B2 Crystal Structure Research Articles

Investigating the impact of atomic positional variations on crystal magnetism: Insights from B2 crystal structure

Grounded in research on B2 crystal structures, this study systematically substitutes the central 8 atoms with non-metallic Si atoms and metallic Fe atoms, exploring the resulting impacts on stress, magnetism, and energy across the entire crystal lattice. The positional permutations of these atoms are meticulously examined to elucidate their influence. Additionally, alterations in crystal parameters are analyzed through the lenses of energy band distribution and electronic density of states. Findings underscore that substituting non-metallic Si atoms with metallic Fe counterparts significantly increases stress, magnetism, and energy levels throughout the crystal lattice. Moreover, a positive correlation emerges between the quantity of replacements and the resulting stress levels, magnetic intensity, and energy. The arrangement of atoms demonstrates a substantial influence on material magnetism, with denser distributions yielding more pronounced effects on the overall system; individual arrangements are capable of increasing magnetism by approximately 23%. These insights are further validated by changes in energy band distribution and electronic state density, which show a continuous enhancement of metallic properties and reactivity as Si atoms are successively replaced by Fe atoms. This study introduces an innovative idea for enhancing material performance.

The effect of grain boundaries and precipitates on the mechanical behavior of the refractory compositionally complex alloy NbMoCrTiAl

Refractory compositionally complex alloys (CCAs) have been found to exhibit promising high-temperature properties. However, the brittleness of most CCAs at room temperature limits their application. To understand the influence of microstructure on the deformation behavior of these alloys, we conducted micromechanical experiments, including nanoindentation and micropillar compression tests, on a representative NbMoCrTiAl alloy at room temperature. The indentation at the grain boundaries showed increased shear stress and hardness compared to the matrix (ordered B2 crystal structure) due to the presence of local intermetallic precipitates (C14 and A15 phases). Although the NbMoCrTiAl alloy exhibits no ductility at the millimeter scale as shown in previous studies, the micropillars at the grain boundaries, which were decorated with precipitates, demonstrated higher yield strength and significant plastic strain >30% at room temperature. Numerous slip lines were observed, while cracks and fracture occurred at higher levels of deformation. The grain boundaries with precipitates increased the probability of crack initiation and propagation, but they did not necessarily lead to catastrophic failure of the pillars.

Design crystallographic ordering in NbTa0.5TiAlx refractory high entropy alloys with strength-plasticity synergy

To make up for the poor strength of high plasticity NbTa0·5Ti refractory medium entropy alloys (MEAs), light metal Al was introduced as alloying element. In this work, the NbTa0·5Ti-Alx (x = 0, 0.2, 0.4, 0.6, 0.8, 1.0) series non-isoatomic refractory high entropy alloys (RHEAs) were prepared by arc melting, and phase equilibrium was predicted by CALPHAD. The effects of Al content and annealing temperature on microstructure and phase evolution, and its mechanical properties were studied. The NbTa0·5Ti-Alx alloys changed from single phase BCC to two-phase A2+B2 crystal structure after adding Al. The hardness and strength of the as-cast alloys are increased by the solution strengthening and precipitation strengthening effect, but the brittleness is increased. The precipitation of Laves of plate-like NbAlTi2 and finer-scale A15 of (needle, particle)-like AlTi3 precipitates at and near grain boundaries after annealing. Higher annealing temperature is beneficial to eliminate dendrites formed by element segregation in the arc melting cooling process and promote grain growth (up to ∼200 μm). This work designed a new alloys with excellent compression plasticity and enriched the field of composition design and aging treatment of the Al-containing second generation RHEAs， so that their microstructure can be better controlled to achieve a balance of strength and plasticity.

Exploring the Magnetic Behavior of a Magnetic High-Entropy Alloy with Dual-Phase B20 Crystal Structure

Exploring the Magnetic Behavior of a Magnetic High-Entropy Alloy with Dual-Phase B20 Crystal Structure

Microstructure characterization and mechanical properties of AB-typed high-entropy intermetallics with high strength and thermal stability

In this work, we developed a new system of AB-typed high-entropy intermetallics (HEIs) based on CoHf-like B2 crystal structure. The microstructure and mechanical properties of the targeted CoxCuxTiZrHf alloys were detailedly investigated. With the increase of CoCu content, the alloys’ structure changes from multiphase to single phase. In particular, the multi-principal Co1.7Cu1.7TiZrHf alloy with single B2 crystal structure shows ultrahigh compressive yield strength of 1.77 Gpa and fracture strength of 2.15 Gpa at room temperature, as well as high elastic strain limit of approximately 1.5%. Moreover, it can maintain single-phase B2 structure and high strength after annealing at 900 °C.

Critical Evaluation and Thermodynamic Optimization of the Cu-Zn, Cu-Se and Zn-Se Binary Systems

In our study, a complete review of the literature, critical evaluation and thermodynamic assessment of the Cu-Zn, Cu-Se and Zn-Se binary systems were carried out. The modified quasi-chemical model (MQM) was applied to describe the Gibbs energy of the liquid phase. The Gibbs energies of all intermetallic compounds and terminal solid solutions were described using the compound energy formalism (CEF) model. The re-optimization of the Cu-Zn binary system was carried out by considering the ordered bcc_B2 crystal structure of the β’ phase. Moreover, the β and δ phases in the Cu-Zn binary system with the same bcc_A2 crystal structure were modeled as one single phase in the present work. A self-consistent thermodynamic database was constructed for the Cu-Zn, Cu-Se and Zn-Se binary systems, work that formed part of a comprehensive thermodynamic database development project researching zinc-based biodegradable materials.

Tuning skyrmions in B20 compounds by 4d and 5d doping

Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the $3d$ sublattice of the B20 structure by substitution of heavier $4d$ and $5d$ elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for $5d$-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal the spin-spiral nature of the magnetic ground state and field-induced skyrmions for all these systems. Especially small skyrmions $\ensuremath{\sim}50\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ are predicted for ${\mathrm{Co}}_{0.75}{\mathrm{Os}}_{0.25}\mathrm{Si}$, compared to $\ensuremath{\sim}148\phantom{\rule{0.16em}{0ex}}\mathrm{nm}$ for ${\mathrm{Fe}}_{0.75}{\mathrm{Co}}_{0.25}\mathrm{Si}$. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.

Plasma diagnostics and film growth of multicomponent nitride thin films with magnetic-field-assisted-dc magnetron sputtering

During direct current magnetron sputtering (dcMS) of thin films, the ion energy and flux are complex parameters that influence thin film growth and can be exploited to tailor their properties. The ion energy is generally controlled by the bias voltage applied at the substrate. The ion flux density however is controlled by more complex mechanisms.In this study, we look into magnetic-field-assisted dcMs, where a magnetic field applied in the deposition chamber by use of a solenoid coil at the substrate position, influences the energetic bombardment by Ar ions during deposition. Using this technique, CrFeCoNi multicomponent nitride thin films were grown on Si(100) substrates by varying the bias voltage and magnetic field systematically. Plasma diagnostics were performed by a Langmuir wire probe and a flat probe. On interpreting the data from the current-voltage curves it was confirmed that the ion flux at the substrate increased with increasing coil magnetic field with ion energies corresponding to the applied bias. The increased ion flux assisted by the magnetic field produced by the solenoid coil aids in the stabilization of NaCl B1 crystal structure without introducing Ar ion implantation.

High-entropy intermetallics with striking high strength and thermal stability

In this work, we report on a serious of high-entropy intermetallics (HEIs) based on CoHf-like B2 crystal structure. The microstructure evolution and mechanical properties of the targeted TiZrHfCoxNixCux alloys were detailedly investigated. In particular, the equal atomic ratio TiZrHfCoNiCu intermetallic with single-phase B2 structure shows ultrahigh compressive yield strength of 1.58 GPa and fracture strength of 2.48 GPa. It is striking that TiZrHfCoNiCu intermetallic exhibits a certain toughness and very high elastic strain limit of approximately 1.5%. Furthermore, it can maintain single-phase B2 structure and high strength after annealing at 900 °C, indicating its exceptional elevated temperature phase stability. This work not only broadens the family of high-entropy materials, but also provides a new perspective for the design of superalloys based on multi-principal element intermetallics.

Effects of precipitate on the phase transformation of single-crystal NiTi alloy under thermal and mechanical loads: A molecular dynamics study

Molecular dynamics simulations have been carried out in this study to investigate the stress-induced and temperature-induced phase transformations in NiTi shape memory alloy in the presence of Ni4Ti3 precipitate. NiTi alloys form the most popular class of shape memory alloys that have many applications in various industries. Therefore, computational simulation of the material behavior can extend our understanding of the material properties and help the development of alloys with tailored properties. In the simulations, an Ni4Ti3 precipitate (with a rhombohedral crystal structure) was generated and embedded into the NiTi matrix (having a B2 crystal structure). The transformation temperatures were computed by applying a cooling-heating cycle starting from a high temperature. At the austenitic state (at a temperature above Af), a monotonically increasing uniaxial compressive force was applied to the models with and without precipitate and, as expected, both models exhibited superelastic behavior. Furthermore, similar models were made for different directions of the compressive force relative to the crystallographic directions to study the direction dependence of the mechanical response. For all the loading directions that phase transformation happened, a larger hysteresis occurred for the precipitated model compared to the pristine NiTi due to the hindrance of reverse phase transformation in presence of the precipitate. The results show that the precipitate has a pronounced effect on the superelastic behavior in 〈100〉 loading directions.

Grain boundary segregation induced precipitation in a non equiatomic nanocrystalline CoCuFeMnNi compositionally complex alloy

Compositionally complex alloys (CCAs) in a nanocrystalline state often involve complex and poorly understood phase transformations which can consequently result in grain growth even at low temperatures. A detailed study of the microstructure and phase stability in CCAs is challenging due to the presence of multiple principal components. In view of these challenges the objective of the present study is to establish a systematic understanding of the phase evolution in a face centered cubic non equiatomic nanocrystalline CCA (CoCuFeMnNi). To accomplish this objective, we employed in-situ transmission electron microscope heating in combination with automated crystal orientation mapping (ACOM) and energy filtered transmission electron microscopy (EFTEM) to elucidate the sequence of phase decomposition of the high-pressure torsion (HPT) processed CoCuFeMnNi. Our analysis reveals a complex succession of grain boundary segregation and depletion steps leading to the formation of a FeCo-rich secondary phase. Our results show that prior to the formation of the secondary phase, Cu, Ni and Co segregate and Fe and Mn deplete at the grain boundaries. After the FeCo precipitation is triggered, Mn segregates to the grain boundaries along with Ni and Cu, whereas Fe and Co are depleted. The FeCo precipitates have a B2 crystal structure and typically exhibit a Kurdjumov-Sachs (K-S) and/or Nishyama-Wasserman (N-W) orientation relationships with adjacent fcc grains. Ex-situ heat treated CoCuFeMnNi analyzed by atom probe tomography (APT) revealed a highly heterogeneous segregation of the different elements to different grain boundaries. The FeCo-rich precipitates contain trace amounts of Ni, whereas Cu is rejected leading to the formation of a separate Cu rich phase. This complex segregation phenomenon is assisted by the high fraction of grain boundaries and triple junctions in the nanocrystalline material, which are critical for the phase evolution in this alloy, which is not frequently observed in the corresponding coarse-grained material.

Critical evaluation and thermodynamic modeling of the Ag-X (X=Mn, Y, Sr) binary systems

A complete literature review, critical evaluation and thermodynamic assessment of the Ag-X (X = Mn, Y, Sr) binary systems have been conducted. The Modified Quasi-chemical Model in the Pair Approximation (MQMPA) was applied to describe the Gibbs energy of the liquid phase. The Gibbs energy of all intermetallic compounds and terminal solid solutions was described by the Compound Energy Formalism (CEF). The critical literature assessment and thermodynamic modeling of the Ag–Mn binary system were carried out for the first time. The Differential Scanning Calorimetry (DSC) measurement was carried out to resolve the discrepancy of liquidus in Ag-rich region of the Ag–Sr binary system. Thermodynamic re-optimization of the Ag–Sr binary system was performed based on reported literature data and our new measured results using DSC technique. Thermodynamic re-optimization of the Ag–Y binary system was also carried out with considering the ordered bcc_B2 crystal structure of the AgY compound. All reliable experimental data are well reproduced within the experimental limits. A self-consistent thermodynamic database of the Ag-X (X = Mn, Y, Sr) binary systems was constructed for the development of new biomaterials.

Equation of State of TiN at High Pressures and Temperatures: A Possible Host for Nitrogen in Planetary Mantles

AbstractNitrogen, the most abundant element in Earth's atmosphere, is also a primary component of solid nitride minerals found in meteorites and on Earth's surface. If they remain stable to high pressures and temperatures, these nitrides may also be important reservoirs of nitrogen in planetary interiors. We used synchrotron X‐ray diffraction to measure the thermal equation of state and phase stability of titanium nitride (TiN) in a laser‐heated diamond anvil cell at pressures up to ∼70 GPa and temperatures up to ∼2,500 K. TiN maintains the cubic B1 (NaCl‐type) crystal structure over the entire pressure and temperature range explored. It has K0 = 274 (4) GPa, K0′ = 3.9 (2), and γ0 = 1.39 (4) for a fixed V0 = 76.516 (30) Å3 (based on experimental measurements), q = 1, and θ0 = 579 K. Additionally, we collected Raman spectra of TiN up to ∼60 GPa, where we find that the transverse acoustic (TA), longitudinal acoustic (LA), and transverse optical phonon modes exhibit mode Grüneisen parameters of 1.66(17), 0.54(15), and 0.93 (4), respectively. Based on our equation of state, TiN has a density of ∼5.6–6.4 g/cm3 at Earth's lower mantle conditions, significantly more dense than both the mantle of the Earth and the estimated densities of the mantles of other terrestrial planets, but less dense than planetary cores. We find that TiN remains stable against physical decomposition at the pressures and temperatures found within Earth's mantle, making it a plausible reservoir for deep planetary nitrogen if chemical conditions allow its formation.

Thermodynamic Descriptions of the Co–Zr and Co–Fe–Zr Systems

The Co–Fe–Zr system and its Co–Zr subsystem were optimized using the CALculation of PHAse Diagram (CALPHAD) approach. The substitutional solution model was used for describing the phases liquid, fcc_A1, bcc_A2 and hcp_A3. Two Laves phases were modeled as (Co,Fe,Zr)2(Co,Fe,Zr)1, and the phases CoFe and CoZr with the bcc_B2 crystal structure were described as the ordered one of bcc_A2 in the formula (Co,Fe,Va,Zr)0.5(Co,Fe,Va,Zr)0.5Va3. With limited solubility ranges, all other phases were treated as the line compounds (Co,Fe)mZrn. An excellent agreement between the reported and calculated results was reached. The reliable thermodynamic parameters of the Co–Fe–Zr system were acquired, which can be well applied to various thermodynamic calculations and materials design.

Copper-graphene composites; developing the MEAM potential and investigating their mechanical properties

Unraveling the deformation mechanisms at the atomistic scale of metal matrix-graphene composites is a key step toward designing and fabricating these materials with exceptional mechanical properties. While these composites can be made by embedding graphene into multiple metallic matrices, it is shown that superior mechanical properties can be obtained by combining copper and graphene. In the past few years, molecular dynamics simulations have been used to investigate the fundamental deformation mechanisms at the nanoscale of Cu-graphene composites to facilitate designing these composites with improved mechanical properties. However, in all of the reported works, Lennard-Jones potential has been used for modeling the interaction between copper and carbon atoms. The complexities in the Cu-C interaction emerges the necessity of utilizing more accurate potentials. In this work, a 2NN MEAM (second nearest-neighbor modified embedded atomic method) potential for the copper and carbon atom interaction is developed. Since crystal structures like B1 or B2 are not experimentally available for the Cu-C system, first-principle calculations are used to determine the reference structure and its elastic constants in this work. It is shown that the B1 and B2 structure of Cu-C has positive formation energy, but B1 is dynamically stable. Accordingly, the B1 crystal structure is used as the reference structure for the Cu-C system to develop the interatomic potential. It is shown that the reported potential agrees reasonably well for phonon dispersion frequencies, stacking fault energies, and the atomic forces with the available experimental data and first-principle calculations. The developed potential is utilized to study the mechanical properties of copper-graphene composites subjected to uniaxial loading. Our results show that adding graphene to a defect-free Cu crystal weakens the metallic matrix’s mechanical properties. However, when the graphene is embedded into a Cu matrix with some defects, e.g., in a polycrystalline Cu, it can significantly improve the mechanical properties.

Thermodynamic re-optimizations of the Ru–X (X=Al, Hf, Mo, Ti) binary systems

The Ru–X (X = Al, Hf, Mo, Ti) systems were re-optimized by means of the CALPHAD method according to the experimental phase equilibria and thermochemical data, as well as updated Gibbs energy function of “GBCCRU” in SGTE Pure 5 database. The substitutional solution model was applied to the phases liquid, fcc_A1, bcc_A2 and hcp_A3. Besides, the phases Al6Ru, Al13Ru4, Al5Ru2, Al2Ru and Al3Ru2 in the Ru–Al system were modeled as stoichiometric compounds and the phase Mo5Ru3 (σ) in the Ru–Mo system was described as (Mo,Ru)10(Mo,Ru)20 and (Mo,Ru)10(Mo,Ru)4(Mo,Ru)16 using the two-sublattice and three-sublattice models, respectively. The phases AlRu, HfRu and RuTi with the bcc_B2 crystal structure were treated as the ordered ones of bcc_A2. The reliable thermodynamic parameters of these binary systems were obtained, which could be used as the basis for thermodynamic calculations of high-order systems and database establishment of Ni-based superalloys.

Chemical bath deposited CdxPb1−xS solid solution films: composition, structure, and optical properties

Films of cubic substitutional CdxPb1−xS solid solutions have been synthesized by the chemical bath deposition with varying cadmium acetate salt Cd(CH3COO)2 in the reaction mixture within 0.01–0.08 mol/l on quartz substrates. The crystal structure, chemical composition and morphology were studied by the X-ray diffraction, scanning electron microscopy, elemental analysis, Auger spectroscopy, and Raman spectroscopy, as well as the evolution of the optical and photoelectric properties was determined. The thickness of the deposited films changes from ~ 0.4 to ~ 1.0 μm, and the content of cadmium x in the CdxPb1−xS solid solution varies from 0.036 to 0.090. All the CdxPb1−xS films have B1 crystal structure (NaCl type), but differ in the morphology (the size, form and predominant orientation of the particles forming the films). Forming the substitution solid solution is confirmed by the shift of the Raman lines at 72 and 134 cm−1 toward higher frequencies. The optical band gap determined from the absorption spectra varies from 0.46 to 0.70 eV. The dependences of the photoresponse value (volt-watt sensitivity) and the dark resistance on cadmium acetate concentration in the solution are represented.

Experimental investigation and thermodynamic description of the Ni–Ti–W system

The liquidus surface projection and isothermal sections at 1173 and 1373 K of the Ni–Ti–W system were constructed on the basis of microstructure and phase constituents of as-cast and annealed alloys, which were obtained by means of scanning electron microscopy (SEM) coupled with energy dispersion spectroscopy (EDS), X–ray diffraction (XRD). Six primary solidification regions were determined in the liquidus surface projection. Five and six three-phase regions were derived in the isothermal sections at 1173 and 1373 K, respectively. No new ternary compounds were found. Based on the present experimental data, the Ni–Ti–W system was optimized using CALPHAD (CALculation of PHase Diagram) method. The solution phases, liquid, fcc, bcc, and hcp, were treated as substitutional solution. Two compounds Ni3Ti and NiTi2 were treated as (Ni,Ti,W)m(Ni,Ti,W)n, and Ni4W was treated as (Ni,Ti)4W1 by a two-sublattice model. NiTi with B2 crystal structure was treated as the ordered phase of bcc solution, and model was (Ni,Ti,W)0.5(Ni,Ti,W)0.5(Va)3. A set of self-consistent thermodynamic parameters was obtained.

Effect of Point Defects on the Tensile and Thermal Characteristics of Nickel–Aluminum Nanowire through Molecular Dynamics

The point defect has profound effect on the tensile characteristics such as yield stress and yield strain of any material. In this study, the influence of point defect (1%, 1.5% and 2%) on Ni–Al (B2 crystal structure) nanowire has been analyzed through molecular dynamics simulation. The uniaxial tensile load of 6 × 108/s has been applied along [001] direction under various temperature conditions such as 300 K, 450 K and 600 K to analyze the resulting outcomes which arise from the point defects and temperature and found that the increase in point defect concentration leads to the decline of the yield stress by around 9.7%, 13.2% and 17.42%, respectively, under ambient temperature condition (300 K). Besides, the increase in temperature under the same point defect concentration leads to the drop of the yield stress significantly. In addition to the above, it has been found that the point defect has a higher influence on the tensile characteristics of nanowire followed by the temperature effect.

Large positive magnetoresistance and Dzyaloshinskii\u2013Moriya interaction in CrSi driven by Cr 3d localization

Spin chiral systems with Dzyaloshinskii–Moriya (DM) interaction due to broken inversion symmetry are extensively studied for their technological applications in spintronics and thermoelectrics. Here, we report an experimental study on the magnetization, magnetoresistance (MR) and electronic structure of a non-centrosymmetric compound CrSi with B20 crystal structure. Both magnetization and MR shows competing ferromagnetic (FM) and antiferromagnetic (AFM) correlations with the FM correlations being comparatively weaker indicating the presence of DM interaction in CrSi. A large positive MRsim ,25% obtained at 5 K and 5 T magnetic field arises due to the stronger AFM correlations. Resonant photoemission shows both localized and itinerant nature of Cr 3d electrons to be present in CrSi and this is supported by the temperature dependence of magnetic susceptibility. Drastic variation in the density of states along with valence band broadening at low temperature indicates the increase in hybridization between Cr 3d and Si 3s–3p states which enhances the localization effects. Spin polarized itinerant Cr 3d electrons give rise to AFM spin density wave in CrSi. Magnetic interaction between the localized and itinerant Cr 3d electrons are found to be crucial for realizing DM interaction in this system. Spectral density of states derived from high resolution valence band measurements provides evidence of electronic topological transition in CrSi. Large density of polarized itinerant electrons which varies with temperature and the large positive MR with AFM correlations suggests CrSi as a potential candidate for both the thermoelectric and spintronics applications.