High Β-phase Research Articles

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.

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.

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.

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.

Diffusion of hydrogen in the β-phase of pd-H studied by small energy transfer neutron scattering

The diffusion of hydrogen in β-PdH x has been studied by quasielastic neutron scattering. It is shown that the diffusion occurs through jumps between adjacent octahedral interstitial sites. The observed integrated quasielastic intensities cannot be described by a simple Debye-Waller factor. The phase transition from the β-phase to the α-phase has also been studied. No dramatic changes in the scattering patterns were observed. It is concluded that the diffusion mechanism is remarkably similar for the low concentration α-phase and the high concentration β-phase.

The dynamics of nucleation and growth of a thermoelastic martensite in a splat quenched Au-47.5 at.% Cd alloy

The dynamics of nucleation and growth of a thermoelastic martensite in a splat quenched Au-47.5 at.% Cd alloy has been studied utilizing electron microscopy. The high temperature CsCl type β-phase is retained at room temperature by splat quenching. A reversible martensite transformation has been induced in the electron transparent regions of the liquid quenched foils by varying the intensity of the illuminating electron beam while the foil was under observation in the electron microscope. This control over the transformation has permitted a study of the nature and types of martensite nucleation sites, as well as, the crystallography, morphology and growth mechanisms of the martensite embryos in this alloy. The nucleation of the 3 R martensite laths is heterogenous. The lath initially appears as a planar feature on a {110}β plane and rapidly increases in thickness until a wedge shape is achieved. Growth of the lath occurs by the propagation of this wedge into the β-phase. The screw dislocation-like nature of this wedge interface is 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.

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 ...