Solidification behaviour of binary organic eutectic alloys

The phase diagrams of binary organic faceted-faceted systems of p-dibromobenzene with acenaphthene, p-chloronitrobenzene and acetanilide show the formation of a simple eutectic with 0.53, 0.61, and 0.64 mole fraction of p-dibromobenzene, respectively. Crystallization data, determined by the capillary method, suggest that the eutectics of p-dibromobenzene with acenaphthene and p-chloronitrobenzene solidify with alternate nucleation mechanism and the crystallization of the eutectic of p-dibromobenzene-acetanilide system takes place by the side-by-side growth of the phases involved. With a view to throw light on the thermodynamic behaviour of eutectic alloys of non-metals, heats of fusion of their transparent organic analogues and their eutectics were determined, and using these values different thermodynamic parameters were calculated. The microstructures of the eutectics show their typical characteristic features.     

Physical chemistry of binary organic eutectic and monotectic alloys; 1,2,4,5-tetrachlorobenzene−β-naphthol and 1,2,4,5-tetramethylbenzene-succinonitrile systems

Phase diagrams of 1,2,4,5-tetrachlorobenzene–β-naphthol and 1,2,4,5-tetramethylbenzene–succinonitrile systems which are organic analogues of a nonmetal–nonmetal and a nonmetal–metal system, respectively, show the formation of a simple eutectic (melting point 103.7°C) with 0.71 mole fraction of β-naphthol in the former case and a monotectic (melting point 76.0°C) with 0.07 mole fraction of succinonitrile and a eutectic (melting point 52.5°C) with 0.97 mole fraction of succinonitrile in the latter case. The growth behaviour of the pure components, the eutectics and the monotectic studied by measuring the rate of movement of the solid–liquid interface in a capillary, suggests that the data obey the Hillig–Turnbull equation, v=u(ΔT)ⁿ, where v is the growth velocity, ΔT is the undercooling and u and n are constants depending on the nature of the materials involved. From the values of enthalpy of fusion determined by the DSC method using Mettler DSC-4000 system, entropy of fusion, interfacial energy, enthalpy of mixing and excess thermodynamic functions were calculated. The optical microphotographs of pure components and polyphase materials show their characteristic features.     

Solidification behaviour of organic eutectics and 1:1 addition compound: p-Phenylenediamine-resorcinol system

The phase diagram of p-phenylenediamine-resorcinol system, determined by thaw-melt method, shows the formation of a 1:1 molecular compound and two eutectics. The linear velocity of crystallization, determined by measuring the rate of movement of growth front in a capillary, indicates that crystallization data obey Hillig-Turnbull equation: Where u and n are constants and ΔT is undercooling. X-ray diffraction data of the molecular compound, eutectics and pure components infer that these eutectics are not simply the mechanical mixture of the two components and there is preferential ordering of atomic planes during their formation. The infrared studies carried out on this system reveal the intermolecular hydrogen bonding between two components forming the molecular compound. Using experimental values of heats of fusion, entropy of fusion and excess thermodynamic functions were also calculated.     

Microstructure and spacings of directionally solidified metal-semiconductor eutectic alloys

Directionally solidified metal-semiconductor eutectic alloys, representing nonfacetted facetted eutectics, show a variety of microstructures caused by the great differences in semiconductor volume fraction (Zn Ge 7,8 %; Ge ZrGe₂ 98.6 %), the influence of growth rate and temperature gradient at the solid-liquid interface. The Al Si, Ag Si, Ag Ge, Zn Ge, Cu₃Si Si, NiGe Ge, CoGe₂ Ge, Mn₃Ge₂ Ge, FeGe₂ Ge, Mn₁₁Si₁₉ Si, Cu₃Ge Ge, Ge TiGe₂ and Ge ZrGe₂ eutectics habe been investigated. The following three models are applicable for the calculation of the spacings as a function of growth rate and temperature gradient at the solid-liquid interface to certain microstructures: diffusion-determined growth, branchinglimited growth and phase-lead-determined growth.     

Metastable microheterogeneity of melts in eutectic and monotectic systems and its influence on the properties of the solidified alloy

Experimental results that are the basis for the existence of metastable microheterogeneities in melts of eutectic and monotectic systems are summarized. Among these, recent results of small angle neutron scattering experiments are discussed in some detail. It is shown that the influence of a homogenizing heat treatment of a melt on the structure of crystalline castings and amorphous ribbons obtained after its solidification is very important.     

Formation of atomic solution in liquid eutectic alloys

The influence of Pb, In, Bi and Ge on structure of tin within 0–12.5 at.% concentration range has been studied by means of X-ray diffraction method. Samples were prepared by melting in vacuum furnace filled with pure argon from ingots of Sn(99.999%), Pb(99.99), In(99.999), Bi(99.99), Ge(99.999) and checked by means of X-ray microanalysis The structure factors and pair correlation functions were analyzed. Concentration dependencies of structure parameters for liquid Sn-enriched melts in binaries with eutectic point are described. It is shown that atomic arrangement in tin-based molten alloy deviates from random atomic distribution within concentration range where these alloys can be considered as diluted solutions. Limited solubility in solid state in eutectic Sn-based systems is a precursor to deviation from random atomic distribution in liquid state.     

Solidification behavior and microstructural evolution of the Cr‐Nb eutectic alloy

A series of microstructures including fully coupled eutectic, both α‐Cr and β‐Cr₂Nb primary dendrites embedded in eutectic and only β‐Cr₂Nb primary dendrites plus eutectic were observed in the arc‐melted Cr‐Cr₂Nb eutectic alloy. By employing EPMA analysis performed at the eutectic regions, the eutectic composition of the Cr‐Cr₂Nb system was indicated to contain less than 18 at.%Nb. Based on the solidification phase selection involving phase competitive nucleation and growth, the α‐Cr phase was predicted to be the primary nucleating phase and the β‐Cr₂Nb the primary growing phase. Under large undercooling conditions, the solidification process was controlled by nucleation, which led to the formation of α‐Cr primary particles. With the decrease in undercooling, the solidification process developed into growth controlling, which caused the occurrence of β‐Cr₂Nb primary phase since the actual solidification path of the alloy lay within the hypereutectic region. The explanation was confirmed by the experimental composition analysis.     

Solidification behaviour of organic nonlinear optical crystals

A representative rather than exhaustive review has been given on the solidification behaviour of organic nonlinear optical crystals. Many of the developments reported here represent quick experimental responses to the rapid theoretical advances made in the field of nonlinear optics. The article is divided into four parts. The first part deals with the characteristics of the organic nonlinear materials and need for large single crystals. The second and third parts are devoted to the thermophysical properties and experimental studies on crystallization processes. Here, recent work by authors is reviewed on the purification of source material, solidification behaviour and crystal growth. Various interactions between morphology, microsegregation, defects, growth velocity and temperature gradient are discussed. Specific examples of substituted anilines are presented. In the fourth section a stronger interaction is suggested between optical physicists and crystal growers to understand the problems of inhomogeneity, stress, damage threshold and doping in the organic crystals.     

Solidification behaviour of BaB2O4 melts

Experimental evidence has been obtained that both α- and β-phase BaB₂O₄ material can be grown directly from pure melts. Growth of α-phase material onto a platinum rod is initiated by α-phase structural centers present in the melts which are stable at temperatures around or just above the melting point. Overheated melts in which solid nuclei are destroyed have modified freezing characteristics and form β-phase material. β-phase material can be produced onto a platinum rod at temperatures well above the reported phase transition temperature.     

On the mechanism of lamellar spacing adjustment in eutectic alloys

Well controlled undirectional solidification experiments with continous increased rate were performed for eutectic alloys in the systems Ag-Cu, Al-Zn, Al-Cu and Al-Ni. In the Ag-Cu and Al-Zn alloys it was observed that discontinous changes in the lamellar spacing occured at low solidification rates. At high solidification rates such discontinuities could not be observed. Anyway the lamellar spacing always larger than observed at constant rate experiments. In Al-Cu alloys discontinous changes in the lamellar spacing with increasing solidification rate were also observed. However, after each change the lamellar spacing increased gradually up to a lamellar distance of 20% lower than before the abrupt decrease. All-Ni eutectic alloys gives a rod like eutectic structure. In this alloy no discontinous changes could be observed. A theoretical analyses of the observation based on well known theoretical models are discussed.     

Density studies of liquid alloys SnAg and SnZn with near eutectic compositions

The density values for SnAg and SnZn alloys with near eutectic compositions were determined by the gamma-absorption method both in the liquid and solid states. Sn-rich melts of the AgSn system were found to be microheterogeneous, whereas SnZn melts demonstrate the behavior typical for true solutions.     

Structural characterization of anthracene-picric acid eutectic system

The eutectic composition has been derived from phasediagram studies and the nature of eutectics has been established by thermal and spectral studies. Diffraction studies reveal the establishment of preferential ordering and the crystalline character of eutectics.     

Investigations on binary p-nitrophenol and trichloroacetic acid eutectic system

p-Nitrophenol and trichloroacetic acid eutectic system has been investigated using phase-diagram, undercooling, linear velocity of crystallization and viscosity measurement methods. Microstructure of the eutectic indicates that it possesses faceted-faceted (f/f) morphology. Phase equilibrium data have been subjected to thermodynamic analysis, and G^E and S^E values for each mixture as a function of composition and temperature have been determined. Thermodynamic analysis shows that the system is non-ideal.     

Structural properties of liquid Au–Si and Au–Ge alloys with deep eutectic region

Au–Si and Au–Ge alloys have a deep eutectic region around 19 at.% Si and 28 at.% Ge, respectively. The metallic glass was prepared by quenching around 25 at.% Si near the eutectic composition for the Au–Si alloys [2] which has stimulated many researchers to study on the structural properties of such liquid alloys. However their results were different from each other, which seems due to the experimental difficulties from heavy absorption of both X-ray and neutron beams by Au atom in diffraction experiments. X-ray diffraction measurements have been carried out for liquid Au–Si and Au–Ge alloys around the eutectic region by the transmission method using high-energy X-rays to investigate the atomic arrangements in the liquid state. From the temperature dependence of the observed structure factors, the partial pair correlation and the detailed atomic arrangements will be discussed by Reverse Monte Carlo analysis. The produced atomic arrangements around the eutectic region suggest a substitutional structure and also an increase in liquid density with decreasing temperature.     

The relaxation behaviour of strenght and fracture toughness of amorphous metal-metalloid alloys

The effect of an annealing treatment on crack and fracture kinetics and on mechanical properties of Fe₄₀Ni₄₀P₁₀B₆ and Fe₅Co₇₀Si₁₅B₁₀ amorphous ribbons — both pure and with defined impurities — was investigated by in-situ tensile experiments in SEM. The effect of relaxation processes on tensile strength, microhardness and fracture toughness was analysed. A result of technological interest is the increase of fracture toughness after an annealing treatment in the temperature range T_ann. = (0.55 … 0.6) · T_cryst.. This effect will be enhanced by defined chemical impurities, and it can be well interpreted by a model of fracture toughness variation during a temper process. In this model the deformation behaviour cannot be described within the scope of a homogeneous solid, but it is determined by a relationship between closely localized deformation instabilities.     

Characteristics of the binary faceted eutectic: benzoic acid – salicylic acid system

The consistent symmetry relations computed from the heterogeneous nucleation data of the non‐ideal benzoic acid – salicylic acid eutectic system verifies the validity of nucleation theory. The kinetics of crystal growth from the molten state of the system follows the dislocation mechanism. Anomalous behaviour of both viscosity and activation energy for the eutectic melt confirms the essence of specific interactions animating molecular clusters rich in predominating eutectic phase. Micromorphology of the system obeys the Hunt‐Jackson model. The plot between a mechanical property and variable anisotropic growth velocity for the eutectic composite evidentially complies with the Weibull probability distribution curve. The curve is perused with two cut‐off points corresponding to a lower strength limit in the slow and fast growth regions and an upper strength limit in the moderate growth region. The latter aspect reveals the theoretical strength of the eutectic crystallites. The strength‐growth relationship explicated thereby is linear, optimum and linear respectively in the slow, moderate and fast growth regions of solidification. The moderate anisotropic growth (∼2.96 X 10^‐7m³s^‐1) is of greater interest by virtue of its attribute to unifying and organizing the crystallites parallel to each other in the growth direction. The eutectic composite material obtained by this process attains remarkable superiority in the mechanical properties over its isotropic growth in an ice bath (∼273 K), and its constituent phases. The co‐relation between excess thermodynamic functions indeed predicting the liquidus character and the microstructural parameters inevitably structuring the morphology is presented. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Influence of various conditions of convective mixing of melt on the structure and magnetoresistance of eutectic InSb-NiSb alloys obtained by directional crystallization

The microstructures are analyzed of the eutectic alloys solidified in the quasi-binary InSb-NiSb system under different conditions of convective mixing of melts grown in an ampoule by the Czochralski and directional crystallization methods under earth and microgravity conditions. It is established that growth of the phases in directional solidification of the InSb-NiSb eutectics is determined not only by the diffusion mechanism of the component redistribution at the crystallization front but also by the diffusion-convection mechanism. The comparative analysis of the influence of the crystal microstructure on the value of the magnetoresistance, R _M , is performed for the first time. It is shown that the R _M value of directionally crystallized InSb-NiSb eutectics depends on the density of the NiSb needles, disorder of the eutectic structure, and electrophysical parameters of the matrix.