Monotectic Growth Research Articles

Monotectic growth evolution and raman scattering of self-assembled ZnO hierarchical micro-nanostructures

The self-assembled hierarchical micro-nanostructures with geometric complexity and/or constituent diversity are becoming a promising class of candidates for the next generation nanotechnology. Different from conventional vapor-liquid-solid (VLS) eutectic liquid droplets for one-dimensional(1D) nanowires, the Zn-Bi VLS monotectic liquid droplets are employed to build 1D, 2D and 3D ZnO into 3D hierarchical micro-nanostructures. As-obtained micro-nanostructures are well defined self-assembly and have not yet been observed in the conventional eutectic alloys. The synergy of monotectic droplets and nonthermal equilibrium is responsible for the evolution of ZnO hierarchical micro-nanostructures. Raman spectra show that E2H and E1L modes are shifted slightly towards the high-wavenumber side by an amount of 3.7 and 2 cm−1 as compared with undoped ZnO nanowires and an additional peak at 519.5 cm−1 is associated with Bi38OZn60. The results manifest that 3D ZnO micro-nanoarchitectures possess a bulk phonon structure. The monotectic liquid droplet route presented in this paper may offer a new opportunity to the fabrication of hierarchical nanostructures for realistic technology platforms with multifunctional coupling and interplay.

The Behavior of Liquid Alloys Visualized by X-Rays

Developments in synchrotron and home laboratory X-ray sources and fast low noise X-ray imaging detectors over the last 15-20 years has enabled real time X-radiography of alloy solidification from the melt. These investigations have been an important tool for in–situ investigations of dendrite-, eutectic and monotectic growth, dendrite fragmentation etc. At the same time, the techniques have allowed studies of phenomena in the melt such as convection, formation of solute boundary layers and minority phase droplet interactions. The article will review the X-radiography techniques and some of the results with emphasis on studies of phenomena in alloy melts.

Thermal, physicochemical and microstructural studies of binary organic eutectic systems

Solid–liquid phase equilibrium data of three binary organic systems, namely, 3-hydroxybenzaldehyde (HB)—4-bromo-2-nitroanilne (BNA), benzoin (BN)—resorcinol (RC) and urea (U)—1,3-dinitrobenzene (DNB), were studied by the thaw–melt method. While the former two systems show the formation of simple eutectic, the third system shows the formation of a monotectic and a eutectic with a large immiscibility region where two immiscible liquid phases are in equilibrium with a liquid of single phase. Growth kinetics of the pure components, the monotectic and the eutectics, studied by measuring the rate of movement (v) of solid–liquid interface in a thin U-tube at different undercoolings (ΔT) suggests the applicability of the Hillig–Turnbull’s equation: v = u (ΔT)n, where v and n are the constants depending on the nature of the materials involved. The thermal properties of materials such as heat of mixing, entropy of fusion, roughness parameter, interfacial energy, and excess thermodynamic functions were computed from the enthalpy of fusion values, determined by differential scanning calorimeter (Mettler DSC-4000) system. The role of solid–liquid interfacial energy on morphologic change of monotectic growth has also been discussed. The microstructures of monotectic and eutectics were taken which showed lamellar and federal features.

Thermal, physicochemical and microstructural studies of organic analog of nonmetal–nonmetal monotectic alloy

The phase equilibrium of an organic analogue of a nonmetal–nonmetal system, involving resorcinol (R)–1,4-diiodobenzene (DIB), was established which shows solid is in equilibrium with liquid as well as two immiscible liquid phases are also in equilibrium with a liquid of single phase. The phase diagram study infers the formation of a monotectic and a eutectic at 0.05 and 0.97 mole fractions of resorcinol, respectively. Using X-ray diffraction technique, the range of solid solubility of R in DIB and DIB in R was studied. The thermal properties of materials such as heat of mixing, entropy of fusion, roughness parameter, interfacial energy and excess thermodynamic functions were computed from the enthalpy of fusion values, determined using differential scanning calorimeter (DSC) method. The solid–liquid interfacial energy shows the applicability of non-wetting condition. The effect of solid–liquid interfacial energy on morphological change of monotectic growth has also been discussed. The microstructures of monotectic, eutectic and pure components were taken and have shown their peculiar characteristic features.

The real-time, high-resolution x-ray video microscopy of solidification in aluminum alloys

The directional solidification of thin alloy sheets in a Bridgman furnace has been studied by x-radiography using high-brilliance synchrotron x-radiation in combination with a low-noise, fast-readout camera. Spatial resolutions down to 1.5 μm and a temporal resolution of about 0.15 s have permitted real-time video microscopy of microstructural evolution during columnar and equiaxed dendrite growth and eutectic and monotectic growth. The technique has also allowed for direct observations of important solidification phenomena such as dendrite fragmentation and porosity formation, primarily in aluminium alloys. As a result, insights have been gained into mechanisms of dendrite fragmentation, criteria for dendrite tip kinetics and interface stability during transient growth, and microstructure formation mechanisms during monotectic solidification. The results are expected to be important for validation of dendrite growth models. This paper presents a review of the technique as well as examples of images obtained during solidification of aluminum alloys.

Monotectic Growth: Unanswered Questions

Early solidification experiments in immiscible alloy systems almost immediately led to conflicting findings between investigators. Investigations revealed that several factors usually considered unimportant, especially the interfacial energy relationships between phases, could have a dramatic influence on the types of microstructures produced in immiscible alloy systems. During the 1980s, work concentrated on the influence interfacial energy on microstructure. However, some findings raised new questions. In the mid 1990s and continuing through today, most efforts have focused on modeling the monotectic growth process and on obtaining steady state coupled growth conditions in hypermonotectic alloys. This paper focuses on some of the advances that have been made to date in understanding solidification in immiscible alloy systems and some of the questions that remain to be answered.

The effect of marangoni convection on fibrous monotectic growth

Directional solidification of monotectic alloys can lead, under certain conditions of growth velocity and temperature gradient in the melt, to composite microstructures with a rodlike appearance. For a theoretical description, most often the Jackson and Hunt model of rod eutectic growth is used. A relation between the mean rod distance R and the solidification velocity v, similar to eutectics, is generally observed in experiments. A comparison between theory and experiments always led to discrepancies not yet fully resolved. In the approach presented here, we propose an additional mode of mass transport ahead of the monotectic solidification front, namely, Marangoni convection. The main difference between monotectic and eutectic solidification seems to be the liquid phase state of the (rod) L2 phase growing simultaneously within a nearly perfectly pure solid matrix. We assume that the thermocapillary effect causes convection at the interface of the fibrous liquid phase. This Marangoni convection induces a flow field in the matrix and has an influence on the solute transport if the fluid flow Peclet number is large enough. First, an analytical model is described using a suitably modified convective diffusion equation for the concentration field; and, second, a numerical model is presented. The main result of both models is a new functional relation between fiber spacing and solidification velocity being explicitly dependent on the temperature gradient ahead of the solidification front.

Aligned monotectic growth in unidirectionally solidified Zn-Bi alloys

The solidification morphology of the Zn-Bi monotectic alloy is studied through directional solidification processing. It is shown that the aligned growth depends on the crystallography of the solid-liquid interface. The best alignment of the monotectic liquid phase (L2) occurs when the growth direction is [0001]Zn followed by that of [1010]Zn. Aligned growth could not be observed in other growth directions. A parametric approach is adopted, which incorporates the mechanical stability of the three phases at the growing interface and Derby and Favier’s criterion for monotectic growth, derived from Jackson and Hunt theory of coupled growth, to identify the possible window of growth conditions and phase spacings where aligned growth is possible.

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.

Monotectic Composite Growth with Fluid Flow

In this paper, we present two different models for the description of composite monotectic growth. We focus our interest on the evolution of a fibrous mircostructure with liquid L 2 fibers. We assume that the thermocapillary forces on the L 1 - L 2 interface induce convection in the liquid phases. This Marangoni convection has a strong influence on the transport of solute depending on the local temperature gradient and the Peclet number. In addition, the density differences of the phases also affect the flow field. The resulting flow field changes the transport of solute and therefore the mean undercooling. First we describe an analytical approach for the calculation of the concentration field and the mean constitutional undercooling. We use a series of approximations in order to get some analytically tractable equations. With this extended.lackson and Hunt theory we get some insight into the effect of the flow field on monotectic composite growth. The second model is more acurate. We solve numerically the diffusion equation coupled with the Navier-Stokes equation in the L 1 phase to find the minimal undercooling for a given velocity and temperature gradient. We neglect diffusion and convection in the L 2 fiber in both models. The minimal undercooling defines the mean rod spacing, that could be plotted in a monotectic Jackson and Hunt diagram.

Monotectic composite growth with fluid flow

It is a well-known fluid-mechanical phenomenon that thermocapillary forces induce surface convection on a fluid–fluid interface. This so-called Marangoni convection depends on the variation of the surface energy along the interface. In our present work we focus our attention on the evolution of a fibrous monotectic microstructure with liquid L 2 fibers. We will show, that the Marangoni convection has a strong influence on the transport of solute in front of the solidification front, despite the flow induced by density differences. The resulting flow field affects the constitutional undercooling and therefore the mean undercooling of a monotectic solidification front. In a previous paper we discussed qualitatively the influence of fluid flow on the microstructure evolution of composite monotetic growth (C. Stöcker, L. Ratke, J. Crystal Growth 203 (1999) 582). We introduced an analytical model that takes the density differences of the phases and the surface convection on the L 1–L 2 surface into consideration. With this extended Jackson and Hunt theory for composite monotectic growth we derived a characteristic equation for the inter-rod distance depending on solidification velocity and temperature gradient. In this paper we develop a more accurate model. We solve numerically the diffusion equation coupled with the Navier–Stokes equation in the L 1 phase to find the minimal undercooling for a given velocity and temperature gradient. We derive a Jackson and Hunt diagram and show that the fluid flow leads to a strong dependence of the inter-rod distance on the temperature gradient opposite to eutectic solidification.

Analysis of monotectic growth: infinite diffusion in the L2 phase

The Jackson-Hunt model of eutectic solidification is applied to monotectic solidification in which a liquid (L1) transforms into rods of a different liquid (L2) in a solid matrix. Limiting cases of no diffusion and infinite diffusion (complete mixing) in the L2 phase are considered. An adaptive refinement and multigrid algorithm (MGGHAT) is used to obtain numerical solutions for the concentration field in the L1 phase; this allows consideration of a general phase diagram. Density differences between the three phases, which cause fluid flow, are treated approximately. Specific calculations are carried out for aluminum-indium alloys. Infinite diffusion in the L2 phase has only a small effect on the relationship between interface undercooling and rod spacing.

Solidification structure of monotectic alloys

The formation manner of the monotectic structure is shown from the observation of the evolution of crystallization and growth of CuPb and some aluminium-based monotectic alloys solidified in a free direction and unidirectionally. The morphological change of the monotectic composite structure is discussed in relation to the solid-liquid interfacial morphology at the monotectic growth front as well as the interfacial energy balance between the solid and two liquids in the monotectic reaction. In free-directional solidification, a characteristic monotectic cell with a spherical shape is formed which is unlike dendritic and lamella morphologies. The monotectic cell consists of a spherical solid and a separated L 2 liquid lying along the radii of the solid sphere. In unidirectional solidification, the monotectic structure of AlPb, AlBi and AlIn alloys changes in the following sequence with decreasing growth rate under a constant temperature gradient; random dispersion of L 2 droplets in the aluminium solid matrix → periodic regular array of L 2 droplets → fibrous L 2 composite → aluminium single-phase region without the L 2 phase. In CuPb monotectic alloy the monotectic structure changes with decreasing growth rates as follows: irregularly shaped rod-like L 2 composite → coalesced coarsened discontinuous L 2 composite → periodic banded structure consisting of L 2-rich regions and L 2-poor regions. These morphological transformations of monotectic structure are strongly affected by the ratio of the temperature gradient to the growth rate, the volume fraction of liquid L 2 separated through the monotectic reaction and the interfacial energies between the solid and two liquids at the monotectic growth front.

アルミニウム基合金の偏晶凝固機構

Monotectic Al-Bi, Al-Pb binary alloys and Al-Bi-Pb ternary alloys were solidified unidirectionally in various growth conditions. The alloys were quenched during growth to observe the monotectic growth front morphology. Attempts were made to produce the instability of the solid-liquid interface by adding 0.025 mass%Fe to Al-Bi alloys. Morphology of monotectic composite varies depending on the ratio of temperature gradient G/growth rate R. Random dispersion of L2 droplets forms at G⁄R below 109 Ks/m2, on the other hand, a regularly arrayed fibrous composite structure forms at G⁄R over 109 Ks/m2. Further increase of G⁄R results in the growth of aluminum single phase structure without L2 phase. The addition of Fe increases the critical value of G⁄R for the structural transitions. Morphological change in the monotectic structure of aluminum base alloys is explained by the interfacial energy balance between a solid and two liquids as well as the interaction between liquid L2 phase and monotectic growth front.