Undercooling and metastable phase formation in a Bi95Sb5 melt

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

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

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

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.

The as-solidified microstructures of Zr 3 Al-Nb alloys with varying Nb concentration, viz., Zr 3 Al, Zr 3 Al-3Nb, and Zr 3 Al-10Nb, have been investigated and the phases occurring in these alloys have been identified in terms of their crystal symmetry and their chemical compositions. The formation of various phases is influenced by solute partitioning. With increasing Nb concentration, the extent of solute partitioning increases, resulting in formation of new metastable phases. The phases that solidify first are rich in Al and lean in Nb, whereas the phases that solidify last are rich in Nb and lean in Al. The formation of these phases has been explained in terms of the driving force for nucleation. For this purpose, the free energy for nucleation has been computed for all pertinent binary intermetallics phases by using thermodynamic data available in the literature. Using solution thermodynamics, it has also been shown that the liquid phase in the Zr-Al-Nb system should exhibit a miscibility gap, and the boundaries of the miscibility gap have been estimated by invoking the regular solution model.

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.

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.

In this work, the glass-forming ability (GFA) of Cu-Hf binary alloys was systematically studied and some metastable phases were observed in the rapidly quenched samples. The microstructure of a Cu 50 Hf 50 as-cast rod between the surface and the central region was carefully examined, and the composition of the unknown metastable phase was identified to be about Cu 42 Hf 58 . The formation mechanism of metastable phases and their effect on the GFA of Cu-Hf binary alloys was also investigated. Taking the metastable phases into account, the parameter γ* was revised, and its prediction of the GFA of Cu-Hf binary alloys was found to be roughly in accordance with the experimental results.

The thermal spike effect dominates rapid heating and quenching at the nanoscale in ion beam assisted materials surface phase engineering. This phenomenon often leads to the formation of metastable nanocrystalline phases in the resulting micro-/nanostructures. However, such a non-equilibrium process cannot be accurately described by the conventional Fourier thermal spike model. In this study, a non-Fourier model is proposed to elucidate the dynamics of heat diffusion in sub-keV Cu ion beam sputter deposition. The effects of beam density and energy on the non-equilibrium thermal and stress fields are quantitatively investigated. The influence of high stress field on nanocrystalline Cu phase content is also studied. It is found that the energetic Cu ion beams produce a rapidly oscillating stress field with a maximum of ∼ 10 GPa within 50 ps, which significantly constrains the growth of nanocrystalline Cu phase at a threshold of ∼ 4 GPa. This understanding provides new insights into the formation of metastable nanocrystalline phases in the fabrication and modification of surface micro-/nanostructures using ion beam assisted techniques.

Ag–Pd alloys are widely used as thick-film conductors and are potential alternatives to the expensive Au bump. In this work, because Sn is the primary element in solders, we investigated Sn/Ag–Pd interfacial reactions at 250°C as a means of assessing the reliability and evaluating reflow reactions at joints between solder and Ag–Pd conductor contacts, and in the Ag bump combined with the solder cap. The experimental results showed that Sn/Ag–Pd interfacial reactions at 250°C are different from those of Sn/Ag and Sn/Pd. A metastable Sn–Ag–Pd ternary phase is formed when the amount of Pd added is 20–40 at.%. Because, in commercial applications, at least 20 wt.% Pd (~20 at.% Pd) is used in Ag–Pd alloys to eliminate the silver-migration phenomenon, assessment of the reliability of Ag bumps and the soldered joints of Ag–Pd thick film hybrid circuits must be based on Sn/Ag–Pd interfacial reactions, not those of Sn/Pd and Sn/Ag.

The superconducting properties of thin films made by mixing alternating layers of Au and Si using ion-beam bombardment correlate with the formation of metastable metallic phases in what is otherwise a simple eutectic system. Transmission-electron-microscopy measurements reveal the superconducting phases to be amorphous. Compound formation and the nature of Au-Si bonding in these metastable phases are demonstrated from x-ray photoelectron spectroscopy and from a previous study of x-ray-absorption spectroscopy. After mixing with a beam of Xe ions, multilayered films with an average nominal composition ${\mathrm{Au}}_{\mathit{x}}$${\mathrm{Si}}_{1\mathrm{\ensuremath{-}}\mathit{x}}$, where x=0.2, 0.4, 0.5, 0.72, and 0.8, exhibited superconducting transition temperatures in the range 0.2--1.2 K. A double transition feature in the magnetic field dependence of the resistivity is attributed to the formation of more than one metastable metallic phase in the same sample as the ion dose increases.

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).

Formation of glassy phases and polymorphism in deep eutectic solvents