Intermediate Metastable Phase Research Articles

STUDIES ON THE PHASE TRANSITIONS OF LITHIUM IODATE CRYSTALS

The phase transitions of LiIO3 crystals have been investigated by the methods of DTA, heat-treatment at constant temperatures, and high-temperature X-ray diffraction. Our experimental results show that the τ phase of the crystal appears as an intermediate metastable phase such that a = τ→β, and it can exist only in the temperature range 230-300℃ within a period of several hours to a few days. Upon crystallization from the melt, a new phase or rather a group of new phases never mentioned previously in the literature has been revealed by X ray diffraction studies. We designate them as d-LiIO3. The difference in their structures is probably related to the cooling condition. After subsequent cooling to 200-180℃, the δ phases transform into the a phase. However, if the temperature is raised again before δ→ a takes place, then d directly transforms into β after reaching the τ→β temperature range. The results of our investigation may be summarized in the following diagram: temperature decreased immediately after endo- thermal peak appears

Vapour-quenched AgCu alloys

Metastable Cr0.8Cu0.2 alloy thin films with nominal thickness of 360 nm have been deposited on Si(100) substrate by co-evaporation of Cu and Cr using molecular beam epitaxy (MBE). Phase evolution, microstructure, stress development, and crystallographic texture in Cr0.8Cu0.2 thin films have been investigated by X-ray diffraction (XRD), atom probe tomography (APT) and transmission electron microscopy (TEM) combined with energy dispersive X-ray spectroscopy (EDS) during annealing of the films in the temperature range 200–450 °C. X-ray diffraction of the as-deposited thin film shows single phase bcc crystal structure of the film whereas APT observation of fine precipitates in the film matrix due to inherent compositional fluctuation indicates onset of phase separation via spinodal decomposition regime. XRD (in-situ) and APT investigation of 300 °C annealed film reveals that the early stage of phase separation involves localized formation of metastable intermediate bcc precipitate phase having 60 at% Cr and 40 at% Cu approximately (~Cr0.6Cu0.4). For longer duration of annealing at temperature ≥350 °C, such metastable bcc precipitates act as heterogeneous nucleation sites for the onset of precipitation of Cu rich fcc Cu(Cr) phase which indicates a change of phase separation mechanism from ‘spinodal decomposition’ to ‘nucleation and growth’. Annealing of the film at temperature ≥400 °C for longer duration leads to the formation of a two phase structure with Cu rich fcc precipitate phase in a Cr rich bcc matrix. Observed phase decomposition is accompanied by significant changes in the microstructure, residual stress and crystallographic texture in the Cr rich bcc film matrix which leads to the minimization of both surface and strain energies and thereby a reduction of total Gibbs free energy of the thin film. Thermodynamic model calculation has been presented in order to understand the nucleation pathway of Cu rich stable fcc Cu(Cr) precipitates via non-classical nucleation of metastable intermediate bcc Cr0.6Cu0.4 phase.

電析Ａｕ‐Ｎｉ合金の微細構造と相に関する電子顕微鏡的研究

The microstructure and phase of Au-Ni alloys electrodeposited from citric acid-cyanide acidified baths were studied by utilizing T. E. M. and S. E. M. and also by measuring microhardness. The electrodeposited alloys were found to form supersaturated solid solutions on the side of Au with a limit up to about 45at. % Ni, and also on Ni side with about 95at. % or more Ni. Over the at. % range of Ni from about 45 to 95, metastable intermediate phases, which probably have similar structures to the phases of AuNi and/or AuNi3 in metallurgically melted Au-Ni alloys, coexisted with the supersaturated solid solutions. Film growth by electrodeposition was of the following process: nucleation→island crystal→network crystal→plate crystal. Also, microcrystallites of 50-200Å in size and many microtwins were observed in the plate crystals of the films. In the initial stage of electrodeposition the film growth took place coherently with Cu substrates. The thick films deposited for 60min, on the other hand, showed smooth surfaces and the smoothness increased with increasing Ni content of the alloys. Formation of supersaturated solid solutions and metastable intermediate phases caused an increase in the film hardness. A maximum value of Hv=400 was obtained at about 80at. % Ni, but the alloys containing about 63-90 at. % Ni were brittle.

Phases ε obtenues par decomposition des spinelles non-stoechiometriques dans les systemes Ga 2O 3-MgO, Al 2O 3-NiO, Al 2O 3-AlN et Al 2O 3-Li 2O

In the systems Al 2O 3-AlN, Al 2O 3-NiO, Al 2O 3-Li 2O and Ga 2O 3-MgO, non-stoechiometric spinels, when decomposed at high temperatures, form an intermediate metastable phase ε. This phase has a one-dimensional periodic antiphase-domain structure. The antiphase boundaries are parallel to the (310) plane and the displacement vector is 1 4 [110] when referred to the spinel structure. Across each antiphase boundary, some octahedral and tetrahedral sites share faces instead of corners. In the case of ε AlN, cationic vacancies occupy these sites; in the other cases, this is likely to go along with a segregation of the more charged ions.

Non-equilibrium phases in Al-rich Al-Pt alloys

The solid solubility of Pt in Al was extended from 0 to 2 at. % by “two-piston” quenching technique. The stability of the Al-Pt solid solution during annealing is discussed. During the decomposition an intermediate metastable phase was detected, the unit cell constants of which were determined.

A metastable phase Al3Cu2

A new metastable intermediate phase having a trigonal unit cell belonging to the space groupP¯3ml has been detected in a rapidly solidified aluminium-45 at. % copper alloy. The unit cell dimensions area=4.106 andc=5.094 a. With five atoms per unit cell, the observed structure can be regarded as an isotype of Al3Ni2.

Study of disordering processes in a copper-palladium alloy

Disordering processes in aCu-40 at.% Pd alloy are studied by the method of successive quenching. For analyzing the transformations, measurements of electrical resistance and microhardness are supplemented by x-ray diffraction. For certain quench temperatures the ordered and disordered phases are present at the same time. On increasing the quench temperature the amount of ordered phase is reduced; the crystal lattice constants of the main and precipitated phases nevertheless remain constant within the limits of experimental error. During the transformation the microhardness diminishes and the electrical resistance increases, constant values being reached at a quench temperature of 760‡C, at which the alloy is completely disordered. The transition from the ordered to the disordered state occurs without the formation of any intermediate metastable phases. An x-ray diffraction analysis of the alloy reveals the orientational relationship between the matrix and precipitated phases. Experiments indicate the following mutual directions [001] bcc ∥[110]fcc and [110] bcc ∥[111]fcc.

Mechanical and structural properties of Al-Ni alloys rapidly quenched from the melt

The solid solubility of nickel in aluminum was extended up to 7.7 at. % Ni by the “two-piston” cooling technique. A strengthening was observed in Al-Ni solid solutions and, less pronounced, in flakes which contained the Al 3Ni phase. The stability of the solid solutions during annealing is discussed. During the decomposition of solid solutions an intermediate metastable phase was detected, the unit cell constants and space group of which were determined.

On the origin of metastable intermediate phases in splat-cooled binary alloys

On the origin of metastable intermediate phases in splat-cooled binary alloys

Calorimetric Study of the Glassy State. VI. Phase Changes in Crystalline and Glassy-Crystalline 2,3-Dimethylbutane

Abstract 2,3-Dimethylbutane has two crystalline phases, high temperature form (crystal-I) and low temperature form (crystal-II). On account of the supercooling effect of crystal-I, its glassy crystalline state (non-equilibrium frozen-in state) is established below Tg (76 K) by rapid cooling. The heat capacities for various phases were determined from 13 to 300 K from principal interest in the thermodynamic properties of the glassy crystal. The transition temperature from crystal-II to crystal-I and melting point were determined to be 136.02 K and 145.05 K, respectively. The heats of transition and of fusion were determined to be 6427 J mol−1 and 788 J mol−1, respectively. From these data, the residual entropy and T2 temperature were determined to be 7.4 JK−1 mol−1 and 65 K, respectively. We also calculated the ratio of Tg to T2 for several glassy crystals known hitherto, which amounts to 1.15–1.20. The stabilization effect was studied by the measurement of the change of the configurational enthalpy in the glassy crystalline state. Also, we discovered a new meta-stable intermediate phase (crystal-III) in the course of irreversible transition from the supercooled crystal-I to crystal-II. The thermal properties of this phase were also studied and its residual entropy was determined to be 2.7 JK−1 mol−1.

The Origin of Pyrophyllite Rectorite in Shales of North Central Utah

AbstractPyrophyllite is widespread in pelitic rocks of the Manning Canyon Shale in north central Utah, and the association of this mineral with other clay minerals, especially rectorite is related to the origin. The regular mixed-layer clay mineral rectorite seems to form as a result of the alteration of muscovite-paragonite during late stages of diagenesis and represents an intermediate metastable phase in the mineral paragenetic sequence. Pyrophyllite subsequently formed from the alteration of rectorite during advancing metamorphism and is the stable end member of the clay mineral assemblage.Structural interpretations of rectorite found in the Manning Canyon Shale shows a regular, alternating sequence which consists of a fixed layer of 9.6 Å and an expandable layer, varying from 10 Å to 17 Å. With ethylene glycol saturation in the natural state a basal reflection of 26.60 Å is recorded.