spin chirality dimer order in the Heisenberg chain with modulated Dzyaloshinskii-Moriya interactions

The ground state phase diagram of a spin $S=1/2$ $XXZ$ Heisenberg chain with spatially modulated Dzyaloshinskii-Moriya (DM) interaction $ {\cal H}= \sum_n J\left[\left(S^x_n S^x_{n+1} +S^y_n S^y_{n+1}+\Delta S^z_n S^z_{n+1}\right)+(D_0+(-1)^n D_1)\left(S^x_n S^y_{n+1} -S^y_n S^{x}_{n+1} \right) \right] $ is studied using the continuum-limit bosonization approach and extensive density matrix renormalization group computations. It is shown that the effective continuum-limit bosonized theory of the model is given by the double frequency sine-Gordon model (DSG) where the frequences i.e. the scaling dimensions of the two competing cosine perturbation terms depend on the effective anisotropy parameter $\gamma^*=J\Delta /\sqrt{J^2+D_0^2+D_1^2}$. Exploring the ground state properties of the DSG model we have shown that the zero-temperature phase diagram contains the following four phases: (i) the ferromagnetic phase at $\gamma^*<-1$; (ii) the gapless Luttinger-liquid (LL) phase at $-1<\gamma^*< \gamma^*_{c1}=-1/\sqrt{2}$; (iii) the gapped composite (C1) phase characterized by coexistence of the long-range-ordered (LRO) dimerization pattern $\epsilon \sim (-1)^n (S_n S_{n+1})$ with the LRO alternating spin chirality pattern $\kappa \sim (-1)^{n}\left(S^{x}_{n}S^{y}_{n+1} -S^{y}_{n}S^{x}_{n+1} \right)$ at $\gamma^{\ast}_{c1}<\gamma^{\ast} <\gamma^{\ast}_{c2}$; and (iv) at $\gamma^{\ast} >\gamma^{\ast}_{c2}>1$ the gapped composite (C2) phase characterized in addition to the coexisting spin dimerization and alternating chirality patterns, by the presence of LRO antiferromagnetic order. The transition from the LL to the C1 phase at $\gamma^*_{c1}$ belongs to the Berezinskii-Kosterlitz-Thouless universality class, while the transition at $\gamma^*_{c2}$ from C1 to C2 phase is of the Ising type.

Shock induced phase transition in SiC polytypes

The effect of shock on 3C and 6H phases of silicon carbide was investigated within our earlier developed scaled binding energy based Debye–Gru¨neisen model. Required input parameters for the equation of state (EOS) model were obtained from density functional theory calculations using PBE-GGA functional as well as a nonlocal exchange-correlation functional B3LYP. In our earlier calculations using PBE-GGA functional, we predicted the phase transition from 3C to B1 phase at about 68 GPa, which deviated significantly from the experimental value of about 100 GPa. An empirical energy correction was applied in that case to obtain the experimental transition pressure. However, in the present work, we observed that if hybrid B3LYP functional is employed to calculate EOS parameters, shock induced phase transitions from 3C to B1 and 6H to B1 phases occur at about 102 GPa and 105 GPa, respectively, in close agreement with experiments without invoking any empirical correction. We obtained good agreement of our predictions with experimental shock data using a simple model of metastability and the hybrid B3LYP functional, which accounts for exchange-correlation effects better than the PBE-GGA functional in the case of SiC.

A new BaCa(CO3)2 polymorph.

A new polymorph of the double carbonate BaCa(CO3)2, `a C2 phase', has been synthesized. Its structure has been obtained by density-functional-theory-based (DFT-based) model calculations and has been refined by Rietveld analysis of X-ray powder diffraction data. The structure of the new polymorph differs significantly from those of the established polymorphs barytocalcite, paralstonite and alstonite. The unit-cell parameters of the new monoclinic (space group C2) compound are a = 6.6775 (5), b = 5.0982 (4), c = 4.1924 (3) Å, β = 109.259 (1)°. The new compound has been further characterized using Raman spectroscopy. This work shows that earlier studies have misidentified the products of an established synthesis route and that findings based on the incorrect identification of the synthesis product concerning the suitability of barytocalcite as a matrix for the retention of radioactive isotopes will need to be reconsidered.

Pressure dependence of the silicon carbide synthesis temperature

The starting temperature for SiC synthesis from elemental silicon, carbon black, and graphite powders was determined for pressures ranging from 0.8 to 11 GPa by in situ X-ray diffraction experiments. The synthesized SiC corresponds to the cubic 3C phase with the presence of stacking faults along the [111] direction. The lowest density of the stacking faults is achieved when black carbon is used instead of graphite. The minimum temperature to start the Si + C → SiC reaction slightly decreases when the pressure is increased up to 6 GPa and the reaction begins before silicon melts. For pressures higher than 8 GPa, the starting temperature increases, and the formation of SiC from the SiII phase requires the complete melting of silicon. Bulk modulus Bo = 236(14) GPa was obtained for the synthesized SiC phase.

Effects of lithium addition in AB2 metal hydride alloy by solid-state diffusion

In this study, microstructures and electrochemical properties of a Zr-based AB2-type metal hydride alloy (Ti12.1Zr21.3V10.5Cr7.5Mn8.1Co8.0Ni31.7Al0.8) doped with 6 wt% LiH by solid-state diffusion at different temperatures were investigated and compared with those of the base alloy sintered at the same conditions. Structural study by x-ray diffraction analysis exhibited a lattice expansion in the main C14 phase of the Li-doped alloys with a relatively low sintering temperature, indicating that Li incorporated into the C14 Laves phase but was evaporated at higher sintering temperatures. Addition of Li deteriorated both the electrochemical capacity and activation easiness; however, it improved the high-rate dischargeability (HRD). The benefit of Li addition on HRD can be associated with the increase in amount of metallic nickel clusters embedded in the surface oxide (estimated by the saturated magnetic susceptibility) and was a strong function of sintering temperature. As the sintering temperature increased, HRD increased due to the increase in amount of surface catalytic metallic nickel.

Capacity degradation of Laves phase-related body-centered-cubic solid solution metal hydride alloys in battery

The nickel/metal hydride (Ni/MH) is a good example of using metal hydride alloy which is cable of reversely storing the hydrogen energy in an electrochemical environment. The failure mode of two Laves phase-related body-centered-cubic metal hydride alloys doped with Y, and Ce separately in sealed Ni/MH batteries is studied by scanning electron microscopy with x-ray energy dispersive spectroscopy and x-ray diffraction analysis. These two alloys come from a series of studies targeting toward using Ni/MH battery as a low-cost and high-durability alternative to rival Li-ion battery in the electric vehicle application. This group of alloys demonstrates a discharge capacity up to 400 mAh g−1 under a C/4 discharge rate. The main failure mode of these two alloys is found to be selective dissolution of C14 phase which contributes to a fast increase in internal resistance. Due to the deprival of Cr from the first solidified BCC phase, the next solidified C14 phase does not have enough Cr to protect it from the KOH corrosion. Rapid quench fabrication method and addition of other corrosion-resistant non-BCC former, such as Cu, Pd, and Hf, in the alloy composition are suggested.

Effect of Hydrogen on the Structure, Magnetic and Magnetocaloric Properties of Laves Phase Type-Compounds RE(Fe&lt;sub&gt;0.25&lt;/sub&gt;Co&lt;sub&gt;0.75&lt;/sub&gt;)&lt;sub&gt;2&lt;/sub&gt;H&lt;sub&gt;y&lt;/sub&gt; with RE = Ho and Er, y = 0.0, 3.0, 3.5

The crystal structure and overall magnetic properties of RE(Fe0.25Co0.75)2Hywith RE = Ho and Er, were analyzed versus y, the hydrogen content. The single phase C15 type compounds, synthesized using cold crucible HF melting, were hydrogenated and controlled using a PCI apparatus. The impact of hydrogen insertion on the cell parameter, the Curie temperature TCand the magnetization saturation were determined. All compounds and hydrides were found to be ferrimagnets, the magnetic moment of RE and 3d elements being opposite since Ho and Er belong to the 2ndrow of rare earth elements. Depending on the formula, a typical compensation point was in evidence. The 2ndorder character of the ferri ↔ paramagnetic transition was established using the Arrott plot method. Magnetization isotherms vs. magnetic field gives quantified results for the magnetic entropy variation by application of the Maxwell relation. Control of y, the hydrogen content, shows it is interesting for improvement of the MCE of the starting compounds. However increasing y leads to decreased TCat the lowest temperatures, suggesting potential cryogenic uses.

Precipitate transformation of τ-AlMn phases to Al8Mn4Ca during solution heat treatment of CaO-added AZ31 alloys

The precipitate transformation mechanism of Al8Mn4Ca and its effect on mechanical properties of CaO-added AZ31 were investigated. Microstructural characterization showed that precipitate transformation occurred by dissolution of the C15 (Al2Ca) phase during solution heat treatment. Then, dissolved Al and Ca solutes have formed segregation layer near metastable τ (Al11Mn14 and Al8Mn11) which leads phase transformation. Depending on different diffusion kinetics of Al and Ca solutes, formation of Al8Mn4Ca or γ2 (Al8Mn5) was determined by distribution of C15 near τ. To this end, complex layered particle structures were frequently found. The precipitate transformation affected mechanical properties of the alloy. Due to fragment behavior during phase transformation and irreversibility of Al8Mn4Ca, duration of solution heat treatment affects hardening behavior. Precise control over the solution heat treatment process is necessary to obtain optimal properties in Ca-bearing Mg-Al-Mn alloys because of the unique conditions required for the formation of Al8Mn4Ca.

Semi-hard magnetic nanocomposites based on out-of-equilibrium Fe2+δNb and Fe2+δTa Laves phases

Melt-spun Fe100-xNbx and Fe100-xTax alloys consisting of a hexagonal (C14) Laves phase and an iron-based bcc phase were found to exhibit room-temperature coercivities up to 0.45 and 0.48 kOe, respectively. The non-equilibrium C14 structures in the melt-spun alloys are characterized by a smaller unit cell volume, a higher Curie temperature and, presumably, a greater concentration of Fe compared to the C14 structures in the alloys annealed at 1533 K. The room-temperature coercivity, which correlates with the C14 lattice contraction, is believed to be caused by the magnetocrystalline anisotropy of the non-equilibrium Laves phase with the latter being magnetically coupled with the Fe phase via an intergranular exchange interaction. On the other hand, a coercivity of around 0.3 kOe persists in the melt-spun alloys above the Curie temperature of the C14 phase. This high-temperature coercivity may originate from the shape or strain anisotropy of the Fe phase particles. The energy density of the reported two-phase alloys is not sufficiently high to consider them potential hard magnetic materials. However, the pure non-equilibrium Fe2+δNb and Fe2+δTa compounds may be of interest if they can be isolated and textured.