Metastable Phase Diagram Research Articles

Neutron diffraction and calorimetric study on Al-based metallic glasses

The kinetics of transformation of the melt spun Al 87Ni 7Nd 3Cu 3 alloy was analysed by differential scanning calorimetry (DSC) and neutron diffraction (ND) experiments. The study of the emerging nanocrystalline grained structure by in situ heat treatment on quenched ribbons of the aluminium rich alloy composition indicates that no detectable nanocrystallisation occurs till temperatures of about 370 K. Changes of the structure occur in two well defined and not overlapped temperature ranges. The thermodynamic optimisation of the Al-rich part of the Al–Ni–Nd–Cu system, by computing calculation of the equilibrium and metastable phase diagrams, is applied to the kinetics of the crystallisation of the face-centered-cubic (fcc)-(Al) for the Al 87Ni 7Nd 3Cu 3 alloy. The fcc-(Al) precipitation occurs in a temperature interval of ∼200 K, although the most rapid structural changes occur at temperatures around 380 K. These results are indicative of the diffusion control of the last stages of the first crystallisation event. Eutectic precipitation of fcc-(Al)+Al 3Ni is promoted when temperature is about 520 K. The overall analysis shows that primary fcc-(Al) crystallisation is kinetically hidden.

A Study of the Crystallization of ZrO2 in the Sol–Gel System: ZrO2–SiO2

ZrO2–SiO2 sol–gel powders were produced using tetraethoxysilane (TEOS) and zirconium propoxide. After gellation, the ZrO2 crystallization process was investigated using X-ray diffraction (XRD), thermal analysis (DTA/TGA), and scanning electron microscopy (SEM). Fresh gels were amorphous. Thermal treatments were carried out from 100 to 1400°C for a total annealing time of 182 h. Tetragonal zirconia, (Z(t)) was the first phase to crystallize, between 300 and 500°C. Crystallization temperature was lower for zirconia-rich compositions, increasing as silica content was raised. DTA analysis showed that Z(t) crystallization occurred in two stages. Complete tetragonal–monoclinic zirconia transformation occurred near 1000°C, and was clearly observed only in ZrO2-rich compositions (>80%). Silica remains amorphous until 1200°C, when ZrSiO4 formation took place. A metastable sol-gel phase diagram was proposed to show the crystallization process between 100 and 1400°C.

Direct determination of metastable phase diagram by synchrotron radiation experiments on undercooled metallic melts.

The phase selection process during the crystallization of undercooled metallic melts is studied in situ by combining the electromagnetic levitation technique with energy dispersive x-ray diffraction of synchrotron radiation. The crystallization of metastable bcc phase in binary Ni-V alloys was identified. A metastable phase diagram of Ni-V alloy is constructed, which shows the primarily solidifying phase as a function of composition and undercooling. The analysis within nucleation theory emphasizes the important role of metal oxide as a heterogeneous nucleation site controlling the phase selection.

Conversion of Al 2 O 3 –SiO 2 powder mixtures to 3:2 mullite following the stable or metastable phase diagram

A model system, composed of powder blends of amorphous isomorphic silica spheres, being 500 nm in diameter, and also monosized crystalline α-Al 2 O 3 powder, was investigated. Two different particle sizes of the corresponding alumina powder were employed: 300 nm and 2 μm. This particular assembly enabled a distinction between amorphous silica and crystalline alumina merely by their difference in particle morphology. The powder blends were sintered at temperatures between 1400 and 1700 C and the microstructure evolution was characterized by scanning (SEM) and transmission electron microscopy (TEM). It is worth noting that upon annealing at 1700°C, both microstructures were indistinguishable. However, depending on the Al 2 O 3 particle size, different conversion mechanisms were monitored. When using the 300 nm Al 2 O 3 powder, fast dissolution of alumina into the coalesced silica glass occurred, followed by homogeneous nucleation and growth of mullite within the glass. Utilizing 2 μm Al 2 O 3 particles, however, resulted in the formation of two Al-containing glasses (phase separation into a Si- and Al-rich glass). In this case, the transformation to mullite can be rationalized by the conversion of the metastable Al-rich transient glass into mullite, which forms an epitaxial, single crystalline coating on the host Al 2 O 3 particle. Therefore, depending on the initial Al 2 O 3 particle size, mullite formation follows either the stable or metastable phase diagram.

The metastable phase diagram of the Blume–Emery–Griffiths model in addition to the equilibrium phase diagram

As a continuation of our previously published works, the metastable phase diagram of the Blume–Emery–Griffiths model is presented in addition to the equilibrium phase diagram by using the lowest approximation of the cluster variation method. We also calculate the phase transitions for the unstable branches of order parameters. The calculated first- and second-order phase boundaries of the unstable branches of the order parameters are superimposed on the equilibrium phase diagram and metastable phase diagram. It is found that the metastable phase diagram and the first- and second-order phase boundaries for the unstable branches of order parameters always exist at the low temperatures, which are consistent with the experimental and theoretical works.

Phase behavior of a monoacylglycerol: (Myverol 18-99K)/water system

The phase behavior of Myverol 18-99 K, a food emulsifier rich in monoacylglycerols, in combination with water has been determined. X-ray diffraction and polarized light microscopy (PLM) were used for phase identification and structure characterization. Phase behavior was established in the temperature range from −15 to 50°C and in the composition range from dry to full hydration. Phases identified include the solid lamellar crystal (Lc) phase, the liquid fluid isotropic phase and three liquid crystal phases, the lamellar liquid crystal, the cubic-Ia3d and the cubic-Pn3m phase. Phase information is reported in the form of temperature-composition phase diagrams. It was collected under equilibrium conditions where measurements were made in the heating direction beginning with the Lc phase at −15°C. Phase metastability was also examined in which the natural tendency of the liquid crystal phases to undercool was facilitated. Under this condition, both cubic phases were found to remain free of the solid Lc phase over a relatively wide range of hydration values down to 0°C. The microstructure of the different phases and its dependence on temperature and hydration has been determined. Compositional analysis using thin layer chromatography and gas chromatography/mass spectrometry shows that Myverol 18-99 K consists of 82% monoacylglycerols (86.6% monoolein, 7.0% monostearin, 3.5% monopalmitin, 0.9% monoarachidin, 2.0% unidentified). The equilibrium and metastable phase diagrams of the Myverol 18-99 K/water system show remarkable similarity to those reported for the monoolein/water system (Qiu, H., Caffrey, M., 2000. The phase diagram of the monoolein/water system: metastability and equilibrium aspects. Biomaterials 21, 223–594.).

Study on phase diagram of Bi 2O 3SiO 2 system for Bridgman growth of Bi 4Si 3O 12 single crystal

Phase relation of Bi 2O 3SiO 2 system was evaluated experimentally from DTA and XRD measurements and its stable and metastable phase diagrams were proposed. Although BSO melts near-congruently at 1025°C in the stable phase equilibrium, its melt crystallizes to form metastable phase Bi 2SiO 5 in accordance with the metastable phase diagram while cooling. Therefore, BSO couldn't nucleate and crystallize spontaneously without crystal seed and only Bi 2SiO 5 crystallized at about 850°C with significant supercooling during Bridgman growth. BSO single crystal with 20×20×100mm 3 was grown in a vertical Bridgman furnace with a BSO seed according to its phase diagram. The measuring results of scintillation properties of BSO specimen show that its decay constant is 91 ns (about 1/3 of BGO) and light output is 23% of BGO.

Supercooling and transient phase induced nucleation in n-alkane solutions

The supercooling exhibited by bulk solutions of the n-alkane tricosane (C23) in dodecane (C12) is measured as a function of concentration using adiabatic scanning calorimetry. For small dilutions where crystallization occurs into the rotator phase, negligible supercooling is observed. For larger dilutions where crystallization is into the herringbone crystal phase, nucleation occurs at the temperature where the rotator phase becomes stable with respect to the liquid. The supercooling in bulk n-alkane solutions is thus determined by the metastable phase diagram. This is a novel, but possibly often occurring mechanism where nucleation is induced by a transient metastable phase.

Ostwald step rule in films of metastable nanocrystalline alloys Fe-C prepared by pulsed plasma vaporization

Ferromagnetic nanocrystalline Fe(C) films were prepared by pulsed plasma vaporization. A comprehensive investigation of the structure and magnetic properties made it possible to identify the type of short-range order here and to establish the sequence of structural states occurring in these films in the process of thermal relaxation: fcc-Fe(C)→hcp-Fe(C)→bcc-Fe+C. On the basis of an analysis of the metastable phase diagrams using Ostwald’s rule, it is shown that the observed scenario of the structural transformations in these metastable nanocrystalline alloys Fe(C) is a natural phenomenon.

Coupled growth in the peritectic Nd-Ba-Cu-O system from highly undercooled melt

In this paper, based on the idea that coupled growth will occur even in the peritectic system if the melt is highly undercooled below the hypothetical congruent melting temperature of the peritectic phase, containerless solidification experiments were carried out on Nd-Ba-Cu-O. The peritectic phase diagrams were categorized into two different types from the point of view of the metastable phase diagram on peritectic phase. The stability of the coupled growth is discussed based on the two types of the peritectic phase diagrams.

A diffusion-kinetic model for predicting solder/conductor interactions in high density interconnections

A combined thermodynamic and diffusion-kinetic approach is very viable for developing microjoining and interconnection materials and processes, in particular, whenever thinner metallizations, coated overlayers, or smaller solder-joint volumes are encountered in very high density electronics. A diffusion-kinetic model based on the utilization of integrated diffusion coefficients and mobilities is introduced and discussed for calculating the layer growth of intermetallic compounds between metal conductors and tin-based solders and is exemplified with a relatively simple ternary Cu/SnBi system. The model has also been used for calculating the local nominal composition of the effective joint or contact region. Moreover, the mobilities of Sn and Cu in Cu6Sn5 and Cu3Sn intermetallic compounds are determined, and the role of both stable and metastable phase diagrams is discussed in predicting the appearance of possible reaction products as well as the driving forces for the dissolution, diffusion, and precipitation processes.

Order-disorder transitions of Cu-Al-Mn shape-memory alloys

The order-disorder transitions in Cu-Al-Mn shape-memory alloys have been studied experimentally by means of calorimetric techniques. Results are compared with Monte Carlo simulations of a simplified Blume-Emery-Griffiths model. A first order ${\mathrm{DO}}_{3}\ensuremath{\rightleftharpoons}A2$ transition is obtained for the stoichiometric ${\mathrm{Cu}}_{3}\mathrm{Al}$ alloy, while for concentrations near ${\mathrm{Cu}}_{2}\mathrm{AlMn}$ two second order transitions are found: ${L2}_{1}\ensuremath{\rightleftharpoons}B2$ and $B2\ensuremath{\rightleftharpoons}A2.$ Despite the simplicity of the model, the agreement between experimental and simulation results is remarkably good. Finally, the metastable phase diagram of bcc Cu-Al-Mn alloys is presented.

Diffusion and phase transformations during interfacial reaction between lead-tin solders and palladium

The interfacial reaction between Pb-Sn solders and bulk Pd substrate is studied both in the liquid- and solid-state of the solder. The interfacial microstructures are characterized by imaging and energy dispersive x-ray analysis in scanning electron microscope. The correlation between the diffusion path and interfacial microstructure in 62Sn38Pb/Pd, 95Pb5Sn/Pd, and Pb/Pd diffusion couples is demonstrated by means of calculated isothermal sections of the Pb-Sn-Pd system and the metastable phase diagram of the Pb-Pd system.

The metastable phase diagram and the kinetics of transient ordered states in a ternary system

The kinetics of a ternary system is investigated using the cluster-variation method and the path probability method in the pair approximation. It is shown that differences between the diffusive jump rates for the three atomic species of a ternary system have a strong effect on the kinetics of the system. The calculated metastable phase diagram of the system shows the existence of transient ordered states during the relaxation both from the completely disordered state to the equilibrium disordered state and from the completely disordered state to the equilibrium ordered state due to the differences between the relaxation times for the three atomic species. The kinetic paths of ordering and disordering are classified according to the metastable phase diagram. From the study of the kinetics of ordering and disordering, it is shown that there are two types of transient ordered state and that there are overshooting effects during the relaxation from the disordered state to the equilibrium ordered state. It is also shown that there is an anomalous effect in the evolution of long-range ordering parameters in the kinetics of disordering for a ternary system.

The use of metastable phase diagrams in primary crystallization kinetics study

Metastable multiphase materials are currently obtained by controlled crystallization of amorphous samples. This paper deals with the use of metastable equilibrium phase diagrams and related thermodynamic quantities to the kinetic analysis of primary crystallization of an undercooled liquid solution as obtained by calorimetric means. The analysis is performed as a function of the processing conditions, assuming thermodynamic and kinetic competition between several crystalline phases for nucleation and growth. The hierarchy of crystalline phases formation on both heating an initial amorphous alloy or on cooling the liquid solution follows a sequence mostly consequent with the different temperature dependence of the several quantities driving crystal formation.

Melting Temperature, Transformation and Metastable Phase Diagram of Rapidly Solidified Ag–Cu Alloys

Cu-Ag samples produced during rapid solidification experiments using a Planar Flow Melt Spinning Process or a levitation casting technique were rapidly heated in a mirror furnace to a variety of temperatures at rates between 100 and 1.6 K/s. The surface of each sample was observed in a confocal laser scanning microscope. Metallographic analyses were performed on the samples by optical and scanning electron microscopy. The rapidly solidified samples melted far below the equilibrium melting point. The melted and resolidified samples mostly consisted of structures similar to those seen in alloys with a miscible gap. It is proposed that this type of behaviour is a result of lattice defects formed during the rapid solidification process. The effect of these defects on the thermodynamic properties was investigated. A metastable phase diagram with a miscibility gap can be constructed below the equilibrium melting point. This could explain the observed phase structure and the melting of the alloy below the equilibrium melting point.

Synthesis of nanodispersed phases during rapid solidification and crystallization of glasses in Ti75Ni25 alloys

The formation of the metallic glass and crystalline phases and related microstructures and the decomposition behavior of rapidly solidified Ti75Ni25 alloys obtained under different processing conditions have been investigated in detail. The competition between glass transition and nucleation of β-Ti during rapid solidification leads to the possibility of synthesizing the nanocomposites of β-Ti and glass. Additionally, it is shown that the presence of a small amount of Si also promotes simultaneous nucleation of fine Ti2Ni intermetallic compound. Thermodynamic calculation of the metastable phase diagram indicates the presence of a metastable eutectic reaction between α-Ti and Ti2Ni. Evidence of this reaction at lower cooling rates has been presented. On heating, the glass decomposes through this reaction. Finally, on the basis of understanding of the microstructural evolution during decomposition, a new approach has been adopted to synthesize a nanodispersed composite of α-Ti in the crystalline Ti2Ni matrix with a narrow size distribution by controlling the devitrification heat treatment of the metallic glass.

Simultaneous interpenetrating polymer networks based on epoxy‐acrylate combinations

Simultaneous interpenetrating polymer networks (SINs) based on epoxy/poly(n-butyl acrylate) systems were synthesized at 120°C. The polymerization kinetics were studied both in situ by Fourier Transform Infrared Spectroscopy (FT–IR). Three key events occurred during the polymerization, namely the gelation of the network I, gelation of the network II, and phase separation of one polymer from the other. Thus, metastable phase diagrams describing the relations between the three events were constructed. Three-dimensional tetrahedrons characterizing the four-component system (the two monomers and the two polymers) allow the visualization of these three key events and also define some critical points, for example, the loci of the points where simultaneous gelation of the two networks occurs. The inside of the tetrahedron was also investigated using partially reacted model compounds. These tetrahedrons can be used as guidelines for setting up a synthesis strategy leading to desired morphologies. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1973–1984, 1997

Simultaneous interpenetrating polymer networks based on epoxy-acrylate combinations

Simultaneous interpenetrating polymer networks (SINs) based on epoxy/poly(n-butyl acrylate) systems were synthesized at 120°C. The polymerization kinetics were studied both in situ by Fourier Transform Infrared Spectroscopy (FT–IR). Three key events occurred during the polymerization, namely the gelation of the network I, gelation of the network II, and phase separation of one polymer from the other. Thus, metastable phase diagrams describing the relations between the three events were constructed. Three-dimensional tetrahedrons characterizing the four-component system (the two monomers and the two polymers) allow the visualization of these three key events and also define some critical points, for example, the loci of the points where simultaneous gelation of the two networks occurs. The inside of the tetrahedron was also investigated using partially reacted model compounds. These tetrahedrons can be used as guidelines for setting up a synthesis strategy leading to desired morphologies. © 1997 John Wiley & Sons, Inc. J Polym Sci B: Polym Phys 35: 1973–1984, 1997

Metastable phase diagrams for simultaneous interpenetrating networks

AbstractA metastable phase diagram for SIN and related compositions is constructed, using a net‐polyurethane‐inter‐net‐poly(methyl methacrylate) system as the model material. In addition, the thermodynamics of having small amounts of remaining monomer in the system was examined, with the conclusion that the partition coefficient for many such systems should be close to unity.