B2 Crystals Research Articles

Core origin of seismic velocity anomalies at Earth's core-mantle boundary.

Seismic studies have found fine-scale anomalies at the core-mantle boundary (CMB), such as ultralow velocity zones (ULVZs)1,2 and the core rigidity zone3,4. ULVZs have been attributed to mantle-related processes5-10, but little is known about a possible core origin. The precipitation of light elements in the outer core has been proposed to explain the core rigidity zone3, but it remains unclear what processes can lead to such precipitation. Despite its importance for the outer core11, the melting behaviour of Fe-Si-H at relevant pressure-temperature conditions is not well understood. Here we report observations of the crystallization of B2 FeSi from Fe-9wt%Si melted in the presence of hydrogen up to 125 GPa and 3,700 K by using laser-heated diamond anvil cells. Hydrogen dramatically increases the Si concentration in the B2 crystals to a molar ratio ofSi:Fe ≈ 1, whereas it mostly remains in the coexisting Fe liquid. The high Si content in the B2 phase makes it stable in a solid form at the outermost core temperatures and less dense than the surrounding liquids. Consequently, the Si-rich crystallites could form, float and be sedimented to the underside of the CMB interface, and that well explains the core side rigidity anomalies3,4. If a small amount of the FeSi crystals can be incorporated into the mantle, they would form dense low-velocity structures above the CMB, which may account for some ULVZs10. The B2 FeSi precipitation promoted by H in the outermost core provides a single core-driven origin for two types of anomalies at the CMB. Such a scenario could also explain the core-like tungsten isotope signatures in ocean island basalts12, after the materials equilibrated with the precipitates are entrained to the uppermost mantle by the mantle plumes connected to ULVZs.

Stability of the B2 CuZr phase in Cu-Zr-Al-Sc bulk metallic glass matrix composites

This study investigates the effect, that replacing Zr in a glass-forming Cu47.5Zr47.5Al5 alloy by Sc (0–2 at.%) has on the phase formation as well as on the thermal and mechanical properties. Even though it is not reflected in the thermal data, the glass-forming ability (GFA) is significantly reduced. This originates from the increased tendency to precipitate the shape-memory phase B2 CuZr(Sc), which, in turn, promotes the formation of bulk metallic glass (BMG) matrix composites. Sc appears to be very effective in stabilizing the B2 crystals because it forms the stable B2 CuSc phase with a similar lattice constant like B2 CuZr. By adjusting the casting parameters, the composite microstructure of Cu47.5Zr46.5Al5Sc1 can be controlled to a certain extent. The yield strength and the plasticity of the present composites depend on the crystalline volume fraction. In-situ high-energy X-ray diffraction reveals that deformation proceeds in three stages: (i) martensitic transformation of the B2 phase, (ii) yielding of the amorphous phase and continuing martensitic transformation, (iii) completion of the phase transformation and plastic deformation of all phases. Our work suggests that Sc is a promising candidate to adjust the microstructure and, thus, the mechanical properties of CuZr-based composites consisting of a glassy matrix and shape-memory crystals.

Electronic and thermoelectric properties of some CuH crystals

The first-principles calculations under the framework of density functional theory are applied to unveil electronic and thermoelectric properties of a few crystals of CuH. Lattice parameters of the B1, B2, B3 and B4 crystals are deduced. The B4 → B1, B3 → B1 pressure induced phase transitions are observed at 42.9 and 64.5 GPa respectively. Electronic bands structure calculations show that B3 and B4 crystals are semiconductors whereas B1 and B2 are metals. We report that the semi-conducting crystals B3, B4 have the indirect band gap. The E-k spectrum calculated from the first-principles method is interfaced with the Boltzmann transport equations to compute the thermoelectric properties. The Seebeck coefficient, electrical conductivity, power factor etc. are calculated. The electrical conductivity in the B4 structure is highly anisotropic. The calculated power factor (2.0–2.7) × 10−3 W/mK2 of B4 is good enough for optimum usage in thermoelectric applications at very cold stations.

In-situ tensile testing of ZrCu-based metallic glass composites

ZrCu-based bulk metallic glass composites (BMGCs) are well known for their plastic deformability, superior to traditional metallic glasses (MGs), which is attributed to a unique dual-phases structure, namely, the glassy matrix and unstable B2 phase. In the present study, in-situ tensile testing is used to trace the deformation process of a ZrCu-based BMGC. Three deformation stages of the BMGC, i.e., the elastic-elastic stage, the elastic-plastic stage, and the plastic-plastic stage are identified. In the elastic-elastic and elastic-plastic stages, the yield strength and elastic limit are major influenced by the volume fraction of the B2 crystals. In the plastic-plastic stage, the B2 phase stimulates the formation of multiple shear bands and deflects the direction of shear bands by disturbing the stress field in front of the crack tip. The deformation-induced martensitic transformation of the metastable B2 phase contributes to the plasticity and work hardening of the composite. This study highlights the formation and propagation of multiple shear bands and reveals the interactions of shear bands with structural heterogeneities in situ. Especially, the blocking of shear bands by crystals and the martensitic transformation of the B2 phase are critical for the mechanistic deformation process and illustrate the function of the B2 phase in the present BMGCs.

Rapid and partial crystallization to design ductile CuZr-based bulk metallic glass composites

Transformation-induced plasticity in CuZr-based bulk metallic glass (BMG) composites opens up a new perspective for the design of ductile BMG composites. In this work, rapid and partial crystallization of BMGs is proposed to tailor microstructures of ductile BMG composites. During the early crystallization stage, the precipitation of B2 CuZr crystals in the Cu42Zr42Al10Co6 BMGs is growth-controlled at low or high heating rates after having passed through a nucleation-dominated temperature region. Abundant nucleation is beneficial for the uniform precipitation of B2 crystals in the glassy matrix. Compared with annealing processes at slow heating, the degree of the free volume annihilation could be reduced to some content by employing a high heating rate. The microstructures of CuZr-based BMG composites can be effectively tuned by changing rapid annealing parameters, which enhances the ductility of the resulting BMG composites. This approach proves to be efficient for the development of ductile metastable alloys and composites.

Dual structure O + B2 for enhancement of hardness in furnace-cooled Ti2AlNb-based alloys by powder metallurgy

Powder metallurgic Ti2AlNb alloys were sintered at 900°C, 990°C, 1060°C, and 1100°C (i.e. in the O+B2, α2+B2+O, α2+B2, and B2 phase region, respectively) for 12h, followed by water quenching and furnace cooling. Quenching was employed to reserve the high-temperature phase and microstructure, and furnace cooling aimed to regulate the room-temperature microstructure for the enhancement of hardness. Widmanstatten B2+O structure, which contributes to the properties, was induced from B2 crystals by sintering, unless the alloy was treated in the α2+B2 phase region. With the elevation of the sintering temperature, the content of α2 phase became lower in the furnace-cooled alloys, and the hardness was improved accordingly. The highest hardness performance, 389±23HV, was obtained in the alloy solution treated in the single B2 region, and the alloy was comprised of complete O+B2 Widmanstatten structure.

Local melting to design strong and plastically deformable bulk metallic glass composites

Recently, CuZr-based bulk metallic glass (BMG) composites reinforced by the TRIP (transformation-induced plasticity) effect have been explored in attempt to accomplish an optimal of trade-off between strength and ductility. However, the design of such BMG composites with advanced mechanical properties still remains a big challenge for materials engineering. In this work, we proposed a technique of instantaneously and locally arc-melting BMG plate to artificially induce the precipitation of B2 crystals in the glassy matrix and then to tune mechanical properties. Through adjusting local melting process parameters (i.e. input powers, local melting positions, and distances between the electrode and amorphous plate), the size, volume fraction, and distribution of B2 crystals were well tailored and the corresponding formation mechanism was clearly clarified. The resultant BMG composites exhibit large compressive plasticity and high strength together with obvious work-hardening ability. This compelling approach could be of great significance for the steady development of metastable CuZr-based alloys with excellent mechanical properties.

Glass-Forming Ability and Early Crystallization Kinetics of Novel Cu-Zr-Al-Co Bulk Metallic Glasses

In recent years, CuZr-based bulk metallic glass (BMG) composites ductilized by a shape memory B2 CuZr phase have attracted great attention owing to their outstanding mechanical properties. However, the B2 CuZr phase for most CuZr-based glass-forming compositions is only stable at very high temperatures, leading to the uncontrollable formation of B2 crystals during quenching. In this work, by introducing Co (i.e., 4, 5, and 6 at. %) and 10 at. % Al into CuZr-based alloys, the relatively good glass-forming ability (GFA) of CuZr-based alloys still can be achieved. Meanwhile, the B2 phase can be successfully stabilized to lower temperatures than the final temperatures of crystallization upon heating CuZr-based BMGs. Unlike previous reported CuZr-based BMGs, the primary crystallization products upon heating are mainly B2 CuZr crystals but not CuZr2 and Cu10Zr7 crystals. Furthermore, the primary precipitates during solidification are still dominated by B2 crystals, whose percolation threshold is detected to lie between 10 ± 2 vol. % and 31 ± 2 vol. %. The crystallization kinetics underlying the precipitation of B2 crystals was also investigated. Our results show that the present glass-forming composites are promising candidates for the fabrication of ductile CuZr-based BMG composites.

Influence of Ag and Co additions on glass-forming ability, thermal and mechanical properties of Cu–Zr–Al bulk metallic glasses

In this study, the effect different amounts of Ag and Co have on thermal stability, phase formation and mechanical properties of the glass-forming alloys (Cu0.5Zr0.5)95−x−yAl5AgxCoy (2≤x≤5 and 0≤y≤2) and Cu47.5−yZr47.5Al5Coy (0≤y≤2) was investigated, respectively. The addition of Ag increases the glass-forming ability (GFA) of the alloys by enhancing the stability of the undercooled melt and by simultaneously reducing the stability of the B2 CuZr high-temperature phase. Co instead, has the opposite effect. It leads to an increased thermal stability of the B2 phase, as well as a decreased stability of the undercooled melt and, consequently, lowers the GFA of the alloys. Moreover, a metastable big cube phase precipitates in some alloys stabilized by oxygen, which is introduced during casting. There is a strong interdependence between the phase formation and the tendency of the present alloys to vitrify. This correlation can be captured by the K-parameter (Song et al. 2011 [1]) calculated from the respective transformation temperatures. The plastic strain of the present ternary, quaternary and quinary alloys reflects the GFA as well: the higher the tendency to form a glass, the smaller the plastic strain. Once B2 crystals precipitate in the glass, the plasticity is enhanced significantly.

Aggregation of binary colloidal suspensions on attractive walls

The adsorption of colloidal particles from a suspension on a solid surface is of fundamental importance to many physical and biological systems. In this work, Brownian Dynamics simulations are performed to study the aggregation in a suspension of oppositely charged colloidal particles in the presence of an attractive wall. For sufficiently strong attractions, the wall alters the microstructure of the aggregates so that B2 (CsCl-type) structures are more likely obtained instead of B1 (NaCl-type) structures. The probability of forming either B1 or B2 crystallites depends also on the inverse interaction range κa. Suspensions with small κa are more likely to form B2 crystals than suspensions with larger κa, even if the energetic stability of the B2 phase decreases with decreasing κa. The mechanisms underlying this aggregation and crystallization behaviour are analyzed in detail.

Deformation-Induced Martensitic Transformation in Cu-Zr-Zn Bulk Metallic Glass Composites

The microstructures and mechanical properties of (Cu0.5Zr0.5)100−xZnx (x = 0, 1.5, 2.5, 4.5, 7, 10, and 14 at. %) bulk metallic glass (BMG) composites were studied. CuZr martensitic crystals together with minor B2 CuZr and amorphous phases dominate the microstructures of the as-quenched samples with low Zn additions (x = 0, 1.5, and 2.5 at. %), while B2 CuZr and amorphous phases being accompanied with minor martensitic crystals form at a higher Zn content (x = 4.5, 7, 10, and 14 at. %). The fabricated Cu-Zr-Zn BMG composites exhibit macroscopically appreciable compressive plastic strain and obvious work-hardening due to the formation of multiple shear bands and the deformation-induced martensitic transformation (MT) within B2 crystals. The present BMG composites could be a good candidate as high-performance structural materials.

Simple bond-order-type interatomic potential for an intermixed Fe-Cr-C system of metallic and covalent bondings in heat-resistant ferritic steels

It is known that M23C6(M = Cr/Fe) behavior in heat-resistant ferritic steels affects the strength of the material at high temperature. The ability to garner direct information regarding the atomic motion using classical molecular dynamics simulations is useful for investigating the M23C6 behavior in heat-resistant ferritic steels. For such classical molecular dynamics calculations, a suitable interatomic potential is needed. To satisfy this requirement, an empirical bond-order-type interatomic potential for Fe-Cr-C systems was developed because the three main elements to simulate the M23C6 behavior in heat-resistant ferritic steels are Fe, Cr, and C. The angular-dependent term, which applies only in non-metallic systems, was determined based on the similarity between a Finnis-Sinclair-type embedded-atom-method interatomic potential and a Tersoff-type bond-order potential. The potential parameters were determined such that the material properties of Fe-Cr-C systems were reproduced. These properties include the energy and lattice constants of 89 crystal structures; the elastic constants of four realistic precipitates; the bulk moduli of B1, B2, and B3 crystals; the surface energies of B1 and B2 crystals; and the defect-formation energies and atomic configurations of 66 Fe-Cr-C complexes. Most of these material properties were found to be reproduced by our proposed empirical bond-order potentials. The formation energies and lattice constants of randomly mixed Fe-Cr alloys calculated using the interatomic potentials were comparable to those obtained through experiments and first-principles calculations. Furthermore, the energies and structures of interfaces between Cr carbide and α-Fe as predicted through first-principles calculations were well reproduced using these interatomic potentials.

Thermal stability and mechanical properties of Cu46Zr46Ag8 bulk metallic glass and its composites

The glass-forming ability (GFA), the thermal stability and the crystallization behavior of Cu46Zr46Ag8 metallic glass and the thermal stability of the competing metastable B2 CuZr phase were investigated in order to fabricate the corresponding shape memory bulk metallic glass (BMG) composites based on a new prediction strategy. A rather large supercooled liquid region and a relatively low liquid fragility indicate a quite high thermal stability of the present glass. With the addition of Ag to Cu50Zr50 alloy, it becomes more difficult for the metastable B2 phase to decompose into the room-temperature equilibrium phases and to transform into martensitic phases during quenching. Based on these insights, a Cu46Zr46Ag8 BMG and its composites with different volume fractions of in situ precipitated B2 crystals were fabricated by carefully controlling the casting process. The BMG composites exhibit a larger plasticity and obvious work hardening compared to the monolithic BMG, which can be attributed to the formation of multiple shear bands and a deformation-induced martensitic transformation (MT) within the B2 crystals.

Reversible stress-induced martensitic phase transformations in a bi-atomic crystal

In an earlier work, Elliott et al. [2006a, Stability of crystalline solids—II: application to temperature-induced martensitic phase transformations in bi-atomic crystals. Journal of the Mechanics and Physics of Solids 54(1), 193–232], the authors used temperature-dependent atomic potentials and path-following bifurcation techniques to solve the nonlinear equilibrium equations and find the temperature-induced martensitic phase transformations in stress-free, perfect, equi-atomic binary B2 crystals. Using the same theoretical framework, the current work adds the influence of stress to study the model's stress-induced martensitic phase transformations. The imposition of a uniaxial Biot stress on the austenite (B2) crystal, lowers the symmetry of the problem, compared to the stress-free case, and leads to a large number of stable equilibrium paths. To determine which ones are possible reversible martensitic transformations, we use the (kinematic) concept of the maximal Ericksen–Pitteri neighborhood (max EPN) to select those equilibrium paths with lattice deformations that are closest, with respect to lattice-invariant shear, to the austenite phase and thus capable of a reversible transformation. It turns out that for our chosen parameters only one stable structure (distorted α IrV ) is found within the max EPN of the austenite in an appropriate stress window. The energy density of the corresponding configurations shows features of a stress-induced phase transformation between the higher symmetry austenite and lower symmetry martensite paths and suggests the existence of hysteretic stress–strain loops under isothermal load–unload conditions. Although the perfect crystal model developed in this work over-predicts many key material properties, such as the transformation stress and the Clausious–Clapeyron slope, when compared to real experimental values (based on actual polycrystalline specimens with defects), it is—to the authors' knowledge—the first atomistic model that has been demonstrated to capture all essential trends and behavior observed in shape memory alloys.

Concentration of point defects in binary NiAl

A mean-field-type model was established to describe the concentration of point defects in binary NiAl by taking into account the effects of crystal expansion and off-stoichiometry. Different from some recent studies, this analysis shows that constitutional vacancies are present only in Al-rich NiAl at T=0 K but that the vacancy concentration decreases with increasing temperature until diffusion is activated and thereafter becomes nearly independent of temperature. Such vacancy formation behaviour in Al-rich NiAl belongs to the category of metastable constitutional type according to a method of classifying vacancy formation in B2 crystals proposed in this letter.

Growth Conditions, Electrical Resistivity, Microhardness and Thermal Properties of PrRh4.8B2 Single Crystals Synthesized from High-Temperature Copper Solutions.

PrRh4.8B2 single crystals were prepared from high temperature copper solutions by slow cooling method, using praseodymium tips, rhodium powders and boron powders as starting materials under helium flow. PrRh4.8B2 belongs to the orthorhombic system with the space group Fmmm; a=0.9697(4)nm, b=0.5577(2)nm and c=2.564(3)nm. Vickers microhardness and electrical resistivity of the compound were measured, and oxidation resistivity of the compound at high temperature in air was studied. Vickers microhardness (HV) and the electrical resistivity (ρ) value on (001) plane with the orthorhombic system of PrRh4.8B2 were in the range of HV=6.12-7.32GPa and ρ=670×10-6Ω·cm, respectively. The oxidation in air of PrRh4.8B2 crystals began to proceed at about 590°C. Weight gain of the sample heated up to 1200°C in air is 13% and the identified oxidation product consisted of the PrBO3 (orthorhombic) and Rh (cubic)

Complex formation and intermolecular electron transfer between flavocytochrome b2 in the crystal and cytochrome c.

The present study addresses the question whether tetrameric flavocytochrome b2 in the crystal is catalytically competent and, if so, whether it is possible to prepare a functional complex of the crystalline enzyme with its physiological electron acceptor cytochrome c. By single crystal microspectrophotometry we show that the native reduced enzyme can be oxidized by oxygen or ferricyanide and that the oxidized enzyme can be reduced by the electron donor L-lactate. Reduced cytochrome c appears to diffuse through the liquid channels of flavocytochrome b2 crystals and, at low ionic strength, to accumulate in amounts stoichiometrically equivalent to the enzyme protomers. Both cytochromes can be oxidized by ferricyanide. In the presence of L-lactate, both cytochromes become reduced. Since reduction of cytochrome c by L-lactate requires the catalytic action of flavocytochrome b2, it is concluded that the structure of the crystalline enzyme not only allows for electron transfer from L-lactate to flavin and intramolecular electron transfer from flavin to heme, but also for the formation of a productive complex with cytochrome c.