B2 Crystal Structure Research Articles

Effect of thermodynamic properties on the infrared radiation behavior of Ti-based solid solutions

Powdered Ti-based carbonitride and carboxide solid solutions were produced from milled mixtures of graphite and TiO2 via carbo-nitridation. The thermodynamic interactions in the solid solutions were calculated and compared with those of the corresponding TiC–TiO and TiC–TiN mixtures. Carbon and oxygen had low mutual affinity in the Ti(CO) B1 crystal structure, inducing strong positive deviation from ideal mixing behavior in their activity curves and high infrared radiation. In contrast, carbon and nitrogen showed high mutual affinity in the Ti(CN) B1 crystal structure. Strong negative deviation from ideal mixing behavior in their activity curves and low infrared radiation were caused. Consequently, the infrared radiation is significantly affected by the thermodynamic activity.

Microstructural Development and Ternary Interdiffusion in Ni-Mn-Ga Alloys

NiMnGa alloys functioning as either ferromagnetic shape memory alloys or magnetocaloric materials have both practical applications and fundamental research value. In this study, solid-to-solid diffusion couple experiments were carried out to investigate the phase equilibria, microstructural development, and interdiffusion behavior in Ni-Mn-Ga ternary alloys. Selected diffusion couples between pure Ni, Ni25Mn75 and four ternary off-stoichiometric NiMnGa alloys (i.e., Ni52Mn18Ga30, Ni46Mn30Ga24, Ni52Mn30Ga18, Ni58Mn18Ga24) were assembled and annealed at 1073 K, 1123 K, and 1173 K (800 °C, 850 °C, and 900 °C) for 480, 240, and 120 hours, respectively. At these high temperatures, the β NiMnGa phase has a B2 crystal structure. The microstructure of the interdiffusion zone was examined by scanning electron microscopy and transmission electron microscopy. Concentration profiles across the interdiffusion zone were determined by electron probe micro analysis. Solubility values obtained for various phases were mostly consistent with the existing isothermal phase diagrams, but the phase boundary of the γ(Mn) + β two-phase region was slightly modified. In addition, equilibrium compositions for the γ(Ni) and α′ phases at 1173 K (900 °C) were also determined for the respective two-phase region. Both austenitic and martensitic phases were found at room temperature in each diffusion couple with a clear boundary. The compositions at the interfaces corresponded close to valence electron concentration (e/a) of 7.6, but trended to lower values when Mn increased to more than 35 at. pct. Average effective interdiffusion coefficients for the β phase over different compositional ranges were determined and reported in the light of temperature-dependence. Ternary interdiffusion coefficients were also determined and examined to assess the ternary diffusional interactions among Ni, Mn, and Ga. Ni was observed to interdiffuse the fastest, followed by Mn then Ga. Interdiffusion flux of Ni also has strong influences on the interdiffusion of Mn and Ga with large and negative cross interdiffusion coefficients, $$ \tilde{D}_{MnNi}^{Ga} $$ and $$ \tilde{D}_{GaNi}^{Mn} $$ . The $$ \tilde{D}_{NiNi}^{Ga} $$ and $$ \tilde{D}_{MnMn}^{Ga} $$ ternary interdiffusion coefficients exhibited minimum values near 52 at. pct Ni concentration.

Metal-insulator transition and local-moment collapse in FeO under pressure

We employ a combination of the \emph{ab initio} band structure methods and dynamical mean-field theory to determine the electronic structure and phase stability of paramagnetic FeO at high pressure and temperature. Our results reveal a high-spin to low-spin transition within the B1 crystal structure of FeO upon compression of the lattice volume above 73~GPa. The spin-state transition is accompanied by an orbital-selective Mott metal-insulator transition (MIT). The lattice volume is found to collapse by about 8.5~\% at the MIT, implying a complex interplay between electronic and lattice degrees of freedom. Our results for the electronic structure and lattice properties are in overall good agreement with experimental data.

Thermodynamic stability of in situ W–ZrC and W–Zr(CN) composites

Powders of W–ZrC and W–Zr(CN) were carbothermally synthesized in situ from milled mixtures of graphite, WO3 and ZrO2. The thermal stability of Zr(CN) in a W matrix was simulated and compared with that of ZrC in W in terms of free energy change and carbide coarsening. Carbon and nitrogen had high mutual affinity in Zr(CN) of B1 crystal structure, which led their activity curves to exhibit strong negative deviation from ideal mixing behavior. Zr(CN) was more stable than ZrC up to 2075 K; however, a microstructural study showed that it became less stable than ZrC at around 1975 K. This result is attributed to the decreasing thermodynamic stability of ZrN with increasing temperature. Other transition metal carbonitrides containing group 4–6 elements are expected to show similar coarsening behaviors at high temperatures.

Solid Solution Cermet: (Ti,Nb)(CN)–Ni Cermet

Solid solution powders without W, (Ti,Nb)(CN) powders with a B1 structure (NaCl like), were synthesized by high energy milling and carbothermal reduction in nitrogen. The range of molar ratios of Ti/Nb for forming complete (Ti,Nb)(CN) phase was broader than that of Ti/W for the (Ti,W)(CN) phase because carbide or carbonitride of Nb had a B1 crystal structure identical to Ti(CN) while WC had a hexagonal crystal structure. The results revealed that the hardness of (Ti,Nb)(CN)-Ni cermets was higher than that of (Ti,W)(CN)-Ni cermets. The lower density of the (Ti,Nb)(CN) powder contributed to the higher hardness compared to (Ti,W)(CN) because the volumetric ratio of (Ti,Nb)(CN) in the (Ti,Nb)(CN)-Ni cermets was higher than that of (Ti,Nb)(CN) in the (Ti,W)(CN)-Ni cermets at the same weight ratio of Ni. Additionally, it was assumed that intrinsic the properties of (Ti,Nb)(CN) could also be the cause for the high hardness of the (Ti,Nb)(CN)-Ni cermets.

Thin film MnGe grown on Si(111)

MnGe has been grown as a thin film on Si(111) substrates by molecular beam epitaxy. A 10 Å layer of MnSi was used as the seed layer in order to establish the B20 crystal structure. Films of a thickness between 45 and 135 Å have been prepared and structurally characterized using reflection high-energy electron diffraction, atomic force microscopy and x-ray diffraction. These studies provided evidence that MnGe forms in the cubic B20 crystal structure as islands exhibit a very smooth surface. The islands become larger with increasing film thickness. A magnetic characterization reveals that the ordering temperature of MnGe thin films is enhanced compared to that for bulk material. The properties of the helical magnetic structure obtained from magnetization and magnetoresistivity measurements are compared with those of films of the related compound MnSi. The much stronger Dzyaloshinskii–Moriya interaction in MnGe results in a higher rigidity of the spin helix.

Synthesis and characterization of Co2FeAl Heusler alloy nanoparticles

Heusler alloy Co2FeAl (CFA) nanoparticles have been synthesized by reducing a mixture of the precursors: CoCl2·6H2O, Fe(NO3)3·9H2O and AlCl3·6H2O under H2 atmosphere. XRD, SEM and TEM techniques have been used for the characterization of the prepared material. XRD and SAED data from TEM show the formation of mixed phases of L21, B2 and A2 type crystal structure of the alloy. The estimated particle size from XRD data and TEM micrograph has been found in the range of 10 nm to 50 nm. The saturation magnetization has been found of 115 emu/g from M-H characteristics which is close to its bulk value of saturation magnetization. Chemical composition of the elements has also been estimated from EDAX, which shows a ratio of Co:Fe:Al as 2.12:1.06:0.81.

Phase relations in the Fe–FeSi system at high pressures and temperatures

The Earth's core is comprised mostly of iron and nickel, but it also contains several weight percent of one or more unknown light elements, which may include silicon. Therefore it is important to understand the high pressure, high temperature properties and behavior of alloys in the Fe–FeSi system, such as their phase diagrams. We determined melting temperatures and subsolidus phase relations of Fe–9wt% Si and stoichiometric FeSi using synchrotron X-ray diffraction at high pressures and temperatures, up to ~200GPa and ~145GPa, respectively. Combining this data with that of previous studies, we generated phase diagrams in pressure–temperature, temperature–composition, and pressure–composition space. We find the B2 crystal structure in Fe–9Si where previous studies reported the less ordered bcc structure, and a shallower slope for the hcp+B2 to fcc+B2 boundary than previously reported. In stoichiometric FeSi, we report a wide B2+B20 two-phase field, with complete conversion to the B2 structure at ~42GPa. The minimum temperature of an Fe–Si outer core is 4380K, based on the eutectic melting point of Fe–9Si, and silicon is shown to be less efficient at depressing the melting point of iron at core conditions than oxygen or sulfur. At the highest pressures reached, only the hcp and B2 structures are seen in the Fe–FeSi system. We predict that alloys containing more than ~4–8wt% silicon will convert to an hcp+B2 mixture and later to the hcp structure with increasing pressure, and that an iron–silicon alloy in the Earth's inner core would most likely be a mixture of hcp and B2 phases.

Nanophase stability in a granular FeRh-Cu system

A granular system of FeRh-based nanoprecipitates (∼10–15 nm diameter) embedded in a rapidly solidified copper ribbon matrix was found to transit from a metastable tetragonal L10 (AuCu-1-type) structure to a stable B2 (CsCl-type) structure upon annealing-induced coarsening to ∼94 nm. The hysteretic magnetic transition observed at ∼100 K develops a gradual broadening that accompanies the L10 → B2 crystal structure transition. It is proposed that the Cu matrix influences the structural and magnetic properties of the FeRh-based nanoparticles through interfacial strain and chemical effects. These results emphasize the sensitivity of the magnetostructural response of FeRh to changes in the nanostructural scale, and provide pathways for tailoring the transition.

Interatomic Bonding Model with Polar Motifs Based on Point Group Theory for B2 and A2 Crystal Structures of Shape Memory Binary Alloys

Interatomic Bonding Model with Polar Motifs Based on Point Group Theory for B2 and A2 Crystal Structures of Shape Memory Binary Alloys

On the crystallographic anisotropy of nanoindentation in pseudoelastic NiTi

We use a nanoindenter with a Berkovich tip to study local mechanical properties of two polycrystalline intermetallics with a B2 crystal structure, NiAl and NiTi. We use orientation imaging scanning electron microscopy to select a relevant number of grains with appropriate sizes and surface normals parallel to 〈001〉, 〈101〉 and 〈111〉. As a striking new result, we find a strong crystallographic orientation dependence for NiTi. This anisotropy is less pronounced in the case of NiAl. For NiTi, the indentation force required to impose a specific indentation depth is highest for indentation experiments performed in the 〈001〉 direction and lowest along the 〈111〉 direction. We consider transmission electron microscopy results from cross-sections below the indents and use molecular dynamics simulations and resolved shear stress calculations to discuss how this difference can be accounted for in terms of elementary deformation and transformation processes, related to dislocation plasticity (NiAl and NiTi), and in terms of the stress-induced formation and growth of martensite (NiTi). Our results show that the crystallographic anisotropy during nanoindentation of NiTi is governed by the orientation dependence of the martensitic transformation; dislocation plasticity appears to be less important.

Thermodynamic re-assessment of Fe–Ti binary system

The Fe–Ti binary system was re-assessed using the CALPHAD method in order to improve the capability of being extrapolated to a ternary or higher-order system. Compared with previous assessments, the main focus was put on the thermodynamic description of the two intermetallic compounds Fe2Ti and FeTi. The C14_Laves phase Fe2Ti was described by the two-sublattice model, which is widely used at present. By checking the homogeneity range on the boundary of the ternary systems involving the binary, the phase boundary of this compound was further confirmed. The FeTi phase with a BCC_B2 crystal structure was treated as the ordered phase of the BCC_A2 phase and a unified Gibbs energy function was used to describe both the ordered and disordered phases. Reproduction of the specific heat capacities of these compounds was another aspect paid particular attention to. Comprehensive comparisons of the calculated and experimental results regarding the phase diagram and thermodynamic properties show a good agreement between them and prove the validity of the present thermodynamic description.

Thermodynamic modeling of the Fe–Ti–V system

The thermodynamic optimization of the Fe–Ti–V system was carried out using the CALPHAD technique. The solution phases (liquid, bcc, fcc and hcp) were modeled as substitutional solution. The compound Fe2Ti with C14 structure had a solubility of 34at% V at 1273K and was described as (Fe,Ti,V)2(Fe,Ti,V) by a two-sublattice model in the Fe–Ti–V system. The compound FeTi with B2 crystal structure had a solubility of 9at% V at 1273K and was treated as (Fe,Ti,V)0.5(Fe,Ti,V)0.5(Va)3 in order to cope with the order–disorder transition between FeTi(B2) and bcc solid solution (bcc-A2). The sigma phase was treated as the formula (Fe,Ti,V)10(Fe,Ti,V)20 in the Fe–Ti–V system. A set of self-consistent thermodynamic parameters of the Fe–Ti–V system was obtained.

Stability of (Ti, M)C (M = Nb, V, Mo and W) carbide in steels using first-principles calculations

The lattice parameters, formation energies and bulk moduli of (Ti, M)C and M(C, Va) with the B1 crystal structure have been investigated using first-principles calculations, where M = Nb, V, Mo and W. The replacement at 0 K of Ti by Mo or W in the TiC lattice is found to be energetically unfavorable with respect to the formation energy. However, it decreases the misfit strain between the carbide and ferrite matrix, a factor which is of critical importance during the early stages of precipitation, thus favoring the substitution of Ti by Mo, as is observed in practice. The effect of Mo in enhancing the coarsening resistance of (Ti, Mo)C precipitates is discussed in terms of its role in the nucleation process, but followed by a more passive contribution during coarsening itself. The role of tungsten has been predicted to have a similar effect to molybdenum on the nucleation and coarsening process. Analysis of precipitates in Ti-, Ti–Mo- and Ti–W-bearing steels shows results consistent with the calculations.

Influences of chromium and hafnium additions on the microstructures of β-nial coatings on superalloy substrates

In this study, Hf-doped and Cr/Hf-modified NiAl coatings were deposited onto René N5® substrates via direct current magnetron sputtering. Microstructural analysis using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), and electron probe microanalysis (EPMA) showed the as-deposited coatings to be single phase with B2 crystal structures. Post-deposition annealing at 1000°C in an Ar+5%H2 atmosphere resulted in the formation of nanometer-sized precipitates along grain boundaries and within grain interiors. TEM analyses showed most of the precipitates to be β′-Ni2AlHf. Three-dimensional atom probe tomography (3D-APT) also revealed the presence of α-Cr precipitates within the NiAlCrHf coatings after annealing. The results have been analyzed and discussed relative to previous research on sputter deposited NiAl–Hf coatings.

The deformation behavior of DyCu ductile intermetallic compound under compression

Abstract In this study polycrystalline specimens of as-cast DyCu with B2 crystal structure were compressed to different strains at room temperature to test if stress-induced phase transformation and twinning occur during deformation. In these tests DyCu exhibited high ductility with plastic strain as high as 35% and ultimate compressive strength of ∼790 MPa. X-ray diffraction and transmission electron microscopy (TEM) showed no indications that DyCu had undergone stress-induced phase transformation or twinning. 〈1 1 1〉-type dislocations have been observed to play an important role for imparting the ductile behavior. TEM analyses showed the presence of second phases, Dy 2 O 3 and DyCu 2 within the DyCu. The mechanisms of ductility and impurity phase formation are discussed.

Ni2MnAl Single Crystals for Magnetic Scattering Neutron Holography

Ni2MnAl phase has the B2 crystal structure when an alloy was prepared at the moderate cooling rate. However, the crystal structure of Ni2MnAl oders from the B2 structure to the L21 structure by annealing at lower temperature than approximately 773 K. Ni2MnAl with the L21 structure exhibits martensitic phase transformation and ferromagnetic phase transition with decreasing temperature. On the contrary, Ni2MnAl with high Al content with the B2 structure exhibit no martensitic transformation but antiferromagnetic transition. With further cooling, Ni2MnAl with the B2 structure exhibit transition from antiferromagnetic phase to spin glass phase. Ni2MnAl with the B2 structure is suitable for developing a method to measure spin state of solids. [DOI: 10.1380/ejssnt.2011.331]

Structural and magnetic properties of ε-Fe1−xCoxSi thin films deposited via pulsed laser deposition

We report pulsed laser deposition synthesis and characterization of polycrystalline Fe1−xCoxSi thin films on Si (111). X-ray diffraction, transmission electron, and atomic force microscopies reveal films to be dense, very smooth, and single phase with a cubic B20 crystal structure. Ferromagnetism with significant magnetic hysteresis is found for all films including nominally pure FeSi films in contrast to the very weak paramagnetism of bulk FeSi. For Fe1−xCoxSi this signifies a change from helimagnetism in bulk, to ferromagnetism in thin films. These ferromagnetic thin films are promising as a magnetic-silicide/silicon system for polarized current production, manipulation, and detection.

Exothermic reactions in Co/Al nanolaminates

Vapor-deposited Co/Al nanolaminates having a net equiatomic stoichiometry exhibited rapid, high-temperature synthesis. When ignited locally, 7.5-μm-thick Co/Al foils were characterized by self-sustained propagating reactions with flame front speeds between ∼0.5 and 9 m/s. Speed was largely affected by bilayer thickness and premixed volume fraction, consistent with previous studies of other exothermic multilayers. Cobalt aluminide foils reacted by self-propagating synthesis developed a variety of rough surface morphologies characterized by a large amplitude (peak-to-valley ∼1.0 μm), multiperiod, wavelike structure. High-temperature reaction was also stimulated by rapid global heating. Tests revealed low ignition temperatures (Tig)∼240–460 °C compared with previous Co–Al powder reactant experiments. Ignition temperature was influenced by bilayer thickness. All foils ignited by global heating and those reacted in a self-propagating mode developed a single-phase B2 (CsCl-type) crystal structure. Thick Co/Al nanolaminates were used successfully to join a few different materials including metalized Al2O3 (by soldering) and polyetheretherketone (by direct attachment).

Single-Crystalline Ferromagnetic Fe1-xCoxSi Nanowires

We report the synthesis of ternary Fe1-xCoxSi nanowires by a vapor transport method. Structural characterization of the nanowires shows a single-crystalline nature with B20 crystal structure. Spatially resolved elemental analysis revealed the homogeneous distribution of all elemental components along the nanowire. The nanowire ensemble shows ferromagnetic properties with a sudden change in magnetization at applied magnetic field near zero. Magnetoresistance measurements on a single-nanowire device show positive magnetoresistance, which does not saturate up to the highest applied field. These ferromagnetic nanowires could find application as spin filters in the new field of nanospintronics.