Bct Phase Research Articles

Optimization of V-Ti-Fe hydrogen storage alloy based on orthogonal experiments

Vanadium-based solid solution hydrogen storage alloys have attracted much attention because of their high hydrogen capacity, mild hydrogen absorption- desorption conditions, and high safety. However, the V-Ti-Fe alloys, developed at low cost, have a relatively low effective capacity and is unsuitable for practical production. In this work, (VxFe100-x)100-yTiy-z%Ce (x=0.83,0.85,0.87; y=16,18,20; z=1,2,3) alloys are designed by orthogonal method to investigate the effect of compositions on the microstructure and hydrogen storage properties. It is shown that the effective hydrogen capacity and desorption plateau pressure of the (VxFe100-x)100-yTiy-z%Ce alloy are related to multiple factors, such as lattice constants, tetrahedral interstitials of the BCC and BCT phases, electron concentration and Vickers hardness. The optimized alloy (V87Fe13)82Ti18-1 %Ce based on the orthogonal experiments shows a reversible hydrogen storage capacity of 2.17 wt% and a desorption plateau pressure of 0.373 MPa at 70°C. The reversible hydrogen capacity of (V87Fe13)82Ti18-1 %Ce is superior to most V-Ti-Fe alloys.

Microstructure and First Hydrogenation Properties of Ti16V60Cr24−xFex + 4 wt.% Zr Alloy for x = 0, 4, 8, 12, 16, 20, 24

In body-centered cubic (BCC) alloys of transition elements, elemental addition or substitution in the vanadium-based alloys can be beneficial for improving the hydrogen storage properties and for reducing the production cost. In this context, the current study focused on the effect of the substitution of Cr by Fe in Ti16V60Cr24−xFex + 4 wt.% Zr alloys where x = 0, 4, 8, 12, 16, 20, 24. The microstructure of each alloy was composed of a matrix having a chemical composition close to the nominal one and a Zr-rich region. From X-ray diffraction patterns, it was found that the matrix has a BCC structure, and the Zr-rich regions present the C14 Laves phase structure. The lattice parameter of BCC phases decreased linearly with x, in accordance with Vegard’s law. The measurement of the first hydrogenation at 298 K under 3 MPa of hydrogen revealed a decrease in the maximum hydrogen capacity: 3.8 wt.% for x = 0, 3.1 wt.% for x = 4 and around 2 wt.% for x = 8 to 24. The XRD patterns after hydrogenation showed a BCT phase for all alloys, with a C14 phase for x = 4, 8, 12 and with C14 and C15 for x = 16, 20 and 24.

Nanoscale microstructures and hydrogenation properties of an as-cast vanadium-based medium-entropy alloy

Vanadium-based hydrogen storage alloys have been widely investigated; however, alloys in the cast state are typically coarse-grained. In this study, an as-cast V45Fe15Ti20Cr20 medium-entropy alloy was prepared by arc melting, and microstructural analysis revealed that the alloy was composed of nanocrystals. The initial pretreatment temperature of the alloy was approximately 100 K lower than that of the as-cast coarse-grained alloy. At room temperature, the time required for the alloy to reach 90% saturation was only 140 s, indicating excellent hydrogen absorption kinetics. The alloy is fully activated after two hydrogen absorption/desorption cycles. The phase transformation of the alloy in the early hydrogenation stage was investigated using X-ray diffraction, and the results showed that the BCC phase was completely transformed into the BCT phase when hydrogen uptake was performed for 6 s. Furthermore, the apparent activation energy of dehydrogenation in the present alloy calculated using the Kissinger method was 69.8 ± 0.8 kJ/mol. The pressure-composition-isotherms tests showed that the hydrogen absorption capacity of the alloy at 295 K was 2.12 wt%. The hydrogenation/dehydrogenation enthalpy change of the alloy was calculated by the Van't Hoff equation, which was 30.90 ± 1.47 and 33.95 ± 0.41 kJ/mol, respectively. The present work demonstrates that nanostructured vanadium-based hydrogen storage alloys can be fabricated using traditional casting techniques. Our study also enriches the understanding of the microstructures of medium-entropy alloys, which may provide positive guidance for the design of novel vanadium-based hydrogen storage alloys.

Microstructure and mechanical property of Al56-xCo24Cr20Nix eutectic high-entropy alloys with an ordered FCC/BCT phase structure

A Fe-free Al56-xCo24Cr20Nix (x = 34, 36, 38, 40, and42, denoted as Nix alloys hereafter) eutectic high-entropy alloys were designed and prepared to investigate the effect of Al and Ni content on the microstructure and mechanical property. It was found that all five alloys exhibited a typical eutectic microstructure and an ordered FCC/BCT phase structure, and the volume fraction of BCT phase reduced with an increase in Ni content. The FCC and BCT phases were enriched in Cr-Co and Al-Ni elements respectively, and the nano-scale precipitates forming in BCT phase also exhibited a BCT phase structure but enriched in Cr element. The Ni38 alloy displayed an entire eutectic microstructure with two types of eutectic lamellas, the long and straight one, and the short and curve one. The irregular lamellas resulted in an unsatisfactory tensile property in Ni38 alloy with a yield strength of 543 MPa and a fracture strain of 8.1 %. To optimize the strength-plasticity configuration, the Ni content was further fine-turned. The Ni38.5 alloy, exhibiting a regularly and fine lamellar hypereutectic microstructure with bits of primary FCC phase, was found presenting an improved fracture strain of 14.5 % and without any reduction in yield strength compared with Ni38 alloy.

Phase evolution process and hydrogen storage performances of V72Ti18Cr10 alloy prepared by Co-precipitation-reduction method

The V72Ti18Cr10 alloy was prepared by a co-precipitation-reduction method in order to consume less energy during whole-life alloy manufacturing, and the phase evolution process in hydrogenation/dehydrogenation process was investigated. The structure refinement analysis of the alloy contented with 1 ​wt% hydrogen after hydrogenation shows that BCC phase, BCC-hydride phase and FCC phase coexisted, but little BCT phase was found. It indicates that the phase evolution process during hydrogenation was BCC→ BCC-hydride→ FCC, nevertheless the formation of BCT phase was restrained. The limited particle sizes of the alloy in the range of 0.1–5 ​μm and fewer defects than the normal alloy ingot contributed to the suppression of BCT formation. The annealed alloy, which had similar particle sizes with the unannealed alloy, has less unevenness of the compositions than the unannealed alloy, and the annealed alloy showed flatter plateau in the hydrogenation PCT (pressure-composition-temperature) curves. However, the BCT phase in the annealed alloy appeared in its dehydrogenation process owing to the produced defects during the following dehydrogenation and the hydrogenation process. The alloy with the limited particle sizes even in the range from 0.1 ​μm to 5 ​μm could not prevent the generation of BCT phase in the dehydrogenation process.

Phase transitions and properties of lanthanum under high pressures

Lanthanum (La), the first member of the lanthanide elements, recently aroused interests due to the discovery of high-T c superconductor LaH10 under high pressures and its unique superconducting properties. Here, we study the phase transitions, superconductivity and mechanical properties of metallic La under high pressures by first-principle calculations. The known face-centered cubic (fcc) phase with space group still exists above 100 GPa. And it transforms into an unprecedented body-centered tetragonal (bct) phase with space group I4/mmm above 180 GPa, which expands the high pressure phase transition sequence. Further calculations show that the superconducting transition temperature T c of fcc phase decreases with increasing pressure with the rate of −0.13 K GPa−1, in good agreement with the experimental results. For the bct phase, the estimated superconducting transition temperature is very low with T c of 0.7 K at 200 GPa. The calculations of mechanical properties show that both of fcc and bct phases are compressible and brittle.

Dynamic mechanical behaviors of metallic glass-shape memory alloy bilayered nanocomposite under shock wave compression

It has been extensively reported that metallic glass-shape memory alloy (MG-SMA) composites possess superior ductility and strength. Here, shock responses of bilayered MG-SMA (Cu50Zr50-B2 CuZr) nanocomposites are investigated by atomistic simulations. The results show that the MG/SMA interface can reflect compression/rarefaction waves, depending on the shock direction. Higher shear stress of the SMA means it has stronger shear resistance than the MG. At the velocity of 0.8 km/s, a B2 to BCT phase transformation is shown in the SMA regions, which is more obvious in the monocrystalline than that in the nanocrystalline regions because the grain boundary compression and grain rotation assimilate substantial plastic strain. At the velocity of 1.6 km/s, a two-stage phase transformation of B2 to BCT to amorphous structure is captured in the monocrystalline SMA. But for the nanocrystalline SMAs, the B2 phase is rapidly amorphized due to the promoting effects of the grain boundary.

Superior energy storage performance coupled with excellent electrical characteristics in lead-free Ba0.8Ca0.2TiO3-(Bi0.80Mg0.20) (Ti0.65Mg0.30)O3 ceramics

Multi-layer ceramic capacitors with high recoverable energy storage density and large energy efficiency play a crucial in modern electronic devices. Various ferroelectric ceramic materials with relaxor characteristics and large dielectric constant are the most favorable candidates for achieving superior energy storage performance. Herein, a novel lead-free (1−x)Ba0.80Ca0.20TiO3-x (Bi0.80Mg0.20)(Ti0.65Mg0.30)O3 (BCT-BMT) ceramic materials were prepared by two-stage mechano-chemical activation technique. BMT substitution in Ca modified BaTiO3 has resulted in substantial improvement in microstructural characteristics and subsequently an improvement in energy storage performance in comparison to pure BCT. The introduction of BMT in BCT has resulted in transformation into relaxor ceramics, which is vital for obtaining high recoverable energy storage density and large energy efficiency. In the present investigation, the composition with 0.65BCT-0.35BMT shows a typical relaxor characteristic, large breakdown strength, and a substantially large dielectric constant. As a result of improvement in electrical properties, the composition 0.65BCT-0.35BMT possesses an ultra-high recoverable energy density of 3.09 J cm−3 along with a moderately high energy efficiency of ∼80.15%. The obtained results indicate the effectiveness of BMT substitution on the energy storage properties and excellent compatibility between BCT and BMT phases.

Electrospinning of multiferroic CoFe2O4@ Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 nano-structured fibers via two different routes

Multiferroic composite materials have attracted a lot of attention for their excellent ferroelectric and ferromagnetic properties. For the first time one-dimensional core-shell structure of CoFe2O4/Ba(Zr0.2Ti0.8)O3–0.5(Ba0.7Ca0.3)TiO3 (CFO@BZCT) multiferroic composite was synthesized by electrospinning. The effect of electrospinning routes on the composite fibers properties was investigated. To do so, two different routes were utilized: the electrospinning of core and shell solutions using coaxial nozzle and the electrospinning of BZCT solution containing CFO fibers (with three different concentrations of 2, 4 and 8 g/100 ml solution). The fiber morphology was maintained for all samples in calcination temperatures up to 900 °C. Spinel cobalt ferrite and perovskite BZT-0.5BCT phases were crystallized for calcined composites. The nano and core-shell structure of composites were confirmed by the TEM analysis. Magnetoelectric coupling of CFO and BZCT phases was investigated by the temperature-dependent SQUID measurement, where a ferroelectric phase transformation at 100 °C induced magnetization changes. Low saturation magnetization and dielectric constant of 6.7 emu/g and 300 ± 10 were obtained for the sample synthesized by the first method while these values rose to 36.8 emu/g and 400 ± 30 respectively for the second method. A comparison of the two methods exhibited a larger fiber diameter, porous fibers and lower ferroic properties for the sample synthesized by the first method.

Effect of particle size, pressure and temperature on the activation process of hydrogen absorption in TiVZrHfNb high entropy alloy

The effect of particle size, pressure and temperature on the first hydrogenation of TiVZrHfNb high entropy alloy were investigated. Three particle size ranges were selected: less than 0.5 mm, between 0.5 mm and 1 mm and between 1 mm and 2 mm. The hydrogen pressure applied during the first hydrogenation was 1 bar, 2 and 20 bars. The temperature of first hydrogenation was varied from 100 °C to 400 °C. It was found that the particle size plays a role in the activation; smaller particle size having faster kinetics. Under 2 bars and 20 bars of hydrogen pressure, the activation curves have the same features. The activation is strongly dependent on the temperature: higher temperature drastically reduces the incubation period. The important parameter for the activation is the temperature rather than the particle size. It was also found that the activation process obeys an Arrhenius relationship. Upon hydrogenation, the BCC phase is transformed to a BCT phase. Also, an amorphous phase seems to be formed. This formation depends on the hydrogenation temperature and particle size.

Structure and Phase Transition Features of Monoclinic and Tetragonal Phases in U–Mo Alloys

Using molecular dynamics simulations, we studied the structural properties of orthorhombic, monoclinic, and body-centered tetragonal (bct) phases of U–Mo alloys. A sequence of shear transformations between metastable phases takes place upon doping of uranium with molybdenum from pure α -U: orthorhombic α ′ → monoclinic α ″ → bct γ 0 → body-centered cubic (bcc) with doubled lattice constant γ s → bcc γ . The effects of alloy content on the structure of these phases have been investigated. It has been shown that increase in molybdenum concentration leads to an increase in the monoclinic angle and is more similar to the γ 0 -phase. In turn, tetragonal distortion of the γ 0 -phase lattice with displacement of a central atom in the basic cell along the <001> direction makes it more like the α ″ -phase. Both of these effects reduce the necessary shift in atomic positions for the α ″ → γ 0 -phase transition.

Coexistence of hcp and bct Phases during In Situ Superlattice Assembly from Faceted Colloidal Nanocrystals.

We study the in situ self-assembly of faceted PbS nanocrystals from colloidal suspensions upon controlled solvent evaporation using time-resolved small-angle X-ray scattering and X-ray cross-correlation analysis. In our bulk-sensitive experiment in transmission geometry, the superlattice crystallization is observed in real time, revealing a hexagonal closed-packed (hcp) structure followed by formation of a body-centered cubic (bcc) superlattice. The bcc superlattice undergoes continuous tetragonal distortion in the solvated state shortly after its formation, resulting in the body-centered tetragonal (bct) structure. Upon solvent evaporation, the bct superstructure becomes more pronounced with the still coexisting hcp phase. These findings corroborate the existing simulations of assembling cuboctahedral-shaped particles and illustrate that we observed the predicted equilibrium states. This work is essential for a deeper understanding of the fundamental forces that direct nanocrystal assembly including nanocrystal shape and ligand coverage.

Investigation of the microstructure, crystal structure and hydrogenation kinetics of Ti-V-Cr alloy with Zr addition

The effect of 4 wt% Zr as additive in TixV70-xCr30 (x = 10 to 50) alloy has been investigated. After casting, all of these alloys were multiphase but with different microstructure and phase composition. From X-ray diffraction, bcc structure of all as-cast alloys was confirmed. For alloys x = 30 and 40, a Ti and a Ti-Zr high pressure-high temperature phase were observed along with the bcc phase. Upon hydrogenation, bcc phase transformed to a body centred tetragonal and a face centred cubic phase. Only alloy x = 40, has shown a complete hydrogenation. For x = 20 and 30, a bct phase was seen along with the bcc and fcc phase. The first hydrogenation kinetics of Zr added alloys were found to be faster than without additive but not as fast as with 7Zr + 10 Ni additive. It was also found that the volume taken by the hydrogen atom in the fcc phase decreases with increasing value of the bcc lattice parameter.

A Metastable Crystalline Phase in Two-Dimensional Metallic Oxide Nanoplates.

A simple method was adopted in which ultrathin cerium oxide nanoplates (<1.4 nm) were synthesized to increase the surface atomic content, allowing transformation from a face-centered cubic (fcc) phase to a body-centered tetragonal (bct) phase. Three types of cerium oxide nanoparticles of different thicknesses (1.2 nm ultrathin nanoplates, 2.2 nm nanoplates, and 5.4 nm nanocubes) were examined using transmission electron microscopy and X-ray diffraction. The metastable bct phase was observed only in ultrathin nanoplates. Thermodynamic energy analysis confirmed that the surface energy of the ultrathin nanoplates is the cause of the remarkable stabilization of the metastable bct phase. The mechanism of surface energy regulation can be expanded to other metallic oxides, thus providing a new means for manipulating and stabilizing novel materials under ambient conditions that otherwise would not be recovered.

Structural, microstructural and hydrogenation characteristics of Ti-V-Cr alloy with Zr-Ni addition

In this paper we report the effect of Ti-proportion and addition of Zr-Ni on the crystal structure, morphology and hydrogen storage kinetics of Ti-V-Cr alloy. For this study, five alloys of compositions TixV70-xCr30 (x = 10, 20, 30, 40, 50) added with 4 wt% of 7Zr+10Ni were synthesized. Scanning electron microscopy revealed that all alloys were multiphase. X-ray powder diffraction confirmed that all the as-cast alloys have the body-centred cubic (bcc) phase as the main phase. The same amount of (7Zr+10Ni) was added in all of the alloys, but each alloy has shown a different elemental concentration in their secondary phases. For alloys x = 10 and 20, a Zr rich secondary phase was formed but for alloys x = 30, 40 and 50, the secondary phase has relatively high concentration of all elements.Upon hydrogenation, the bcc phase transformed into a face-centred cubic (fcc) phase. For x = 20, 30 and 40 a body-centred tetragonal (bct) phase was also observed along with fcc phase. Hydrogenation kinetics of all of these alloys was measured and it was found that absorption capacity increased with Ti-content. The maximum absorption capacity of 3.6 wt% was achieved for x = 50.

Stress-controlled carbon diffusion channeling in bct-iron: A mean-field theory

Diffusion of interstitial carbon atoms in iron is the rate-limiting phenomenon of a number of phase transitions in body-centered (bcc) and body-centered tetragonal (bct) phases such as ferrite and martensite. These phases being rarely stress-free and undeformed, the influence of stress/strain on the diffusivity of carbon is essential, although scarcely documented. We developed a model of carbon elastodiffusion in bct-iron. We combined anisotropic linear elasticity theory of point defects, the dilute approximation of regular solutions and the multisite model of random walk into a coherent mean-field theory. The model allows predicting the effects of composition, temperature and mechanical loading on the anisotropy of carbon diffusion. Density functional theory calculations have provided most of the materials parameters. The predictions were successfully tested against kinetic Monte Carlo simulations. Our results show that compression of the crystal increases carbon diffusivity, while tension has the opposite effect. Axial straining is accompanied by a large anisotropy of diffusion. This effect could be exploited to produce stress-controlled diffusion channeling for the engineering of anisotropic microstructures during thermal ageing of martensitic Fe-C alloys.

Effect of solutes on the lattice parameters and elastic stiffness coefficients of body-centered tetragonal Fe

We compute changes in the lattice parameters and elastic stiffness coefficients Cij of body-centered tetragonal (bct) Fe due to Al, B, C, Cu, Mn, Si, and N solutes. Solute strain misfit tensors determine changes in the lattice parameters as well as strain contributions to the changes in the Cij. We also compute chemical contributions to the changes in the Cij, and show that the sum of the strain and chemical contributions agree with more computationally expensive direct calculations that simultaneously incorporate both contributions. Octahedral interstitial solutes, with C being the most important addition in steels, must be present to stabilize the bct phase over the body-centered cubic phase. We therefore compute the effects of interactions between interstitial C solutes and substitutional solutes on the bct lattice parameters and Cij for all possible solute configurations in the dilute limit, and thermally average the results to obtain effective changes in properties due to each solute. The computed data can be used to estimate solute-induced changes in mechanical properties such as strength and ductility, and can be directly incorporated into mesoscale simulations of multiphase steels to model solute effects on the bct martensite phase.

Thermodynamic optimizations of the Nd-Sn and Sn-Tb systems

The thermodynamic optimizations of the Nd-Sn and Sn-Tb binary systems were carried out by means of the Calculation of Phase Diagram (CALPHAD) method on the basis of the available experimental data including the thermodynamic properties and phase equilibria. The Gibbs free energies of the liquid, bcc, bct, dhcp and hcp phases were described by the substitutional solution model with the Redlich-Kister equation, while all of the intermetallic compounds (Nd5Sn3, Nd5Sn4, Nd11Sn10, NdSn, Nd3Sn5, NdSn2, Nd3Sn7, Nd2Sn5, NdSn3, Sn3Tb, βSn7Tb3, αSn7Tb3, Sn2Tb, Sn5Tb4, SnTb4, Sn10Tb11, Sn4Tb5 and Sn3Tb5) were described by the sublattice model. A set of self-consistent thermodynamic parameters of each phase in the Nd-Sn and Sn-Tb binary systems has been obtained, and the calculated results are in good agreement with the available experimental data.

Composition-dependent properties and phase stability of Fe-Pd ferromagnetic shape memory alloys: A first-principles study

The composition-dependent properties and their correlation with the phase stability of Fe75+xPd25−x (−10.0≤x≤10.0) alloys are systematically investigated by using first-principles exact muffin-tin orbitals (EMTO)-coherent potential approximation (CPA) calculations. It is shown that the martensitic transformation (MT) from L12 to body-centered-tetragonal (bct) occurs in the ordered alloys with about −5.0≤x≤10.0. In both the L12 and bct phases, the evaluated a and c/a agree well with the available experimental data; the average magnetic moment per atom increases whereas the local magnetic moments of Fe atoms, dependent on both their positions and the structure of the alloy, decrease with increasing x. The tetragonal shear elastic constant of the L12 phase (C′) decreases whereas that of the bct phase (Cs) increases with x. The tetragonality of the martensite (|1−c/a|) increases whereas its energy relative to the austenite with a negative value decreases with Fe addition. All these effects account for the increase of MT temperature (TM) with x. The MT from L12 to bct is finally confirmed originating from the splitting of Fe 3d Eg and T2g bands upon tetragonal distortion due to the Jahn-Teller effect.

Dynamic electronic correlation effects in NbO2 as compared to VO2

In this work we present a comparative investigation of the electronic structures of NbO$_2$ and VO$_2$ obtained within the combination of density functional theory and cluster-dynamical mean field theory calculations. We investigate the role of dynamic electronic correlations on the electronic structure of the metallic and insulating phases of NbO$_2$ and VO$_2$, with focus on the mechanism responsible for the gap opening in the insulating phases. For the rutile metallic phases of both oxides, we obtain that electronic correlations lead to strong renormalization of the $t_{2g}$ subbands, as well as the emergence of incoherent Hubbard subbands, signaling that electronic correlations are also important in the metallic phase of NbO$_2$. Interestingly, we find that nonlocal dynamic correlations do play a role in the gap formation of the (bct) insulating phase of NbO$_2$, by a similar physical mechanism as that recently proposed by us in the case of the (M$_1$) dimerized phase of VO$_2$ (\textit{Phys. Rev. Lett. 117, 056402 (2016)}). Although the effect of nonlocal dynamic correlations in the gap opening of bct phase is less important than in the (M$_1$ and M$_2$) monoclinic phases of VO$_2$, their presence indicates that the former is not a purely Peierls-type insulator, as it was recently proposed.