Phase selection during solidification of undercooled Ni 70.2Si 29.8 eutectic alloy

High undercoolings have been obtained in bulk Bi95Sb5 alloy melts by the cyclic superheating and cooling technology. The highest undercooling that was achieved in this paper is 121 K. The influence of various processing factors on the undercooling behavior is examined. Undercooling of 121 K leads to the formation of a metastable solid phase with the tetragonal crystal structure. The phase selection and the metastable phase formation have been discussed based on the classical nucleation theory. A criterion that contains the relative melting temperature, the relative molar volume of the solid, the relative structure-dependent factor, and the undercooling has been developed to interpret the formation of the metastable tetragonal phase.

Phase selection during crystallization of undercooled liquid eutectic lead-tin alloys

Transformation kinetics of the metastable bcc phase during rapid solidification of undercooled Fe-Co alloy melts

The article investigates the influence of high cooling rates during quenching from the melt on the formation of metastable phases and the appearance of metastable equilibrium diagrams. It is shown that with increasing cooling rate, both an increase in the existence region of strongly supersaturated solid solutions and the formation of new metastable phases in binary Fe-P and Sb-Zn alloys are observed. Mechanisms of non-equilibrium crystallization during quenching from the melt are proposed, the influence of large supercooling under high cooling rate conditions on changes in the kinetics of occurring crystallization and the emerging competition during non-equilibrium crystallization of congruent and incongruent phases are demonstrated. The validity of the newly formed metastable phase in the Sb-Zn alloy is confirmed by the correctness of the dependencies of specific atomic volumes of equilibrium phases calculated using Vegard's approximation. The lattice parameters of the emerging stable and metastable phases in the alloys are calculated.

Metastable phase formation in undercooled eutectic Ni78.6Si21.4 melts

Adopting glass fluxing and cyclic superheating, high undercooling up to ∼550 K was achieved in bulk eutectic Ni78.6Si21.4 alloy melt. With increasing undercooling, the as-solidified microstructure shows an interesting evolution, i.e., regular lamellar eutectic, coarse directional dendrite, quasi-spherical dendritic colony, fine directional dendrite, fine quasi-spherical dendritic colony, and superfine anomalous eutectic. In combination with different theories for nucleation and growth, the microstructure evolution was analyzed and described using competitions incorporated in rapid solidification of the bulk undercooled eutectic Ni78.6Si21.4 alloy. For undercooling below and above 180 K, Ni3Si, and α-Ni are primarily solidified, respectively. This phase selection can be ascribed to competitive nucleation. As undercooling increases, a transition of the prevalent nucleation mode from site saturation to continuous nucleation was interpreted in terms of competition of nucleation mode. Accordingly, the superfine anomalous eutectic is obtained, due to the substantially increased continuous nucleation rate, i.e., grain refinement occurring at high undercooling (e.g., ∼550 K).

Thermal behavior of the orthorhombic (α) and triclinic (β) polymorphs of BiNbO4 was studied by the methods of high-temperature powder X-ray diffraction (HTPXRD) and differential scanning calorimetry (DCS) in the temperature range 25-1200 °C. The study revealed the sequence of thermal phase transformations and the new high-temperature modification, γ-BiNbO4, which was formed above 1001 °C and existed up to the melting temperature of BiNbO4. The incongruent melting of BiNbO4 was characterized by the formation of the cubic phase with the approximate composition Bi3NbO7. The HTPXRD method was used in this study to evaluate thermal deformations and to calculate thermal-expansion coefficients (TEC) of the three modifications of BiNbO4 (α, β, and γ). The average volumetric TECs of these three modifications were in the range 19-36 × 10-6 °C-1. The triclinic phase β-BiNbO4 demonstrated the highest anisotropy of thermal expansion. α-BiNbO4 was characterized by the minimal TEC and anisotropy, which indicated its greatest stability. The crystal structure of γ-BiNbO4 was determined at 1100 °C using powder data and was refined using the Rietveld method (the α-LaTaO4 structural type, the space group Cmc21, a = 3.95440(3) Å, b = 15.0899(1) Å, c = 5.65524(5) Å, V = 337.458(5) Å3, Rwp = 4.82, RBragg = 3.61%). The methods of thermal analysis and high-temperature powder X-ray diffraction revealed that, during the heating, bismuth orthoniobate underwent the following sequence of phase transitions: α-BiNbO4 → γ-BiNbO4 → β-BiNbO4 and β-BiNbO4 → γ-BiNbO4 → β-BiNbO4 or, at slow heating, β-BiNbO4 → (α-BiNbO4) → γ-BiNbO4 → β-BiNbO4, where γ-BiNbO4 is the high-temperature phase of bismuth orthoniobate.

Growth related metastable phase selection in a 6xxx series wrought Al alloy

Formation of glassy phases and polymorphism in deep eutectic solvents

Phase selection and metastable phase formation during solidification of undercooled alloy melts have been studied using Bi-Sb alloys. Undercoolings larger than 0.2T(1) were obtained for bulk Bi95Sb5 alloy melts. The factors which influence the undercooling behavior are discussed. Microstructures of the samples obtained at different undercoolings reveal that undercooling can play an important role on the formation of the final phase structure. The specific heat of the undercooled Bi95Sb5 alloy melt is measured by DSC. Results of X-ray diffraction on the samples show that a metastable phase with tetragonal structure, which is similar to that existing stable under high pressure, can form if the undercooling of Bi95Sb5 is 121 K.

The development of infant milk fat similar in its structure and composition to that present in human milk receives a considerable amount of attention and has become a challenge for the world’s food industry. The aim of this work was an attempt to obtain structured lipids resembling human milk fat (HMF) by modifying mixtures of lard (L) and rapeseed oil (RO) catalyzed by Lipozyme RM IM. The enzymatic reactions were carried out at 70 °C for 4, 8, and 24 h. The second objective was to study the properties of obtained substitutes of human milk fat (HMFS) using gas chromatography, thin layer chromatography, and differential scanning calorimetry (DSC) methods. Chromatographic methods were used to determine the composition of fatty acids (FAs) and the regiospecific structure of triacylglycerols (TAG) of obtained HMFS. DSC methods were used to establish the oxidative stability of HMFS and to calculate their kinetic parameters of oxidation. Structured lipids obtained from a mixture of L and RO after 4 h interesterification in 70 °C were characterized by the closest FA content and their distribution in TAG to HMF. Interesterification influenced the decrease of the induction time of obtained structured lipids.

ABSTRACTWe have investigated the solid-state reaction of Ti/Al multilayer films by x-ray diffraction (XRD) and differential scanning calorimetry (DSC), with focus on the early stages of the reaction provided by samples with pair thicknesses in the range 5 - 40 nm. This reaction, which results in formation of TiAl3 with metastable Ll2 structure, can be modeled by a nucleation and growth process on the basis of the Johnson-Mehl-Avrami theory, with a reaction-order parameter n ≈1. These observations indicate the significance of nucleation barriers even at early stages of solid-state reactions, and suggests that the phenomena of phase selection and formation of metastable phases can result from the presence of nucleation barriers.

Metastable phase formation during mechanical alloying of AlGe and AlSi alloys

Pressure‐induced amorphization is a new advanced subject. It is found that the formation of metastable phase is the prerequisite of that of amorphous phase. The metastable phase is formed only in the system which volume of phase transformation decreases during heating. For these systems, the pressure can accelerate nucleation, restrain diffusion, decrease the critical cooling rate for the formation of metastable phase. There exists a critical pressure for the formation of metastable phase. In the end, the metastable phase transforms to amorphous phase.

This paper is concerned with observations on the preparation of phases which may exist in the system CaCO 3 -H 2 0 in the presence o f ‘additives’, with a qualitative explanation of the appearance of unstable phases under such conditions. It has been found that the tendency for metastable phases of calcium carbonate to be precipitated from aqueous solutions is increased not only by raising the rate of precipitation but also by increasing the concentration of certain added substances. Thus in the presence of small amounts of Calgon* or magnesium-ion, and in saturated sodium chloride solution in the presence of magnesium-ion, a range of products is obtained according to the conditions of precipitation, particularly the stirring and the temperature. Calcite (the stable phase), aragonite, vaterite (/^-calcium carbonate) and calcium carbonate hexahydrate have been prepared In this way together with a new hydrate, CaCO 3 .H 2 O . By allowing reaction mixtures containing additive to stand until the first-formed gel has crystallized, metastable phases may be produced with considerable regularity. Metastable phase diagrams can be constructed which show the dependence of the phases on conditions of precipitation, e.g. sodium carbonate concentration and concentration of additive. By exploration using such diagrams, all the previously mentioned phases except vaterite have been prepared in the presence of magnesium-ion, and in the presence of Calgon all but aragonite have been identified. This suggests that to some extent the action of the additives is specific. In stirred conditions of reaction, calcium carbonate may be grown, without fresh nucleation, on seed crystals of calcite and vaterite at a much faster rate in the presence of limited amounts of Calgon than in its absence. At relatively high concentrations other phases may appear and growth of the calcite and vaterite ceases in spite of the presence of seed crystals. Compounds containing calcium carbonate formed in the presence of magnesium-ion or Calgon have been shown by analytical methods to contain magnesium or phosphorus in appreciable quantities. The presence of the additive leads to modification in the crystal habit of calcite and of the hexahydrate. It has also been noted that distortion of X-ray patterns of the crystalline species is common. This is consistent with absorption of the additive in the distorted phase. Absorption of additive seems, therefore, to be important in influencing nucleation and the formation of metastable phases. These observations are explained on the grounds that germs of the crystalline phases are subject to varying degrees of distortion depending on the constitution of the solution. The growth is therefore hindered to different degrees in different cases. In the limit the effect of distortion may be such that growth of nuclei of certain types may be inhibited, and there is a corresponding change in the conditions for the spontaneous crystallization from solution of the various phases.