High-temperature Β-phase Research Articles

Microstructure of as-cast aluminium bronze containing iron

The morphology, crystallography, and chemistry of phases present in an as-cast commercial aluminium bronze containing iron, of nominal composition (wt-%) Cu–10Al–2·5Fe (BS 1400: AB1), and the development of microstructure on cooling from elevated temperatures, have been studied using optical and electron microscopic techniques. The as-cast microstructure consists of α-phase, martensite, and iron-rich intermetallic precipitates. The α-phase is fcc copper-rich solid solution and exhibits a Widmanstätten morphology. The martensitic phase, which is derived from the high-temperature β-phase, has the 9R crystal structure. The intermetallic particles are based on Fe3Al and have the Do3 structure. These iron-rich particles are precipitated in the β-phase and cause a refinement of the microstructure by providing sites for the nucleation of the β-phase, to some extent, by impeding the growth of the α-phase.MST/150

Microstructure of as-cast aluminium bronze containing iron

The morphology, crystallography, and chemistry of phases present in an as-cast commercial aluminium bronze containing iron, of nominal composition (wt-%) Cu–10Al–2·5Fe (BS 1400: AB1), and the development of microstructure on cooling from elevated temperatures, have been studied using optical and electron microscopic techniques. The as-cast microstructure consists of α-phase, martensite, and iron-rich intermetallic precipitates. The α-phase is fcc copper-rich solid solution and exhibits a Widmanstätten morphology. The martensitic phase, which is derived from the high-temperature β-phase, has the 9R crystal structure. The intermetallic particles are based on Fe3Al and have the Do3 structure. These iron-rich particles are precipitated in the β-phase and cause a refinement of the microstructure by providing sites for the nucleation of the β-phase, to some extent, by impeding the growth of the α-phase.MST/150

Dilatometric and Dielectric Studies of an Incommensurate Transition in Quartz

Dielectric and dilatometric measurements were carried out to study the intermediate incommensurate phase of quartz crystal, which is stabilized between the α- and the β-phases. Anomalies associated with the transitions from the high temperature β-phase to the incommensurate phase and then to the α-phase were observed.

Decomposition of the β-Phase in Zr-20%Nb

Decomposition of the β-phase in the Zr-20%Nb alloy for different thermal treatments was studied. The grains of the high temperature β-phase which could be fully retained on quenching showed a distribution of plate-like features in electropolished or chemically etched samples. These plates were identified to be of the γ-hydride phase. The crystallography and the mechanism of formation of γ-hydride in the β-phase have been discussed. The formation of the ω-phase during isothermal ageing at temperatures between 548 K and 698 K was studied. The appearance of ω-particles of cuboidal shape in periodic arrays along <100> β directions and the pronounced tendency of alignment of clusters of ω-particles of the same variant have been explained in terms of a possible spinodal decomposition in the β-phase prior to ω-precipitation and the strain energy minimization tendency. Precipitation of α-plates was studied in two set of samples given the following thermal treatments: (i) quenched from 1173 K to 300 K followed by ageing at 773 K and (ii) rapidly cooled from 1173 K to 773 K and isothermally held. The former set showed a more homogeneous distribution of widmanstatten a-plates while in the latter, a majority of α-plates grew from the grain boundaries. Copious nucleation in the former set has been attributed to the presence of athermal co-particles or ω-like defects in samples quenched to 300 K. The periodic array of internal twins and the {334} β habit plane associated with the α-plates in the former set suggested the possibility of the operation of a shear in the α-precipitation process.

Transition ordre-desordre dans le spinelle deficitaire Ga 1,33Cr 4S 8

The spinel compound Ga 1.33Cr 4S 8 may be prepared in two different varieties. The low-temperature α-form exhibits a well-defined gallium occupancy on the tetrahedral sites, while the high-temperature β-phase is a disordered form. Single-crystal structural studies show a continuous order-disorder transition between these two limits, as a function of the synthesis temperature. Magnetic studies have been done by statical and dynamical methods.

Mechanisms of the isothermal decomposition of ?-solid solution in two-phase martensitic titanium alloys

1. The kinetics of isothermal decomposition of high-temperature β-phase for two-phase martensitic titanium alloys after undercooling from different temperatures are described by typical C-shaped curves; this decomposition may have one or two stages above the MS point. The type of isothermal decomposition diagram for this phase is governed by the alloy heating temperature and the temperature for the start of the martensite β→α″-transformation. 2. An increase in alloy heating temperature into the two-phase region leads to a shift in C-shaped curves in the direction of higher temperatures and shorter exposures, i.e., the decomposition process is accelerated. 3. With isothermal exposure for undercooled alloys in the martensite range it is possible for the martensitic β→α″-transformation to occur. With undercooling to a temperature above MS high-temperature β-solid solution is stabilized and the martensitic β→α″-transformation does not occur during subsequent cooling. 4. Mechanical properties of the alloys are goverved by the mechanism of high-temperature β-phase decomposition. Decomposition by an intermediate mechanism provides high alloy strength, although the ductility is lower. As a result of β-solid solution decomposition by a diffusion mechanism, an alloy has higher ductility with quite high strength. The optimum combination of strength and ductility as well as high property stability is obtained as a result of isothermal exposure for an alloy undercooled from the two-phase region to a temperature corresponding to the maximum rate for β-solid solution decomposition in the first stage.

A single crystal study of the defect chemistry and transport properties of silver selenide, Ag 2 + δ Se

Electronic and ionic conductivities of silver selenide crystal (Ag 2 + δ Se) have been measured over a range of stoichiometry through the α-β transition by using solid state electrochemical techniques. In the high temperature β-phase Ag 2 Se shows metallic behaviour of electronic conductivity for high values of δ ; with decrease in δ, the conductivity of the material exhibits a transition. The magnitude of change in electronic conductivity at the α -β transition is also determined by stoichiometry. Ionic conductivity of the β-phase does not vary significantly with stoichiometry. A model to explain the observed transport properties has been suggested.

Variation in the shape recovery temperature in NiTi alloys

In the near-equiatomic NiTi alloy, certain processing factors significantly affect the shape recovery temperature T f and the extent of shape recovery while others do not. The lowest recovery temperature and the best shape recovery were obtained by annealing between 450 and 500 °C. Varying the annealing temperature resulted in the largest change in these two parameters, causing an increase of 20 °C in T f and a decrease of 13% in the shape recovery (in one sample batch) if the annealing temperature was increased much above 500 °C. Increasing the maximum strain (in the range between 0% and 8%) induced an increase of 10 °C in T f at 500 °C with larger increases at other annealing temperatures but without any effect on the extent of shape recovery. Altering the annealing time had a relatively small effect. A recovery-resistant physical constraint raised the effective T f but if the constraint was maintained at a sufficiently high temperature the transformation occurred, plastic deformation resulted and permanent partial loss of shape memory was suffered. T f extrapolates to nearly the same temperature, at zero strain, for all the annealing temperatures near 500 °C. Similarly, the final temperature of the transformation from matensite to β phase (zero strain) is the same for all the annealing temperatures. This strongly suggests that each annealing temperature yields a high temperature B2 phase with a slightly different structure which interacts differently with stress. Although this study is based on only two batches of NiTi wires of similar composition, it is expected that qualitatively similar variations with processing factors occur in all NiTi alloys which exhibit shape memory.

Phase diagram and some properties of Cu2?xSe(2.01?2?x?1.75)

The temperatures of phase transformation from low-temperature α-phase to high-temperature β-phase were measured for copper selenide Cu2−x Se in the composition range 2.01≥2−x≥1.75. The α-phase was found to be unstable in air, and further experiments should be performed in a good vacuum. The phase diagram of Cu2−x Se was redetermined by the X-ray parametric method and the possible reasons why the phase diagrams reported by several authors do not coincide with each other are discussed.

Die Kristallstruktur von β-Li3VF6

Abstract The crystal structure of the metastable high temperature β-phase of Li3VF6 has been determined from 1404 independent reflections (R = 0.035). The space group is C2/c, Z = 12, a = 1440.5(6)pm, b = 868.8(3)pm, c = 1008.2(4) pm, β = 95.84(3)°, dc = 2.949 g cm−3, dm = 2.94 g cm−3. The hitherto unknown β-form of Li3CrF6 has been prepared using a mineralisator [lattice constants: a = 1438.2(5) pm, b = 859.4(3) pm, c = 1001.1(2)pm, β = 94.81(2)°]. In contrast to the cryolithe related room temperature α-form β-Li3VF6 represents a new structure type. While the V-atoms and 7/9 of the Li-atoms have octahedral coordination, the remaining 2/9 of the Li-atoms show a distorted tetrahedral environment and form binuclear [Li2F6]-groups. The octahedra are linked to units of one [VF6] and two [LiF6] groups having one common vertex and sharing three edges like the octahedra in some heteropoly anions. These units form a 3-dimensional network by additional edge and corner sharing with each other and the [Li2F6]-groups. The unusual polymorphism of the isotypic Li3MF6 compounds (M = Al, Ga, Cr, Fe, V, Ti) is discussed on the base of these new results.

Structural phase transition in ferroelectric NH4103

Abstract The structural phase transformation between the ferroelectric and paraelectric form of NH4IO, near 83°C is dominated by tilt motions of IO6-octahedra within a perovskite-like framework. Simultaneously, small displacements of NH4 along the polar axis give rise to a switchable spontaneous polarization at low temperatures. As the transition mechanism is governed by the tilting of octahedra and not by the appearance of ferroelectricity, NH4IO3 is characterised as an improper ferroelectric material. Although the space group of the high-temperature β-phase is polar, the polarization moment is zero. Raman-active thermal soft modes appear in the ferroelectric phase. The phase transition is first order with anomalies of dielectric, piezoelectric and elastic constants.

Ionic Motion and Phase Transition in Potassium Hydrogendifluoride KHF2 Studied by Pulsed NMR

Abstract Nuclear spin-lattice relaxation times, T1 and T1ρ, have been measured above room temperature for 1H and 19F nuclei in powdered KHF2 and its deuterated analogue. Below 400 K and in the high-temperature β-phase (Tt=469.8 K), the magnetization recovery for both nuclei in KHF2 is strongly non-exponential owing to the heteronuclear H–F interaction. In the low-temperature α-phase the T1 vs. 1/T curves show broad minima ascribable to the 180°-flip motion of linear [FHF]− anions. The activation energy Ea of this motion was determined to be 50.5±2 kJ/mol. The phase transition, which is defined by discontinuous changes in both T1 and T1ρ, is associated with diffusional disorder superimposed on rapid isotropic reorientation of the HF2− ion. The relaxational process in the β-phase is governed by self-diffusion of the anions with an Ea of 80±6 kJ/mol deduced from the T1ρ data. In contrast to the earlier suggestion from electrical measurements, the NMR results indicate that the dominantly mobile species in the conducting high-temperature β-phase are not single F− ions but complex [FHF]− anions.

On the equilibrium temperature in thermoelastic martensitic transformations

The shape memory effect (SME) and superelasticity (SE) of Ni50.3Ti49.7 alloy single crystals were studied after quenching and marforming (deformation in the martensite phase, followed by annealing at 713 K for 0.5 h) along the [001] direction, under compression, in the high-temperature B2-phase. It was shown that marforming leads to the following: strengthening of the high-temperature B2-phase; an increase in the temperature range of SE; a decrease in the mechanical Δσ and thermal ΔТh hysteresis; and an increase in the transformation hardening coefficient Θ = dσ/dε, in comparison with the crystals after quenching. The reasons for improvements in the functional properties of [001]-oriented single crystals after marforming are discussed.

Mixed ionic conduction in solid KHF 2

From EMF measurements it can be concluded that there is no significant electronic conductivity in KHF 2. In the relatively good conducting high temperature β-phase the following transport numbers are found: t K≈0,25 and t F≈0,75 while t H≈0 as was concluded from changes in chemical composition on electrolysis in Hittorf-Tubandt type experiments. From current-time characteristics it follows that in the low-conducting low temperature α-phase, the proton is the only mobile charge carrier in this phase, i.e. t H≈1. It also proved that the d.c. conductivity of single crystals measured with hydrogen permeable electrodes coincides with the zero-frequency conductivity found by extrapolation of a.c. data.

The tempering of the strain induced martensitic phase in zirconium 3 wt % molybdenum 1 wt % aluminum

A study has been made of certain morphological and crystallographic changes associated with the tempering of the strain induced α D 1 phase in Zr-3 wt % Mo-1 wt % Al. It has been found that tempering at 500 °C of the α D 1 phase produced a transformation to an \\ ̄ ga phase which was also h.c.p. but which is assumed to contain less molybdenum in solution than α D 1 . Concurrent with the \\ ̄ ga formation, evidence is seen of another phase, probably ZrMo 2, which changes from a fine globular coherent precipitate to an incoherent acicular precipitate as tempering proceeds. The c/ a ratio for the α D 1 phase initially increases to a maximum after 8 h at 500 °C and then decreases. This change in c/ a ratio seems to be closely connected with the hardness which follows a similar trend. Tempering at temperatures of 600 °C and above produced a partial transformation to the high temperature β-phase. Production of the β-phase caused a reduction in the c/ a ratio of the \\ ̄ ga phase as well as considerable softening of the material.

Martensitic transformation in a splat cooled Au-50 at.% Cd alloy

Martensitic transformations have been studied in splat cooled samples of an Au-50 at.% Cd alloy. A vertical shock-tube apparatus was used to quench the molten alloy from various controlled temperatures in the liquid state to room temperature, the quenching rate being of the order of 10 7°C/sec. The splat cooled samples were studied by the use of X-ray and electron diffraction as well as transmission electron microscopy. It was found that quenching, even at this high speed, from temperatures as high as 300°C above the melting point of this alloy did not affect its long range order. Furthermore, the martensitic transformation temperature was suppressed in the splat cooled samples, and the high temperature β-phase CsCl type structure persisted. This metastable structure was then found to decompose isothermally at room temperature into three morphologically and crystallographically different martensitic phases. Excellent resolution of the martensitic fine structure was obtained by this splat cooling technique.

Evolution structurale, au cours de trempes ou de revenus, d'alliages zirconium-cuivre a teneur en cuivre inferieure a 5% pds

Various zirconium-copper alloys with 1.1–1.6 or 2.4 wt % copper were quenched from the hightemperatureβ phase and their associated phase transformation studied. According to the quenchingrate, the β phase gives rise to various metastable structures which can be subsequently decomposed by tempering. The fastest quenching-rates give rise to a ω phase of hexagonal structure ( c/ a = 0.624) which forms martensitically. If the ω phase is reheated, a linear expansion anomaly of + 0.34% is found in the range 418–455 °C approximately; this corresponds to the decomposition of the ω phase into an α s solid solution, which is a supersaturated solution of copper in hexagonal zirconium, similar to the α equilibrium solid solution. On isothermal tempering or on heating to higher temperatures, α s decomposes into equilibrium α and a precipitate of Zr 2Cu. Only the most zirconium-rich alloys among those studied give rise to the metastable ω phase. The TTT curves for the decomposition of ω have been determined over a wide temperature range.

On the duplex nature and dislocation structure of the massive Cu–Ga phase

Abstract The massive phase transformation which occurs in Cu–Ga alloys on quenching from the high temperature β-phase has been studied using the technique of electron microscopy. The transformed alloys consist of a fine lamellar structure of h.c.p. or h.c.p. plus f.c.c. phases. The lamellae are twisted with respect to one another along the close-packed planes and are grouped together into packets or blocks which are joined by sub-grain boundaries that are similarly twisted. In regions where the f.c.c. lamellae are dense, their presence can be easily demonstrated by selected area diffraction through characteristic diffraction spots of the f.c.c. structure. However, in regions where the f.c.c. lamellae are less dense, the f.c.c. diffraction spots are too weak to be detected. It then becomes necessary to make a detailed study of the diffraction contrast effects. From such an analysis it was found that the observed fringe effects could not be identified with the presence of stacking faults. An examination of ...

Paramagnetic resonance of Mn2+ in KNO3 single crystal: Motional effects

The electron paramagnetic resonance of Mn2+ in KNO3 single crystal is investigated over a temperature cycle through transition temperatures. The hyperfine coupling constant, A, half width,Δ H, and the line intensity, I, are found to show sudden changes at the transition temperature, at whichα-KNO3 changes intoβ-KNO3. The lines are much sharper in the high temperatureβ-phase than inα-phase of the crystal. They are explained, qualitatively, in terms of structure change and rotation of NO 3 − ions. The spectra in the two phases,α andβ, are analysed in terms of usual spin-Hamiltonian. A search of metastableγ-phase is also made and probable indications for the same are found.