High-temperature Β-phase Research Articles

Determining orientation relationships for the B2 → B19′ transformation in single crystal titanium nickelide according to the texture of B19′ martensite

A precision method for determining orientation relationships upon the B2 → B19′ transformation in titanium nickelide has been developed. The method is based on analyzing the martensite texture formed in the initial high-temperature B2 phase of a single crystal having a high degree of perfection. The orientation relationships between the B19′ lattice and the initial B2 lattice of the TiNi single crystal were established from the B19′ martensite texture formed in single crystals of titanium nickelide upon the B2 → B19′ transformation and from measurements of the lattice parameters. Crystal mechanism of B19′ martensite was proposed which included the shear in the (21-1) B2 plane in the direction [−11-1] B2 by 10°. Such a shear system is typical of the bcc crystals at deformation by twinning. Absolute shear values are in a ratio of 1:4 for B2 → B19′ transformation and for twinning, respectively. Martensite deformation at an invariant lattice is accompanied by small rotations of martensite crystals (±1.6°), that increases the quantity of martensite orientations from 12 to 24.

Mechanisms of plastic deformation in microcrystalline and nanocrystalline TiNi-based alloys

Mechanisms of plastic deformation of a high-temperature B2 phase that act upon tension, compression, and high-pressure torsion in TiNi-based single crystals have been studied depending on the crystal orientation. For the crystals with orientations located near the [ $$ \bar 1 $$ 11] and [ $$ \bar 1 $$ 12] poles in the standard stereographic triangle, multiple dislocation slip prevails upon both compression and tension. In “hard” crystals with the deformation axis close to the [001] direction, in which the Schmid factors for dislocation slip are close to zero, the main deformation mechanisms are the mechanical twinning in the B2 phase and the stress-assisted B2 → B19′ martensitic transformation. All the above listed mechanisms take part in the formation of the {111}〈hkl〉 texture. The mechanism of the change in the orientation of “hard” polycrystalline grains upon the formation of a nanocrystalline and amorphous-crystalline state has been demonstrated on the example of the evolution of the structure of [001] crystals upon severe plastic deformation in a Bridgman cell.

Structural analysis of a new precipitate phase in high-temperature TiNiPt shape memory alloys

Aging of the high-temperature shape memory alloy Ti 50Ni 30Pt 20 (at.%) results in precipitation of a previously unidentified phase, which plays a key role in achieving desirable shape memory properties. The precipitate phase has been analyzed with electron diffraction, high-resolution scanning transmission electron microscopy and three-dimensional atom probe tomography. The experimental observations show that the precipitates have unique crystallography due to their non-periodic character along one of the primary crystallographic directions. It will be shown that the structure can be explained in terms of crystal intergrowth of three variants of a monoclinic crystal. The monoclinic crystal structure is closely related to the high-temperature cubic B2 phase; the departure of the structure from the B2 phase can be attributed to ordering of Pt atoms on the Ni sublattice and relaxation of the atoms (shuffle displacements) from the B2 sites. The shuffle displacements and the overall structural refinement were deduced from ab initio calculations.

Examination of Multiphase (Zr,Ti)(V,Cr,Mn,Ni)2 Ni-MH Electrode Alloys: Part II. Solid-State Transformation of the Interdendritic B2 Phase

Solidification microstructure of multicomponent (Zr,Ti)-Ni-(V,Cr,Mn,Co) alloys intended for use as negative electrodes in Ni-metal hydride (Ni-MH) batteries was studied in Part I of this series of articles. Part II of the series examines the complex internal structure of the interdendritic grains formed by solid-state transformation and believed to play an important role in the electrochemical charge/discharge characteristics of the overall alloy composition. By studying one alloy, Zr21Ti12.5V10Cr5.5Mn5.1Co5.0Ni40.2Al0.5Sn0.3, it is shown that the interdendritic grains solidify as a B2 (Ti,Zr)44(Ni,TM)56 phase, and then undergo transformation to Zr7Ni10-type, Zr9Ni11-type, and martensitic phases. The transformations obey orientation relationships between the high-temperature B2 phase and the low-temperature Zr-Ni–type intermetallics, and consequently lead to a multivariant structure. The major orientation relationship for the orthorhombic Zr7Ni10 type is [011]Zr7Ni10//[001]B2; (100)Zr7Ni10//(100)B2. The orientation relationship for the tetragonal Zr9Ni11 type is [001]Zr9Ni11//[001]B2; (130)Zr9Ni11//(100)B2. Binary Ni-Zr and ternary Ti-Ni-Zr phase diagrams were used to rationalize the formation of the observed domain structure.

Electronic and crystal structure of α- and β-CeIr2Si2

Abstract We present the results of XRD, magnetization, resistivity and specific heat measurements of CeIr2Si2 single crystals for both, the low-temperature α-phase and the high-temperature β-phase, respectively. The α-phase adopts the tetragonal ThCr2Si2-type whereas the β-phase forms in the CaBe2Ge2-type structure. Both the phases remain paramagnetic down to low temperatures, nevertheless both, the magnetization and resistivity exhibit pronounced anisotropy in the whole temperature range of measurements (2–300 K). Results of fitting the temperature dependence of the susceptibility within the interconfiguration-fluctuation model point to the Ce valence fluctuating between 3+ and 4+. The α-phase behaves as a Fermi-liquid (FL) at low temperatures whereas the β-phase exhibits non-Fermi-liquid (NFL) features. The results are discussed in context of other similar polymorphic compounds.

Insight into Mg(BH4)2 with Synchrotron X-ray Diffraction: Structure Revision, Crystal Chemistry, and Anomalous Thermal Expansion

Pure Mg(BH4)2 has been characterized by single-crystal and powder synchrotron X-ray diffraction and by vibrational spectroscopies. The earlier reported P61 structure of the α-phase is revised in the space group P6122. Location of the H-atoms from powder data in the published P61 models posed the main problem for the identification of the correct symmetry: wrongly determined orientations of some BH4 groups hampered a successful detection of the true P6122 symmetry. Four nearly ideally tetrahedral BH4 groups form a dodecahedral MgH8 coordination around each Mg atom, which can be described as a slightly distorted snub disphenoid, where two nearly planar BH2−Mg−H2B fragments are situated at ∼90° dihedral angle. The H···H distances between two BH4 groups within the BH2−Mg−H2B fragments are among the shortest (2.18−2.28 Å) in the structurally characterized metal borohydrides. α-Mg(BH4)2 contains an unoccupied void, accounting for 6.4% of space in the structure. It is large enough (37 Å3) to accommodate a small molecule, such as H2O. The high-temperature β-phase is less dense by ∼3% but contains no unoccupied voids. The α-phase transforms into the β-phase above 490 K; the latter is quenched (metastable) on cooling. The anomalous cell expansion of the β-phase down to 100 K may be related to the evolution of the free energy profile far from the phase transition temperature.

Decomposition of the ω-phase in the equiatomic TiZr alloy under high pressure

The phase decomposition phenomenon is found in the hexagonal ω-phase of the Ti—Zr system under high pressure. The ω → ω1 + ω2 decomposition of the equiatomic TiZr alloy occurred due to long thermobaric treatment at P = 5.5±0.6 GPa and T = 710±30 K. The chemical compositions of the ω1- and ω2-phases recovered to ambient conditions were estimated from the X-ray data to be around Ti20Zr80 and Ti83Zr17. The experimental data were used to calculate the mixing energy and the top of the decomposition curve in the isobaric T-C diagram of this system. We find that the equilibrium T-C phase diagram of the Ti-Zr system at pressures above ∼8 GPa is of the eutectoid type with the high-temperature β-phase and the low-temperature ω1- and ω2-phases.

Development of texture inhomogeneity in sheets from Zr-based alloys under high-temperature rolling

Deformation processes under hot rolling of sheets from Zr-based alloys for chemical and nuclear industries are considered by data of the layer-by-layer X-ray texture analysis. An essential texture inhomogeneity of studied sheets testifies, that the deformation conditions in successive layers prove to be rather various depending on the distance of layer from the surface and promote operation of different mechanisms of plastic deformation, including crystallographic slip in the low-temperature α-phase with HCP crystalline lattice and in the high-temperature β-phase with BCC crystalline lattice as well as noncrystallographic mutual shifts of grains by their boundaries. The latter is conditioned by development of dynamic recrystallization or phase transformations, observed by texture features of corresponding layers. The real temperature of concrete layers deviates from the nominal one within (α+β)-region of Zr-Nb phase diagram due to the heat sink to rolls and local heating by deformation. Changes in reductions per pass and in the total number of passes allow to weaken the texture component, produced by rolling in β-phase and unfavorable for mechanical properties of the sheet.

Phase formation in Ti/Ni dissimilar welds

We explore phase formation in Ti/Ni dissimilar welds using a combination of microscopy and composition analysis (TEM, SEM and EDS). Main microstructural features are NiTi dendrites and Ti 2Ni grains in the inter-dendritic space. The high temperature B2 phase of NiTi is found to transform to trigonal ‘R’-phase, B19 ′martensite, and rhombohedral Ni 4Ti 3 phase; these different transformation products highlight the composition inhomogeneity in the parent B2 phase and probable non-equilibrium solidification events during rapid cooling of the weld. Solidification sequence of NiTi and Ti 2Ni was found to vary depending on local conditions in the weld. Formation of impurity nitride phases of titanium is observed which signify incomplete shielding during welding.

Enantiotropic Polymorphism in Di-indenoperylene

The enantiotropic polymorphic phase transformation of di-indenoperylene (DIP) at 403 K has been structurally characterized, using single-crystal X-ray diffraction. Both the low temperature α- and the high-temperature β-phase have a herringbone-type structure, with the α-phase being triclinic with doubled unit cell volume compared to the monoclinic β-phase. In the latter, the molecules have a more upright orientation in the herringbone plane. The epitactic transformation from β- to α-phase involves strong shearing displacements as well as bending and torsional deformations of the DIP molecules.

Phase stability and ionic conductivity in substituted La 2W 2O 9

Different substitutions, i.e. Sr 2+, Ba 2+, K +, Nb 5+ and V 5+, have been performed in the triclinic α-La 2W 2O 9 structure in order to stabilise the high temperature and better ionic conductor cubic β-phase. This approach has been used to try to obtain a new series of ionic conductors with LAMOX-type structure without molybdenum and presumably better redox stability compared to β-La 2Mo 2O 9. Nanocrystalline materials obtained by a freeze-drying precursor method at 600 °C exhibit mainly the β-La 2W 2O 9 structure, however, the triclinic α-form is stabilised as the firing temperature increases and the crystallite size grows. Only high levels of Ba 2+ and V 5+ substitutions retained the cubic form at room temperature after firing above 1100 °C. However, these phases are metastable above 700 °C, exhibiting an irreversible transformation to the low temperature triclinic α-phase. The synthesis, structure, phase stability, kinetic of phase transformation and electrical conductivity of these materials have been studied in the present report.

Laser Processing of Net-Shape NiTi Shape Memory Alloy

Fabrication of bulk NiTi alloy parts by conventional processing contains undesirable intermetallic phases in varying proportions depending on the composition and requires postfabrication heat treatments, which necessitate development of new processing routes. We have fabricated net-shape samples, with and without designed porosity, from equiatomic NiTi alloy powders using laser engineered net shaping (LENS), a commercially available rapid prototyping technique. Laser-processed NiTi samples retained the desired phases of initial powder and showed higher hardness due to the retention of high-temperature cubic B2 phase. Thermally induced stresses and defects, due to high cooling rate in laser processing, increased the reverse transformation temperatures of laser-processed samples. LENS-processed samples also showed high compressive strengths (plastic flow stress) in the range of 890 to 1,050 MPa due to finer grain size. This article discusses the influence of laser parameters on microstructure, mechanical properties, and phase transformation behavior of fully dense NiTi parts produced via LENS.

Electro-optical switching of a thin antiferroelectric liquid crystal film in the planar geometry with evaporated top electrode

An antiferroelectric liquid crystal phase formed by bent-shape molecules was investigated in a geometry typical of solid-state technology. A thin Langmuir film of the compound was transferred onto a conductive glass and supplied with a top electrode evaporated in vacuum. Upon heating the sandwich keeps its integrity even in a high-temperature B2 phase which, despite strong confinement, manifests antiferroelectric and electrooptic properties similar to those in bulk samples. This approach fills the gap between investigations of solid and liquid crystal ferroelectrics and open new ways for studying dielectric, ferro- and antiferroelectric properties of thin and ultrathin films of liquid crystals.

Crystal structure of non-stoichiometric copper selenides studied by neutron scattering and X-ray diffraction

Structural characteristics of non-stoichiometric copper selenides were studied by the elastic neutron and X-ray scattering techniques. Rietveld analysis was used to refine the structure of the high-temperature β-phase of the Cu1.75Se, Cu1.78Se, and Cu1.83Se samples. The homogeneity ranges of the cubic phase were determined. The modification of the crystal structure accompanying the β-α phase transition was studied for Cu1.75Se and Cu1.98Se compounds within the 443-10 K temperature range. It was shown that the phase transition is accompanied by distortions of the fcc lattice and the ordering of copper ions.

Non-Contact Measurements of the Thermophysical Properties of Hafnium-3 Mass% Zirconium at High Temperature

Several thermophysical properties of hafnium-3 mass % zirconium, namely the density, the thermal expansion coefficient, the constant pressure heat capacity, the hemispherical total emissivity, the surface tension and the viscosity are reported. These properties were measured over wide temperature ranges, including overheated and undercooled states, using an electrostatic levitation furnace developed by the National Space Development Agency of Japan. Over the 2220 to 2875 K temperature span, the density of the liquid can be expressed as ρL(T)=1.20×104−0.44(T−Tm) (kg⋅m−3) with Tm=2504 K, yielding a volume expansion coefficient αL(T)=3.7×10−5 (K−1). Similarly, over the 1950 to 2500 K span, the density of the high temperature and undercooled solid β-phase can be fitted as ρS(T)=1.22×104−0.41(T−Tm), giving a volume expansion coefficient αS(T)=3.4×10−5. The constant pressure heat capacity of the liquid phase can be estimated as CPL(T)=33.47+7.92×10−4(T−Tm) (J⋅mol−1⋅K−1) if the hemispherical total emissivity of the liquid phase remains constant at 0.25 over the 2250 K to 2650 K temperature interval. Over the 1850 to 2500 K temperature span, the hemispherical total emissivity of the solid β-phase can be represented as eTS(T)=0.32+4.79×10−5(T−Tm). The latent heat of fusion has also been measured as 15.1 kJ⋅mol−1. In addition, the surface tension can be expressed as σ(T)=1.614×103−0.100(T−Tm) (mN⋅m−1) and the viscosity as h(T)=0.495 exp [48.65×103/(RT)] (mPa⋅s) over the 2220 to 2675 K temperature range.

Enthalpy measurements of the solid high-temperature β-phase and the liquid phase of hafnium (3 wt.% Zr) by levitation drop calorimetry

The molar enthalpy, Δ H, of hafnium (3 wt.% Zr) has been measured by levitation drop calorimetry for the solid β-phase in a temperature range from T=2096 to 2455 K and for the liquid phase from T=2339 to 2988 K. The temperatures of the α to β and the β to liquid transition of this material are 2012 and 2471 K, respectively. The enthalpies obtained are represented by the following enthalpy-temperature functions: Solid β-Hf ΔH /J mol −1=−8299+31.79 T /K+1.42×10 −3 T 2 /K 2. Liquid Hf ΔH /J mol −1=99171+37.51(T−2471) /K. The experimental values scatter within a relative error of ±4.8%. The average deviation between the values and the curves given by the two functions is ±1.06% for the measurements on the solid phase and ±1.27% for the liquid phase. The resulting values for the enthalpy of fusion, Δ fus H, and the entropy of fusion, Δ fus S, are 20.247±2.1 kJ mol −1 and 8.19 J mol −1 K −1, respectively. The results are compared with enthalpy estimates given in the literature where only two experimental data points are available for the solid β-phase.

Enthalpy measurements of the solid high-temperature β-phase of titanium and zirconium by levitation drop calorimetry

The molar enthalpy, Δ H, of β-titanium and β-zirconium has been measured by levitation drop calorimetry in a temperature range of T=1637–1908 K for Ti and T=1821–2105 K for Zr. The melting temperatures are 1939 K for Ti and 2125 K for Zr. The values obtained are represented by the following enthalpy–temperature functions in J·mol −1: β-titanium: Δ H=−348+21.47· T+3.91·10 −3· T 2; β-zirconium: Δ H=24 556−2.63· T+9.17·10 −3· T 2. The experimental results of the enthalpy are tainted with a relative error of ±2.0% for Ti and ±3.1% for Zr. The average deviation between the experimental values and the curves given by the two functions is ±0.53% for Ti and ±0.25% for Zr. In combination with enthalpy data of the liquid phases the values for the enthalpy of fusion can be obtained: Δ H f=13 017±649 J·mol −1 for Ti; Δ H f=17 282±695 J·mol −1 for Zr. The resulting values for the entropy of fusion are Δ S f=6.71 J·mol −1·K −1 for Ti and Δ S f=8.13 J·mol −1·K −1 for Zr. The results are compared with those provided by the literature where just one experimental enthalpy study is available for β-titanium in the temperature range investigated. The available literature data for the β-zirconium are derived from measurements of the enthalpy at lower temperatures and heat capacity studies.

Crystal structure of the high-pressure monoclinic phase-II of cristobalite, SiO2

AbstractThe crystal structure of the high-pressure phase-II of cristobalite has been solved by neutron diffraction (space group P21/c, a = 8.3780(11) Å, b = 4.6018(6) Å, c = 9.0568(13) Å, β = 124.949(7)°, at P = 3.5 GPa). This phase corresponds to a distortion of the high-temperature cubic β-phase, rather than of the ambient temperature and pressure tetragonal α-phase.

Structural and electrical behavior of Bi 2VO 5 with temperature. : Stabilization at room temperature of the high temperature polymorphs

Mixed bismuth(III)–vanadium(IV) oxide Bi 2VO 5 was prepared as a very crystalline powder by solid-state reaction under vacuum and N 2 flow. Thermal analysis and powder X-ray diffraction at high temperatures confirmed the existence of two phase transitions, α → β at 397°C and β → γ at 778°C due to the progressive loss of oxygen vacancy ordering. Impedance spectroscopy measurements showed that this material is a good ionic conductor. This is particularly true of the β-phase, which exhibits a conductivity as high as 5 × 10 –2Ω –1·cm –1 at 700°C. Three solid solutions Bi 2V 1–xGe xO 5 (0 ≤ x < 0.5), Bi 2V 1–yTi yO 5 (0 ≤ y ≤ 0.1) and Bi 2V 1–zFe zO 5–z/2 (0 ≤ z ≤ 0.4) were studied. The X-ray powder diffraction patterns show that partial substitution of isovalent Ge(IV) and Ti(IV) or aliovalent Fe(III) on the V(IV) site stabilizes the high-temperature β-phase (x < 0.5, z ≤ 0.4) and γ-polymorph (y ≤ 0.1).

Reorientation behaviour of bent core molecules in response to an external electric field as detected by time-resolved FTIR spectroscopy

Time-resolved polarized Fourier transform infrared spectroscopy (FTIR) is employed to analyse the segmental orientation and mobility of achiral bent core molecules in response to an external electric field. By shearing the substance between ITO coated CaF2 windows two types of domain, racemic and homochiral, are formed in the high temperature B2 phase. Each of these domains is characterized by two spontaneous symmetry-breaking instabilities which yield a symmetric and an antisymmetric electro-optical response, respectively. Taking advantage of the specificity of IR spectroscopy, this switching behaviour is analysed on a molecular level for the moieties of the bent core liquid crystal materials. In this way, the electrically induced reorientation of the different segments on a cone and the suppression of the antiferroelectric structure at higher frequencies can be followed in detail. Furthermore the biased rotation of the two carbonyl groups around the molecular long axis is determined. It is shown that all molecular units move synchronously on the time scale of the experiment (10mus).