Monotectic Temperature Research Articles

Solidification and thermal behaviour of binary organic eutectic and monotectic; succinonitrile–pyrene system

Transparent binary alloy models are important in metallurgical and materials science, as phase transformations can be observed during solidification. This communication concerns the solidification and thermal studies of succinonitrile (SCN)–pyrene (PY) system, which is an organic analogue of a metal–nonmetal-type system. Phase diagram of the SCN–PY system, determined by the thaw-melt method shows the formation of a monotectic and a eutectic at 143.3°C and 55.3°C with 0.025 and 0.744 mole fractions of SCN, respectively. The critical solution temperature of the system lies 48.7°C above the monotectic temperature. The growth velocity ( v) data at different undercoolings obtained from the capillary method, obey the Hillig–Turnbull equation, v= u(Δ T) n . The heats of fusion of the binary as well as single materials were obtained from the DSC (Mettler DSC-4000 system) from which the entropy of fusion, enthalpy of mixing, Jackson's roughness parameter, excess thermodynamic functions, interfacial energy and radius of the critical nucleus were calculated. The optical microphotographs of the eutectic and monotectic show their characteristic features.

Microstructural evolution of Sn–Ag–Cu–Al solder with respect to Al content and heat treatment

The Pb-free Sn–Ag–Cu–Al solders were investigated for microstructural evolution with respect to Al% and heat treatment. The Al% varies from 0.1% to 0.45% while the contents of Ag are 3.1%–2.53% and of Cu are 0.41%–0.33%. Differential scanning colorimetry (DSC) was applied to identify the melting behavior. A monotectic temperature of 224 °C and a eutectic temperature of 220 °C are deduced from the DSC results. The microstructure was characterized with x-ray diffraction and scanning electron microscopy-energy dispersive spectroscopy. Ag3Sn and the γ2 phase of Al–Cu system are the intermetallic compounds formed in the as-cast solders. Cu6Sn5 formed upon heat treatment for 1000 h at 150 °C in the 0.45 Al-containing solder, while not found in the other solders or other heat treatment conditions. Sn whiskers were detected in a 0.45 Al-containing specimen after aging for 50 h.

Wetting behavior at the free surface of a liquid gallium–bismuth alloy: an X-ray reflectivity study close to the bulk monotectic point

Abstract We present X-ray reflectivity measurements from the free surface of a liquid gallium–bismuth alloy (Ga–Bi) in the temperature range close to the bulk monotectic temperature Tmono=222 °C. Our measurements indicate a continuous formation of a thick wetting film at the free surface of the binary system driven by the first order transition in the bulk at the monotectic point. We show that the behavior observed is that of a complete wetting at a tetra point of solid–liquid–liquid–vapor coexistence.

Phase Diagram and Growth Behaviour of Durene–resorcinol System

Phase diagram of durene–resorcinol system, determined by the thaw-melt method, shows the formation of a monotectic (0.109 mole fraction of durene) and an eutectic (0.964 mole fraction of durene) with a large liquid miscibility gap in the region from 0.109 to 0.964 mole fraction of durene. The eutectic, monotectic and consolute temperatures are 78.4, 107.8 and 165.0°C, respectively. The growth behaviour studied by measuring the linear velocity of crystallization (v) in a capillary at different undercoolings (ΔT) suggests that the data obey the Hillig–Turnbull equation,v=u(ΔT)n, where u and n are constants depending on the nature of materials involved. From the values of enthalpy of fusion of the pure components, the eutectic and the monotectic determined by the DSC method using Mettler DSC-4000 system, entropy of fusion, enthalpy of mixing, Jackson’sroughness parameter, size of the critical nucleus interfacial energy and excess thermodynamic functions were calculated. The microstructures of the eutectic, and the monotectic, determined by the Leitz Laborlux D optical microscope show their characteristic features.

Tetra point wetting of liquid K–KCl mixtures: Spectroscopic characterization of mesoscopic wetting and prewetting films

The wetting and prewetting transitions at the metal-rich K–KCl melt–sapphire interface have been investigated by spectroscopic ellipsometry in combination with normal incidence reflectivity in the spectral range 0.8⩽ℏω⩽2.2 eV at temperatures up to 730 °C. Along the coexistence curve a salt-rich liquid wetting film is observed which is identified by the spectral features of the liquid state F-center. Unusually thick wetting films are found ranging from 30 nm near 540 °C to 300 nm approaching the monotectic temperature of 751 °C. Their composition has been determined from the absorption coefficients of the F-center band and it corresponds to about 90 mole % salt. At conditions off coexistence and near the prewetting line, similar mesoscopically thick wetting films exist. Crossing the prewetting line towards metal-rich solutions, the optical properties at the interface agree with those of the nearly free electron metal. The high thickness of the prewetting films is qualitatively explainable by charging and double layer formation at the interface. The occurrence of liquid F-center-like states up to 200 K below the monotectic temperature gives evidence of a strong undercooling of the wetting films with respect to the bulk phase. These characteristics of the wetting transition in a metal–molten salt solution can be described by the tetra point wetting scenario for binary fluid mixtures.

Processing of immiscible metallic alloys by rheomixing process

Mixing immiscible alloys has been a long standing challenge to both materials scientists and processing engineers. Despite great efforts made worldwide, including extensive space experiments, no casting techniques so far can produce the desirable fine and uniform dispersed microstructure. Based on extensive experience in mixing immiscible organic liquids offered by the polymer processing community, the authors have successfully developed a rheomixing process for mixing immiscible alloys. The rheomixing process utilises first the intensive shear stress-strain field offered by a twin screw extruder to create a fine homogeneous liquid dispersion within the miscibility gap and then the viscous force offered by a semisolid slurry at a temperature below the monotectic temperature is used to counterbalance the gravitational force and the Marangoni effect. A laboratory scale rheomixer has been designed and constructed to realise this two step mixing strategy. The Ga–Pb and Zn–Pb systems were selected to demonstrate the principles of rheomixing. The experimental results showed that the rheomixing process developed is capable of creating a fine and uniform microstructure from immiscible alloys. This paper describes the rheomixing process in detail and the preliminary experimental results on rheomixing in the Ga–Pb and Zn–Pb systems are discussed.

Thermodynamic modeling of the miscibility gaps and the metastability in the R 2O 3–SiO 2 systems (R=La, Sm, Dy, and Er)

A range of compositions and temperatures below the monotectic temperature exists where there are thermodynamic restrictions that prevent the equilibrium solid from forming directly from the undercooled homogeneous liquid. In this region the solid can form only after liquid–liquid phase separation has occurred. As suggested by earlier research, the thermodynamic restrictions on the crystallization process may be useful to control the crystallized grain structure in glass–ceramic systems. Thus, understanding the thermodynamic limitations on the formation of the solid in monotectic systems could have commercial significance. In the present paper, the metastable liquidus boundaries, liquid miscibility gaps, and spinodal curves in the binary La 2O 3–SiO 2, Sm 2O 3–SiO 2, Dy 2O 3–SiO 2, and Er 2O 3–SiO 2 systems were calculated using analytical expressions for the Gibbs free energies of the liquid phases.

Influence of growth direction on the microstructure of unidirectionally solidified Cu–Pb monotectic alloy using zone-melt technique

The influence of growth direction on the monotectic structure of the Cu–Pb alloy is studied. In order to examine the influence under a 1 g environment, both the upward (opposite to the direction of gravity) and downward (the direction of gravity) unidirectional solidifications (UDS) are carried out. In the case of the upward UDS, a banded structure, which consists of Pb-rich and Cu-rich layers, is observed. The L 2 droplets pile up in front of the solid/liquid interface. On the other hand, in the downward UDS, the irregularly shaped L 2 phase uniformly disperses in the specimen and no banded structure is found. The gravity macrosegregation of the L 2 liquid is observed at the bottom of the molten alloy in the downward solidified specimen. This is caused by the difference in the density between the L 1 and L 2 phases. Furthermore, a mechanism for the formation of a banded structure is suggested. This mechanism suggests that the coalesced L 2 phase covers the solid/liquid interface by producing a Pb-rich layer that permits an increase in the undercooling of the L 1/L 2 interface compared to the monotectic temperature. As nucleation of the α-Cu phase occurs on the Pb-rich layer, the coexisting three phases are then restored. The temperature at the growth front is also returned to the monotectic temperature. The repetition mentioned above will result in the banded structure found in the upward UDS.

Study of the containerless undercooling of Ti-Ce immiscible alloys

Ti-Ce immiscible alloys of compositions across the miscibility gap were containerlessly processed in both a low-gravity and a unit-gravity environment. Although undercooling of the single-phase liquid into the miscibility gap could not be observed, undercooling did occur across the miscibility gap for the separated liquid Ti-rich phase. The low gravity, quiescent environment favored higher undercooling over the unit-gravity samples. Every undercooled sample had massive separation of the liquid phases. Metallurgical analysis of samples undercooled in unit-gravity showed signs of vigorous convective stirring and shearing of the L1 Ti-liquid by the applied levitation electromagnetic field. In low-gravity processed samples, the L1 liquid formed a near-concentric sphere within a Ce shell with some residual smaller spherical particles dispersed throughout the Ce. This configuration is predicted from wetting theory and from Marangoni separation. Plots of both the melting and solidification temperatures indicate that the monotectic temperature is 1831 ± 12°K rather than the 1723°K as reported in the literature. From chemical and diffraction analysis, the solubility of Ce in the Ti-rich phase was found to be extended; also, some cerium oxide precipitates formed but no perceptible dissolved oxygen within the Ce or Ti phases was found which indicates that the higher monotectic temperature reported here is probably not an oxygen effect.

Phase-field model for solidification of a monotectic alloy with convection

In this paper we discuss two phase-field models for solidification of monotectic alloys, a situation in which a liquid phase L 1 may simultaneously transform into both a new liquid phase L 2 and a solid phase S via the reaction L 1→L 2+S. The first model uses three different phase-fields to characterize the three phases in the system and, in addition, a concentration field. This construction restricts the validity of the model to describe phase transitions within the vicinity of the monotectic temperature. In contrast, the second model distinguishes the two liquid phases by their concentration using a Cahn–Hilliard type model and employs only one phase-field to characterize the system as solid or liquid. This formulation enables the second model to represent a wider temperature range of the phase diagram including the miscibility gap where the spinodal decomposition L→L 1+L 2 occurs. Both our models permit the interfaces to have temperature-dependent surface energies which may induce Marangoni convection at L 1–L 2 interfaces in non-isothermal systems. By deriving a generalized stress tensor including stresses associated with the capillary forces on the diffuse interface, we extend the two monotectic phase-field models to account for convection in both liquid phases. Together with a generalized set of Navier–Stokes equations, we give a complete set of dynamic field equations to describe monotectic systems with fluid flow. Finally, we present numerical simulations of lamellar monotectic growth structures which exhibit wetting phenomena as well as coarsening and particle pushing.

Microscopic structure of the wetting film at the surface of liquid Ga-Bi alloys

X-ray reflectivity measurements of the binary liquid Ga-Bi alloy reveal a dramatically different surface structure above and below the monotectic temperature T(mono) = 222 degrees C. A Gibbs-adsorbed Bi monolayer resides at the surface in both regimes. However, a 30 A thick, Bi-rich wetting film intrudes between the Bi monolayer and the Ga-rich bulk for T>T(mono). The wetting film's internal structure, not hitherto measured, is determined with A resolution, showing a concentration gradient not predicted by theory and a highly diffuse interface with the bulk phase.

Wetting, spreading and Marangoni convection in gallium-bismuth alloys

The present study reports several novel results pertinent to the wetting transition in Ga-Bi. These results show that complete wetting depends crucially on the boundary conditions between the liquid sample and the container. If liquid Ga does not wet the container wall a thick Bi film of about 50 µ spreads over the Ga surface at nearly room temperature due to the Marangoni effect. If Ga wets the wall the wetting transition occurs near to the monotectic temperature of this system. The interfacial tension inferred from the surface light scattering data displays non-monotonic behaviour on heating.

Thermodynamics of the miscibility gap in the Ag-Se system

The miscibility gap in the Ag-Cu-Se system plays a central role in the smelting of copper anode slimes for the recovery of silver; however, its position in the Ag-Se and Cu-Se component binary systems is poorly defined. In this work, the miscibility gap in the Ag-Se system has been accurately defined by holding samples at selected temperatures, then quenching them and analyzing the two phases. The miscibility gap has been shown to extend from 6.7 to 25.0 pct at the monotectic temperature of 890 °C. As the temperature increases to 1150 °C, the boundaries do not converge very rapidly. The limiting activity coefficient of Ag2Se in molten silver was calculated to be 7.8 at 1100 °C, confirming earlier work. The limiting activity coefficient of hypothetical liquid selenium at the same temperature was found to be approximately 0.008. Finally, the activity of hypothetical liquid selenium in the liquids on either side of the miscibility gap, from the monotectic temperature to 1150 °C, was mapped.

Thermodynamic Modeling of the Miscibility Gaps and the Metastable Liquidi in the MgO‐SiO2, CaO‐SiO2, and SrO‐SiO2 Systems

A range of compositions and temperatures below the monotectic temperature exists where there are thermodynamic restrictions that prevent the equilibrium solid from forming directly from the undercooled homogeneous liquid. In this region, the solid can form only after liquid‐liquid phase separation has occurred. As suggested by earlier research, the thermodynamic restrictions on the crystallization process may be useful to control the crystallized grain structure in glass‐ceramic systems. Thus, understanding the thermodynamic limitations on the formation of the solid in monotectic systems could have commercial significance. In the present paper, the metastable liquidus boundaries, liquid miscibility gaps, and spinodal curves in binary MgO‐SiO2, CaO‐SiO2, and SrO‐SiO2 systems are calculated by using analytical expressions for the Gibbs free energies of the liquid phases. Calculating the metastable liquidus rather than using a simple extrapolation as originally proposed in the aforementioned previous research provides greater control of the heat‐treatment processes and, thus, greater control over the resulting microstructure.

Electrical conductivity measurements for immiscible In–Se–Te alloys

Abstract The electrical conductivity of liquid In 0.80 (Se x Te 1− x ) 0.20 alloys has been measured by a contact method in the temperature range from the monotectic temperatures up to about 1200 K under pressures of argon gas (up to 50 MPa). It was revealed that the ternary In 0.80 (Se x Te 1− x ) 0.20 system may be considered as a set of quasibinary In–(Se/Te) alloys of almost critical concentrations. It was found that variation of Se–Te ratio at a constant content of In changes the properties of coexisting liquids and affects the phase separation temperature T c which decreases in a nonlinear manner from 949 K for In 0.80 Se 0.20 to 817 K for In 0.80 Te 0.20 . A brief comparison analysis of our results with available data for binary and ternary immiscible alloys has been performed.

The phase diagram of cyclohexane–methanol: a challenge in chemical education

Cloud point and cooling curve studies of the cyclohexane–methanol system were ideally suited as a laboratory exercise for a physical chemistry curriculum. Within 3–4 h, students can obtain a sufficient number of relatively precise experimental data (temperature vs. mol fraction of methanol) to construct the major part of the solid–liquid phase diagram. The experimental procedure is based on three different methods: (1) cloud points were detected using an H–Ne laser; (2) temperatures of the methanol-rich part below the monotectic were taken when the first crystals of cyclohexane become visible; and (3) cooling curves were recorded in order to determine the melting point of C 6H 12 and the monotectic temperature. The results of the system exhibiting an upper critical solution temperature, UCST, agreed reasonably with the most reliable data presently available. Assuming a tetrameric methanol species, the experimental T− x data of the freezing-point depression and the sub-monotectic curve were fitted to a sub-regular model with three adjustable parameters for the liquid phase. However, only an extended Ising model agreed with the experimental curve near the critical temperature.

Interfacial properties of liquid alloys: An experimental study on Ga‐Bi and Ga‐Ge

AbstractInterfacial phase transitions like wetting and prewetting transitions are of considerable interest in physics and chemistry of condensed matter since they represent phase transitions in reduced dimensionality. Besides this interfacial properties are of profound practical and technological interest. Most systems studied experimentally in this respect are characterized by Van der Waals intermolecular interactions. However, in the last few years it was shown that Coulomb liquids like liquid alloys or metal molten salt solutions exhibit interfacial phase transitions similar to those known in Van der Waals systems.In order to get more insight into these phenomena the fluid‐vapor interface of two different alloy systems have been studied using ellipsometry. Gallium‐bismuth is a binary alloy with large positive deviations from Raoult's law, exhibiting a distinct miscibility gap. Approaching liquid‐liquid coexistence a Bi‐rich film completely wets the fluid‐vapor interface. As can be estimated from the ellipsometric results the film thickness jumps at the monotectic temperature to a value of about 50 Å. In contrast, gallium‐germanium shows continuous miscibility and deviates much less from ideal mixing. As the Ge concentration in liquid Ga increases along the solid‐liquid coexistence curve the optical properties at the surface also vary continuously, which can be modelled within a simple effective medium approach.

On some physical properties of nanostructured Cu-Pb alloy prepared by mechanical alloying

Cu-Pb alloys have no solubility in the whole solid state and their physical properties are very different from each other. In the present study, nanostructured Cu-Pb alloy powders were synthesized by the mechanical alloying process, and their nanostructural characteristics were evaluated in order to elucidate the relationship between structure and properties. By appropriate control of mechanical alloying process variables, the Pb solid solubility in Cu matrix was increased up to 10 wt.%. The monotectic temperature of Cu-Pb alloy was also decreased by decreasing the crystalline size. The relation between the structure and properties of this nanostructured Cu-Pb alloy is discussed on the basis of the experimental results.

Investigation of critical phenomena in the miscibility gap region of the In-Te system

The shear viscosity of In 100 − x Te x melts in the miscibility gap region (4.9 ≤ x ≤ 27.8 at.% Te) was measuredwith an oscillating cup viscometer. The shape of the liquid-liquid coexistence curve was determined by fitting experimental data to a polynomial. The coordinates of the critical point were found to be T c = 527 ± 1 °C and x c = 18.6 ± 1.3 at.% Te. The monotectic temperature is T m = 418.3 ± 0.7 °C. The coexisting concentrations and diameter were analysed in terms of the reduced temperature ɛ, giving the critical indexes β = 0.331 ± 0.002 for ɛ > 10 −3, β = 0.501 ± 0.002 for ɛ < 10 −3 and (1 − α) = 0.81 ± 0.02. In the vicinity of the critical point, the viscosity can be represented by a power law η ~ ɛ − y with values for y ranging from 0.026 to 0.034.

Experimental evidence for a wetting transition in liquid GaPb alloys

The surface composition of Ga-rich liquid in equilibrium with Pb-rich solid has been investigated between 508 K and the monotectic temperature (586 K) of the GaPb system using Auger spectroscopy. A Pb-rich layer is shown to thicken up to 9 equivalent monolayers of Pb at the surface of the liquid as the temperature increases. The thickness of the surface layer diverges logarithmically as the composition of the liquid approaches the monotectic composition. This is the signature of the existence of complete wetting in the GaPb system occurring in the metastable region of coexistence of the two liquids.