High-temperature Β-phase Research Articles

Thermodynamic evaluation of the Germanium — Tellurium system

In order to optimize the binary Ge-Te system available experimental data were critically compiled from the literature. The cubic high-temperature β-phase and the rhomboedric room-temperature α-phase were described by a two-sublattice model with one kind of defect (vacancies on germanium sites). The room-temperature orthorhombic γ-phase was treated as a stoichiometric compound, as there is a lack of information about its solubility. The liquid phase was modelled by the the associate model with one kind of associate, namely ‘GeTe’. A set of thermodynamic parameters was obtained and the calculated phase diagram is presented.

Phase stability of bismuth lead antimony oxide

Phase equilibria in the system (Bi 1− x Sb x ) 8Pb 5O 17 ( x=0–0.3) was studied using high temperature X-ray diffraction in combination with differential thermal analysis. Five phases were identified in this system: tetragonal Bi 8Pb 5O 17(β 2), tetragonal Bi 1.23Pb 0.77O 2.62(β 3), intermediate Φ-phase, body-centred cubic Bi 8Pb 5O 17(β), and orthorhombic Bi 3SbO 7(χ); the temperatures at which these different phases occurred were thus determined. Of particular interest in relation to undoped Bi 8Pb 5O 17, were the temperature ranges (on heating): below 420°C, pure β 2-phase; 420–460°C, coexistence of β 2-phase and Φ-phase; 460–590°C, Φ-phase; 590–615°C, a mixture of Φ-phase and β-phase; above 615°C, pure β-phase. On cooling, the high temperature β-phase transfers to the β 2-phase in the temperature range 615–590°C. Increasing the Sb concentration, x, in (Bi x Sb 1− x ) 8Pb 5O 17 caused a decrease in the temperature at which pure β-phase formed on heating. At 10 mol% Sb dopant level, the Φ-phase appears at much lower temperatures than the others. Two hypothetical binary diagrams (of Bi 8Pb 5O 17–Bi 3SbO 7) were constructed for heating and cooling regimes, respectively, which show the phases present in (Bi x Sb 1− x ) 8Pb 5O 17 over a range of different Sb concentrations ( x=0–0.3) and temperatures (room temperature to 630°C).

Sol-gel processing of silica supported Ni and Co molybdate catalysts used for isoC 4 alkane oxidative dehydrogenation

A novel sol-gel route has been developed to prepare silica supported Ni and Co molybdate catalysts. The metal oxides obtained are homogeneously dispersed in the silica matrix and specifically for NiMoO 4 the high-temperature β-phase which is catalytically more active has been found to be stable at room temperature. The catalytic activities of the silica supported stoichiometric Ni and Co molybdates prepared by sol-gel method have been tested continuously in the oxidative dehydrogenation of isobutane.

Improvement of Hardness and Microstructures by Ageing in Shape Memory CuAlNi Alloys

The shape memory CuAlNi alloys, having a composition near that of Cu 3 Al, exhibit disorder-order transition from the disordered bcc β-phase to the ordered β 1 -phase (DO 3 type) at high temperatures. Although the β 1 -phase is a non-equilibrium phase, it nonetheless remains stable at room temperature upon quenching from the high temperature β-phase. At the lower temperatures, the β 1 -phase itself undergoes martensitic transformation and displays layered structure called M18R. The influence of ageing on two CuAlNi β-phase alloys was studied by measuring the time dependence of hardness, lattice parameters and by-optical microscopy observation at room temperature. Hardness shows a trend to decrease with the ageing duration due to the formation of precipitation. and grain size is also affected by ageing.

Two New Phases in the System Cobalt-Tin: The Crystal Structures of α- and β-CoSn3

The tin-rich portion of the binary system cobalt-tin was reinvestigated and the existence of the new dimorphic compound CoSn 3 was established. The high-temperature β-phase is formed peritectically by reaction of CoSn 2 with the tin-rich melt at 345±2°C. The low-temperature α modification is stable below 275±5°C. Single crystals of both modifications were obtained from samples with excess of tin by dissolving the tin-rich matrix in diluted hydrochloric acid. Their crystal structures were determined from single-crystal X-ray diffractometer data ; β-CoSn 3 : I4 1 /acd, a = 627.5(1) pm, c = 3374(1) pm, Z = 16, R = 0.017 for 557 structure factors and 21 variable parameters; α-CoSn 3 : Cmca, a = 1686.4(3) pm, b = 626.8(1) pm, c = 627.0(2) pm, Z = 8, R = 0.035 for 401 F values and 22 variables. The β modification represents a new structure type, while α-CoSn 3 is isotypic with PdSn 3 . The two structures are stacking variants of each other. The β modification is a metallic conductor and Pauli paramagnetic.

A high-resolution transmission electron microscopy study of the precipitation process in a dilute Ti-N alloy

The precipitation processes in dilute nitrogen alloys of titanium have been examined in detail by conventional transmission electron microscopy (CTEM) and high-resolution electron microscopy (HREM). The alloy Ti-2 at. pct N on quenching from its high-temperatureβ phase field has been found to undergo early stages of decomposition. The supersaturated solid solution (α″-hcp) on decomposition gives rise to an intimately mixed, irresolvable product microstructure. The associated strong tweed contrast presents difficulties in understanding the characteristic features of the process. Therefore, HREM has been carried out with a view to getting a clear picture of the decomposition process. Studies on the quenched samples of the alloy suggest the formation of solute-rich zones of a few atom layers thick, randomly distributed throughout the matrix. On aging, these zones grow to a size beyond which the precipitate/matrix interfaces appear to become incoherent and theα′ (tetragonal) product phase is seen distinctly. The structural details, the crystallography of the precipitation process, and the sequence of precipitation reaction in the system are illustrated.

Characterisation and stability of the fast ion conductor (Bi 2O 3) 1 − x(PbO) x

β-(Bi 1 − x Sb x ) 8Pb 5O 17 has been shown to have very high oxygen-ion conductivity at temperatures as low as 600 °C; 1 s/cm has been reported in the literature. This paper describes how the material can be characterised using high temperature Raman spectroscopy. The effect of increasing x allows for a broadening of the peaks in the Raman spectra and a shift towards lower wavenumber. This can be explained in terms of the Sb atom deforming the room temperature tetragonal β-structure and an incipient phase change in the high temperature β-phase. This paper also describes the effect of low oxygen partial pressure on the sample. In general, as x increases, the width of the ionic domain, and hence the low P o 2 stability, increases down to approximately P o 2 = 10 −13 atmospheres. Below this dopant concentration, the stability again reduces.

Enhancing the mechanical properties of titanium alloys with rapid heat treatment

This article shows that a significant improvement in the mechanical properties of high-strength titanium alloys can be reached by using a special heat treatment based on rapid heating into the β-field. Such a treatment allows the production of a high-temperature β-phase with a fine-grained microstructure and optimal chemical inhomogeneity, which strongly determines the mechanism and kinetics of phase transformations on cooling and further annealing or aging. A wide range of final microstructures with an enhanced combination of mechanical properties can be obtained.

Shape Memory Effect and Pseudoelasticity in Ti-40%Ni-10%Cu (At.%) Single Crystals

ABSTRACTThe dependence of shape memory effect (SME) and pseudoelasticity (PE) on crystal orientation and on sign of applied stresses (tension/compression) has been investigated on Ti-40%Ni-10%Cu (at.%) single crystals. It have been shown that both SME and PE depend on an orientation and an applied stress sign. Asymmetry effects were found at deformation in high-temperature B2 phase and in the range of stress-induced martensite transformations.

Effect of χ-Phase Precipitation on Elastic Modulus of Cu–Al–Ni–(Ti)–(Mn) Shape Memory Alloys

Cu-Al-Ni shape memory alloys present a minimum in their elastic modulus during the martensitic transformation, and the values in both the martensite and high temperature β-phase, follow the same trend. The addition of Ti, as grain refiner, produces x-phase precipitates which substantially modify the elastic modulus profile of the alloy. The elastic modulus values of both martensite and β-phase increase to a different level, and no minimum during the transformation is observed. The specific changes in the elastic modulus depend upon the size distribution of the x-phase precipitates, which, in turn, is a function of the thermomechanical history of the material. In the present article, a dislocation string model is used to qualitatively explain the elastic modulus profiles, making use of the coherency degree of the precipitates.

Synthesis, Structures, Magnetic Properties, and Phase Transition of Manganese(II) Divanadate: Mn2V2O7

The α–β phase transition of Mn2V2O7has been studied with the use of single crystals grown from hydrothermal reactions. The structure of the high temperature β-phase (thortveitite) was previously determined from neutron powder diffraction data. Here we report the structure determination of a new phase, α-Mn2V2O7, and the structure refinement of β-Mn2V2O7from single-crystal X-ray diffraction data. α-Mn2V2O7crystallizes in the space groupP1(no. 2) witha= 6.868(2) Å,b= 7.976(2) Å,c= 10.927(2) Å, α = 87.81(1)°, β = 72.14(1)°, γ = 83.08(1)°,V= 564.5(5) Å3,Z= 4 atT= 293 K.β-Mn2V2O7crystallizes in the space groupC2/m(no. 12) witha= 6.7129(6) Å,b= 8.7245(5) Å,c= 4.9693(4) Å, β = 103.591(8)°,V= 282.88(4) Å3,Z= 2 atT= 323 K.β-Mn2V2O7adopts the thortveitite structure containing edge-sharing MnO6octahedra and corner-sharing V2O4−7divanadate groups, which have staggered conformation and linear V–Ob–V. The structure of α-Mn2V2O7, differs from that of β-Mn2V2O7mainly by the bending of V–Ob–V moieties, as observed in the low-temperature forms of other thortveitite-like compounds. Differential scanning calorimetry (DSC) indicated that a reversible first-order phase transition occurs at ∼296 K. The low-temperature phase exhibits a paramagnetic–antiferromagnetic transition atTN=16.0(5) K and follows Curie–Weiss behavior (μeff= 5.9 μB/Mn atom) at higher temperature. A change of Weiss constants (θp= −28 K for α-Mn2V2O7; −4 K for β-Mn2V2O7) relevant to the α–β phase transition was observed between 290 and 300 K.

Study of phase transitions in K2ZnBr4by NQR and DSC methods

Abstract The sequence of the steady-state transitions from high-temperature β-phase, as well as the metastable sequence, observed on heating of supercooled β-phase were studied on K2ZnBr4 by means of 79,81Br NQR and DSC methods. The low-temperature phase of the metastable sequence is confirmed1 to be C1c1 (Z = 48), the metastable ferroelectric phase is P21 cn (Z = 12), the transition into incommensurate phase occurs at about 18°C.

ω-phase formation in V[sbnd]Al and Ti[sbnd]Al[sbnd]V alloys

Abstract ω-phase formation in V-50 at.% A1 alloy and Ti[sbnd]Al[sbnd]V alloys has been studied using electron diffraction. It is shown that both electron and size factors are important in affecting the magnitude of the deviation of diffuse ω maxima from the commensurate ω maxima positions. It is suggested that the addition of the electron-deficient element Al to the later transition metal V will destabilize the high-temperature β-phase and favour ω formation. It is expected by analogy that the w transformation may occur in other later transition-metal based alloy systems such as Nb[sbnd]Al, Mo[sbnd]Al, Cr[sbnd]Al and Mn[sbnd]Al. Structural models have been proposed to interpret the heavy streaking effects in β + ω and B2 + ω structures.

Neutron diffraction studies of the premartensitic transformation B2 ? B19? in Ti49Ni51 single crystals

We have studied structural changes in the high-temperature B2-phase in a large single crystal at temperatures near the premartensitic transformation B2 → B19′. We are the first to observe an extra 1/2 (110) reflection in neutron diffraction patterns taken along the [110]B2 direction as the sample is cooled below 420 K, but still far from the martensite start temperature (Ms=180 K). This extra reflection heralds the formation of long-range order in atomic displacements with wave vectorq=(1/2±δ)[110]2Θ/a. Premartensitic diffraction effects (caused by the development and correlation of lattice waves of atomic displacements with wave vectorsq1′≊2Θ/a[1/3, 1/3, 0] andq1″≊2Θ/1[1/3, 1/3] that were clearly visible in this same single crystal before the martensitic transformation B2 → R, appeared at even lower temperatures with substantially lower intensities.

The Influence of Boron Additions in Cast Ti-47AL-2Cr-2Nb-0.8B

AbstractMorphology, fine structure, and chemistry of phases formed in cast Ti-47Al-2Cr-2Nb-0.8B due to boron addition were examined in detail. Boron acts an inoculant, refining and stabilizing the cast grain size. A curving ribbon-shaped phase was present throughout the material, related to the ordered phase Ti2AlCr. The ribbon phase had the B2 structure, with a0∼0.318nm. Many ribbons had a low degree of order, i. e. the material was a BCC mixture of Ti, Al, Nb, and Cr. Ribbon phase composition was variable, measured to be: Ti-(20–30)Al-(2–4)Nb-(5–16)Cr. TiB2 was present in small amounts, always in intimate contact with the ribbon phase. A (100)ribbon ‖ {1100}TiB2, (011)ribbon ‖ {1122}TiB2 orientation relationship was observed. It is proposed that the ribbon phase is a remnant of the high-temperature titanium aluminide β-phase, stabilized by dissolved boron.

A Molecular Dynamics Study of Zirconium Based on An N-Body Potential: HCP/BCC Phase Transformation and Diffusion Mechanisms in the BCC-Phase

ABSTRACTWe propose a simple interatomic potential model for zirconium which successfully reproduces the phonon dispersion curves in both the α- and the β-phases as well as the temperature-induced phase transformation. Two interesting phenomena that help to understand the anomalous diffusion in the high-temperature β-phase are observed: the spontaneous formation of Frenkel-pairs and the highly correlated walk of the vacancy.

Omega-related phases in a Ti-Al-Nb alloy

Study of phase equilibria in the Ti-Al-Nb system is often complicated by the possibility of rapid ordering reactions of a chemical or displacive type during cooling from high temperatures. In the present study we have investigated the decomposition of the high temperature B2 phase of composition Ti-37.5at%Al-12.5at%Nb into “omega-type” phases during either cooling or low temperature annealing. Formation of an “omega-type” phase from the high temperature B2 matrix of composition Ti-27.8at%Al-11.7at%Nb was observed previously by [1]. If the “omega-type” phase forms by pure displacive ordering, i.e. by collapse of (111) plane pairs of the B2 phase, the resulting structure will be as shown in Fig. 1a (termed ω′). The chemical order of the ω′ phase is directly inherited from the B2 structure. The space group of the ω′ phase is P3ml (#164). The same authors suggested a secondary transformation of the ω′ phase to another omega-type phase with the B82 structure shown in Fig.1b The space group for the B82 structure is hexagonal P63/mmc (#194) with reflection conditions such that the 1100 reflection is present, but the 0001 (or 000ℓ,ℓ=2n+l) is absent.

Structure of the α-phase of solid methanol

The crystal structure of α-methanol at 15K has been determined from neutron powder diffraction measurements. The structure is orthorhombic, space group P212121. The molecular geometry is found to be very similar to that in the gas phase, but the methyl group no longer has ideal 3-fold symmetry. The crystal is formed by infinite hydrogen-bonded chains of molecules with adjacent chains ‘pointing’ in opposite directions. The O-H … O hydrogen bonds are almost linear. No phase intermediate between the low temperature α-phase and the high temperature β-phase was found, but a new, metastable phase was discovered.

Repeated realization of the shape memory effect in TiNi alloy

1. The change in strain parameters during thermal cycling of TiNi alloy under load through the martensitic transformation range governs the tendency of the alloy towards socalled martensitic transformation creep (MTC). Consideration of MTC on the basis of ideas about the instability of crystal lattices makes it possible to interpret residual strain accumulation in the material with a reduction in activation energy for operation of dislocation sources in the temperature range of martensitic transformations. 2. Strain hardening (for example, during wire drawing with a high-temperature B2-phase structure) promotes an increase in yield strength of the matrix, which prevents development of thermoelastic martensitic transformation and thus suppresses the tendency towards MTC for alloy TiNi, it shifts the martensitic transformation temperature range into the region of lower temperatures, and it reduces the amount of SME. A subsequent softening anneal restores SME in the alloy. The optimum temperature for the softening anneal providing a high set of temperature and deformation-force parameters for SME is 450–500°C. These parameters are stable during thermal cycling on a base of 104 cycles under stress not exceeding 300–400 N/mm2. 3. The thermomechanical treatment suggested makes it possible over wide ranges to vary mechanical properties, temperature and deformation-force parameters for SME of TiNi alloy, and mainly to optimize them with the aim of improving reliability and stability for operation of elements of this alloy under conditions of repeated SME realization.

Effect of quenching rate on the physicomechanical properties of copper-aluminum-zinc alloys with reversible martensite in the structure

1. An increase in the zinc content in alloys based on copper-aluminum increases the thermodynamic stability of high-temperature β-phase as a result of an increase in the electron concentration. 2. A reduction in the cooling rate (water→air) for Cu−Al−Zn (α+β)-alloy with low β-phase stability leads to diffusion decomposition, changing the alloy phase composition and martensite morphology, which markedly increases the ductility properties and sharply reduces the level of mechanical vibration energy dissipation. 3. Quenching in air for β-single-phase Cu−Al−Zn alloys with high β-phase stability is not accompanied by diffusion decomposition and there is a marked increase in damping capacity without a change in mechanical properties.