B2 Structure Research Articles

Thickness Effects on the Martensite Transformations and Mechanical Properties of Nanocrystalline NiTi Wires.

This paper studied the features of the martensitic transformations and mechanical properties of 40, 60, and 90 µm thick NiTi wires with nanocrystalline B2 structures. It was established that the wires were composites and consisted of a TiNi matrix and a TiO2 + TiNi3 surface layer. Structural methods showed that the wire matrix was formed by grains of up to 20 nm in size. The method of measuring the electrical resistivity during cooling and heating revealed a two-stage nature of the martensitic transformation. Cyclic loading-unloading demonstrated that all the samples exhibited superelasticity effects and completely restored their shape when unloaded from a 4-8% relative strain at room temperature. An increase in mechanical characteristics with respect to the wire thickness was experimentally established. This was due to the change in the composition of the TiNi matrix during drawing.

The mechanical, dynamical, thermodynamical properties and elastic anisotropies of cubic YbAu compound under pressure

The structural, elastic, vibrational, electronic and thermodynamic properties of thermodynamically important YbAu in B2 structure under pressure have been investigated by performing first principles method using DFT. The calculated lattice parameter and equilibrium volume are in good agreement with the experimental values. Zero pressure elastic constants and pressure-dependent elastic constants are positive up to 30 GPa. The resulting elastic constants are stable mechanically as they meet the Born stability condition, which makes the YbAu compound stable in nature. B/G ratio shows that the YbAu has a brittle nature while Poisson’s ratio (ν) values show that an ionic bond nature. The presented phonon dispersion curves and one-phonon DOS also confirms that this compound is stable dynamically. The electronic structure and metallic nature have been analysed quantitatively from band structure and density of states. To better illustrate the nature of bonding and charge transfer, we have studied the charge density plots and mulliken charge analysis. Also, the thermodynamic properties have been investigated under temperature and high pressure. The pressure effects on the volume of YbAu are found to be greater than the temperature effects. • Pressure effect on the structural, elastic, vibrational and electronic properties of YbAu compound was investigated using DFT. • Electronic band structure and density of state calculations revealed that YbAu is metallic in nature. • The elastic anisotropy was visualized in detail by plotting the directional dependence. • The thermodynamic properties were investigated under temperature and high pressure.

Entropy-stabilized silicides: Expanding the B20 single-phase region from mono-silicide to high-entropy silicide

The formation of a single phase is an important requirement for high-entropy ceramics (HECs) because precipitation of unwanted phases generally degrades their functional properties. This paper provides a useful guideline for the single-phase formation of HECs. First, metal elements constituting HECs can be divided into two groups: elements that have a parent phase as a stable phase and elements that have a phase with the same stoichiometry as the parent phase but a different crystal structure. Second, even when the latter elements are added in an HEC, we can stabilize the parent phase if stabilizing energy by configurational entropy is larger than the difference in formation energy due to their stable phase, which can be quantitatively calculated through first-principles calculation. Interestingly, based on these guidelines, (CrMnFeCoNi)Si HE silicide with a single B20 structure was sequentially developed from mono-silicide. In particular, the HEC with maximized configurational entropy was searched in our HEC system by adding NiSi to (CrMnFeCo)Si, which is stable in B31 and B20 structures. This study offers a chance to increase the structural and compositional complexity in HECs, enabling the expansion of the single-phase region in HECs.

Prediction of the melting curve and phase diagram for CaO using newly developed interatomic potentials

A comprehensive investigation of the melting curve and pressure–temperature (P–T) phase diagram for CaO, a candidate mineral in the Earth's lower mantle, is performed via atomistic simulations applying newly developed interatomic potentials. The efficiency and reliability of the new potentials under high temperature and pressure are verified with lattice dynamics, molecular dynamics, and first–principles approaches, where the fitting values of the potentials are extracted from the lattice parameters, elastic constants, and energy differences of rock–salt (B1) and cesium chloride (B2) phases in CaO. The melting temperatures of CaO with B1 structure at ambient pressure obtained from the single–phase method, void method, and two–phase method are 3080 K, 2775 K, and 2767 K, respectively. The structure and diffusion are evaluated by radial distribution functions, mean square displacements, and self–diffusion coefficients in the solid and liquid states for CaO. Simultaneously, the P–T phase diagram of CaO is calculated up to the core–mantle boundary pressure of 135 GPa, in which the Clausius–Clapeyron relation is introduced to determine the slope of the phase boundary between B1 and B2 phases. Other physical properties, including phase transition pressure, thermal expansion coefficient, isovolumic heat capacity, and entropy, are investigated. The limitations of the new potentials are discussed.

Study on Reaction Sites of Active Structures in Coal Based on the ReaxFF Reactive Force Field.

To study the reaction paths and reaction mechanisms of the active structures in coal during the oxidation process, the oxygen-free pyrolysis and oxygen-containing combustion were simulated for nine active structures in coal based on the ReaxFF MD method. A separate simulation analysis of the active structure yielded that O2 inhibited the reaction of H1. As the branched chain grows, the reaction paths of C2 and Q2 follow the direction of the reaction of carboxyl and aldehyde groups to CO2 and CO. Considering the reaction rates and reaction process products of A1, B1, and B3 structures, it is obtained that O2 has the greatest contribution to the decomposition reaction of aliphatic hydrocarbon structures. This is due to the strong electron-absorbing property of O2 that attracts H radicals to generate HO2, which in turn affects the reaction path of the active structure. Tracing the reaction process reveals that OH and oxygen-containing radicals under oxygen-free conditions greatly influence the active structural reaction.

Crystal structure of potassium chloride monohydrate: water intercalation into the B1 structure of KCl under high pressure.

A new hydrate form of potassium chloride, KCl·H2O, is identified for the first time by in situ single-crystal X-ray diffraction under high pressure. It has a monoclinic structure with lattice parameters of a= 5.687 (7), b= 6.3969 (3), c= 8.447 (3) Å and β= 107.08 (8)° at 2.23 (4) GPa and 295 K. The structure of this hydrate has K-Cl alignments similar to the B1 phase of anhydrous KCl, while water molecules intercalate among the ionic species. The coordination structures of the K and Cl atoms can be regarded as the intermediate states between the B1 and B2 phases of KCl. This finding provides a perspective on the structural interpretation of multicomponent materials and an additional candidate for bound water in salt-water systems under high pressure, such as inside of icy bodies.

Structural ordering, magnetic and electrical transport properties in Ni60-xFe13+xGa27 Heusler alloys

The evolutions of phase transformation, magnetic and electrical transport properties with the change of Fe concentration have been systematically investigated in a series of Ni60-xFe13+xGa27 (x = 0–10) Heusler alloys. The structure and heat flow measurements reveal every sample undergoes a disordered-ordered transition from a disordered B2 structure to a L21 ordered structure, and a first-order structural transformation from the L21 cubic structure to a L10 tetragonal structure is corresponding to the martensitic transformation (MT). With increasing Fe concentration, the disordered-ordered transition and ferromagnetic transition shift to higher temperature, while the MT shifts to lower temperature. According to phase diagram established in the present investigation, it is also found that the increase of Fe content can convert the first-order MT from a paramagnetic type to a ferromagnetic one, which is accompanying by an obvious increase of both effective and spontaneous magnetic moments. Furthermore, the calculated magnetocrystalline anisotropy constant (K1) of the studied alloys monotonically increases as Fe increases. It is evident that the magnetic degrees of freedom in these two MTs remain almost unchanged so that the electron-spin scattering doesn't change, which results in the resistivity difference between the austenitic and martensitic states is feeble. Such an intrinsic nature leads to a weak magnetoresistance (MR) and baroresistance (BR) effects in the studied samples because of only contribution of electron-lattice scattering change on the resistivity. This demonstrates from a negative perspective that the electron-spin scattering change originated from the variation in the magnetic degrees of freedom acts as a dominant role in improvement of the MR and BR effects for the Heusler system possessed the first-order MT.

Balance between oxidation and tribological behaviors at elevated temperatures of Hf1-xWxN films by optimizing W content

Wear resistance and anti-oxidization are required for the binary transitional metal nitride coatings when they are applied as protective coating on cutting tools. The dope with other transitional metal elements is one of the effective strategies to enhance the wear resistance of binary transitional metal nitride coatings. In this study, Hf1-xWxN ternary coatings with various W content were prepared by DC magnetron sputtering. The hardness, toughness and tribological properties of Hf1-xWxN coatings varied with W doping amounts were investigated. The results show that the Hf1-xWxN coatings with the x range of 0–1 maintain B1 structure (FCC single solid solution phase). High hardness (35.3 GPa) and enhanced toughness (3.5 MPa m1/2) are obtained from Hf1-xWxN coating with x = 0.73 (Hf0.27W0.73N). The friction coefficient and wear rate of Hf0.27W0.73N are as low as 0.38 and 2.48 × 10−6 mm3/Nm at room temperature, while they are 0.42 and 8.12 × 10−6 mm3/Nm at the temperature of 400 °C. The excellent friction and wear properties of Hf1-xWxN coatings at elevated temperatures are mainly attributed to the enhanced mechanical properties and the lubrication of tungsten oxides (WO3). The balance between oxidation and tribological behaviors of Hf1-xWxN coatings are achieved by optimizing W content.

Hard and tough sub-stoichiometric B1 Ta-Mo-Nx films by regulating N content

Ternary transition metal nitrides (TMNs) films with B1 structure (NaCl type) have been widely explored as hard protective materials. Their structure and mechanical performances are influenced by the stoichiometric coefficients (TM: N), while the films with high hardness yet sound toughness could be obtained by regulating the nitrogen content. In this case for the sub-stoichiometric B1 Ta-Mo-Nx films, the microstructure, mechanical properties and the toughening mechanisms by N atoms were investigated. The NaCl-structure sub-stoichiometric Ta0.44Mo0.56Nx films with N concentrations at 0.42 ≤x ≤ 0.73 grown by reactive magnetron sputter deposition. The film with x = 0.59 shows higher hardness (∼36.4 GPa), combined with shorter cracks around indentation and higher scratch resistance, reveals the enhanced fracture toughness (KIC=∼1.67 MPa*m1/2, H/E = 0.14); the film simultaneously obtained a lower wear rate (∼6.6 × 10−7 mm3/N * m) and friction coefficient (∼0.4). The formed point defects in sub-stoichiometric Ta-Mo-Nx films are responsible for the enhanced toughness, while the film with x = 0.59 exhibit an increased p(N) - d-eg(Me) strength character accompanied with high d-t2 g(Me) - d-t2 g(Me) metallic states for the TM-N bonds according to the XPS results.

Cr-promoted formation of B2+L21 composite nanoprecipitates and enhanced mechanical properties in ferritic alloy

The critical role of Cr on nanoprecipitates and the mechanical property of Fe-Ni-Al-Mn ferritic steel were systematically studied in this research. The two types of nanoprecipitates in the Cr added alloy were characterized through a combination of aberration-corrected scanning transmission electron microscopy and atom probe tomography techniques. The atomic-scale structure and chemistry analysis reveal that fine globular-shaped precipitates have a B2-structure, while coarse elongated precipitates have B2+L21 composite structures. The first-principles calculations reveal that the segregation of Cr at the L21/bcc interface reduces the interface and strain energy for the nucleation of the L21-type phase. With the increasing precipitate size, the B2 structure is gradually transformed to L21 to reduce elastic strain, thereby promoting the formation of B2+L21 composite nanoprecipitate. The addition of 10 wt% Cr results in an increase of ∼275 MPa in yield strength without obvious loss of ductility. The effect of Cr on the strength mechanisms were quantitatively analyzed, revealing that the strength of the ferritic alloy mainly improved by the formation of B2+L21 composite nanoprecipitate, which is more effective than solid solution strengthening.

Microstructure evolution and compressive property variation of AlxCoCrFeNi high entropy alloys produced by directional solidification

The AlxCoCrFeNi (molar radio, x = 0.3, 0.6, 0.9 and 1.2) high entropy alloys (HEAs) were directionally solidified at the withdrawal rate of 150 μm/s. The effects of Al content on microstructure evolution, element segregation as well as compressive property were investigated. It was found that the increase of Al content resulted in the formation of BCC crystal structure. Meanwhile, the microstructure of AlxCoCrFeNi HEAs transformed from dendrite to equiaxed dendrite. The Al0.3CoCrFeNi HEA formed a single FCC solid solution phase, the Al0.6CoCrFeNi HEA consisted of CoFeCr-enriched FCC solid solution phase + NiAl-enriched BCC solid solution phase, the Al0.9CoCrFeNi and Al1.2CoCrFeNi HEAs were composed of NiAl-enriched B2 solid solution phase + CoFeCr-enriched BCC solid solution phase. Morphology of solid-liquid interface was coarse dendrite or typical dendrite. As the Al content increased, the compressive strain of directionally solidified AlxCoCrFeNi HEAs decreased from 21.26% to 18.05%, and the ultimate compressive strength increased first and then decreased. The improvement of strength results from crystal structure transition and solution strengthening. As the Al contents increased from 0.6 to 0.9, the crystal structure of AlxCoCrFeNi HEAs transformed from CoFeCr-enriched FCC structure + NiAl-enriched BCC structure to CoFeCr-enriched BCCstructure + NiAl-enriched B2 structure. The strain of Al0.3CoCrFeNi HEA was higher than others, and Al0.9CoCrFeNi HEA exhibited the maximum ultimate compressive strength of 2114.57 MPa.

Photocatalytic applicability of HfN and tuning it with Mg and Sc alloys: A DFT and molecular dynamic survey

In this work, we study the phononic, electronic, elastic, thermodynamic, and optical properties of two rock salt (B1) and zinc blend (B3) structures of Hafnium Nitride (HfN) and its alloys with Mg and Sc. Our calculations were performed using the density functional theory (DFT) method and molecular dynamic (MD) simulations. Raman and IR active modes for the B1 structure show high quality for hard materials and the powerful bond between different types of atoms. Electronic band structures exhibit semiconductor behavior for Mg alloys that could be appropriate for semiconductor devices. Moreover, the spatial dependence of elastic modulus for B3 structure represents lower anisotropy than rock salt. The absorption spectrum of the MD method for B1 at 100 K shows a higher amount of absorption than the lower temperatures but, after that, it tends to constant values for 300, 400, 500, and 600 K.

Ion irradiation effect on B2 single phase AlFeCoNi alloy

The radiation behavior of single phase B2 AlFeCoNi alloy was studied using Transmission Electron Microscopy (TEM). Same samples were characterized prior and post irradiation. TEM samples were irradiated by Ar + ions at 1016, 1017 and 1018 ions/cm2 doses, keeping the rate of 6.2 *1015 ions/cm2s. At all stages, AlFeCoNi samples remained crystalline and preserved the initial B2 structure. As the dose increased, multiplication of dislocations occurred, elevating the density and forming new dislocations with different Burgers vector. At the higher dose, dislocations rearranged into walls and networks, and recrystallization occurred.

Microstructure and properties of the solid solution ceramic coating by high speed laser cladding

It is facing challenge to prepare protective ceramic coatings on the surfaces of metal parts. In this study, two types of ceramic coatings, ZrB2 and ZrB2 + TiB2, were prepared on 304 stainless steel using high speed laser cladding (HSLC) technology, and a comparative study was conducted on them. The results show that additional TiB2 were melted sufficiently during the HSLC process and reacted with ZrB2 to generate acicular (Zrx, Ti1-x)B2 structure. The existence of (Zrx, Ti1-x)B2 resulted in the increase of hardness from 719 HV0.5 to 1130 HV0.5, and the decrease of wear rate from 8.89 × 10-6 mm3 / N·m to 5.0 × 10-6 mm3 / N·m of the coating. It can be attributed to the large lattice distortion inside (Zrx, Ti1-x)B2, which improved the hardness and wear resistance compared with ZrB2. In addition, the friction coefficient of the coating decreased from 0.73 to 0.68 due to lubricating effect of the oxide debris generated by (Zrx, TI1-x) B2 in the friction process.

Abnormal notch brittleness induced by short-range ordering in low-cobalt iron-cobalt alloys under tensile and impact loading: A combined experimental and molecular dynamics simulation study

According to the binary Fe-Co phase diagram, Fe-21.5/23 wt% Co alloys undergo an order-disorder transition in the temperature range of 430–470 °C. Above the order-disorder temperature, Fe-21.5/23 wt% Co alloys are generally considered as disordered solid solutions with excellent ductility and toughness. In this work, short-range ordered (SRO) structures in Fe-21.5 wt% Co alloy quenched from α phase region were directly observed via scanning transmission electron microscope (STEM). SROs are mainly the B2 structure, with a very small amount of L60 Fe3Co structure. The mechanisms of SRO-induced notch brittleness were systematically investigated by experiments and molecular dynamics (MD) simulations. Under tensile loading, SROs change slip mode from cross slip to planar slip, which significantly leads to the strengthening effects and large work hardening rates. For the notch tensile tests, different notch depths were set to investigate the effects of SROs on the notch brittleness. The Fe-23 wt% Co samples quenched from 550 °C were surprisingly found to exhibit the notch brittleness induced by SROs. Under impact loading, SROs significantly lead to impact brittleness with the cleavage fracture. MD results of notch tensile and impact tests showed that SRO-induced poor dislocation and twinning plasticity leads to larger concentrated stresses and stress concentration factor near the notch, directly resulting in the easier nucleation and rapid propagation of intergranular and intracrystalline microcracks. Our findings suggest that SRO-induced notch brittleness must be considered in engineering applications for the low-cobalt iron-cobalt alloys.

Insights on mechanical properties of dual-phase high entropy alloys via Y introduction

AlCoCrFeNi high entropy alloy (HEA) with a dual-phase A2+B2 structure has received significant research interests due to its favorable mechanical properties. In order to further optimize the mechanical properties of the dual-phase AlCoCrFeNi HEA, we design several types of (AlCoCrFeNi)100-xYx (x = 0, 0.05, 0.1, 0.5 and 1, at%) HEAs and explore the microstructure evolution as well as mechanical behaviors as a function of the Y addition. Adding Y element into the (AlCoCrFeNi)100-xYx HEAs tailors the phase composition, where Y-0.05 HEA is of the single BCC solid solution, similar to Y-0 HEA. And when Y addition is equal to or higher than 0.1 at%, (Ni, Y)-rich HCP phase is identified except for the BCC phase, coinciding with a δ-Ω phase selection model. Meanwhile, Y introduction can lighten the intragranular dendrite segregation. The volume fraction of HCP phase follows a growing trend from 0 vol% to 10.33 vol% with an increment of Y introduction, which is beneficial to enhancing the yield strength (from 1302 to 1447 MPa) and microhardness (from 517 to 597 HV). Furthermore, the fracture mechanism from a transgranular fracture mode to an intergranular fracture mode is also revealed.

Theoretical study of the M7X6 type of vacancy ordered phases in nonstoichiometric hafnium and tantalum carbides

Carbides of IV and V group transitional metals with B1 structure contain large amounts of vacancies in the carbon sublattice. Ordering of the vacancies leads to formation of new phases with various stoichiometry. Ordered structures were discovered and studied for all the IV and V group transitional metal carbides except carbides of hafnium and tantalum. Nevertheless, recent theoretical works have predicted large variety of ordered structures in non-stoichiometric HfCy and TaCy including unusual Hf7C6 and Ta7C6. Such a stoichiometry is not typical for the ordered carbides. In this work, we study in detail the new M7X6 type of vacancy ordered structures using ab initio methods. For both compounds Hf7C6 and Ta7C6, we have found two B1-derived vacancy-ordered phases: trigonal (space group R3¯) and monoclinic (space group C2/m). The comparison of their formation enthalpies with the disordered B1 structure has showed that the only ordered phase Hf7C6 can be trigonal phase while in Ta7C6 both trigonal and monoclinic structures are possible. Using the predicted monoclinic superstructure, we explain some peculiarities of the neutron diffraction pattern of the annealed partially ordered samples of TaC0.83–0.85. The theoretical and experimental data on the short range structural order in superstructures is discussed. We also study the influence of the M7X6 type of ordered structure on mechanical properties. The highest Vickers hardness ∼27.1 GPa is expected for the trigonal ordered Hf7C6 phase.

Understanding formation mechanisms of intermetallic compounds in dissimilar Al/steel joint processed by resistance spot welding

This paper confirmed the formation mechanism of intermetallic compounds (IMCs) in Al/steel resistance spot welds with transmission electron microscopy, electron backscatter diffraction, nanoindentation and thermodynamic calculations. In particular, the formation of AlFe with BCC_B2 structure, which is not commonly seen in welds, was identified. The formation mechanism of IMCs at the high welding energy region is described as follows. Al13Fe4 first nucleates from the Al side, followed by Al5Fe2 growth with Fe atoms accumulating at the Al13Fe4 grain boundaries. Then, Al5Fe2 grains grow continuously to coarse columnar grains, and AlFe forms at the interface of Al5Fe2 grains and the ferrite phase. Lastly, the needle-like Al13Fe4 forms in the cooling process. At the middle welding energy region, only equiaxed Al5Fe2 and small Al13Fe4 grains are formed at the interface because of the lower diffusion rates of Al and Fe, hence postponing the growth of Al5Fe2 and Al13Fe4. At the low welding energy region, only sporadic Al5Fe2 and Al13Fe4 grains are formed surrounded by the Al phase.

Ambient Pressure Synthesis of Re-Substituted MnGe and Its Magnetic Properties

Due to their non-centrosymmetric structure, B20-type compounds have intriguing properties of chiral magnets and are the objects of study of topological spin textures. Among them is a high-pressure phase MnGe, which demonstrates properties of magnetic skyrmions. We report on the synthesis of an Mn1−xRexGe solid solution with the B20 structure, which can be prepared without the application of high pressure. Mn1−xRexGe (x = 0.169(6)) shows unconventional magnetic behavior, where the Neel temperature is only slightly reduced compared to a chiral-lattice helimagnet MnGe.

Elemental Doping and Interface Effects on Spin–Orbit Torques in CoSi‐Based Topological Semimetal Thin Films

AbstractThis work reports on Ni and Fe doping and interface effects on spin–orbit torques (SOTs) generated from a topological semimetal CoSi. CoSi thin films grown on Al2O3(0001) substrates by magnetron co‐sputtering show the B20 structure with a texture in the [210] orientation even after Ni or Fe doping. The SOTs from the films exerted on the magnetization of a CoFeB layer are evaluated by harmonic Hall and spin‐torque ferromagnetic resonance measurements. The spin Hall efficiency ξSH of the textured B20‐CoSi at room temperature is determined to be 9.6%, which decreases to 1.8% for Ni0.15Co0.85Si and to 5.5% for Fe0.26Co0.74Si. The electrical conductivity dependence of the spin Hall conductivity is assigned to the regime of intrinsic mechanism of spin Hall effect, suggesting that the reduction of ξSH with the element doping can be due to the degradation in topological electronic structures of CoSi. Furthermore, inserting a Cu layer at the Co(Ni, Fe)Si/CoFeB interface results in an increase of the ξSH up to 10.9% for CoSi, 4.0% for Ni0.15Co0.85Si, and 8.3% for Fe0.26Co0.74Si. These enhancements of the ξSH can be attributed to the improvement in the interfacial spin transparency between the Co(Ni, Fe)Si and CoFeB layers.