Pressure-induced amorphization mechanism of Cd-Sb system

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

Six multilayered films with overall compositions of Co10Zr80Nb10, Co10Zr70Nb20, Co20Zr70Nb10, Co30Zr60Nb10, Co20Zr60Nb20and Co10Zr60Nb30were first prepared by alternatively depositing high pure metals onto the NaCl single crystal substrates and then irradiated by 160 keV xenon ions with the doses ranging from 2×1014Xe+/cm2to 9×1015Xe+/cm2. It turns out that several metastable phases, e.g. amorphous phase, were formed in these metallic multilayered films upon irradiation. Meanwhile, some interesting phase transformations were also observed with increasing irradiation doses. Besides, the phase formation and transformation mechanism of non-equilibrium alloy phases are discussed in terms of thermodynamics of solids and atomic collision theory. KEYWORDS: Zr-Co-Nb system, metastable alloys, phase transformation, ion beam irradiation

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.

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 during crystallization of undercooled liquid eutectic lead-tin alloys

A time-resolved x-ray diffraction technique is employed to monitor the structural transformation of laser-shocked bismuth. Results reveal a retarded transformation from the shock-induced Bi-V phase to a metastable Bi-IV phase during the shock release, instead of the thermodynamically stable Bi-III phase. The emergence of the metastable Bi-IV phase is understood by the competitive interplay between two transformation pathways towards the Bi-IV and Bi-III, respectively. The former is more rapid than the latter because the Bi-V to B-IV transformation is driven by interaction between the closest atoms while the Bi-V to B-III transformation requires interaction between the second-closest atoms. The nucleation time for the Bi-V to Bi-IV transformation is determined to be 5.1±0.9 ns according to a classical nucleation model. This observation demonstrates the importance of the formation of the transient metastable phases, which can change the phase transformation pathway in a dynamic process.

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

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.

This paper presents the results of a study of crystallization processes in amorphous metallic alloys of various compositions. Structural changes within the amorphous state prior to crystallization, the formation of a heterogeneous amorphous phase, and the multistage transition to an equilibrium state are considered. The mechanism of nanocrystal nucleation in ordered regions and the simultaneous formation of stable and metastable phases (including quasicrystalline ones) are discussed. Examples of changes in the crystallization mechanism (eutectic crystallization instead of polymorphic) and structure-property correlations are provided.