Transformation Of Metastable Phase Research Articles

The roles of water and polyelectrolytes in the phase transformation of amorphous calcium carbonate

Amorphous calcium carbonate (ACC) is a metastable phase of calcium carbonates and it is a precursor during the biomineralization of calcium carbonate in nature. Different ACC in the absence and presence of the additives of poly(sodium 4-styrene sulfonate) and poly(acrylic acid) were prepared and their transformation processes under controls of water were investigated. It was found that ACC powders generally contained about 15% water. Two different transformations via thermal and solution pathways were studied in the present work. We found that water was released from the ACC samples when the temperature was above 100 °C and the calcium carbonate began to crystallize at around 270–400 °C, which resulted the calcite crystals. The involvement of the polymer additives could inhibit this crystallization process and the crystallization temperature shifted to the higher values. During the solution transformation, the evolution steps of the ACC in different water–ethanol solution were monitored by FT-IR. In this case, water could accelerate the transformation and crystallization of ACC. An increasing of the water amount in the mixed solvents always led to the promotion of the transformation kinetics. Interestingly, the other metastable crystal phases of calcium carbonates, vaterite and aragonite, were readily to be induced during the solution transformation of ACC when the low water amounts were applied. However, water could still make them turned into the stable calcite phase with the experimental time prolonged. Since the additives influenced the transformation of ACC and inhibited the transformation of metastable crystal phase, they could be used to stabilize different phases of calcium carbonates in the solutions. The current experimental results confirmed that the controls of water amount and additives played important roles in the transformation of ACC.

Na+-fast ionic conducting glass-ceramics of silicophosphates

The Na+-fast ionic conducting glass-ceramics with Na5YSi4O12 (N5)-type structure were successfully synthesized using the composition formula of Na3+3x-y R1-x P y Si3-y O9 for a variety of rare earth ions, R, under the appropriate composition parameters. In the crystallization of N5-type glass-ceramics, its relatives (Na3YSi3O9 (N3)- and Na9YSi6O18 (N9)-type glass-ceramics) structurally belonging to the family of Na24−3x Y x Si12O36 were found to crystallize as the precursor phase at low temperatures. In order to produce N5 single phase glass-ceramics, the concentration of both phosphorus and rare earth was found important. The meaning of the composition was evaluated by thermodynamic and kinetic studies on the phase transformation of metastable N3 or N9 phases to stable N5 phase with Na+-fast ionic conductivity. The possible combinations of x and y became more limited for the crystallization of the fast ionic conducting phase as the ionic radius of R increased, while the Na+ conduction properties were more enhanced in the glass-ceramics of larger R. These results are discussed in view of the structure and the conduction mechanism. Also studied were the microstructural effects on the conduction properties, which were dependent upon the heating conditions of crystallization. These effects were understood in relation to the grain boundary conduction properties as well as the transmission electron microstructural morphology of grain boundaries.

Age-hardening by the formation of metastable phases in an In-added Au–Ag–Cu–Pd alloy

The age-hardening of an In-added Au–Ag–Cu–Pd alloy was examined by characterizing the hardening behavior, phase transformations and changes in microstructure, and elemental distribution during aging over the temperature range 200 to 500 °C. The major product phases, Ag-rich α 1 and AuCu I, were formed homogeneously from the parent α 0 phase by the transformation of precursor metastable phases in the grain interior. This led to a significant age-hardening response. A minor product phase, InPd, containing Cu and Zn, contributed to the overall hardness increase by initiating a grain boundary precipitation reaction. Continued long-time aging led to a hardness decrease due to overaging, which involved the formation of a lamellar microstructure composed of the stable Ag-rich α 1 and AuCu I phases. At the longest aging time at 400 °C, the lamellar structure consumed the matrix α 1 and AuCu I phases.

Phase transformation of the A15 metastable phase of Fe-Cr thin films prepared by ion-beam sputtering

Thermal stability of metastable A15 Fe-Cr phase is investigated through the study of its magnetic and structural properties. This phase presents very interesting mechanical properties suggesting that A15-structured films might be of great interest for tribological applications when considering the hardness H over Young’s modulus E ratio i.e., a description in terms of “elastic strain to failure” for wear resistance. Indeed, H is multiplied by a factor 2 with respect to the value measured for the bulk cubic centered α phase whereas E remains identical. Then, an improvement by a factor 8 of resistance to plastic deformation may be expected since predictive models stand that this quantity is proportional to the H3∕E2 ratio. However, heating problems due to sliding during tribological tests may lead to structural transformation in the film and then a loose of mechanical performance. The formation and the stability of the A15 cubic structure (δ phase) in centered-cubic refractory metals are generally attributed to the presence of oxygen atoms in the unit cell. For equiatomic Fe-Cr thin films elaborated by physical vapor deposition techniques, residual oxygen atoms present in the deposition chamber would be absorbed during the deposition process. In this work, the δ-phase transformation has been studied ex situ in the temperature range 400−650 °C; structural changes have been accurately investigated thanks to the combination of x-ray diffraction and Mössbauer spectroscopy techniques. Thin films were deposited onto quartz substrates and then annealed ex situ under secondary vacuum. From 400 °C, a “structural relaxation” occurring in the δ phase precedes and accompanies the beginning of the phase transformation. Finally, the partially ordered metastable δ-phase transforms into a stable α-phase presenting the precipitation phenomenon at temperature above 550 °C and the presence of a τ-carbide phase is clearly visible from 600 °C. The δ-phase transformation is completed at 650 °C. The stability of the A15 type structure up to 550 °C is a promising indication of good tribological performance of such coatings.

Mechanically induced cyclic metastable phase transformations ofZr2Nialloys

Cyclic crystalline-glassy-crystalline phase transformations have been investigated during high-energy ball milling of tetragonal-${\mathrm{Zr}}_{2}\mathrm{Ni}$ alloy powders using mechanical disordering (MD) method. The results show that the metastable $\mathrm{fcc}\text{\ensuremath{-}}{\mathrm{Zr}}_{2}\mathrm{Ni}$ alloy powders which is obtained after $173\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$ of MD time, transforms into a new metastable phase of nanocrystalline big cube-${\mathrm{Zr}}_{2}\mathrm{Ni}$ upon milling for $360\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$. The lattice parameters ${a}_{0}$ for the metastable-fcc and big-cube phases were calculated and found to be $0.451\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$ and $1.226\phantom{\rule{0.3em}{0ex}}\mathrm{nm}$, respectively. The obtained big-cube phase subsequently transforms to a single glassy phase after $540\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$ of MD time. This obtained glassy phase, however, devitrificated into the same metastable big-cube phase upon increasing the MD time to $720\phantom{\rule{0.3em}{0ex}}\mathrm{ks}$. As the MD time increases $(806--900\phantom{\rule{0.3em}{0ex}}\mathrm{ks})$, the obtained nanocrystalline big-cube phase transforms into the pervious metastable $\mathrm{fcc}\text{\ensuremath{-}}{\mathrm{Zr}}_{2}\mathrm{Ni}$ phase, indicating a cyclic-phase transformations between the three obtained metastable phases. Such cyclic phase transformations are observed several times during the milling procedure. On the basis of our results, the destabilizing effect of the defects created by the milling media (balls) is responsible for such mechanically induced solid-state devitrification that leads to the cyclic transformations.

Study of secondary intermetallic phase precipitation/dissolution in Zr alloys by high temperature–high sensitivity calorimetry

It is well established that in Zr alloys, secondary precipitate phases (SPP) have a major influence on the materials properties (corrosion and mechanical behaviour). Thus, it is of great importance to know the SPP formation and dissolution temperatures and their possible metastability as a function of the alloy chemical composition, the thermal treatments and the SPP characteristics (crystallographic structure and chemical composition). The aim of the present paper is to give an overview of the studies performed recently on different alloy families, i.e., Zr–Fe–V, Zr–Fe–Mo, Zr–Nb–Fe and Zircaloy (Zr–Sn–Fe–Cr) types. High temperature–high sensitivity calorimetry (Setaram HF-DSC) has been extensively applied on these alloys. From the experimental thermograms, we have derived the SPP fraction evolution as a function of the temperature, and also the enthalpy associated with their dissolution upon heating and precipitation upon cooling. It has thus been possible to study the reversibility of the reactions involved. Depending upon the thermal diffusivities of the alloying elements, we have shown that the alloys studied can be classified into two sub-groups: (1) alloys with fast-diffusing elements, such as Cr, Fe, V, showing reversible phase transformations upon heating and cooling; (2) alloys with slow-diffusing elements, such as Nb or Mo, showing metastable phase transformations, that are characterised by a partial Zr-Beta decomposition and metastable SPP precipitation upon cooling. In addition, the sensitivity of calorimetric measurements to low fractions of SPP (less than 1%) is demonstrated.

Investigation of scale inhibition mechanisms based on the effect of scale inhibitor on calcium carbonate crystal forms

To probe the scale inhibition mechanisms, calcium carbonate scale occurring before and after the addition of scale inhibitors was collected. The results from scale SEM confirm that, without scale inhibitor, calcium carbonate scale shows rhombohedron and hexagon, which are the characteristic feathers of calcite. After addition of inhibitors, morphology of scale is changed, and the more efficient the scale inhibitor is, the more greatly the morphology is modified. To elucidate the scale constitute, they were further analyzed by FT-IR, XRD. Besides calcite, vaterite and aragonite occur in calcium carbonate scale after addition of inhibitors, and the higher scale inhibition efficiency is, the more vaterite presents in scale. It can be concluded that the alteration of morphology is ascribed to the change of crystal form. There are three stages in the crystallizing process including occurrence and disappearing of unstable phase, occurrence and disappearing of metastable phase, development of stable phase. Without scale inhibitors, metastable phases usually transform into stable phase, thus the main constitute of formed scale is calcite. When scale inhibitors are added, both formation and transformation of metastable phases are inhibited, which results in the occurrence of aragonite and vaterite. From the fact that more vaterite presents in scale with a more efficient scale inhibitor added, we can see that the function of scale inhibitor is realized mainly by controlling the crystallizing process at the second stage.

地球マントル鉱物の高圧相転移カイネティクス

Kinetics of high-pressure transformations in Earth's mantle minerals have been studied up to 35 GPa and 2,000 K based on time-resolved X-ray diffraction measurements with energy-dispersive method by combining Kawai-type high-pressure apparatus and synchrotron radiation. Kinetic behaviors are different among various types of transformations occurred in the mantle transition zone, and the rates of transformations depend on not only temperatures but also water contents. These metastable phase transformations would affect the density and viscosity of the down going cold current of mantle convection.

In situ x-ray study of the γ- to α-Al2O3phase transformation during atmospheric pressure oxidation of NiAl(110)

The oxidation in air of NiAl(110) was investigated in the temperature range from 870 °C–1200 °C by in situ x-ray diffraction and transmission electron microscopy. Oxidation at 870 °C and 1 bar oxygen leads to the formation of an epitaxial layer of γ-alumina showing an R30° orientation relationship with respect to the underlying substrate. At oxidation temperatures between 950 °C and 1025 °C, we observed a coexistence of epitaxial γ- and polycrystalline δ-Al2O3. The α-Al2O3starts to form at 1025 °C and the complete transformation of metastable phases to the stable α-alumina phase takes place at 1100 °C. The fcc-hcp martensitic-like transformation of the initial γ-Al2O3to epitaxial α-Al2O3was observed. X-ray diffraction and cross-section transmission electron microscopy proved the existence of a continuous epitaxial α-Al2O3layer between the substrate and the polycrystalline oxide scale, having a thickness of about 150 nm. The relative orientation relationship between the epitaxial alumina and the underlying substrate was found to be NiAl(110) || α-Al2O3(0001) and [110] NiAl || [1120].

Microstructures and phase formations in the surface layer of an AISI D2 steel treated with pulsed electron beam

The nanostructures and metastable phase transformations in the surface layer of an AISI D2 steel treated with high current pulsed electron beam (HCPEB) were investigated. The surface structure is marked by two distinct features, i.e. the formation of sub-micrometer fine austenite γ grains (50–150 nm), and the disappearance of carbides via dissolution and crater eruption. The γ phase directly grows from the melt and is retained down to room temperature. Although the cooling rate is as high as 10 7 K/s in our case, the martensitic transformation could completely be suppressed. Such an effect is due to the increased stability of the austenite phase through grain refinement and chemistry modification.

Precipitation kinetics and transformation of metastable phases in Al–Mg–Si alloys

In situ synchrotron small-angle X-ray scattering (SAXS) and transmission electronic microscopy were used to study the precipitation and transformation behaviors of metastable β″ and β′ phases of naturally aged Al–Mg–Si alloys, AA 6022, during continuous heating, and subsequent isothermal aging. A new SAXS analysis approach for the complex SAXS profiles enabled a quantitative analysis of the structural evolution and the corresponding kinetics of the concurrent needle-like β″ and rod-like β′ phase transformations, of different size characteristics. The stable cross-section and rapid length growth along the needles of β″ phase, and the radius growth of β′ phase during the temporal evolution were characterized well by the concomitant nucleation, growth and coarsening stages. For an alloy aged at 260 °C, the β″ precipitates (∼3.5 nm in radius) and β′ precipitates (∼10.5 nm in radius) reached the coarsening stages at t = 30 and 210 min, respectively. During the coarsening stage, the β″ precipitates dissolved partially into the matrix and transformed partially to the β′ phase, and then stabilized with the residual ∼65% precipitates. The same methodology was also applied to an Al–Mg–Si alloy, AA 6111, containing Cu. The β″ precipitates of AA 6111 alloy aged at 180 °C showed an inert radius (∼1.8 nm) and a growing length with aging time, and the longer incubation period of ∼60 min observed in the early stage of precipitation, relative to AA 6022, was attributed to the more natural clusters.

Metastable phases, phase transformations, and phase diagrams in physics and chemistry

Concepts of a 'phase' and a 'phase transition' are discussed for stable and metastable states of matter. While condensed matter physics primarily considers equilibrium states and treats metastable phases as exceptions, organic chemistry overwhelmingly deals with metastable states. It is emphasized that many simple light-element compounds — including most hydrocarbons; nitrogen oxides, hydrides, and carbides; carbon monoxide CO; alcohols and glycerin — are also metastable at normal pressure in the sense that they do not correspond to a minimum Gibbs free energy for a given chemical composition. At moderate temperatures and pressures, the phase transformations for these metastable phases are reversible with the fulfilment of all laws of equilibrium thermodynamics over the entire range of experimentally accessible times. At sufficiently high pressures (> 1–10 GPa), most of the metastable molecular phases irreversibly transform to lower-energy polymer phases, stable or metastable. These transitions do not correspond to the equality of the Gibbs free energy for the involved phases before and after the transition and so they are not first-order in the 'classical' sense. At normal pressure, the resulting polymer phases can exist at temperatures above the melting point of the original metastable molecular phase, as the examples of polyethylene and polymerized CO dramatically illustrate. As pressure is increased further to 20–50 GPa, the PV contribution to Gibbs free energy gives rise to stable high-density atomic phases. Many of the intermediate-energy polymer phases can likely be synthesized by methods of 'classical' chemistry at normal pressure.

Ageing of isotactic polypropylene due to morphology evolution, experimental limitations of realtime density measurements with a gradient column

Ageing in crystalline polymers is responsible for the deterioration of physical properties leading, for example, to a decrease in toughness and to dimensional changes that are to some extent responsible for warpage and scrap production in injection molding. Since, it depends on the mutual transformation of stable and metastable phases, being always related to changes in morphological organization, it is here preferred to call it ‘Morphological ageing’.Although, one would expect the ageing regime to be determined by the complex morphology with amorphous phases of different mobility and eventually multiple crystalline phases, transformed into each other at an associated transition, existing literature always shows a more trivial linear with the logarithm of time dependence of every probe used to describe ageing.Existing literature always overlooks the initial morphology, often complex, or, adopts well equilibrated samples, not representative of processing conditions. In this work, ageing of iPP was addressed producing samples characterized by an homogeneous morphology even at the largest cooling rates adopted using a CCT approach. This paper describes few of the many attempts undertaken to quantitatively relate ageing to initial morphology in iPP melt solidified under conditions emulating polymer processing.Ageing was monitored by measuring the density time dependence offline, i.e. separately applying the ageing protocol, and online in a gradient column conditioned at the ageing temperature following samples' apparent density evolution. The offline method can give hints about the role of temperature and initial morphology but the results so obtained suffer from significant data scattering. The online method on the other hand, can provide a more accurate interpretation of ageing if due account is made of all the features of the fluid dynamic transient superimposed on the densification due to morphological ageing.Densification during ageing of the iPP samples used, solidified between 1 and 100°C/s, takes place as a superposition of two phenomena: one is the linear increase of density with the logarithm of time, which is quantitatively related to the initial morphology, i.e. the cooling rate, and the other is a density jump that takes place at times much smaller than the available resolution and thus not quantitatively accessible.

Solidification microstructure and dynamics of metastable phase transformation in undercooled liquid Cu–Fe alloys

The solidification behavior of an atomized droplet of a metastable, immiscible Cu–Fe alloy has been investigated experimentally and by numerical simulation. A model is presented to analyze quantitatively the microstructural evolution during the metastable liquid–liquid phase transformation. The model takes into account not only the common actions of nucleation, diffusional growth, and spatial motion of the minority phase spheres, but also the repulsive interaction between the spheres and the solid/liquid interface. The effect of the interface morphology on the collision and coagulation is first discussed. The formation mechanism of an Fe-poor layer and the kinetics of the metastable phase transformation in the undercooled liquid Cu–Fe alloy are sufficiently clarified.

Na+ SUPERIONIC CONDUCTING GLASS-CERAMICS OF SILICOPHOSPHATES: CRYSTALLIZATION, MICROSTRUCTURE AND CONDUCTION PROPERTIES

Based on the composition formula of Na3+3x-yR1-xPySi3-yO9, Na5YSi4O12 (N5)-type glass-ceramic Na+ superionic conductors were successfully produced for a variety of rare earth ions, R, under the appropriate composition parameters. In the crystallization of N5-type glass-ceramics, its relatives (Na3YSi3O9 (N3)- and Na9YSi6O18 (N9)-type glass-ceramics) structurally belonging to the family of Na24-3xYxSi12O36 were found to crystallize as the precursor phase at low temperatures. In order to produce N5 single phase glass-ceramics, the concentration of both phosphorus and rare earth was found important. The meaning of the composition was evaluated by thermodynamic and kinetic studies on the phase transformation of metastable N3 or N9 phases to stable N5 phase with Na+ superionic conductivity.The possible combinations of x and y became more limited for the crystallization of the superionic conducting phase as the ionic radius of R increased, while the Na+ conduction properties were more enhanced in the glass-ceramics of larger R. These results are discussed in view of the structure and the conduction mechanism. Also studied were the microstructural effects on the conduction properties, which were dependent upon the heating conditions of crystallization. These effects were understood in relation to the grain boundary conduction properties as well as the transmission electron microstructural morphology of grain boundaries.

Metastabil'nie fazi, fazovie prevrasheniya i fazovie diagrammi v fizike i himii

Metastabil'nie fazi, fazovie prevrasheniya i fazovie diagrammi v fizike i himii

Thermodynamics of Liquid Alloys and Metastable Phase Transformations in the Copper - Titanium System

We have used solution calorimetry at temperatures of 1573 K and 1873 K over broad concentration ranges to study the mixing enthalpy of Cu - Ti liquid alloys. The molar mixing enthalpies of the system are significant negative values. We have established the temperature dependence of the molar mixing enthalpies of the system: there is an increase in their exothermicity as the temperature is lowered. The significant negative mixing enthalpies of the system allow us to conclude that the chemical bonds are localized in the studied melts and consequently associates form. We tested this conclusion within ideal associated solution theory, which describes well the results obtained with a set of CuTi and CuTi2 associates. Using the model obtained, we have calculated the excess thermodynamic functions of mixing (enthalpy, Gibbs free energy, heat capacity) for the liquid alloys. We estimated the Gibbs energies of fcc, bcc, and hcp solutions in the system by the CALPHAD method, using data from the initial sections of the phase diagrams and from the corresponding thermodynamic data. We have calculated the metastable phase equilibria between the limiting solid solutions and the liquid or supercooled liquid phase. It was shown that for the supercooled liquid and the amorphous phase, a broad concentration range of relative thermodynamic stability can be obtained. The concentration range of amorphization of Cu - Ti melts corresponds to the position of the metastable liquidus line and the T0 line at temperatures close to the temperature range of amorphous solidification.

Effects of ultrasound on zeolite A synthesis

Synthesis of zeolite A from a clear-to-the-eye sodium aluminosilicate solution was carried out in the presence of ultrasound at different temperatures and times and the results were compared to those obtained by performing conventional static syntheses under similar conditions. It was possible to obtain highly crystalline zeolite A in the presence of ultrasound, which influenced the types and stability areas of the phases that formed during metastable phase transformations. Nucleation and crystallization rates, as well as the yield of zeolite A increased as a result of the application of ultrasound. Thermogravimetric analyses and water adsorption experiments revealed that the development of micropore adsorption capacity, which started very early in the X-ray amorphous samples obtained from this so-called “clear” solution, was also speeded up with the application of ultrasound.

Study of Ti-Si in situ composite processing by multi-stage eutectic solidification

Abstract A titanium composite alloy (Ti–Al–Si–Zr) manufacturing process using eutectic solidification was studied by simultaneous thermal analysis up to 1480°C. Phase transformations predicted by sections of the phase stability diagram (ThermoCalc optimisation) have been observed. The structure of the alloy was also studied by scanning electron microscopy (SEM), light optical microscopy (LOM) and X-ray microanalysis. Several patterns, unexplained on the basis of the stable phase diagrams of the system, are revealed to be associated with metastable phase transformations. The results can be used for a determination of composite applications and the optimisation of its casting technology.

Temperature Dependence of the Surface Topography in Dimyristoylphosphatidylcholine/Distearoylphosphatidylcholine Multibilayers

Simple lipid binary systems are intensively used to understand the formation of domains in biological membranes. The size of individual domains present in the gel/fluid coexistence region of single supported bilayers, determined by atomic force microscopy (AFM), generally exceeds by two to three orders of magnitude that estimated from multibilayers membranes by indirect spectroscopic methods. In this article, the topography of equimolar dimyristoylphosphatidylcholine/distearoylphosphatidylcholine (DMPC/DSPC) multibilayers, made of two superimposed bilayers supported on mica surface, was studied by AFM in a temperature range from room temperature to 45°C. In the gel/fluid phase coexistence region the size of domains, between ∼100 nm and a few micrometers, was of the same order for the first bilayer facing the mica and the top bilayer facing the buffer. The gel to fluid phase separation temperature of the first bilayer, however, could be increased by up to 8°C, most likely as a function of the buffer layer thickness that separated it from the mica. Topography of the top bilayer revealed the presence of lipids in ripple phase up to 38–40°C. Above this temperature, a pattern characteristic of the coexistence of fluid and gel domains was observed. These data show that difference in the size of lipid domains given by AFM and spectroscopy can hardly be attributed to the use of multibilayers models in spectroscopy experiments. They also provide a direct evidence for metastable ripple phase transformation into a gel/fluid phase separated structure upon heating.